This article is the seventh installment of a 10-part series and was written by:

Eugenia Marcus, MD, Pediatric Health Care, Newton-Wellesley Hospital, Newton, MA

Leonard C. Marcus, VMD, MD, Travelers' Health and Immunization Services, Newton, MA

The Companion Animal Parasite Council Guidelines, first presented to the veterinary community in January 2004, were created to protect pet — and human — health. Many parasite species potentially threaten canine and feline health, and since many are zoonotic, pet caregivers share their animal charges’ parasiticdisease risks. However, some individuals are at greater risk than others.

The question of who is most susceptible has two components: “Who is most likely to acquire zoonotic parasites?” and “Who is most likely to suffer severe disease?” How parasitism can be prevented is a third component that both veterinary and human health care professionals should be prepared to discuss with clients and patients.

Who is most likely to acquire zoonotic parasites?

People at greatest risk of acquiring parasitic zoonoses are those who are most exposed to pets and their feces.

The risk of acquiring canine and feline endoparasitic infections is greatest in those who contact or ingest fecally contaminated soil. Plumbers, gardeners and sunbathers are most likely to suffer cutaneous larva migrans caused by hookworms (usually Ancylostoma braziliense) or Strongyloides species. Pica and geophagia, most often seen in children and people who are mentally disabled, are predisposing factors for visceral larva migrans (Toxocara canis, T. cati and Baylisascaris procyonis), toxoplasmosis and hydatid disease (Echinococcus granulosus and E. multilocularis).

Flea bites can occur in anyone in contact with an infested dog, cat or home. Some petinfesting mites (Cheyletiella and Sarcoptes spp.) are transmitted between animal and human companions through direct contact. Ticks such as Ixodes, Dermacentor and/or Amblyomma spp. will feed on humans, dogs and cats, although these infestations are almost always picked up from the environment. Contrary to popular belief, ticks rarely move from pets to humans or vice versa to feed.

Who is most at risk for severe disease?

Immunocompromised individuals — including fetuses, infants, the elderly, and those with immunosuppressive diseases or taking immunosuppressive medications can suffer severely from opportunistic infections.

While people are more likely to get toxoplasmosis from eating undercooked meat or gardening than from contact with pet cats, the risk from cats still exists. A woman who acquires toxoplasmosis during pregnancy may experience little or no clinical illness; instead, her unborn child is most vulnerable. If transplacental transmission occurs, the fetus may suffer severe damage to the brain, eyes and other organs.

Other immunocompromised individuals include patients with AIDS; certain types of hematopoietic system tumors; and those receiving radiation, corticosteroids or immunosuppressive agents for organ transplantation or various autoimmune, inflammatory or neoplastic conditions. These individuals can suffer severe, potentially fatal toxoplasmosis, strongyloidiasis and flea-borne infections such as bartonellosis. Asplenic patients are likely to be severely, often fatally, ill with babesiosis, a tickborne infection.

While usually only an uncomfortable nuisance, flea bites can cause serious irritation and annoying rashes in people with hypersensitive immune systems.

How can zoonotic infections be prevented?

Zoonoses can be prevented through simple control measures, including:

• regular deworming programs (CAPC recommends lifetime, year-round prevention with broad-spectrum anthelmintics effective against gastrointestinal parasites as well as heartworm)
• annual pet fecal examination, followed by appropriate antiparasitic treatment
• treatment of pets and their environments to control fleas and ticks
• daily disposal of canine and feline feces, and compliance with pooper-scooper laws
• covering children’s sandboxes when not in use
• preventing predatory behavior (e.g., keeping cats indoors)
• feeding cooked, canned or dry pet food
• washing or cooking vegetables for human consumption
• adequate hand washing after contact with anything contaminated with dog or cat feces.

Daily disposal of canine and feline feces can help reduce parasite infections because some zoonotic parasites need time to develop to the infective stage. For example, hookworm and ascarid eggs require more than one week to embryonate and produce infective larvae. Toxoplasma oocysts need at least two days to sporulate and thus become infective. Other parasitic stages, such as Echinococcus eggs, Giardia cysts and Cryptosporidium oocysts, are immediately infective when passed in feces, posing potential occupational hazards in kennels, catteries and veterinary hospitals.

Pets are not the source of most human cases of giardiasis and cryptosporidiosis. Most canine and feline strains of these parasites are not readily transmitted to people. To reduce the risk of infecting dogs or cats with human-adapted strains, pets should not be permitted to drink from toilet bowls. People can reduce flea and tick bites by applying permethrin to clothing and DEET to exposed skin.

For more comprehensive information about prevention, transmission and treatment of parasites, visit http://www.capcvet.org