Conference Coverage

Truths and Consequences: The Parasites of Dogs and Cats

Problems in human health when pets contaminate the environment.

By Maria D. Mileno, MD, and Frank J. Bia, MD, MPH

An intriguing symposium was presented at the 52nd Annual Meeting of the American Society of Tropical Medicine and Hygiene, which addressed various enigmatic parasitic infections of animals, and the serious problems that animal parasites may cause in humans.1 The chair for this excellent symposium was Dr. Leonard Marcus, VMD, MD, who has previously highlighted numerous infections for the medical community that are transmitted by animals and their parasites. They can reach humans in a variety of ways, including nosocomial transmission.2 The Companion Animal Parasite Council (CAPC) is currently writing guidelines for diagnosis, treatment, prevention, and control of internal and external parasites in dogs and cats. One of Dr. Marcus’ major objectives in organizing this symposium was to introduce CAPC to a medical audience, explain its significance for practicing physicians, and enhance cooperation between MDs and veterinarians. The web site will be and on it will appear the long and short guidelines that can be downloaded, biographies of participants, and links to other organizations such the American Association of Veterinary Pathologists and the American Heartworm Society.

This symposium began with an excellent summary of the importance of flea control in cats and dogs by Dr. Michael Dryden of Kansas State University. The common cat flea, Ctenocephalides felis, not only affects cats but also accounts for most flea infestations of dogs. It is associated with infestation of about 50 other animal species. The consequences of animal flea infestations have implications for both animals and humans. These include anemia in animals and their general annoyance to humans, along with flea allergy dermatitis, delusional parasitosis, and the potential for transmission of human diseases such as murine typhus. Occasionally, ova of the dog tapeworm, Dipylidium caninum, are ingested by larval fleas in which they develop into cysticercoid larvae. Infection of children may occur following accidental ingestion of an infected flea. Dr. Dryden noted that cat and dog fleas are somewhat unique, since they become permanent ectoparasites of animals unless effectively treated. The home environment of the animal then becomes heavily flea contaminated. These premises are difficult to eradicate fleas from, and resistance to available agents has even become a problem in this context. Flea infestation of a home will generally require extensive treatment of the premises.

Although the cat flea is a major problem for both dogs and cats, the rat flea offers other possibilities. A recent CDC report in MMWR indicates that 47 cases of murine typhus, the zoonotic disease caused by Rickettsia typhi, were identified in 5 islands of Hawaii during 2002: Maui had 35 cases, Molokai had 6, Oahu reported 3, Kauai had 2, and Hawaii had just 1. Symptoms and their frequency (%) included fever (98), malaise (89), headache (87), myalgia (81), loss of appetite (81), chills (81), arthralgias (72), nausea (60), vomiting (54), backache (53), abdominal pain (51), stiff neck (42), and skin rash (40). Severe disease complications occurred in 10 of these cases including acute renal failure, gastrointestinal bleeding, meningitis, encephalitis, pneumonitis, and CHF with pleural effusion. Peridomestic rodents and the oriental rat flea (X cheopis) have maintained the endemicity of murine typhus on the Hawaiian Islands. Before World War II, this disease was widespread in the continental United States, but improved rodent and ectoparasite control practices decreased transmission, with fewer than 50 cases reported annually from Hawaii, California, and Texas combined. While rodent depopulation programs are important, insecticides, which prevent arthropods from transmitting disease to humans, must also be considered and used.

The scheduled use of a 30-day residual adulticide, such as fipronil, on pet animals is suggested before the home becomes infested and presents owners with a more difficult problem in eradication. Topical application of these agents on the animal’s coat will kill newly emerged adult fleas before they can lay eggs, and also eliminate all stages of various tick species, including deer ticks that transmit Lyme disease, babesiosis, and ehrlichiosis. Fipronil acts by collecting within the oils of animal skin and hair follicles. Because it continues to be released from hair follicles onto the skin and coat over time, it has prolonged action for both tick and flea control. Travelers who know they will be staying in homes where dogs and cats are commonly present may want to invest in a supply of one of these very effective agents for animals and bring it with them on their travels to prevent discomfort and disease transmission to both themselves and their children.

The female sand flea presents other possibilities for human infections. A brief article appeared recently in Infections in Medicine and described a 25-year-old researcher who was studying bats in the Comoros Islands. Frequently not wearing shoes and crawling on his hands and knees, he developed itchy erythematous papules that eventually looked like typical lesions of tungiasis, containing a gravid female, which appeared as a central punctum that eventually blackened. They were asymmetrically located on palms and soles, and the fleas were removed with a sterile needle. The sand fleas, or Tunga penetrans jiggers, are about 1 mm and reside in earthen floors of homes, sandy soil, and beaches. Common hosts are humans, dogs, cats, horses, pigs, and birds. Complications can include secondary bacterial infection, autoamputation, tetanus, and even gangrene.

Although the focus of this symposium was on dogs and cats in North America, the scope of these problems will generally encompass most travelers as well. A review of the 4 major infections discussed included toxocariasis, hookworm infections, and Baylisascaris procyonis epidemiology and infection, in addition to a potentially emerging pathogen, Echinococcus multilocularis.

Peter Schantz reviewed the life cycle and clinical implications of Toxocara infections. Unless dams are treated for Toxocara infections, ova will be passed into the soil where they larvate, and these larvated ova of the parasite are quite hardy. They survive best when protected from sun in dry climates and, at times, have been documented to overwinter. Following ingestion, migrating larvae can penetrate the gut wall and travel to the liver and blood and disseminate throughout the body. The tissue phase of animal infections may last for years, and these organisms can be transmitted transplacentally. Interestingly, the parasites shed a glycoprotein that protects them from immunologic attack. There is a strong tropism of Toxocara organisms for the neurological system, particularly the eyes, in paratenic hosts. Paratenesis refers to the passage of an infective agent by one or a series of hosts, in which an infectious agent is transported between hosts but does not undergo further development. Four distinct categories of clinical infections include 1) visceral larva migrans (VLM); 2) ocular larva migrans (OLM); 3) neurological larva migrans; and 4) covert toxocariasis. Patients with low-level exposures to ova may present only with eosinophilia, while children and others who display pica may ingest thousands of eggs simultaneously. Hence, children with pet dogs are the most likely to present with VLM.

Of note, 58% of all US households have a dog or cat residing with them. Travelers to the United States, or other locations in which working or pet dogs are present around children, must be aware of their potential to infect young children with Toxocara ova. Children may present with retinal granulomas, including a white pupil, which is a serious finding and indicates infection will likely result in visual loss. Diagnosis can be made serologically by appropriate ELISA testing. Note that these ocular manifestations of toxocariasis have occasionally been mistaken for ocular malignancies.

In endemic countries, humans who are infected with hookworm have increasing rates of infection with age, as opposed to other infections such as ascaris and whipworm infection. The latter tend to peak in childhood, which implies some degree of immune regulation upon repeated exposures. Ancyclostoma spp. other than duodenale, such as A caninum or A braziliense, represent hookworm parasites of dogs and cats, which can potentially be shed by these animals and spread to humans. They are the cause of cutaneous larva migrans, which so commonly affects travelers to tropical countries. Unlike toxocariasis, canine hookworm infection can also be transferred by direct skin contact with contaminated soil, after which larvae may migrate for days and weeks, occasionally reaching ectopic sites in the human intestine, causing eosinophilic enteritis. Negative stool examinations and the presence of only a single adult male or nongravid female may characterize these human GI infections. Prevention during travel can minimize risk. Picking up after dog excrement, treating house pets with an antihelminthic agent on a regular basis, as well as excluding dogs and cats from playgrounds, will make a significant effect on prevention of childhood infection, particularly during travel.

There is considerable environmental contamination with eggs of the raccoon ascarid or roundworm, Baylisascaris procyonis, found fairly commonly in the United States, Europe, and Japan. This topic was nicely reviewed by Dr. K. Kazacos from the Purdue University School of Medicine. Unfortunately, the helminth can infect both dogs and humans in addition to raccoons, although there is no evidence for transplacental or mammary infection as a route for vertical transmission. Within the United States, this large roundworm is prevalent in the Midwest, the Northeast, and West Coast areas. Ova are commonly found in the environment. Although human cases have been diagnosed since the early 1980s, it is suspected that unrecognized cases have probably occurred since the 1950s. Raccoons and foraging dogs leave behind their feces, which may contaminate backyards, house surfaces, roofs, or even firewood bark. This infected fecal material may find its way into the home, and in one case a child was infected by licking contaminated wood bark.

This parasitic roundworm may first insidiously affect muscle, followed by central nervous system infection with ensuing encephalitis. It is estimated that Balisascaris-associated encephalitis has already affected approximately 100 different species of animals. There is a lag time or latent period between infection and severe CNS disease, which lasts from 1 to 3 weeks. Once neurological symptoms appear, some clinical cases have suggested rabies at first presentation, but on autopsy, infection was shown to be caused by Balisascaris procyonis. On MRI scanning, there may be characteristic deep white matter lesions and periventricular enhancement. This parasitic infection has also been associated with ocular larva migrans. One might suspect disease in persons with clinical neurological symptoms and a history of exposure, peripheral eosinophilia, and an eosinophilic pleocytosis in the CSF. Diagnosis can be made using IFA and ELISA techniques; antibodies may be found in the serum and/or CSF, and one can distinguish this infection from toxocariasis using ELISA techniques. Treatment should be instituted immediately if the disease is suspected. Treatment options include albendazole, although no regimen is particularly efficacious.2 Corticosteroid therapy may be important as part of therapy for ocular or CNS disease in order to quell the severe inflammatory response associated with treatment.

On a final note for this symposium, the tapeworm of coyotes and red foxes, Echinococus mulitlocularis, now has established a focus of infection of dogs in central North America, including the Dakotas. Two human cases were described during the symposium. Farm dogs and cats will consume wild rodents, and prevention of both animal and human disease is accomplished by avoiding animals’ consumption of such rodents, hand washing of fruits and vegetables, and the deworming of both dogs and cats with praziquantel.


1. Marcus L (chair). Parasites of Dogs and Cats: Problems in Human Health. Symposium #41, 52nd Annual meeting of the American Society of Tropical Medicine and Hygiene. Am J Trop Med Hyg. 2003;69:116.

Participants and Topics

• Marcus L. Establishing guidelines for parasite control in dogs and cats: Principles and problems.

• Dryden M. Tick and flea control in dogs and cats: Implications for human health.

• Schantz P. Zoonotic helminths of dogs and cats: Transmission, disease manifestations and prevention.

• Kazacos K. Zoonotic helminths from wildlife.

• Wilson J. Ethical and medico-legal issues involving zoonotic parasites.

2. Marcus LC, Marcus E. Nosocomial zoonoses. N Engl J Med. 1998;338:757-759.

3. Drugs for Parasitic Infections. The Medical Letter.

4. Staufer WM, et al. Images in Infectious Disease. "I’ll be Jiggered":Tungiasis. Infections in Med. 2003;20:230.