There are the known problems common to herbal treatments such as unknown and.or variable concentrations of active compounds in herbs from different sources and even from the same source but harvested at different times and under different conditions. Sometimes demand for herbs or other natural products can lead to damage or even extinction to rare species used in various healing traditions. There are many examples of contamination of herbal products, and some traditional remedies that may be represented as natural or herbal may be neither. When demand for a certain herbal product rises quickly (Such as the use of Chinese Astragalous for influenza viral infections) there may be a strong motivation to use different portions of the plant than usual or to substitute other related plants or to adulterate the product with something else to make up the volume.
These types of problems are common to many if not all herbal remedies sold and are what I consider known unknowns when dealing with these types of products in veterinary medicine.
When herbal products that may have been used traditionally for humans (it is fairly common for veterinarians to use Chinese herbal medicines on their patients) unknown unknowns come into play.
Often different species absorb and metabolize the same plant or drug very differently, and sometimes unexpected toxicities can arise in different species. Even in herbivores, different species can react differently to toxic plants, and one species may be able to consume a given plant safely, while another species may not.
Once you give a carnivorous animal (especially cats) herbs other problems arise. Cats have much lower levels of some liver enzymes such as glucuronosyltransferase(1) which converts toxic metabloites of drugs such as acetaminophen to less toxic compounds. One reason cats may have lower levels of such enzymes is that cats are obligate carnivores, and eat less plant material than other carnivores such as canines and bears, and probaly have less need to metabolize plant toxins.
Cat's red blood cells are also more sensitive than those of other species to oxidative damage, and small amounts of onion or garlic can cause Heinz body anemia in cats. when I was in veterinary school in the late 1980's, it was common to syringe or tube feed sick cats baby meats. Some baby meats have onion powder added for flavor, and there were cases of cats developing heinz body anemia after a few days of feeding baby meats containing onion powder. Now special diets are available for critically ill pets and this problem is not so common. Cats are also more sensitive than dogs to plant-based insecticides such as Pyrethrins and "natural" insecticides such as essential oils. Both dogs and cats have been poisoned by the use of pennyroyal oil, a popular substitute for flea and tick control products (2).
While some of these examples are well known and documented, I call this type of problem "unkown unkowns" because these issues are often unanticipated and are discovered when someone gives their pet a product and then discovers that the animal is not responding well. Other examples of this type of discovery include grape/raisin toxicity in dogs, and Lilly toxicosis in cats (3).
Because of the variable nature of herbal remedies and the potential for unknown and unforeseen toxicity, I think veterinarians and pet owners should be very cautious when using these types of products in animals. Unfortunately, the primary reference for veterinary herbal medicine, while giving lip service to these issues, also seems to put great value in untestable concepts such as Chinese and Ayurvedic forms of vitalism, and repeats standard CAM claims that natural products are always safer and less toxic than standardized pharmaceuticals. While they do state that essential oils should be avoided, they often state that many of these products "appear to be safe", without much consideration for the variables and unknowns I have discussed here. The book does not inspire confidence in me at least.
- Sellon, R.K. Acetaminophen, pp. 550-558 in Small Animal Toxicology, 2nd ed. 2006, Peterson and Talcott, eds, Elsevier/Saunders, St. Louis.
- Poppenga, R.H. Hazards Associated with the Use of Herbal and Other Natural Products, pp. 312-344 in Small Animal Toxicology, 2nd ed. 2006, Peterson and Talcott, eds, Elsevier/Saunders, St. Louis.
- Mason, J.A., Khan, S.A. and Gwaltney-Brant, S.M. Recently Recognized Animal Toxicants pp. 138-143 in Current Veterinary Therapy XIV, 2009, Bonagura and Twedt, eds.Elsevier/Saunders, St. Louis.