Parasitic zoonoses: What you don't know can hurt you (Proceedings)


Parasitic zoonoses: What you don't know can hurt you (Proceedings)

Aug 01, 2008

Parasitic Zoonoses

Unlike bacterial infections, parasites are much easier to prevent. There are a wide variety of effective deworming medications that can eliminate or at least significantly decrease the chances of contracting parasites. Unfortunately, the indications are that we as veterinarians do a relatively poor job of this. In 1991 a survey of veterinarians showed that recommendations in regard to parasite control were inadequate.9 Only approximately ⅓ of veterinarians routinely discussed the zoonotic risk of parasites with owners. Almost ⅔ of the veterinarians incorrectly recommended treating intestinal parasites beginning at an age over 4 weeks. Less than half gave preventive anthelmintics to pups and dogs and about ⅓ did not recommend routine testing and treatment of nursing dogs. Approximately three fourths of the veterinarians tested for helminths in pubs. Unfortunately, over half of the respondents did not initiate therapy unless there was a positive fecal test. This is not appropriate since fecals can be negative even when the dogs are harboring adult parasites. Even more shocking was that almost half of the veterinarians surveyed considered roundworms or hookworms of little or no concern as a zoonosis. The majority of veterinarians in this survey did not even come close to properly addressing parasite burdens. More recently a survey was carried out of Connecticut pediatricians and veterinarians.10 The only comfort we can get from this study is that veterinarians seem to do a better job of discussing zoonosis than physicians. Interestingly veterinarians thought physicians or public health departments were responsible for public education whereas physicians thought it should be public health departments or veterinarians, which means both groups did not feel they had ultimate responsibility in this regard. Considering that ten years had elapsed between studies there was little indication that prevention strategies had improved. Only about 12% of veterinarians began deworming puppies at 2 to 3 weeks of age. The greatest majority (78%) didn't begin treatment till the puppies were over 6 to 7 weeks of age. Slightly more than 50% of veterinarians carry out prophylactic deworming of puppies and kittens, unfortunately almost half of them use an interval greater than 2 weeks as recommended by the Centers for Disease Control. Parasite control is an area where small animal practitioners need to do a much better job in the interest of the pets and the general public.


Roundworms are still of major zoonotic concern. Prevalences vary from 3%8 to 33%11 . Prevalences tend to be higher in kittens and puppies, though this may be misleading in that shedding is more intermittent in adults. Zoonotic transmission to humans does occur, with children being especially susceptible to the negative affects. Visceral larva migrans can be a devastating disease. In Connecticut 10.2 to 27.9% of children were seropositive for exposure to roundworms.10 Thousands of cases are diagnosed each year. Pups acquire Toxocara canis in utero, transmammary or fecal-oral. The adults can become patent and begin shedding eggs at 3 weeks of age. In cats in utero infection does not appear to occur, infection is transmammary or fecal-oral. The eggs are very resistant and can persist for years in the environment.


In dogs and cats it is inappropriate to test for intestinal parasites and then not treat because of a negative fecal. Shedding is intermittent and can be low grade. Routine prophylactic deworming is ideal in all patients; the current dewormers available are highly efficacious with a low incidence of side effects. Puppies should be started at 2 weeks of age and should be dewormed every 2 weeks thereafter until 8 weeks old. Kittens can be started at 6 weeks and repeated at 8 and 10 weeks. Four to 6 weeks after anthelminthic therapy is completed it would be advisable to perform a fecal exam to make sure the parasites have been eradicated. Treating the nursing dam is also indicated, as they will often begin to shed heavily around the time of parturition. Many heartworm preventives can also aid in reducing parasite burdens in adult dogs, although the 4 week interval will allow shedding and may not be adequate for heavily exposed dogs. If one of these heartworm preventives is not used then at least annual deworming is recommended, more frequently if indicated by exposure risk (puppies, kennel, etc.). Extensive recommendations for prevention of zoonotic transmission of intestinal parasites from pets to humans can be found on the CDC website at Another part of the CDC website that is well worth surfing is as there are great handouts available that explain about the parasites that can be very informative for clients.


Hookworms can be found in many areas of the country though the southeast appears to have a greater prevalence of Ancylostoma than other regions. Hookworm larvae can penetrate the skin of humans leading to cutaneous larva migrans. More severe manifestations are also possible with skeletal muscle involvement, visceral migrans and human intestinal involvement. Prevention programs are the same as for roundworms.


The most common tapeworm in dogs and cats, Dipylidium caninum, is not zoonotic. There is increasing concern however about Echinococcus multilocularis. Originally limited to Alaska it is now enzootic to northcentral US and southcentral Canada, though it has been identified in Wyoming, Nebraska, Iowa, Ohio, Indiana and Illinois.12 The main reservoirs are foxes and coyotes. Ingestion of eggs from this parasite by a human lead to alveolar hydatid disease, a potentially fatal occurrence. The liver and other organs are usually affected, surgery is often not curative. To date only a few people have been diagnosed with this disease, given its spread it is likely this will increase. Since this is a highly fatal disease preventive measures should be instituted. In areas where E. multilocularis is known to be dogs and cats that are predatory should routinely be dewormed with a medication that eliminates tapeworms as well as the other common intestinal parasites.

Given the central role veterinarians play in the healthcare of pets we have a vital role in minimizing the risk of zoonotic disease. We also need to be especially vigilant with these diseases as they can spread to other animals we are caring for or potentially to us or the people that work for us. The surveys that have been done to date do not suggest that we are doing the best that we could. Recognizing the importance of zoonotic disease and providing educational information to pet owners will increase the quality of medicine we practice and good medicine is always good for a practice.

References available upon request by the author.