Copyright 2000 - 2014 Sarah Hartwell


What are Zoonoses?
Fungal Infections
Bacterial Infections
Protozoal Diseases (Including Toxoplasmosis)

Prion Disease
External Parasites



Zoonoses (also called zoonotic diseases) are diseases that can be passed from animals to man. Most diseases are species specific. Humans don't catch cat flu despite urban legends to the contrary. Most zoonotic diseases can be avoided through good hygiene - disinfecting areas which have become contaminated, washing hands after handling contaminated items (including cats) and wearing rubber gloves to prevent infectious matter entering skin wounds. The majority of diseases pose no threat to humans and can be treated by your doctor. A few are more dangerous to those with poor immune systems and a very small minority are unquestionably dangerous e.g. rabies. Cats can either be a reservoir for infection (they are hosts/carriers of the disease itself) or they might be incidental in the transmission of a disease (they physically carry an infected flea or tick on their fur, but the cat does not harbour the disease in its body).

Common examples of zoonoses are toxoplasmosis (a protozoan), psittacosis (virus) and ringworm (fungus). Toxoplasmosis is a protozoan which can be spread from cats and a wide variety of other sources to humans. A pregnant woman who contracts toxoplasmosis risks having a baby with congenital defects. Psittacosis is an upper respiratory infection usually caught from cage birds e.g. parrots. It is a chlamydial infection of birds which can cause pneumonia in humans. Although easily cured in healthy adults, it is dangerous to the very young, very elderly and to those with a poor immune system. Psittacosis also causes chlamydial eye and respiratory infections in cats. Ringworm is a fungal skin disease which for most people is more irritating than dangerous, though it can cause scarring (due to constant scratching) and may be of concern to individuals with poor immune systems.

This a short and simple owner's guide to common zoonoses. A web-search will provide in-depth information on any of these infections. If you have any concerns, your veterinarian and your doctor will be able to provide information. Remember - zoonoses are only newsworthy because dangerous ones are so rare! Many pet owners have probably had zoonotic illnesses without even noticing the symptoms.

Important note: Some of the photos in this article show cats in inappropriate situations e.g. sharing human food or on food preparation areas. In all cases, the cats were healthy and their owners had good immunity to most minor infections and understood the risks. Surfaces were wiped clean afterwards.

Good hygiene remains the best defence against zoonoses!
Photo: Sarah Hartwell



Fungal infections of humans are surprisingly common: thrush, athlete's foot etc. The most common fungal infection spread from cat to human is ringworm.

Ringworm (Dermaphytosis) is probably the most common zoonosis of cats and can be spread by direct contact or by spores in the environment. Vets who treat ringwormy cats are used to seeing owners with red, scaly, itchy patches of skin. Ringworm is not a worm but a fungus; it normally causes circular scaly patches (which often fluoresce under a Woods Lamp [UV light]). Some strains do not fluoresce and must be cultured in the laboratory, or examined under a microscope, to provide a diagnosis. Unfortunately up to 40% of cats may have the fungus without showing any symptoms. it may be hard to detect in longhaired cats as their fur hides the scaly patches.

It is a nuisance to eradicate from a household because of fungal spores. Cat treatment is usually by tablet (sometimes by dips or baths). Human treatment is usually by anti-fungal creams; feline treatment is by oral drugs. The human version is probably frequently dismissed as eczema by cat owners and left untreated. As with other skin diseases, the red scaly circular patches may be itchy and continued scratching may cause scarring. It used to be very common in poverty-stricken areas where it was transmitted directly from person to person. This means there is a social stigma to ringworm infection. The author of this piece and several of her co-workers at cat shelters have had ringworm infection and it may be seen as an occupational hazard for cat workers!

Sporotrichosis is a fungal disease commonly found in the soil in parts of the southern USA. Most infection comes directly from the soil or from plants, but cats can also be infected (when the fungus gets into fight wounds or fresh flea bites) and can shed the fungus from weeping skin wounds. Humans with skin wounds can be infected by contact with the cat's weeping sores. It can be treated. It can also be prevented by wearing gloves if handling cats with weeping sores and by using topical fungicides (scrubs, handwashes) after handling infected cats. Dermatophilosis can be contracted by handling an infected cat. The cat's sores tend to be crusty. Human infection tends to be a pustular dermatitis. In healthy people, this usually resolves on its own after a couple of weeks. It can be treated with iodine solutions or certain antibiotics (in cases where a person has a poor immune system). Enterocytozoon is a species of microsporidia that can cause disease in humans and animals. These are now identified as fungi, though older books may still refer to them as protozoa. This can cause diarrhoea and gall-bladder disease, especially in people with poor immune systems. Enterocytozoonosis can be transmitted from cat to human, but is also contracted from pigs and cattle.


Bacteria are usually secondary infectious agents that follow viral or mycobacterial infections. Bacteria may be present in diarrhoea which is an effective means of cross-infection (solid stools buried in litter are less infectious). Common zoonotic bacteria include species of campylobacter, streptococcus and staphylococci; these generally respond to antibiotic. However, some cat breeders now routinely dose their cats on antibiotics in an attempt to reduce disease in the cattery. This is leading to an increase in antibiotic-resistant strains of bacteria. The problems of resistant bacteria have already been encountered in the livestock/poultry industry, caused by too many animals in too little space and routine dosing with 'preventative' antibiotics.

Feline conjunctivitis can be caused by a variety of conditions including bacterial or viral infections; conjunctivitis caused by a foreign body may lead to a secondary bacterial infection. Since some of these germs can also infect humans, it is wise to observe basic hygiene precautions when handling cats with conjunctivitis. Most cases are easily treated in both cats and humans by eye drops and eye ointments - often containing the same active ingredients!

Persistent streptococcal infections in cats can cause tonsilitis and pharyngitis in the owners.

Pasteurella is possibly the most common bacterial zoonosis, carried in the mouths of up to 75% of cats. Cat bite wounds should be always be cleaned carefully with antiseptic or antibiotic cleansers and an antibiotic ointment applied. Any sign of wound inflammation, persistent swelling or fever should be referred to the doctor as it may require oral antibiotics. Most healthy adults will recover without treatment; but you may not wish to risk your health. Pasteurella has been in the news because owners have contracted the disease from kissing dying cats which may explain why women are more susceptible to infection.

Salmonella bacteria is more common in the faeces of cats fed raw meat or those that catch wild birds. Infection follows a faecal-oral route (you clean the litter tray and scratch your lip without first washing your hands).

It may be cute, but cats should not be allowed on food preparation or serving areas!

"Tommy" wants a piece of Irene's burger and is standing right next to a tray of shortcake.

Pet owners are often extremely indulgent of their pets. If you do allow your cat on or around food preparation areas, you should be aware of the risks.

Photo: Sarah Hartwell

Note: The food shown was discarded and the surface cleaned, in addition Irene has lived and worked with cats for long enough to have built up an immunity to most of the minor infections he might be carrying.


Bordetella (kennel cough, snuffles) is more likely to be found in a shelter or cat hoarder situation than in low density cat households. As one of the common names suggests, it is a snuffly illness with nasal discharge. People with poor immune response may get upper respiratory illness (which can lead to pneumonia in HIV+ individuals). It is treatable with antibiotics.

Q Fever (Coxietta burnetii) infects cats in the north-eastern region of North America. Most cats have no symptoms, but pregnant cats may abort. It can be transmitted directly to humans and causes a flu-like illness (sometimes pneumonia). It can cause hepatitis and in chronic cases, endocarditis, but is treatable with antibiotics.

Yersiniosis (Y pseudotuberculosis, more rarely Y enterocolitica) can carried by cats and shed in their faeces. Cats rarely show any symptoms, or may appear to have a tummy bug (vomiting, diarrhoea, not eating) and/or jaundice. It can cause abdominal symptoms in humans. More rarely it leads to septicaemia and arthritis. Anaerobiospirillum is another bacteria carried in cats' intestines which can cause fever, vomiting, diarrhoea and abdominal pain in humans. Both are treatable with antibiotics and preventable through good hygiene e.g. washing hands after petting cats and before handling food.

Cat Scratch Disease (Cat Scratch Fever, Bartonellosis)

Cat Scratch Disease (called Cat Scratch Fever in the UK) is caused by the bacteria Bartonella henselae (formerly called Rochalimaea henselae). Despite its British common name, it does not usually cause fever. The bacteria is carried by cat fleas. Cat Scratch Disease causes systemic illness and lymph node lesions and can be very serious in individuals with poor immune response. Antibiotic therapy usually cures the disease without complications in healthy young adults. Clearing the bacteria from infected cats requires long term antibiotic treatment, however, and cats may be continuously or intermittently infected indefinitely. Studies in the United States suggest 20-40% of cats carry the disease. The figures will be different in other countries due to different cat-keeping habits.

Cat Scratch Disease was first described in 1950 and aroused considerable interest for the next twenty years, with hundreds of case reports in medical literature. Early attempts to identify the causal agent failed and the diagnosis was made on clinical criteria such as lymphadenopathy and a history of animal contact, positive skin test results, negative laboratory findings for other causes of lymphadenopathy and/or characteristic histopathology of lymph node biopsy. In healthy humans, Cat Scratch Disease is usually a benign self-limiting infection and is usually diagnosed too late for any action to be taken (i.e. the body already has it under control by the time a doctor takes samples).

There are at least eight species of Bartonella of which four (B. bacilliformis; B. elizabetbae; B. henselae and B. quintana) are known to cause disease in humans. B. henselae is thought to be the cause of Cat Scratch Disease although French studies also implicate B. quintana. Bartonella is a successful parasite of a wide range of mammals in that it usually causes no ill effects to its host. It is only when they get into the wrong host (e.g. humans) that disease is seen. Cats are unlikely to be the only source of infection. Other small mammals carry Bartonella, but humans are more likely to interact with cats than with other carriers so the cat is generally blamed. In up to 10% of cases there is no recognised cat contact

Helicobacter pylori

Helicobacter pylori is also found in cats. This organism causes gastric ulcers in humans and the cat-human link caused panic among cat owners in the mid 1990s. It is found in the stomach. It has been suggested that it may be transmitted to humans if a cat walks across a food preparation area which is then not disinfected before food is next prepared on it. Simple hygiene, including keeping cats off of counters, is recommended though it isn't known what percentage of cats carry helicobacter.


Feline strains of coccidia and giardia do not appear to be directly infectious to humans. Giardia cause diarrhoea in cats and normal precautions should be followed when cleaning up; normally cats and humans are infected by the same source (contaminated water) not by each other. Cats can also carry cryptosporidium which causes watery diarrhoea in both cats and humans, and may cause gastroenteritis, cramps and fever in humans. Cryptosporidium is hard to treat, but can be prevented by avoiding poorly washed food and properly treating drinking water. These protozoa are also be found in poorly processed domestic water supplies which means reinfection is common. Media attention is sometimes focussed on Toxoplasmosis, a type of coccidia that can cause birth defects in unborn babies. Toxoplasmosis, a disease of cats and other mammalian species, is caused by the protozoan parasite Toxoplasma gondii. Protozoa are single-celled animals and though infection with Toxoplasma is fairly common, actual disease caused by the parasite is relatively rare.

Sarcocystis (Sarcoplasma) is related to Toxoplasma. Cats, dogs, humans (and other carnivores) acquire Sarcoplasma infection by ingesting infected meat. It multiplies in the intestines and is shed in the faeces; the spores may be eaten by domestic livestock. The parasite forms cysts in the intermediate host's muscle tissue and when infected meat is eaten, the lifecycle starts again. Undercooked meat can infect humans with Sarcosporidiosis. It is not passed directly from cat to human. Sarcosporidiosis has been reported in continental and central Europe, South and Central America and the western USA and is endemic in parts of Asia. Many people show no symptoms of Sarcoplasma infection so it is probably a common parasite that rarely causes human illness. In some people it can cause gastroenteritis which usually clears on its own, and occasionally causes more serious illness in susceptible people. Human disease can be prevented by avoiding undercooked meat or by deep-freezing meat for several days. It is rarely feasible to remove cats (etc) from the vicinity of livestock, especially free-range livestock, but they should be prevented from defecating near to livestock feed and from eating raw meat.


Cats acquire Toxoplasma infection by ingesting infected prey. The organisms multiply in the wall of the small intestine and produce oocysts, which are then excreted in the faeces for 2-3 weeks. Most cats shed oocysts only once after infection and are then effectively immune. Within 5 days the shed oocysts become infectious to other animals and to humans. Infectious "sporulated oocysts" are highly resistant to environmental conditions and can survive in moist shaded soil or sand for many months. Some Toxoplasma organisms migrate from the cat's intestine to muscle and brain but usually remain dormant. The same migration can also occur in humans. Congenital infection (transmission from mother to foetus) occurs in sheep, goats, and humans. Repeated congenital transmission is found in rodents and probably serves as a sort of back-up mechanism until a predator arrives to allow the parasite to complete its lifecycle.

In humans, toxoplasmosis symptoms may be flu-like, in most cases there are no symptoms. Infection is more serious in individuals with poor immune response. If such people contract toxoplasmosis for the first time it can run riot in the body with the immune system unable to keep it in check. In people who have previously been infected, the dormant tissue cysts are activated. Stress can also activate dormant cysts. Activated tissue cysts can produce millions of toxoplasmas which can lodge in the heart or brain and other major organs.

Surveys have shown around 20-60% of cats tested are infected with T gondii. Infection is more common in stray and feral cats that eat infected prey. However, surveys found that less than 1% of infected cats actually shed oocysts. Around 30% of UK adults tested had antibodies i.e. had been infected by Toxoplasmosis at some point. In France and Germany up to 80% of humans test positive. In most humans the infection is mild, or may have no symptoms.

There are many scare stories about Toxoplasmosis in humans - oddly, many seem to come from the Czech Republic and others are picked up by the media whose headlines are "cats make you mentally ill." So far, despite over 3 decades of study, the jury is still out as to whether there is a direct link. For most owners, the benefits of cat companionship outweighs the possible risk of developing mental illness. Toxoplasmosis can be contracted from many non-feline sources that it may be an environmental risk rather than a cat-ownership risk.

A study called "Is childhood cat ownership a risk factor for schizophrenia later in life?" by E. Fuller Torrey, Wendy Simmons, Robert H. Yolken was published in April 18, 2015. Despite the media headlines, the study indicates that the link is still a hypothesis not firm fact. Torrey and Yolken have been studying the possible link between Toxo and schizophrenia for around 30 years, but they are careful to state this is "possible" and "needs clarification." it's unethical to deliberately infect children with Toxo and see what happens in later life (and rodents are not perfect substitutes for humans in research), the team compared 2 previous studies that linked childhood cat ownership to adulthood schizophrenia later in life, and an unpublished mental health survey from 1982. They concluded that childhood cat ownership may be one of many risk factors for developing mental disorders. Ive highlighted important text that most newspapers glossed over. "Abstract: Two previous studies suggested that childhood cat ownership is a POSSIBLE risk factor for later developing schizophrenia or other serious mental illness. We therefore used an earlier, large NAMI questionnaire to try and replicate this finding. The results were the same, suggesting that cat ownership in childhood is significantly more common in families in which the child later becomes seriously mentally ill. IF TRUE, an explanatory mechanism MAY BE Toxoplasma gondii. WE URGE OUR COLLEAGUES TO TRY AND REPLICATE THESE FINDINGS TO CLARIFY WHETHER CHILDHOOD CAT OWNERSHIP IS TRULY A RISK FACTOR FOR LATER SCHIZOPHRENIA."

In a study (published in Acta Psychiatrica Scandinavica) by A.L. Sutterland from the Department of Psychiatry at the Academic Medical Centre in Amsterdam, researchers analysed the findings of 50 published studies to confirm a link between Toxo and mental illness. The wording of the news article is misleading - it makes it sound like the link is already established. It found an apparently "overwhelming" association that a person with Toxo infection was almost twice as likely to develop schizophrenia than an uninfected person. But it also said THE FINDINGS SHOULD BE APPROACHED WITH CAUTION. Torrey's press release said "Cat ownership in childhood has now been reported in three studies to be significantly more common in families in which the child is later diagnosed with schizophrenia or another serious mental illness." Saying it is "more common" is not the same as saying it is "cause and effect." Torrey also says "I like cats. Unfortunately, IF WE ARE CORRECT that they transmit infections . . .

Obstacles to Toxoplasmosis Research Outside of a Laboratory

A good researcher challenges his hypothesis rather than just looking for data to support it. If his hypothesis stands up to challenge then he's onto something. For challenging the link between Toxo and schizophrenia, you need at least 4 sample sets. These sets must be large - at least 100 people in each - and geographically widespread to rule out local variations (e.g. sheep/goat farming), human dietary habits (eating undercooked meat) and cat-feeding habits (e.g. feeding raw meat). Researchers need to investigate:
  • People who owned cats/had close contact with cats as a child and later developed mental illness.
  • People who owned cats/had close contact with cats as a child and has not developed mental illness.
  • People who didnt own cats/have close contact with cats and have developed mental illness.
  • People who didnt own cats/have close contact with and have not developed mental illness.
There are still major problems in establishing a link. Its almost impossible to rule out all environmental and genetic factors related to toxoplasmosis infection or to mental illness.

Temporary (12 weeks) behavioural changes have been seen in mice infected with Toxoplasmosis (Hrda et al 2000, Webster 2001), however humans do not necessarily react in the same way as mice. Infected mice were less active, especially when infection was at its peak (peak of tissue cysts developing in their brains). This made them more vulnerable to predation by cats and aids the transmission of the parasite back to cats. The infected mice showed increased levels of dopamine in their brains which led to the controversial suggestion that toxoplasmosis could cause of schizophrenia in humans (Flegr et al 2003) though there is currently no evidence for this. Fuller Torrey (Stanley Medical Research Institute, Maryland, USA) found that schizophrenics were statistically more likely to be cat owners and to have latent toxoplasmosis. Test-tube studies indicated that drugs used to control schizophrenia affected the toxoplasmosis parasite and could explain how the drugs worked. Sceptics point out that the drugs might affect the parasite as a side-effect, rather than a primary effect and that persons genetically pre-disposed to schizophrenia (it is a combination of genetic predisposition and environmental triggers) might also be pre-disposed to liking cats; in addition many people with latent toxoplasmosis do not develop schizophrenia.

Latent Toxoplasmosis infection has been implicated as a cause of slower reaction times (Havlicek et al 2001) that could put infected people more at risk of road traffic accidents (Flegr et al 2002). The higher the antibody count, the greater the risk. These studies involved relatively small numbers of people and results should be interpreted cautiously. A further study by the same scientist, involving 857 Czech military conscripts, reported decreased IQ and verbal intelligence in people who had had Toxoplasmosis infection (Flegr et al 2003). However, a different study found that pregnant women that had previously had toxoplasmosis were more intelligent (Flegr & Havlicek 1999). These results conflict so greatly that it is questionable whether Toxoplasmosis was actually responsible. The Czech researchers suggested that Toxoplasma gondii may make women reckless and friendly and make men jealous and morose. They suggested that toxoplasmosis in infected women more than doubled their risk of causing a traffic accidents by prolonging their reaction time. Meanwhile, they claimed that infected men became quiet, withdrawn, suspicious, jealous and dogmatic (men and women have different brain wiring leading to different effects). None of this has been corroborated, but if it did turn out to be true, toxoplasmosis could be responsible for up to one million road deaths worldwide, making it second only to malaria in deadliness. It could also be responsible for domestic problems between infected parties. The rest of the scientific world remains highly sceptical since the causes of domestic disputes and road accidents are too many and varied to be pinned down on a single cause.

Congenital infection is of greatest concern in humans; especially in pregnant women who contract toxoplasmosis for the first time while they are pregnant. They may show no symptoms themselves, but the foetus may suffer birth defects including blindness and brain damage. Since most cat-owning women have already been exposed to toxoplasmosis (and hence do not become reinfected), this is uncommon which is why it causes headlines. Most infected babies are born healthy, but with dormant toxoplasmosis cysts, in later life these may lead to reduced vision and loss of sight. In the UK, the total number of babies with congenital toxoplasmosis is approximately ten (out of many thousands).

Although cats are the "definitive hosts" for the parasite (the only animals in which it reproduces), they are not the main source of toxoplasmosis infection in humans. The parasite can infect any warm-blooded animal as an intermediate host. As the cat becomes immune to the parasite and stops shedding oocysts, it instead develops tissue cysts containing slowly replicating forms of the parasite. Intermediate hosts do not shed oocysts, but produce tissue cysts that remain infectious for the life of the animal and are infections to cats and other intermediate hosts, including humans. The oocysts can be moved to cat-free areas by moving soil (e.g. in plant tubs or turf), by earthworms churning the earth and by the wind moving spores when it carries dust particles (aerosols). it is therefore an environmental hazard.

industrialised nations, poorly cooked meat is the most significant source of infection, particularly goat, mutton, lamb and pork. In many areas of the world, approximately 10% of lamb and 25% of pork products contain Toxoplasma cysts. In the UK and other industrialised countries, human Toxoplasmosis is mainly seen in those who eat undercooked meat - the vogue for eating meat "rare" or "blue" carries risks. Contact with contaminated soil is a major means by which toxoplasmosis is spread - including soil of vegetables. The organism may be present in some unpasteurized dairy products, such as goat's or sheep's milk. Some American meat producers take the extreme approach of eradicating cats around intensive farms or pregnant animals, but this extreme measure is not practised in Britain where animals are farmed less intensively (open pasture, stalls or barns rather than high density "feed lots") and where there is less artificial feed to attract the vermin (intermediate hosts) that attract cats. Thorough cooking of the meat is a more practical and common-sense way to remove the risk of human infection.

Good hygiene can prevent infection. Avoid rare/undercooked meat and unpasteurised goats milk products. Garden vegetables should be thoroughly washed to remove soil which may be contaminated. Chopping boards which have been used for raw meat should be washed in hot water before they are used for foods to be eaten raw or for ready-to-eat cooked foods. Meat should be cooked to a minimum of 58 Centigrade for 10 minutes or 61 Centigrade for four minutes to kill the tissue cysts. Thorough cooking at 70 Centigrade/158 Fahrenheit for 15-30 minutes is recommended. Microwaving is not a safe way to kill tissue cysts as the heating is uneven. T gondii oocysts can remain infectious when stored in a refrigerator (4 Celsius) for up to 54 months. Freezing meat at -12 Celsius to -20 Celsius for three days kills tissue cysts as does curing or smoking. Gamma irradiated food is free from any risk of infection (gamma irradiation is used to sterilise meat products imported into Australia).

Other precautions are important to pregnant women (or those trying to become pregnant) and those with poor immune systems. Avoid contact with potentially contaminated soil, or wear rubber gloves. Wash with soap and water afterwards. Do not touch mouth with soil-tainted fingers. Cover children's sandboxes to prevent contamination by cats. Clean out litter trays daily i.e. before oocysts can become infectious. Wear rubber gloves for this task (if pregnant, you may wish to get another family member to empty the litter trays). Preferably disinfect litter tray with boiling water. Disinfect potentially contaminated litter boxes with scalding water or with dry-heat sterilization (55C, 131F). Chemical disinfection does not reliably destroy oocysts. Wash hands after handling litter trays; even if you have worn gloves.

Left: Wear rubber gloves when handling soiled litter trays.
Right: The litter tray has been thoroughly disinfected in a sodium hypochlorite bleach solution and is safe to handle.
Photos: Sarah Hartwell


Avoid rare or undercooked meat, unpasteurized dairy products and unwashed garden vegetables. Vegetables should be scrubbed and preferably not eaten raw. All food preparation areas should be kept disinfected and cat-free. Keep surfaces on which raw meat has been prepared scrupulously clean, preferably use a separate chopping board for raw meat. Disinfect these surfaces after preparation of raw meat and scrub hands thoroughly. It is possible to test cats and humans (especially women contemplating pregnancy or already pregnant) to see whether they have antibodies. Those without antibodies are more susceptible to infection and preventative measures should be taken. Those with antibodies are unlikely to be reinfected.

Protect cats from infection (or reinfection) by preventing access to birds, rodents, uncooked meat, and unpasteurized dairy products. Food preparation areas can be infectious to cats! If pregnant (or trying) avoid handling free-roaming cats as their fur or paws may be contaminated; this also means keeping indoor-outdoor cats off of bedding, pillows etc and kitchen counters. Avoid cuddling a cat with diarrhoea or bowel incontinence or one which has an illness (FIV, FeLV) which might reactivate tissue cysts.

Shepherdesses and cats alike should avoid contact with sheep placentas or aborted lambs and placentas during the lambing season as these may harbour the toxoplasma germ. Cats in rural areas are often attracted to placental material as a food source.

Toxoplasmosis may also enter the water cycle through flushable cat litter or through infected soil being washed into drains. The oocysts are extremely reistant to water treatment and in 1995, a large human outbreak of toxoplasmosis was traced to the municipal water supply of the Greater Victoria area of British Columbia, Canada. Elsewhere, the parasite is flushed out to sea and has been responsible for the deaths of Sea Otters. Cat owners in coastal areas of the North America are advised to incinerate used cat litter (complete with faeces) or to bag it and dispose of it with household rubbish.



Mycobacterium tuberculosis and related tuberculosis-causing mycobacteria can infect cats and be transmitted to humans. It is particularly dangerous to humans with poor immune systems e.g. with HIV. The infection route appears to be through cat interaction with infected cattle, infected wildlife or other infected cats. Since there is no fully effective treatment in cats, euthanasia is usually recommended to reduce the risk to humans. It is, however, a rare disease in cats and is newsworthy because of its rarity. Most human cases of TB are from human-to-human contact in unvaccinated individuals.


Most viruses are extremely species specific. Feline Immunodeficiency Virus, Feline Infectious Peritonitis, and Feline Leukaemia Virus can not cause illness in people. A cat can not catch a human common cold, although there is indication that canine coronavirus infection may potentiate FIP in cats and human influenza can infect cats. As well as rabies, there are a few feline viral infections which can cause illness in humans. At the end of this section there is a specific note about feline coronaviruses to clear up the misunderstanding which arose when some newspapers mistakenly attributed the SARS coronavirus to cats (felids) instead of to civets (viverrids).

Rotaviruses cause diarrhoea in a number of species; the infection is usually transient and remedied with diarrhoea treatments. Rotaviruses can cross species and though affected cats are not considered infectious to humans, rubber gloves should be worn when clearing up diarrhoea and the affected area (litter trays, floors, carpets etc) disinfected as a precaution.

Borna virus (Borna Disease) is found in horses, ruminants, rabbits and cats and is transmitted by saliva, tears and nasal mucus. It has been found in continental Europe and Japan, but not in the UK. In cats it causes a progressive "staggering disease": abnormal behaviour, increasing inco-ordination, paralysis and death. Borna virus antibodies have been detected in the cerebro-spinal fluid of some humans with psychiatric disease. Hantavirus (Puumala virus and Hantaan) has been detected in cats in the UK and though cats may be considered a risk factor, they don't appear to be a major reservoir of infection. West Nile Fever has also been detected in cats (not in the UK) but was not a source of human infection.

It is not recommended that you share your ice cream with your cat - her mouth may contain viruses and bacteria. That tongue may have been licking her bottom shortly before licking your ice cream.

Photo: S Hartwell and "Sappho" Hartwell

Notes: Many cats cannot digest the milk in ice cream. The owner lives and works with cats and has built up a good immunity to most of the minor infections they carry.



Rabies (Lyssavirus) is a viral infection spread through the saliva of an infected animal e.g. via biting or by saliva entering an open wound. It attacks the central nervous system causing a variety of symptoms (including behaviour changes) and is almost always fatal. Documented cases of human recovery are extremely rare. Carriers do exist but are uncommon. In areas where rabies is endemic, cats should be routinely vaccinated. In areas where it is prevalent, it may be necessary to keep cats indoors away from other carriers and also to vaccinate humans at risk of coming into contact with infected animals. Rabies vaccines are available for cats and for humans, but post-exposure treatment is also required. Oral vaccines may be given to wildlife to limit the spread of rabies in the mammal population. An infected animal which has bitten a human is destroyed and presented for necropsy to confirm whether it was infected by rabies. Rabid cats may exhibit the passive form of the disease and not always become furious biters (the classical image of the disease) hence cat to human transmission is not common. Excepting isolated cases of infected bats, quarantine and the Pet Passport Scheme has kept Britain rabies free. Other island nations impose strict quarantine of imported animals.

Feline Leukaemia (FeLV)

The potential transmissibility of FeLV was highlighted by a Cambridge researcher (Jarrett) in 1994 in a work related to cancer-causing viruses. In cats, FeLV causes leukaemia, lymphoma (a cancer), anaemia and immunosuppression (immune system failure). Most cats die due to immunosuppression. According to the Vet Record in 1989, in the UK, about 18% of sick cats and 5% of healthy cats are FeLV positive.

The idea of an infectious agent being the cause of a childhood leukaemia led researchers to wonder whether acute lymphoblastic leukaemia in children was caused by FeLV through close contact between children and cats. They had several reasons for suspicion. Firstly, FeLV is spread "horizontally" i.e. from cat to cat through close contact and interaction. Secondly FeLV can infect and multiply in cells from other species in laboratory conditions - in the laboratory, FeLV was able to multiply within human bone marrow cells. Thirdly there have been reported clusters of cancers or leukaemia in human patients and simultaneously in their pets.

FeLV is found in large quantities in saliva, blood and urine of infected cats. It is contagious among cats living in close contact or crowded conditions. It is spread from cat to cat through licking, scratching and biting. Cats may also lick, scratch or bite humans, exposing humans to the virus. Can the virus multiply in human tissue or cause outside of laboratory conditions?

Human bone marrow cells can be deliberately infected with FeLV in the laboratory. There are 3 strains or Subgroups of FeLV: A, B and C. In 1993 it was demonstrated that FeLV-B and FeLV-C can be grown in human bone marrow cells. FeLV-A cannot grow in human cells in the laboratory. FeLV in the cat population is spread by FeLV-A. FeLV-B and FeLV-C occur in cats by mutation or recombination of FeLV-A but are not easily transmissible among cats. Even though two types of FeLV were made to cross species barriers and infect cells in petri dishes, these were the relatively uninfectious strains and there is no evidence that infection can occur in nature.

Where there were clusters of lymphomas developing simultaneously in people and their pets there was no evidence that this was anything more than coincidence. Researchers attempted to investigate (using epidemiology and immunology) whether people who regularly handled infected cats had a higher incidence of cancer than those who did not interact so closely or frequently with cats. They examined the mortality of vets whom they considered a high risk group. Although vets come into contact with a wide variety of zoonotic diseases, there was no evidence that exposure to FeLV was a cause of death. Epidemiological surveys of cat-owning households also failed to find a link between cat ownership and leukaemia. One 1970 study appeared to show an increase in the risk of childhood leukaemia in children exposed to "sick" cats. There was no definition of "sick" and no apparent link between sick cats and adult leukaemia.

Immunological (serological) testing provided no evidence of transmission of FeLV from cat to human. Another approach was to test for FeLV antibodies in patients with various malignant cancers. No evidence of FeLV in humans was found, not even where patients with acute lymphocytic leukaemia had lived with FeLV positive cats which had developed lymphosarcoma.

All of the studies had a small sample size which means no reliable conclusions can be drawn from any single study. When all of the studies are considered together, there is no evidence that FeLV is infectious to human beings. Although there is no evidence that FeLV can infect or cause disease in humans, from a public health perspective it is often recommended that immunosuppressed people and possibly also children should have limited contact with infected cats.  Where there are worries, cats should be tested for FeLV and, if they do not already have the disease, vaccinated against it.

This cat should not be on a food preparation surface and certainly should not be drinking milk from a mug which is about to be used.

She may be carrying all manner of germs on her paws, particularly if she has access to outdoors or has just used her litter tray. Her mouth may contain germs from where she has just washed her bottom.

Photo posed by "Aphrodite" Hartwell.

Notes: The owner lives and works with cats and has built up a good immunity to most of the minor infections they carry.


Feline Immunodeficiency Virus (FIV, Feline AIDS)

A comparison between FIV and HIV shows that the two viruses differ virologicallv, pathologically, and epidemiologically. Although there is no evidence that FIV can infect or cause disease in humans, from a public health perspective it is often recommended that immunosuppressed people should have limited contact with infected cats. However, the risks are considered extremely small and for most patients, the benefits of feline companionship greatly outweighs any risk.

In 1999 it was reported that the FIV virus was helping researchers in developing a potential treatment for drug-resistant HIV. A new chemical active against FIV was also active against HIV in test-tube experiments. However, that same year researches at the University of Calgary reported that FIV could infect primates (macaques) injected with the FIV virus (mimicking a cat bite scenario). This did not prove that FIV could infect humans, but means FIV might not be suitable for use in gene therapy (where modified viruses are used as carriers for genes). They already knew FIV could infect human blood cells in test tubes, but wanted to find out if living primates could be infected. The monkeys lost up to a fifth of their bodyweight and lost up to 80% of crucial immune cells. Though one appeared to regain its health, when it was infected with tetanus 9 weeks later, FIV was found in its blood - showing that the virus stayed dormant in the macaque.

On a more positive note, an article in 2002 reported that the US Department of Agriculture had approved a FIV vaccine developed at the University of Florida's College of Veterinary Medicine. it gives about 60% protection for a year and vaccinated FIV-negative cats would further reduce the already small risk to vulnerable humans.

Chlamydia and Psittacosis

Chlamydia causes conjunctivitis in cats and, less commonly, respiratory symptoms. In humans it can cause pneumonia; however human infection (psittacosis) is generally from parrots and cage birds. There are only isolated cases of chlamydia being spread from cat to human.

Chlamydia psittaci was renamed chlamydophilia felis in 2002, reflecting its role in causing conjunctivitis in cats and its prevalence. However, the old name is still widely used and probably more familiar to doctors and veterinarians.


Feline Cowpox is an uncommon virus which causes skin lesions in cats. There is no vaccine and treatment involves antibiotics to treat secondary bacterial infections. Cats are the source of about half of all human cowpox virus infections; there are about two or three reported cases of human cowpox per year in the UK. Cat to human transmission can be prevented by basic hygiene precautions. Human infection is often limited to a single lesion on the hand or face (after touching it with an infected hand). Widespread infection and severe disease can develop in individuals with poor immune systems or pre-existing skin disease. Human cowpox can cause systemic illness with flu-like symptoms, which may require hospitalization. Occasional human deaths have also been reported, but these are extremely rare. Smallpox vaccination may only provide partial protection. Those handling infected cats should wear gloves and take care not to allow infected material into wounds or the eyes. Young children, the elderly and those with a existing skin condition or with poor immune systems should avoid contact with the cat until its scabs have completely healed. The poxvirus is extremely hardy, and may remain infectious in dry material kept cool for several months or even years. It is susceptible to most disinfectants.


This section is included because of media confusion over the cat's role in SARS and headlines such as "SARS triggered by cat eating" which fail to distinguish between civet cats and true cats.

Spring 2003 saw the emergence of a previously unknown atypical pneumonia called Severe Acute Respiratory Syndrome (SARS) which had originated in Guangdong Province, China and had a fatality rate of approx 20%. The causal agent, SARS coronavirus (SARS CoV), is new member of a large and widely distributed group of RNA viruses. The SARS CoV was linked to the Masked Palm Civet which is farmed for food in Guangdong province. Coronavirus samples from Palm Civets are genetically very similar to the SARS CoV. Researchers believe that a strain of Palm Civet coronavirus jumped from civets to humans. Inadequate preparation of the meat is implicated in this zoonosis as is poor hygiene since the virus is found in Palm Civet faeces and spittle.

The Palm Civet (Paguma larvata), a relative of the mongoose (viverrid family) and not a cat at all. The term "Civet Cat" describes a variety of cat-like creatures such as the North American Civet Cat (Bassariscus astutus) or Ringtail, related to the racoon, and the musk-producing African Civet Cat (Civettictis civetta), another viverrid. The term "Civet Cat" is sometimes used for the true cats Asian Leopard Cat (Felis bengalensis) and African Wildcat (Felis libyca). Although true cats are not implicated in the spread of SARS, the Singapore government has been rounding up and destroying stray cats as a hygiene precaution.

There are cat-specific coronaviruses, but these are not linked to SARS or to other zoonoses. True feline coronaviruses (FCoV) are Feline Infectious Peritonitis viruses (FIPV) and Feline Enteric coronaviruses (FECV). Although these two coronaviruses are closely related, FIPV causes a fatal disease in cats while FECV usually only causes milder, short-term enteritis ("tummy upset"). Neither is known to cause disease in humans.

Following a report that cats were found infected with SARS in a Hong Kong apartment complex, medical researchers at the University of Texas Medical Branch, Galveston, found that cats can catch and transmit SARS under laboratory conditions. The researchers inoculated six cats and six ferrets with the virus cultured from a SARS victim, infecting them via the trachea, eyes and nose. The animals showed signs of infection two days later and produced antibodies within 28 days. The cats did not appear to be affected by the virus, but developed mild pneumonia. Scientists also placed two healthy cats and two healthy ferrets with the infected animals; the ferrets became infected within 2 days and died after 14 days. When the experimental cats were killed by the researchers, the virus was found in their respiratory tract.

The Galveston researchers suggested cats could be a reservoir for the virus and suggested quarantining cats as well as humans. However, the World Health Organisation's chief SARS scientist, D. Klaus Stohr, said: "These animals in all likelihood did not play a significant role in the spread of SARS to humans." SARS may be versatile enough to between human and animal hosts, but while cats can be infected under laboratory conditions there is currently no evidence that they are a vector for the virus outside of the laboratory.

Asian Bird Flu & Feline H5N1

In February 2004, scientists in Thailand confirmed the first cases of Asian Bird Flu in cats. The H5N1 virus is most common in intensively farmed chickens, but also affects pigeons, ducks and other wildfowl. It had killed 22 people across several Asian countries. Thai veterinarian Teeraphon Sirinauemit confirmed that H5N1 bird flu had been found in domestic cats and a white tiger (by 2006, 147 tigers in a Thai Zoo had died after eating infected chicken). The discovery was significant because every time the virus jumps species, the risk of mutation into a human form increases. Autopsies and virus tests found H5N1 in 3 cats at Kasetsart University's animal hospital. They were among 15 cats owned by a man living near an infected chicken farm in Nakorn Pathom, 37 miles (60 Km) west of Bangkok. At least one cat had eaten an infected chicken carcass and 14 of the 15 cats died. A white tiger at Khao Khiew private zoo in Chonburi province tested positive for H5N1, but recovered. H5N1 killed a rare Thai clouded leopard at the same zoo in January 2004. The cause was raw chicken fed to the big cats.

The occurrence of H5N1 in domestic cats is due to eating raw chicken, proximity to infected farms, through contact with infected bird carcasses or through close contact with cats already infected. All the affected domestic cats had a high degree of exposure to the virus. Almost all cases of H5N1 in humans are traceable to infected chickens or infected wildfowl. Although cats are susceptible to the virus, they have not been found to pose a danger to humans. Thai pet owners took immediate precautions, switching meals from cooked chicken to beef. Owners were advised not to let their cats eat dead chicken carcasses, dead birds, or any dead animals found in infected areas. Cat lovers who feed the strays and semi-feral cats around temples were asked not to feed raw chicken. This aims to prevent H5N1 from jumping to other cats and reduces the risk of mutation.

By April 2006, there are still no known cases of bird flu transmitted from cats to humans. However, it is recommended that cats be kept indoors (and dogs be kept on the leash) in areas where H5N1 is confirmed. There is no data on how easily cats become infected or whether they can excrete the virus in a form infectious to humans or back to birds once infected. Infected cats get lung or gut infections. In areas where H5N1 is not endemic there is little cause for owners to be concerned. Cat welfare groups fear a witch-hunt against feral cats and an increase in abandoned or unwanted cats. Preventing cats from coming into contact with infected birds keeps cats and their owners safe. As with any other illness, sick cats should be isolated from other cats and gloves should be worn when emptying litter trays.

Feline H5N1 has been found in other areas where H5N1 is prevalent in poultry: Thailand, Indonesia (where the disease is common among cats) and Iraq. In March 2006, 3 cats were infected in Germany on an island where wildfowl were infected. Being predators, cats pick off weak and sick birds; birds weakened by the virus are easy prey. Any animal that hunts birds or scavenge dead birds are potentially at risk e.g. dogs, foxes and seals. On the Baltic Sea island or Ruegen it has been found in a stone marten. There are reports of H5N1 in pigs, weasels and civets.


Feline Spongiform Encephalitis is a prion disease rather than a virus. As with human vCJD, it may be related to consumption of infected cow products although another chemical agent (producing BSE-like symptoms) has been implicated in the past. Since transmission is believed to be by eating contaminated food, an infected cat does not pose a threat to its owners. Veterinarians handling FSE-infected cats (especially post mortem) should take standard precautions to avoid tissue or fluids coming into contact with open wounds.


Certain parasites may be transferred from cat to human. Fleas are the most common and cause itchy, sometimes inflamed, spots. Some people are more prone to flea bites than others and some people react worse than others. Ticks may also attach to humans but are picked up from long grass where a gorged tick has fallen off of its former host; a tick attached to a cat stays there until it is gorged or until it is removed by the owner/veterinarian.

In some countries around the world, infected fleas or ticks can spread an animal disease from its normal host (rats, rabbits etc) to humans. Free-ranging or hunting cats sometimes play a role in physically transferring a flea or tick from its prey to a human. If the flea or tick is indiscriminate in its eating habits, it may bite and infect either the human or the cat (or both).

Plague (Yersinia pestis) is primarily carried by rats and their fleas. Cats are also susceptible to plague in the same way as humans and can transmit the pneumonic form. Plague has several types: bubonic (black pustules), septicaemic (blood poisoning), pneumonic (respiratory), pharyngeal (throat infection) and meningeal (brain infection). Although it has a notorious reputation, it is treatable with antibiotics. It is endemic in some parts of the world and is a notifiable disease in the UK.

Tularaemia (Rabbit Fever, Francisella tularensis) is endemic to North America and northern Eurasia, but not found in the UK. It is spread by infected ticks, contaminated water and aerosol (sneezes, coughs). Cats are also susceptible to the disease themselves. In humans it can cause fever, swollen glands and septicaemia.

Rickettsia typhi (endemic/murine typhus) - this is transmitted by rat fleas that are sometimes picked up by cats. In most healthy people it produces a flu-like illness, sometimes with a skin rash, and is treatable with antibiotics. It is endemic to parts of the USA and is not found in the UK.

Rickettsia felis or (cat-flea typhus) causes flea-borne spotted fever in cats. The bacteria can also infect humans and cause illness. fleas are the usual vector carriers of the bacteria and it has been found in cat flea populations around the world. Mosquitoes are also carriers and R felis is now found on every continent except Antarctica. The host is usually infected by flea faeces coming into contact with scratched or broken skin. Human cases of Rickettsia felis, whether contracted from infected cat fleas or mosquitoes, have poorly defined symptoms such as fever and a rash. In some individuals this can lead to pneumonia. Despite mosquitoes being a vector, countries with strong anti-cat factions emphasise the link with cat fleas. This is another reason for keeping cats free of flea and ticks.

Lyme disease (borrelia) is another tick-borne disease with a variety of hosts depending on the region and the species of bacteria (ranging from deer to rodents, birds and lizards). It is endemic in some parts of the USA and is increasing in the UK. Between blood meals, the ticks remain in long grass and may attach to foraging cats. Cats may contract Lyme disease and may also physically transfer the ticks into the home. It is treatable with antibiotics. If left untreated it may result in chronic disease with joint pain and neurological symptoms.

Shampoo shares Philip Harvey's glass of water - cute, but not hygienic!
Photo: PJ Harvey

Some feline intestinal parasites can also be transmitted to humans. Roundworm eggs infect humans, particularly children, through a faecal to oral route. Tapeworm eggs are not directly infectious to people, but need an intermediate host.

Tapeworm The most common tapeworm is cats is Dipylidium canium. Tapeworm eggs or egg-filled tapeworm segments are passed in the faeces and resemble grains of rice around the anus but can be seen to squirm. These can remain in the environment for days or months until eaten Cat flea larvae infest the tapeworm eggs which remain inside them when the larvae become adult fleas. Cats - or humans - are infected when they ingest the fleas. These are treatable and also preventable through good hygiene and flea control. Toddlers are most often at risk as they tend to put their fingers or foreign objects in their mouths.

Echinococcus tapeworms can be picked up by cats from cysts in the tissues of prey animals (wild herbivores and rodents are the intermediate hosts). The cats then shed eggs in their faeces. Humans can become infected through eating poorly washed food that is contaminated with the eggs. In areas where this tapeworm is endemic, all fruit and veg should be thoroughly washed before eating; cats should be wormed regularly. In humans, infection can have no symptoms for many years until the cysts in internal organs grow large enough to be discovered. Large cysts can cause serious liver disease requiring surgery as well as drug treatment. It is found in South and Central America, parts of the Western USA, and in China and the Middle East.

Roundworms (Toxocariasis)(Toxocara cati, Toxoscaris leonina) are common feline parasites picked up from the environment. The eggs hatch in the cat's intestine and migrate to the liver and lungs. Parasites that are coughed up and swallowed then mature in the gut and the eggs are excreted in faeces. They can survive in the soil for several months. Infected cats often show no symptoms or suffer gastroenteritis. Severely infected cats may suffer stunted growth or obstructed gut. In humans, the larva may migrate to the internal organs, eyes or skin. In humans, the larvae cause damage as they migrating through tissues. Roundworm infection in humans is not uncommon (especially in small children) and there are numerous medications available. It is preventable by good hygiene and ensuring humans don't directly handle cat faeces (e.g. in the garden or sandpits).

Heartworms (Dirofilariasis immitis) are found in cats in many parts of the world. Although known as heartworm, the parasite spends much of its life in the arteries of the lungs. They can be transmitted to humans by mosquito bites and may cause lesions and nodules e.g. of the lungs. The parasites cannot mature in cats or humans and end up in the arteries where they can form an obstruction or respiratory problems. Most human cases are reported from the USA, Japan, and Australia. There are heartworm preventative drugs available for cats in areas where the parasite is prevalent.

Liver flukes (Opisthorchiasis) are sometimes transmitted from cat to human. Cats and their owners are both at risk of infection through eating raw or poorly cooked fish in areas where the liver fluke is endemic. The eggs are excreted in cat faeces and ingested by a freshwater snail (intermediate host) which releases the larvae into the water where they are picked up by fish (second intermediate host). When an infected fish is eaten by a mammal, it completes its lifecycle and produces eggs that re-enter the environment.

In the USA, there are a number of parasite larvae that are excreted in cat faeces and can infect humans through wounds in the skin. These are generally treated with ointments or skin powders. They can be avoided by not walking barefoot in areas where cats (or other carriers) are likely to defecate.