West LaFayette, Ind. — Following an aggressive deworming schedule for dogs and cats could have a significant impact on the zoonotic transmission
of parasitic diseases, according to one expert.
"The focus in the last few years has been on more regular deworming, monthly and year-round," says Kevin R. Kazacos, DVM,
PhD, professor of veterinary parasitology and director of the Clinical Parasitology Laboratory at the Purdue University School
of Veterinary Medicine in West Lafayette, Ind. Kazacos is past president of the Companion Animal Parasite Council (CAPC).
The increasing presence of animals as part of the family unit brings more opportunities for transmission of disease from animals
to humans. Although there has been a strong public focus on the dangers and prevention of heartworms in animals, intestinal
parasites such as roundworms (Toxocara canis and T. cati), raccoon roundworms (Baylisascaris procyonis) and hookworms (Ancylostoma spp.) are, in many cases, more common and may be transmitted directly to humans.
The CAPC recommends:
Because puppies and kittens are especially vulnerable to such infections, their popularity means children can be at the most
risk for zoonotic transmission. Recent studies document a national seroprevalence of dog and cat ascarid larvae in children
of 14 percent, and it approaches 28 percent in some areas of the United States and Canada.
"We see human cases in this country all the time," Kazacos says. "These are more common than people realize, but they are
not reportable diseases so they're hard to track. But it doesn't mean that they don't occur."
Toxocara in animals and humans
Toxocara (roundworm) infections are common in animals throughout the world, with some studies showing that over 30 percent of dogs
younger than 6 months in the United States shed eggs in their feces. Infections in dogs and cats result from the ingestion
of eggs from fecal contaminated environments, transmission of larvae to the fetal or young animal via transplacental or transmammary
routes or ingestion of larvae contained in the tissues of other animals. Infected animals may present with potbellies, poor
coat, GI signs and a failure to thrive. Severe infections in very young animals may even be fatal.
Zoonotic transmission of roundworms to humans is well-documented and, by no means, rare. It is estimated that up to six million
people are infected each year with Toxocara larva migrans (Schantz, PM. Toxocara larva migrans now. Am J Trop Med Hyg 1989;41(Suppl):21.). Especially at risk are children whose activities bring them into
contact with dirt or other areas that may be contaminated with eggs from the stool of infected animals. High prevalences of
toxocariasis have been documented both in rural and urban environments, wherever animals roam freely and/or regular deworming
schedules are not instituted or maintained.
In humans, larval infections can be located in the eye (ocular larva migrans, or OLM) resulting in permanent vision loss; in the viscera (visceral larva migrans, or VLM) where damage and inflammation
are located in the liver, lungs and other organs; or in the nervous system (neural larva migrans, or NLM), causing clinical
neurologic disease. Young children with heavy infection are at the greatest risk of developing VLM, with liver swelling and
inflammation, respiratory disease from lung migration and high, persistent eosinophilia. However, many cases are mild or subclinical
(covert infection), related to lower levels of infection and the presence of nonspecific clinical signs.