For many decades it was believed that cats could transmit diphtheria to people. This led to many cats being killed if a family member contracted the disease. Experiments in the 1920s showed that, cats did not contract or transmit the disease. Diphtheria-like symptoms in cats are caused by feline-specific pathogens. An unwell-looking cat and a person developing diphtheria were coincidental. Interestingly, the bacillus that causes diphtheria only does so when the bacteria itself is infected by a bacteriophage.

DIPHTHERIA SPREAD BY CATS
New York Times, July 9, 1888 (reprinted from the London Sanitary record.)

Domestic animals have often not only suspected by found guilty of spreading infection. In his report on the recent sustained prevalence of diphtheria in Enfield, Dr Bruce Low of the Medical Department of the Local Governance Board incidentally states that during the continuance of the epidemic cats were observed to suffer in considerable numbers from illness, and in December 1887, and January, 1888, there was a large mortality among those animals, so much so that the attention of the dust contractor was directed to it. He stated that never in his previous experience had he seen so many dead cats in the dust heaps. Some households, seeing their cats ill, destroyed them. Though there were no known cases of diphtheria occurring in the practice of the veterinary surgeons at Enfield, yet they saw many cases of "influenza" at this time among the animals. The following is an illustration of the possible connection between diphtheria in children and in cats: A little boy was taken ill with what turned out ultimately to be fatal diphtheria. On the first day of his illness he was sick and the cat, which was in the room at the time, licked the vomit on the floor. In a few days (the child meanwhile having died) the animal was noticed to be ill and her sufferings became so severe, and so similar to those of the dead boy, the owner destroyed her. During the early part of its illness this cat had been let out at nights in the back yard as usual. A few days later the cat of a neighbour who lived a few doors further off was noticed to be ill. It had also been out in the back yards at night. The second animal, which however recovered, was the pet and playfellow of four little girls, who grieved at the illness of their favourite, nursed it with great care. All four girls developed diphtheria, their mother being convinced that they cat it from the cat; and, indeed, no other known source of contact with infection could be discovered. It is easy to imagine cats catching an infectious disease like diphtheria when n we remember how often milk and other unused food from the sick room is given to the cat, or by some people thrown out into the back yard for the benefit of their neighbors' cats if they have none of their own. It is a frequent occurrence to see children carrying cats in their arms, and even kissing them. it is obvious that if the cat were ill with diphtheria the children under such circumstances would almost inevitably contract the disease.

THE ETIOLOGY OF DIPHTHERIA.
Birmingham Daily Post, 27th May 1890

At the ordinary meeting of the Royal Society, a paper entitled "A Contribution to the Etiology of Diphtheria." was read by Professor Klein, F.R.S. It contained the substance of a research which had been undertaken for the Medical Department of the Local Government Board, and was communicated to the society with the permission of the medical officer. The paper commenced by stating that the microbe of diphtheria had been first described Klebs in 1883, and first grown in artificial cultures by Loeffler, but that the author, in the report of the medical officer of the Local Government Board for 1889-90, had shown that two kinds of bacilli occur in diphtheritic membranes, the second of which is the more virulent of the two. This more virulent bacillus acts very powerfully upon guinea-pigs by subcutaneous injection, producing a local tumour which precisely resembles, both in pathology and in microscopic section, the diphtheritic membrane of the human subject. In human diphtheria the bacillus is present only in the diphtheritic membrane, but neither in the blood nor in the diseased viscera; and the same holds good with inoculated guinea-pigs. In subcutaneous inoculations with artificial cultures, although it causes in these animals disease and death, and although the lungs, intestines, and kidneys are greatly congested, the diphtheritic bacillus remains limited to the seat of inoculation; from whence it has been concluded that the diphtheritic membrane is the sole seat of multiplication of the bacillus, but that this produces a chemical poison which is absorbed into the system, and which causes the generally diseased condition, and often death. Roux and Yersin have separated from artificial broth cultures the chemical products alone, and by injection of the latter into guinea-pigs have produced a general effect. In this year's report of the medical officer to the Local Government Board the author has shown that such injection of cultures into guinea-pigs produces an active multiplication of the bacilli at the seat of injection, and that they can be obtained in pure culture on gelatine both from the local tumour and from the nearest lymph glands.

On various occasions during the last three years the author has received information from health officers of a curious relation existing between a disease in cats and human diphtheria; such that a cat or cats were taken ill with a pulmonary disease, and when ill were nursed by children, and then these latter sickened with well-marked diphtheria. Or children were taken ill with diphtheria, and, either at the same time or afterwards, the cat or cats sickened. The disease in the cat was described as an acute lung trouble; the animals were quiet, did not feed, and seemed unable to swallow. In some cases they recovered, in others they became emaciated, while the lung trouble increased, and they ultimately died. In one instance the cat malady, occurring where children were soon after attacked by diphtheria, was of a widespread nature; and post-mortem examination of the affected cats showed severe lung disease, broncho-pneumonia, and large white kidneys due to fatty degeneration of the entire cortex; a similar condition being also met with in the human subject as an effect of diphtheria. Subcutaneous inoculations of cats were carried out with particles of fresh human diphtheritic membrane and with cultures of the diphtheritic bacillus, producing a local diphtheritic swelling at the seat of inoculation and a general visceral disease. In the cases in which death followed after a few days the lungs were found much congested; when death followed after one or more weeks the lungs showed broncho-pneumonia, and the kidneys were enlarged and white, the cortex being in a state of fatty degeneration. If the disease lasted beyond five to seven days both kidneys were found uniformly white in the cortex; if of shorter duration the fatty degeneration was sometimes only in patches. Although in these experiments the bacillus was recoverable by cultivation from the swelling at the seat of inoculation, no bacilli were found in the lungs, the blood, or the kidneys; and hence the conclusion is justified that, as in human diphtheria and in that produced by the inoculation of guinea-pigs, so in these experimental cats the visceral disease was a result of the action of a chemical poison, produced by the growth of the bacillus at the seat of inoculation. From this it is seen that the similarity between the natural and the artificial disease in the cats very great, and the question that arises is as to the manner in which the cat either receives or imparts the diphtheritic contagium in the natural disease. This natural disease in the cat is in its symptoms and pathology a lung disease, and it is reasonable to suppose from analogy that the lung is the organ in which the diphtheritic process in the cat has its seat. The microscopic examination of the diseased lungs of cats dying from the natural disease bears out this supposition, the correctness of which has also been proved by direct experiment. Broth culture of the special bacillus was introduced into the windpipe in cats without injury to the mucous membrane. The animals became ill with acute pneumonia, and on post-mortem examination, from two to seven days later, there was found extensive pneumonia and fatty degeneration of the kidneys. The air passages contained an exudation like that of human diphtheria, and the bacilli were present in large numbers.

During the last ten or twelve years several epidemics of diphtheria have been traced to milk, but the way in which the milk became contaminated with the diphtheritic virus was not ascertained, although the evidence was very strong that it had not been from a case of the disease in the human subject. The cows which yielded the milk were not reported to be unhealthy, except for having sores or chaps on their teats. The author has made experiments which appear to him to throw much light upon these outbreaks. Two perfectly healthy milch cows, which had been kept under observation for ten days prior to the experiment, were inoculated with a broth culture of the bacillus derived from human diphtheria. On the second and third days there was a soft tender swelling at the place of inoculation, which reached its maximum at the end of a week and then gradually became smaller and firm. The animals had a raised temperature, and left off feeding on the second or third day, then to all appearance recovered. But on the eighth or tenth day they were attacked by slight cough, which gradually increased. Both became emaciated; one died on the fifteenth day, the other was killed (being very ill) on the twenty-fifth day. During the illness both animals had an eruption on the teats and skin of the udder, which appeared in successive crops. From one of the cows on the fifth day milk was drawn from a healthy teat, the outside of the teat and the milker's hand having first been thoroughly disinfected. From this milk cultivations were made, and it was found that thirty-two colonies of the diphtheria bacillus, without any contamination, were obtained from a single cubic centimetre. Contrary, therefore, to what happens in the guinea-pig and in the cat, the diphtheria bacillus passes from the seat of inoculation into the system of the cow, and makes its appearance in the milk. The presence of the bacillus in the eruption on the udder was also demonstrated, both by microscopic examination and by experiment. Two calves were inoculated with matter from this eruption, and both developed a similar one, besides becoming affected with severe broncho-pneumonia and fatty degeneration of the kidney. It therefore appears that a definite disease can be produced in the cow by the diphtheria bacillus, and the contamination of the milk shows that the bacillus, as such, invades the general system.

At the beginning of the month of April two cats died at the Brown Institution, after having been ill for several days, with symptoms like those of natural cat diphtheria. Between the beginning of April and the beginning of May fourteen cats became similarly affected, some more severely than others, and some died with the characteristic morbid changes. This epidemic, as it may be called, commenced with the illness of the first two cats about the end of March; and the question arises as to how the disease originated in these two animals. No cats had been ill in their shed, and the two affected ones were healthy when received at the institution some weeks before. But during the latter half of March there were in the stables of the institution two milch cows ill with diphtheria induced by inoculation with the human diphtheria bacillus - in fact, the two cows already referred to. The diphtheria bacillus was found in the milk drawn from one of these animals on the fifth day after inoculation, and orders were given to the attendant that the milk of both cows was to be thrown away. This order was not obeyed, for part of the milk was given to the two cats above mentioned, and they sickened as described within a day or two afterwards. It ought to be mentioned that the man in attendance on the cows had also charge of the cats; but, in view of the fact that he was himself free from the disease, the possibility of his having conveyed it from the cows to the cats may be disregarded.

[Footnote: It is recognised by veterinary authorities that cats suffer from illness with symptoms which in some respects are analogous to those of human diphtheria but which are certainly not cases of diphtheria or due to B. diphtheriae. - Cats and Human Diphtheria, William G. Savage, Journal of Hygiene (Lond). 1920 Feb; 18(4): 448–462, reproduced in full on this page. ]

CATS CARRY DIPHTHERIA.
The New York Times, January 7, 1894 (reprinted from The Spectator)

Horses, and even sheep, occasionally suffer from a form of sore throat resembling diphtheria, and Dr. Turner, in a. report made to the Local Government Board in 1887, records his surprise at discovering the number of diphtheria victims in country districts, whose occupation had been the management of animals. Commonly they were grooms, blacksmiths, and shepherds. But the animal now almost universally suspected is the household cat. The old saying that if the cat had a cold it would “go through the family,” seems true of a. far more serious charge. The instances collected are so many that it seems impossible to doubt the connection between diphtheria in man and a very similar disease, with almost identical symptoms, in cats, as well as with another, which, as in the case of the cow, is less obvious and less serious in the animal. In addition to the cases reported by Dr. Bruce Low, at Hendon, and quoted in The Times, cases of cat diphtheria have been noticed in Hertfordshire, at Aldershot. Farnham, and Yately, in Surrey; at Moulton, in Suffolk, where the conditions were reversed, and milk used by a sick child gave diphtheria to the cat; and at Petersfield, in Hampshire. In this case, the evidence was very clear. The children in a row of cottages were attacked by diphtheria, and the cats soon contracted a similar disease.

Chicken diphtheria is now thought to be biologically distinct from the human disease. But the coincidences of the two occurring simultaneously are very strange. In the Greek island of Skiatos, for instance, where diphtheria was unknown, a sudden epidemic attacked the town, which was traced to a house to which turkeys had been brought from Salonica. All these turkeys but one had died from fowl diphtheria, and the survivor was found suffering from paralysis of the feet. But the part played in the spread of diphtheria by animals must be small in comparison to the direct contagion conveyed by human beings to each other.

CATS AND DIPHTHERIA - Evidence that the Animals May Spread the Infection
From The British Medical Journal, reprinted by The New York Times, July 14, 1895

The cat is acquiring a bad reputation in Brighton. Dr, Newsholme, in his recently issued quarterly report, devotes a separate section to a description of an outbreak of suspicious illness among cats in a particular district of the town, and to a warning against keeping cats which are suffering from certain enumerated symptoms. Dr. NewshoIme‘s attention was called to cats by the fact that in the neighborhood between Elm Grove and Southover Street- apart of Brighton inhabited almost solely by the laboring classes - there had been notified a group of cases of diphtheria in the course of a single fortnight, which pointed distinctly to the operation of some local cause.

The patients comprised both children and adults. They did not attend any particular school; there was no community of milk supply; personal infection from case to case could not be traced, and no sanitary defects were found in the affected houses. But in each instance there was a history that the household cat had been ill, and in several families the child who was specially fond of the cat was the sole victim of diphtheria The illness of the affected cats had not been carefully observed, but it included one or more of the following symptoms: a bad cough, difficulty in swallowing, discharge from the nose, and marked emaciation. In some of the houses the cat had simply been observed to be wasting; and in several instances the head of the household volunteered the surmise that “the cat had been poisoned."

In one house, in the centre of the affected neighborhood, nine live cats were found, and the neighbors stated that in the previous week a dead cat lay in the yard attached to this house, with discharge oozing from its nostrils. In another house a mild case of diphtheria was attributed to the smell arising from a cat, which had died in a garden adjoining the house. Four of the emaciated cats referred to above were secured, and necropsy, including a bacteriological examination was made, but with entirely negative results. The illness of the cats in question dated from at least a month before the opportunity for examining them arose, so that the negative result is not surprising.

It will be remembered that Dr. Klein, in his investigation into cat diphtheria found that the diphtheria infection produced in the cat an acute lung inflammation, the kidneys becoming degenerated in the manner known in man as the “large white kidney." The condition of the household cat is worthy of inquiry in all such local outbreaks as the one briefly described by Dr. Newsholme, and it may be well to remember that if the cat can be secured for anatomical examination, even in the acute stage of the disease, there will probably be no exudation in the throat, but only marked pneumonia and possibly also renal inflammation. The public warning given in Brighton as to cats has had the desired effect, the small outbreak having come to an abrupt termination with the destruction of suspected cats, and of many others whose career has been shortened in consequence of the publicity given to the facts of the case.

DIPHTHERIA THROUGH CATS - A Serious Charge Against These Household Pets.
The New York Times, March 8, 1896

Mistress Pussy, who has been making such an attractive show of herself at Madison Square Garden all the week, is not quite all that fancy paints her under these excellent auspices. Over the water they are laying serious happenings at her door. In England, for example, where every possible means to ferret out causes of seemingly inexplicable disease is taken, heavy suspicion has fallen on the sleek creature that purrs at the hearthstone. She is found to be subject to a distemper whose symptoms are very similar to diphtheria and, in localities where cats are so afflicted, an epidemic of that terrible disease has followed among children. No other cause could be found for the infection except that in each case a cat in the house had first showed peculiar symptoms. These were emaciation, a cough, difficulty in swallowing, and discharge from the nose. Whenever a cat died there was sure to be found soon after one or more cases of diphtheria Dr. Newholm of London, whose investigating efforts have been very thorough, considers that ample evidence of pussy’s malevolent influence has been got and thinks she should be very carefully watched. In several cases of an increasing epidemic, warning was given and cats destroyed, with a result to arrest at once the spread of the disease.

It is unwise, according further to Dr. Newholm, to allow children to fondle these pets after they have roamed about outside and mingled with other cats. Even if our especial pussy is not sick she can easily bring home germs in her fur that she got from other cats not so healthy as she. The cat is a loved pet, but she were better a dead cat than that she risk the lives of precious children.

CATS CARRY DISEASE.
From The London Globe (Reprinted in The New York Times, March 28, 1899)

Cats will become unpopular, if the correspondence in several of our contemporaries regarding their dissemination of disease among human beings continues much longer. The cat, so the alarmists are persistently assuring us, catches diphtheria, influenza, and pneumonia, and imparts them to its household. This is a. work of supererogation on the part of the cat. A man can always catch as much diphtheria as he is likely to want for himself; and a very small modicum of Influenza will, with ordinary care, last any reasonably economical household for a very long time. But veterinary surgeons and others are writing to the papers to narrate authentic cases of those diseases being introduced in a lavish and extravagant manner into domestic circles by the common or household cat. And the cat, regarded as a centre of infection, is peculiarly dangerous: for when Pussy is ill all the juvenile members of a family begin to pet he; and make much of her, with, as it now appears, dangerous, if not fatal, results to themselves. It is an old saying that when the cat has a cold it will go through the house; and influenza may be regarded as a cold with all its bad points magnified a hundred times. Also, it is noteworthy that in many counties of England the tradition exists that persons who play much with cats never have good health; and the cruel practice of our ancestors who drowned sick cats, be- cause it was “ unlucky ” if a cat died in the house, may have had sound hygienic origin. At present the weight of testimony is certainly against the cats; and if unpopularity should result in a diminution of the legions in London no one would grieve.

BOY GETS DIPHTHERIA FROM CATS
New York Times, May 16, 1915

ROCKAWAY, NJ, May 15 - Richard Allen, the four year old son of Mr and Mrs Thomas Allen of Franklin Avenue, is ill at his home with diphtheria, which it is believed he contracted from two cats with which he played, and which have been pronounced as suffering from that disease. The animals will be killed today.

Here are some excerpts from a 1920s study into whether cats could contract or transmit diphtheria.

CATS AND HUMAN DIPHTHERIA.
BY William G Savage, M.D.,
County Medical Officer of Health, Somerset.
journal of Hygiene (London). Feb 1920; 18(4): 448–462.

IT is a widely accepted belief amongst medical men, particularly those who are Medical Officers of Health, that cats may suffer from diphtheria and convey it to human cases and that they are a not uncommon source of infection. It is of considerable practical importance to determine to what extent this belief is based upon reliable scientific data or whether it is another of those opinions, still far too prevalent, which arise from the acceptance of insufficiently tested and incorrectly interpreted observations. In dealing with this subject I propose to give first a brief account of the available evidence upon which the association of cats with diphtheria has been built, then a summary of my own investigations and lastly a critical consideration of the whole of the data available.

EVIDENCE ASSOCIATING CATS WITH HUMAN DIPHTHERIA.

This is both experimental and epidemiological.

Of the experimental work the most considerable in volume and of most importance as regards its influence upon contemporary and later medical opinion is the work of Klein (1888, 1889, 1890). This investigator inoculated eight cats subcutaneously and two intravenously with pure cultures of diphtheria bacilli. Four were unaffected but the other six showed swelling at the site of inoculation followed by death after five to eleven days. Post-mortem all the fatal cases showed extensive haemorrhagic oedema locally and certain special kidney changes . . . Klein describes this condition of the kidney as extremely characteristic of diphtheria disease in the cat.

He also caused death or paralysis in two cats by injecting diphtheria bacilli direct into the trachea by a syringe pushed through the anterior wall. One kitten and two cats fed with agar cultures in milk remained well, but two other cats fed on three separate occasions became very thin, although showing no other symptoms, and were killed three and four weeks after the start of the experiment. Both showed livers much enlarged, in one the kidneys were large with white cortex, in the other the kidneys were not conspicuously large but their cortex was pale with a few whitish patches. Not the slightest bacteriological proof is given or evidence supplied of the presence of any clinical or pathological features of diphtheria but Klein remarks "From these experiments it is seen that by repeated feeding with cultures of diphtheria bacilli distributed in milk, unquestionable diphtheria disease can be produced in the cat."

Klein also laid stress upon certain cases of accidentally acquired diphtheria (so called) in cats. The symptoms were an acute catarrhal affection of the conjunctiva and of the respiratory passages. The diagnosis of diphtheria was based upon the pathological appearances of the kidney and the appearance in one of the cats of a grey membrane covering the lower part of the larynx and upper part of the trachea in which " the microscope showed crowds of diphtheria bacilli as smaller or larger groups in the necrotic mucous membranes." Diphtheria bacilli were also demonstrated in the bronchial and tracheal exudation of another cat. Nine months later a somewhat similar outbreak occurred with similar pathological lesions and the presence of bacilli morphologically like diphtheria bacilli.

Klein records his examination of three cats sent to him from different sources and all associated with human cases of diphtheria. All three cats had been ill. The kidney condition which Klein records as typical of diphtheria was found in each case but no diphtheria bacilli were cultivated. On the basis of the pathological appearances and their likeness to the conditions induced in cats by inoculation with Klebs-Loffler bacilli Klein considered these were cases of cat diphtheria.

Renshaw (1885) in 1874 administered diphtheric membrane from human cases to a number of cats. Most died after illness and Renshaw reported finding diphtheritic membrane lining the fauces, bronchial tubes, etc. No evidence was adduced that the lesions were due to diphtheria bacilli and not associated with pathogenic cocci, etc.

Welch and Abbott (1891) inoculated into the trachea of a half-grown kitten a platinum loopful of a pure culture of the diphtheria bacillus. The animal died on the third day with the production of a diphtheritic membrane, containing diphtheria bacilli, in the trachea and larynx. No other noticeable lesion was found "unless it be a greater degree of fatty metamorphosis of the renal epithelium than is normal in kittens."

The above constitutes the only experimental work I have been able to trace. Evidence is also available on the epidemiological side in the form of instances in which it is reported that the disease has been transmitted to human cases through the agency of cats. Without making a thorough search of the literature I have collected particulars of a number of such cases.

Turner (1886) appears to have been the first to report the association of illness amongst cats with cases of human diphtheria and he drew attention to their presence in a number of small outbreaks. The evidence was entirely circumstantial and consisted of the concurrent presence of human cases with cats ill with such symptoms as swelling of the neck; foul discharge from the
nostrils, eye inflammation, etc.

Bruce Low (1888) reported a similar relationship in an outbreak at Enfield.

Williams (1895) reported that in the same house as a case of human diphtheria three cats were taken ill with wasting, loss of appetite, inability to swallow, cough and expectoration and all three died. One cat was sent to Klein for examination who reported that there was extensive disease of the lung not uncommon amongst cats.

Gwynn (1893) reported at Hampstead the association of a case of human diphtheria with an ailing cat suffering from a bad swollen throat. Later it showed an unhealed abscess in the vicinity of the throat. No bacteriological examinations were made.

Dowson (1895) examined a cat which became ill in a house in which a child had died 14 days earlier from diphtheria. Cultures from the lung gave a pure culture of an organism morphologically resembling the diphtheria bacillus which was not further investigated.

Synies (1896) mentions a kitten which was in close and constant contact with a fatal case of diphtheria and which subsequently was attacked with vomiting and diarrhoea with slight swelling of the neck and with patches of yellow exudation on the fauces. These- patches extended, the cat became much emaciated and died. Unfortunately the cat was buried before bacteriological examinations could be made.

Barras (1905) reports that in two cases of diphtheria in Govan a history of illness affecting the cats of the families attacked was ascertained. The throats of two of these cats were examined in the laboratory and micro-organisms were cultivated on artificial media which were found to correspond ill every respect to those of the human diphtheria bacillus. No particulars are given of the tests employed for the identification nor are any details furnished as to the degree or nature of the illness from which the cats suffered. Barras also mentions another cat in Govan-a stray cat-which had obtained a home in one of the families affected and which was found to be suffering from post-diphtheritic paralysis of the legs. Apparently no bacteriological examinations were made.

Porter (1908) records a number of cases of diphtheria in a household containing two petted and favourite cats. The cats were not ill. Swabs were rubbed over the fur of each cat. One yielded negative results but smears from the culture from the other cat showed a considerable number of organisms identical in appearance with the Klebs-Loffler bacillus when stained. Porter was unable to isolate the organism in pure culture.

Mapleton (1913), in connection with an outbreak of diphtheria amongst the children living in the Cottage Home, Newton Abbot, reported that there were "three cats in this Home in somewhat intimate association with the children and a bacteriological examination of these proved that two of them were infected." Further inquiries showed that the examinations made were ordinary swabbings grown on blood serum and that the bacilli were not isolated in pure culture nor were any animal inoculations made. In this case the cats were not definitely ill although one was obviously not in health and were not suspected of having conveyed any infection. They were examined to prevent the possibility of their carrying infection elsewhere.

Webb (1914) reported an interesting outbreak at Leigh (Lancashire) amongst cats to which his attention was directed in connection with a case of diphtheria in a child. In the house with this human case the cat had been ailing for more than two weeks, had a peculiar cough, could not miaw properly, refused food, was very thin and could hardly move. During the previous week a cat next door had died presenting the same symptoms and a cat across the way had also died with the same symptoms. The throat of the cat in the house with the diphtheria case was in a dirty sloughy condition and cultivations showed "a bacillus having the characteristics of the Klebs-Loffler bacillus." The organism was not isolated in pure culture and no cultural or inoculation tests were carried out. Webb states that five other cats in the street were found to be ailing and were destroyed.

Priestley (1915) investigating a number of cases of diphtheria at the Stockwell Orphanage found nine cats in that institution. All were bacteriologically examined although none of them were apparently ill. He reports "three of them in the boys' department and one belonging to the girls' department carried the diphtheria bacillus." In a personal communication he was kind enough to inform me that the bacilli were morphologically and culturally undistinguishable from the true Klebs-Loffler bacillus. No inoculation experiments were carried out nor is any information available as to what cultural tests were employed but apparently they were not isolated in pure culture and then tested.

The significance of the above data will be considered after my own experiments have been recorded.

THE AUTHOR'S INVESTIGATIONS.

A. EXAMINATION OF HEALTHY CATS. (I have summarised this)

Eight cats and 12 kittens were examined, the throat being examined in every case while for most of the kittens and for a few of the cats the nose was also swabbed. One cat was examined twice. All the 12 kittens failed to show any bacilli which at all resembled diphtheria bacilli. The eight cats were very different. Swabs from three of them showed no bacilli which might be taken for Klebs-Loffler bacilli. The results from the other five were as follows.

(1) Short bacilli closely resembling Klebs-Loffler bacilli. Bacillus isolated in pure culture.
(2) A number of bacilli present closely resembling Klebs-Loffler bacilli. Isolated in pure culture.
(3) A large number of bacilli present resembling Klebs-Loffler bacilli. A bacillus resembling this organism isolated in pure culture.
(4) Numerous bacilli present closely simulate Klebs-Loffler bacilli. Isolated in pure culture.
(5) A very mixed growth but contains a number of bacilli which very closely resemble true diphtheria bacilli. Differ from typical Klebs-Loffler bacilli size and absence of club-shape. Was unable to isolate in pure culture.

No. 2 is of particular interest as it was the cat in a large girls' school. This cat was re-examined exactly three months later. = 2 a.

(2 a) Many bacilli closely resembling Klebs-Loffler bacilli. Isolated in pure culture.

The bacilli isolated from cats (1), (2) and (3) resembled true diphtheria bacilli less when isolated in pure culture although still superficially resembling it. Culturally they were distinct, the agar and blood serum growths being definitely yellow or yellow-white in colour while they produced no acid in glucose media and in litmus milk produced definite alkali after four to five days' growth. The bacillus from cat (2 a) was extremely like the true diphtheria bacillus both morphologically and culturally. It was only distinguished so far as it was tested by the blood serum colonies having a distinct yellow tinge. Unlike true diphtheria it was non-pathogenic to a guinea pig.

It may be added that the films from the serum cultures made direct from the swabs were shown to the county bacteriologist in the laboratory who was examining several thousand suspected diphtheria swabs every year and he as well as myself was unable to distinguish the films from those from swabs from human throats containing true diphtheria bacilli. We have therefore the striking fact that two experienced bacteriologists were unable to distinguish with any certainty these bacilli in mixed smears from true diphtheria bacilli although in several instances we were in some doubt and could not pronounce them typical bacilli since minute differences were apparent. In several cases the resemblance was extremely close. All of them however (with the possible exception of the strain from cat (2 a)) were definitely not true diphtheria bacilli. Present in 66 per cent. of the adult cats it is interesting to note their entire absence from all 12 kittens.

B. EXAMINATION OF CATS ASSOCIATED WITH HUMAN CASES. (I have summarised these)

Although through Public Health medical officers of health were specially asked to send me such cases, none reached me in this way and the few cases investigated were all obtained through my personal efforts. Four were investigated in 1916 and one in 1919.

Case 1. Case of severe diphtheria in a house. A favourite cat much in the room with this case and with two children who suffered from severe colds but whose throats showed no diphtheria bacilli when swabbed. The cat showed no signs of sore throat or other illness. Swabs from its throat were cultured. The films showed numerous bacilli which resembled true diphtheria bacilli. A Isolated in pure culture the bacillus less closely resembled the Klebs-Loffler bacillus.. Cultures clearly
showed it was not that organism. If morphological appearances alone had been relied upon undoubtedly this cat would have been classed as infected with diphtheria bacilli and would probably have been credited as the source of the infection.

Case 2. A human diphtheria case (a woman of 21) in a house. The house cat came into close association with this woman who was very fond of the animal. The cat was said to have been ailing for some days before the case was notified but no very clear particulars as to symptoms were forthcoming. The cat when examined showed no local or general symptoms and the bacteriological examination of the throat swab showed a few bacteria suspiciously like diphtheria bacilli but nothing more definite.

Case 3. The cat in the house was closely associated with a case of human diphtheria. The animal remained throughout quite well and the cultures from the swab from its throat showed no bacilli at all like true Klebs-Loffler bacilli.

Case 4. A boy of eight years developed diphtheria on Nov. 22nd, 1916. The house cat was said to be "off its food " for about a week before that date and subsequently showed difficulty of swallowing as if it had a sore throat. The patient was very fond of this cat and fondled and nursed her regularly. The cat was swabbed Dec. 6th. The films made from the cultures showed a few clusters of bacilli which resembled diphtheria bacilli. Impossible to exclude as not Klebs-Loffler bacilli. They were only present in very small numbers. I was fortunate in being able to secure the cat and re-swabbed it in the laboratory but the cultures from this swab failed to show any bacilli suspicious of Klebs-Loffler bacilli. The cat was killed and a post-mortem examination made. There were no pathological lesions, the kidneys and other organs being quite healthy. Further swabs from the back of the throat and from the upper part of the trachea showed culturally no bacilli resembling diphtheria bacilli. It may be mentioned that the cat when the first swab was taken and when later on examined at the laboratory showed no symptoms at all of ill health.

Case 5. A child, aged five, was removed to the isolation hospital suffering from diphtheria the diagnosis being bacteriologically confirmed. The child was said to have been ailing for at least ten days before removal. An elder sister (16 years) also had a sore throat before the notified case and was possibly the source of infection, but when swabbed the day the notified case was removed to hospital showed no diphtheria bacilli. According to the mother of the case the house cat was ill for 3-4 weeks before the child of five was notified, refusing to eat and with some discharge from the nostrils and mouth. The
cat however gave birth to some kittens soon afterwards and then recovered and was quite well at the time of the removal of the case to hospital. The cat was brought to the laboratory April 16th, i.e. six days after the removal to hospital of the case, and swabbed. It showed no signs of any illness or nasal discharge. The animal was killed and post-mortem showed no membrane in the throat or any lesions anywhere. The films from the cultures made showed (mixed with abundant cocci) bacilli which could not be distinguished from diphtheria bacilli. With great difficulty this organism was isolated in pure culture and investigated. Morphologically it could not be distinguished from the diphtheria bacillus. Culturally it grew like diphtheria bacilli on but unlike our ordinary diphtheria strains did not grow well on blood serum. It produced acid in glucose broth but none in lactose or mannite broth while it produced a trace of acid in saccharose broth. To test its pathogenicity a guinea pig was inoculated subcutaneously with a very heavy dose but was unaffected. The cultural characters agree with the diphtheria bacillus for the most part and this may possibly have been a non-pathogenic Klebs-Loffler bacillus.

C. EXPERIMENTAL INVESTIGATIONS WITH KITTENS. (I have summarised these)

If, as has been so often asserted, cats suffer from diphtheria or even if it be merely advanced that they act as carriers of the diphtheria bacillus in their throat or nose, it should be possible to infect them artificially and set up either condition. The possibility of this was tested in a long series of experiments. Kittens were used in preference to adult cats as both more easily handled and as likely to be more susceptible to infection than the grown up animals. In every case the throat and in nearly all cases the nose of the animals were swabbed before the experiments started to ascertain if diphtheria-like bacilli were present or absent.

The kittens were all kept in separate cages and very carefully examined for any signs of ill health. Weight observations were recorded but temperature testings were not found very helpful. The kittens were usually from four to six weeks old when the experiments started.

Exp. 1. Throat swabbed with a culture of B. diphtheriae transferred on a sterile swab. Immediately re-swabbed in the same wav with a second culture of another strain. Throat swabbed after three and seven days and cultured. No diphtheria bacilli found. Animal showed no signs of any illness, quite lively and gained in weight. No local throat-lesions.

Exp. 2. Throat of same kitten re-swabbed a month after onset of Exp. 1 with a culture made direct from a swab taken from the throat of an acutely ill case of diphtheria. This culture showed about half the colonies B. diphtheriae and half streptococci and it was used as possibly the mixed growth might favour infection. Animal remained quite well and lively and a swab taken six days after the inoculation showed no diphtheria colonies. Post-mortem eight days later showed no evidence of diphtheria in the organs. No trace of any lesions in the throat. The virulence to guinea pigs of these three strains was not tested.

Exp. 3. Kitten about four weeks old fed on two consecutive days with milk containing cultures of two different strains of B. diphtheriae, one of which was fully virulent to a guinea pig. Animal showed no symptoms whatever and the throat swabbed six days after the first feeding yielded no diphtheria bacilli.

Exp. 4. Throat of the same kitten swabbed with two cultures of B. diphtheriae (plus a small number of staphylococcus colonies) grown direct from the throat of an acute case of diphtheria. Throat swabbed seven days later showed no diphtheria bacilli. Animal showed steady increase in weight and no local or general symptoms. Post-mortem 15 days after the throat swabbing showed no trace of diphtheria membrane on throat, trachea or larynx. Internal organs normal and sterile.

Exp. 5. Throat of a kitten about five weeks old swabbed with a fifth strain (strain B) taken directly from a swab from an acute case and showed B. diphtheriae in almost pure culture. Swabs from the throat taken 4 and 10 days after inoculation showed no diphtheria bacilli. The animal increased in weight and exhibited no signs of ill health.

Exp. 6. Thirteen days after Exp. 5 started this kitten was injected subcutaneously with a pure culture of the same strain (strain B) of diphtheria bacillus. The animal was obviously ill within 24 hours of the injection and remained very quiet. The temperature which for the two days before the experiment had fluctuated between 99 degs and 100 degs rose to 104 degs F. then rapidly dropped to below 100 degs F. and was 96 degs on the ninth day when the animal was moribund and was killed. The day before death very definite paralysis of the hind legs was observable. The post-mortem examination showed no oedema or inflammation at the site of inoculation, no fluid excess in the thoracic cavity. The internal organs, apart from the kidneys and supra-renals, appeared healthy. Both kidneys were greatly enlarged and abnormal. This experiment is important as showing that the same strain which was incapable of infecting or affecting the kitten by means of the throat was highly pathogenic when injected into the same animal.

Exp. 7. An older kitten (about two months) and weighing 830 grms. used for a combined throat swabbing and feeding experiment. Throat swabbed with B. diphtheriae strain (strain J). Throat examined after two, five, and nine days showed no diphtheria bacilli. Five days after the third inoculation fed with milk containing massive dose of strain J culture. Feeding with the same massive dose repeated on the following three days. Throughout the animal remained quite well and gained steadily in weight.

Exp. 8. The same kitten some weeks later injected subcutaneously with culture of strain J. The kitten showed a slight rise of temperature, but no other symptoms and increased in weight. Post-mortem 14 days after the injection showed no macroscopic or microscopic lesions. The culture was scanty and the kitten weighed when inoculated 1020 grms so the dose was probably too small and this experiment hardly indicates the strain was non-virulent.

Exp. 9. The nose of a young kitten about four weeks old inoculated heavily with a pure culture of B. diphtheriae (strain W). Previous swabbings of nose (both sides) and throat showed absence of any diphtheria bacilli-like organisms. Nose swab examined 24 hours after the inoculation failed to show any diphtheria bacilli when cultured. Both sides of the nose then again inoculated with B. diphtheriae and each nostril examined after 48 hours and eight days showed no diphtheria bacilli. The animal remained perfectly well with no nasal discharge or any symptom.

Exp. 10. It may be argued that the failure to infect the kittens by the throat was due to the smooth healthy mucous membrane failing to afford a nidus for growth and the following and some other experiments were devised to deal with this objection.

Thirteen days after the onset of Exp. 9 the throat of this kitten was painted on 2 consecutive days with a 20 per cent. solution of silver nitrate. After second day there was definite redness with much mucous and some whitened areas on the palate. The throat was then inoculated with a culture of B. diphtheriae (strain W). Swabs from the throat 24 hours and 48 hours after the B. diphtheriae implantation showed no diphtheria organisms. The animal remained fairly well but was quiet and did not put on much weight but gradually recovered completely.

Exp. 11. Eight days after the start of Exp. 10 this kitten was injected subcutaneously with a culture of the same B. diphtheriae strain (strain W). Animal obviously ill 24 hours later. After 48 hours very ill, quiet and refused food. Three days about the same and after four days appeared better but died on fifth day. Post-mortem the animal showed the signs noted above as present in the other injected kitten except that the kidneys were especially engorged and enlarged.

Exps. 10 and 11 show that this strain while highly pathogenic when injected was unable to establish itself upon the throat of the same animal even when a definite unhealthy and pathological nidus was provided.

Exp. 12. The throat of a young kitten swabbed with a two days old culture of B. diphtheriae (the virulent strain W being used). Nose (left side) also inoculated from another serum culture of the same organism. No diphtheria bacilli found in the throat when examined after 24 and after 48 hours. Diphtheria bacilli found in the nose at the end of 24 hours but not after 48 hours or subsequently. Animal remained quite well.

Exp. 13. Nose and throat of another quite young kitten inoculated heavily from another B. diphtheriae strain (strain V). Both nose and throat were free from diphtheria bacilli when examined after 24 and 48 hours.

Exp. 14. This same kitten was then fed on eleven occasions over 14 days with a broth culture of B. diphtheriae (strain W). This broth gave abundant growth and over this period as much as 100 c.c. of broth culture was administered. No illness or symptoms of any kind. The animal steadily increased in weight and the post-mortem examination 31 weeks after the commencement of the feeding showed no abnormalities.

Exp. 15. The hard palate (as far back as possible) of another young kitten was scarified by a scalpel making a number of superficial lesions. A B. diphtheriae (strain W) culture was then well rubbed into these superficial lesions. The animal remained quite well and examined 24 and 48 hours after showed no membrane or other lesions nor could diphtheria bacilli be cultivated from the swabs taken despite many attempts.

Exp. 16. Throat of a young kitten swabbed with a strong solution of silver nitrate. Examined 24 hours after there was a well marked slough. The throat was inoculated with a blood serum culture of diphtheria bacilli. The animal was very ill and as it was in pain was killed. Here the throat treatment was too severe but swabs after 24 hours showed no diphtheria bacilli and post-mortem there was no evidence of any true membrane although sloughs over the hard palate. Films from throat and trachea showed no bacilli like B. diphtheriae.

Exp. 17. The nose of another young kitten inoculated heavily with B. diphtheriae (strain W). Nose swabs taken previously showed no bacilli at all like diphtheria organisms. Nose examined after 24 hours and after 48 hours showed diphtheria bacilli colonies, apparently more abundant after the longer period. None found after four and five days. Unfortunately not examined after three days. The animal remained perfectly well, gained in weight, had no nasal discharge and post-mortem showed no lesions.

Exp. 18. Nose of another kitten inoculated heavily with two separate strains of B. diphtheriae (strains W and V). After 24 hours diphtheria bacilli readily grown from the nose, but less readily after 48 hours. Examined after four days few bacilli were found. Could not be found at all after five and six days. The kitten remained perfectly well throughout, increased in weight and showed no nose discharge or other local lesions.

Exp. 19. The nose of another kitten inoculated heavily with another B. diphtheriae strain (strain R) isolated a day or two previously from an acute case of diphtheria. After 24 hours a few diphtheria bacilli were grown from the nose but none after 48 hours, four and five days. Animal remained perfectly well without any local or general symptoms.

REMARKS ON THE EXPERIMENTS. (summarised)

The results obtained with these young kittens are exceptionally uniform and concordant. It was found impossible to infect them by throat swabbing although massive doses were invariably used. As many as 10 different strains were employed and all of them were quite recently isolated from acute human cases of diphtheria. Four of the strains were proved to be of high virulence and two definitely killed but only when subcutaneously injected, young kittens that could not be infected by natural channels of entry (nose, throat). The same failure to infect these animals with diphtheria or any local lesion was experienced when mixed cultures direct from human throats were used or when a favourable site for local growth was provided by chemical or mechanical means. Not only did the bacilli not infect but they failed to survive and although massive doses were inoculated they invariably disappeared after as short a period as 24 hours.

The implantations of vast numbers of diphtheria bacilli into the nasal cavities were equally ineffective in setting up any local or general lesions and (what I did not expect to find) were unable even to survive beyond a very short period. In only one experiment was there possible evidence of increase and only in one instance could the most diligent examination trace their persistence beyond four days. The feeding experiments were also unsuccessful and the kittens consumed vast numbers of virulent diphtheria bacilli without the slightest evidence of harm.

These experiments so far from supporting the view that diphtheria is a naturally occurring disease of cats suggests that the secretion from the mucous membranes of the cat are peculiarly unfavourable to the growth of the diphtheria bacillus and will not even permit it to remain as a saprophyte.

CRITICAL CONSIDERATION OF THE AVAILABLE FACTS.

There are two separate possibilities. On the one hand there is the suggestion that cats themselves may be infected with the disease, which may then be appropriately designated as cat diphtheria. On the other hand we have to consider the possibility that cats may act as carriers of diphtheria infection by harbouring the bacilli in their nose or throat or by their fur becoming infected.

As regards the first possibility, the work of Klein and others and corroborated by my own experiments, shows that cats succumb to the toxins of B. diphtheriae when these are direct injected into the trachea or subcutaneously. This does not prove or suggest that cats suffer from diphtheria. A study of the experimental data in favour of cats suffering from diphtheria detailed in the first part of this paper shows that the evidence is extremely weak and unconvincing and does not warrant the interpretation placed upon it.

My own experiments, testing by many different ways the possibilities of natural infection, are in direct opposition to the widely accepted view that cats can suffer from a kind of diphtheria. The evidence which has been adduced from the association of human diphtheria cases with cases of illness amongst cats whose throats are said to show diphtheria bacilli must be regarded as valueless in almost every case, for the following two reasons.

In all the instances that I have been able to find, with the exception of one case recorded by Barras and possibly Case 5 of my series, the diagnosis rests entirely upon the presence in the throat of the cats of bacilli resembling B. diphtheriae in shape. Barras does not mention the steps he took to identify his bacillus and one cannot exclude the possibility of it being a pseudo-diphtheria bacillus. The results recorded above show that in normal cats no less than 66% bacilli which, without isolation and study in pure culture, I was unable to exclude as not true diphtheria bacilli. In my series of cats associated with cases of human diphtheria both Cases 1 and 4 would without doubt have been classed and accepted as proved cases of diphtheria in cats although a more complete investigation quite refuted this assumption.

The second reason is that the fact that the cat or cats were ill does not supply even primea facie evidence for suggesting that the human and cat conditions were related. It is recognised by veterinary authorities that cats suffer from illness with symptoms which in some respects are analogous to those of human diphtheria but which are certainly not cases of diphtheria or due to B. diphtheriae.

Gray (1896) states that the so-called diphtheria in the cat is a contagious disease of the cat, characterised by the presence of diphtheritic membranes in the throat and due to some micro-organism not yet determined. The disease has a mortality of quite 90 per cent.

Gofton (1913) says that the kidney condition described by Klein as pathognomic is a common condition of the cat's kidney. He adds affections presenting diphtheritic characters are met with in the cat and occur independently of human diphtheria, but the bacteriological investigation of these affections has up to the present always resulted in a failure to demonstrate the presence of the human bacillus. With a prevalent disease such as human diphtheria and a common illness of cats with similar symptoms it is obvious that the two will be associated together in several cases. I investigated my first cases of supposed cat diphtheria (with negative results) just over 20 years ago and I have been on the look out for associated cases ever since; mathematical analysis shows the association of the two conditions is uncommon. There are therefore no grounds for the common assumption that when the two conditions do happen to coincide there is a cause and effect relationship between them.

The interesting cases of illness in cats recorded by Webb (loc. cit.) are clearly examples of this infectious disease in cats and there is no justification for dragging in the human factor to account for them. My experimental work does not support the suggestion that cats act as carriers by the bacilli living in their throat or nose. On the contrary, the mucous membranes of these animals appear inimical to these bacilli; even under the most favourable circumstances they are unable to multiply or survive for long. That the fur of these animals may be infected with these bacilli is possible and there is some experimental evidence in its favour (e.g. Remlinger, 1906) but in view of the low resistance powers of B. diphtheriae this is probably an unimportant factor in the spread of the disease.

Summing up the matter I am of opinion that the common and widely accepted view that cats can suffer from a naturally acquired disease caused by the diphtheria bacillus is entirely without foundation. The reported cases of such an association are based upon insufficient examination and differentiation of the bacilli due to a failure to realise that a large proportion of healthy normal cats contain in their throats bacilli which closely resemble and are difficult to distinguish from the true B. diphtheriae.

REFERENCES.

BARRAS, W. G. (1905). Lancet, n. 1869.
(1906). Public Health, xvm. 709.
DOWSON, W. (1895). Report of Medical Officer of Health, Bristol.
GOFTON, A. (1913). In Hoare's System of Veterinary Med. I. 310.
GRAY, H. (1896). Journ. of Comp. Path. and Therap. Ix. 46.
- (1913). In Hoare's System of Veterinary Med. i. 317.
GWYNN, E. (1893). Brit. Med. Journ. ii. 1013.
KLEIN, E. (1888). Report of Medical Officer, Local Government Board, 435.
(1889). Ibid. 156.
(1890). Ibid. 229.
Low, BRUCE (1888). Ibid. 131.
MAPLETON, H. B. (1913). Annual Report Newton Abbot Urban District Council, and personal
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SYMES, J. 0. (1896)X Ibid. i. 1385.
TURNER, G. (1886). Report of Medical Officer, Local Government Board, 307.
WEBB, F. (1914). Journ. Royal San. Inst. xxxv. 63, and personal communication.
WELCH and ABBOTT (1891). Johns Hopkins Hosp. Bull. ii. 25.
WILLIAMS, W. (1895). Brit. Med. Journ. n. 74.