Pau D’arco, the extract of a South American tree bark, is often touted as a safe natural alternative to antibiotics and antifungal medicines in treating skin infections. Oral Pau D’arco is also touted as an immune booster.

The problem is, many infections can take a long time to resolve. Antibiotics often kill off beneficial bacteria and hence fungal infections can increase. With the concern of antibiotics entering the environment and leading to resistant strains of bacteria, pet owners have become increasingly interested in natural remedies. Owners often figure that if a remedy has been used safely for years by native people it must also be safe for pets. Not so!

Pau d’ arco is an inner bark of the Tabebuia impetiginosa tree of South America. It is a naturally occurring organic antibiotic and antifungal. Tincture of pau d’ arco mixed with an equal quantity of mineral oil, given twice or three times daily, is recommended as a topical treatment for skin problems and ear problems in cats. Some owners are also seduced into administering it orally (mixed in food) in the belief it is an immune booster.

ACTIVE INGREDIENTS & ADULTERANTS

The Tabebuia impetiginosa tree is now on the endangered species list. There is concern, and some evidence, that bark from other types of trees (e.g. Brazilian mahogany sawmill shavings) is sometimes sold as pau d’arco. At least 10 species of Tabebuia are logged commercially in South America. The different species are identified by their leaves and flowers; parts that are stripped after felling. Once in the sawmill there is no way to tell if the bark is from Tabebuia impetiginosa or another species. If the Pau d’arco isn’t properly tracked through processing or quality controlled, you are playing Russian roulette.

The active ingredient is lapachol and it also contains hydroquinone (a benzene compound), which is much more toxic to cats than it is to most other species. Although Pau d’arco is claimed to be less toxic than caffeine, that fact applies to humans rather than cats or dogs.

TOXICITY

Topically applied preparations can be absorbed through the cat’s skin and accumulate in the tissues; in cats these especially accumulate in the liver. Studies in rodents found that higher doses of lapachol can promote cancer metastasis (spread), as well as cause changes in DNA that can lead to cancerous tissue changes. Ironically, it is often considered a cancer cure! Lapachol is also toxic to foetuses and affects male fertility (Fetal growth in rats treated with lapachol ).

Pau D'arco contains hydroquinone. The oral LD50 values (the dosage at which 50% of the test subjects are fatally poisoned) for several animal species range between 300 mg/kg body weight to 1300 mg/kg body weight. That means it is relatively non-toxic in most species.

Cats are a special case. Because they are obligate carnivores they have poorly functioning livers that can’t cope with many plant toxins. Cats rely on their prey to have already broken down plant toxins. The hydroquinone LD50 values for the cat range from a very low 42 mg/kg body weight to 86 mg/kg body weight because the cat's liver is poor at metabolising it. Acute high-level exposure to hydroquinone causes severe effects on the central nervous system including hyperexcitability, tremor, convulsions, coma, and death. At sublethal doses, these effects may be reversible (IPCS International Programme On Chemical Safety, Health and Safety Guide No. 101: Hydroquinone).

Being water-soluble, hydroquinone is quickly absorbed in the gut if it is eaten, and in the trachea (windpipe) if inhaled. Absorption through the skin is slower, but may be speeded up if the hydroquinone is dissolved in alcohol. Once inside the body, it is distributed rapidly. In animals equipped to metabolise and detoxify it, the metabolites include p-benzoquinone which is excreted by the kidneys. In high doses, and in animals not equipped to metabolise it, it causes kidney damage which may prevent it being excreted. This means the hydroquinone and p-benzoquinone accumulate in the body.

As with many things, natural does not mean "safe" and "safe for humans" does not mean "safe for cats".