The popularity of dogs and cats as pets in the United States continues to increase. Recent surveys estimated that there were 73 million owned dogs and 90 million owned cats in the United States, and almost 60% of U.S. households owned a pet.1 The highest rates of dog and cat ownership occur in households of families with young children. Dogs and cats are hosts to many intestinal parasites that may be transmitted to humans through direct contact with infected pets or exposure to environments contaminated with infected animals' feces (Table 1). Children are often at greatest risk of zoonotic infections because of their play habits and affection for pets.
Table 1: Gastrointestinal Parasitic Infections in Dogs and Cats in North America That Are Transmissible to People
Veterinarians in practice are on the front lines in preventing transmission of pet-associated zoonotic parasite infections because of their knowledge of the potential risks and through their contact with pet owners. Practicing veterinarians' services should include preventive treatments to eliminate parasites as well as advice to owners on minimizing the risk of zoonotic transmission. This article summarizes data on the modes of transmission of potentially zoonotic intestinal parasites of dogs and cats and the available data on the frequency at which they infect and cause disease in humans in the United States.*
Potentially zoonotic gastrointestinal parasites of dogs and cats include the maternally transmitted intestinal roundworms and hookworms whose infective stages may contaminate and persist in the peridomestic environment (i.e. in proximity to humans).
Peter M. Schantz, VMD, PhD
Infection of humans by Toxocara canis and Toxocara cati, the common roundworms of dogs and cats, respectively, cause larva migrans syndromes (visceral and ocular larva migrans and covert toxocariasis) in humans who accidentally ingest infective eggs from contaminated environments.2 Toxocariasis ranks among the most common of all zoonotic infections; the results of numerous published surveys document seroprevalences in humans ranging from 1% to 20%, depending on the age, socioeconomic status, and pet ownership status of the tested populations.3-5 Toxocaral larva migrans, or infection by the common ascarid worms of dogs and cats, is arguably the most common zoonotic infection associated with pets in the United States and other industrialized countries.4 It has been estimated that every year in the United States this infection causes hundreds of cases of unilateral blindness and uncountable numbers of less permanent forms of illness in children.3,4 The severity and type of disease in humans produced by Toxocara species infection depend on how many larvae are ingested, the frequency of reinfection, and other factors still poorly understood. Most human infections with Toxocara larvae are well-tolerated, even asymptomatic; however, a proportion of infected people develop larva migrans syndromes that may be systemic or confined to the eye. When a larva invades the eye, it almost always leaves the individual partially or totally blind in that eye. Based on data obtained from the Centers for Disease Control and Prevention (CDC) serologic diagnostic reference service, an estimated minimum of 750 cases of ocular larva migrans are diagnosed by physicians every year in the United States.4 Chart review of patients with a diagnosis of uveitis at the University of California Medical Center determined that ocular larva migrans accounted for 1% of cases of uveitis seen between 1977 and 1996.5 All cases were associated with vision loss in the affected eye. The number of cases of toxocaral visceral larva migrans syndrome is much greater; however, estimates of these are quite imprecise.4
Epidemiologic investigations have consistently determined that the principal risk factor for infection was the presence of a household dog, particularly a pup, in a patient's household within six months of onset of illness.2,3,6 When this condition is combined with pica, especially dirt eating, the statistical association becomes very strong.