CHAPTER 46 Selected Zoonotic Diseases
Puppies and Kittens
Puppies and kittens in their first year of life may become significant carriers of zoonotic diseases, which they acquire through various routes. Zoonosis is a communicable disease that humans may acquire from insect vectors, food, and direct and indirect contact with animals. Historically, more than 200 zoonotic viruses, rickettsiae, bacteria, fungi, and parasites have been identified throughout the world. Zoonotic infections are often acquired orally either by food or fecal-oral transmission. Children are at a higher risk of contracting zoonotic diseases because of their curious nature than adults or elderly persons. Therefore it is important for veterinarians to be aware of zoonoses, especially those that are related to direct pet contact, since 63% of households in the United States have at least one pet.
Ringworm (see Chapter 17) is the most common zoonotic disease in the United States. Ringworm (dermatophytosis) is a fungal disease of cats, dogs, sheep, rabbits, cattle, and horses that invades keratinized structures (e.g., hair, horn, nails, feathers, and skin). Although it is self-limiting, clearing without treatment may take several months. Cats, especially young cats, can harbor the fungus without any noticeable clinical signs, thereby serving as an unsuspecting reservoir of infection.
Ringworm is readily transmitted to people by direct contact with infected animals, fomites (e.g., bedding), or contaminated environment such as the soil. The infections are spread mainly by spores, which are very long lived and can persist in the environment for years in blankets, clothing, bedding, combs, and other grooming tools.
Species that cause ringworm belong to the genera Microsporum and Trichophyton. These genera of fungi are somewhat unusual in that they produce asexual spores; sexual spores are produced so infrequently that they have not been observed. The most common source of ringworm in people is Microsporum canis, a species most often found on cats and dogs. When diagnosed, ringworm should be treated to limit the transmission of the fungus to humans.
The classic lesion is a circular patch characterized by broken stubby hair, scaling or crusting, and redness. The lesion will appear to spread outward, often with a central area of healing. Animals may have just a single lesion or many.
Diagnosis is usually made by microscopic examination of hairs from the site of the lesion for the characteristic spores lining the outside of the follicles. For a definitive diagnosis of ringworm, a fungal culture should be done as this is the most reliable diagnostic test for ringworm; however, it may take up to 3 weeks before any growth is seen. Single lesions (mild infection) can be spot treated with topical antifungal creams. Oral medications are available for more severe infections.
Because of its zoonotic potential, the best ringworm prevention is treating infected animals as soon as possible to avoid its spread to other animals or humans and to improve the likelihood of recovery without reinfection. Pet owners (particularly those with young children or immune-compromised individuals in the home) should be advised of the human heath risk and referred to their physician if lesions appear on any humans in the household. Because ringworm spreads easily, it is best to keep infected animals away from others until the infection has cleared.
Although a more comprehensive discussion of enteric infections may be beyond the scope of this review, several points related to animal-associated infections are relevant. Household pets may become ill with Campylobacter and Salmonella, but transmission from pets to humans is not common compared with person-to-person and food-borne spread. The majority of Giardia infections in children are spread from person to person, especially in daycare settings; however, animal reservoirs of Giardia and other parasites may also contaminate surface water and may cause human disease.
Visceral larval migrans can occur in humans after infection by Baylisascaris procyonis, Toxocara canis, or Toxocara cati eggs. Following ingestion of infectious eggs, larvae penetrate the intestinal wall and migrate through the tissues, leading to eosinophilic granulomatous reactions involving the skin, lungs, central nervous system (CNS), and eyes. Ocular larval migrans most commonly involves the retina and can cause reduced vision, strabismus, uveitis, and endophthalmitis. Because the disease is transmitted by the fecal-oral route, human cases of Baylisascaris infection typically occur in younger age groups, mainly infants, who often engage in oral exploration of their environment and therefore are more likely to ingest Baylisascaris eggs. No effective therapy exists for the visceral form of B. procyonis larval infection. Adult T. cati has also been detected in some infected children.
The prevalence of B. procyonis in raccoons living in Portland, Oregon was assessed in 69 raccoons collected from wildlife control agencies. Infection with B. procyonis was assessed through the harvesting of adult worms from the intestines of raccoons. Fifty-eight percent of sampled raccoons were found to be infected with B. procyonis. Juveniles had the highest prevalence (70%) and heavier adult worm burdens (mean = 35 worms). The data suggest that juvenile raccoons are the major potential source of B. procyonis in Oregon.
Cutaneous larval migrans can be induced with infection by three species of hookworms infecting dogs and cats in the United States: Ancylostoma caninum, Ancylostoma braziliense, and Uncinaria stenocephala. Larvae are released from eggs passed into the environment in feces; infectious larvae infect humans by skin penetration. Larval migration results in the development of an erythematous, pruritic cutaneous tunnel. Occasionally larvae will reach the lungs and cornea. A. caninum has also been linked with eosinophilic enteritis in humans.
Prevention revolves around controlling animal excrement in human environments. Because hookworm and roundworm infections are sometimes occult, anthelmintics such as pyrantel pamoate should be routinely administered to all puppies and kittens at least twice, 14 to 21 days apart. In puppies, pyrantel could be given every 2 weeks between 2 and 8 weeks of age. Pyrantel could be given to high-risk kittens at 6, 8, and 10 weeks of age. Fecal centrifugation and flotation and direct fecal examination should be performed twice yearly on all dogs and cats to check for other parasites, particularly if they go outdoors.
The molecular characterization of Giardia and Cryptosporidium has given rise to a more epidemiologically meaningful taxonomy. More important, molecular tools are now available for further typing of isolates of the parasites directly from clinical and environmental samples. As a consequence, information on zoonotic potential has been obtained, although the frequency of zoonotic transmission is still not completely understood.
Giardia, a flagellate with worldwide distribution, may cause significant gastrointestinal disease in dogs, cats, and people. The organism has a wide host range; mammalian isolates are all currently classified as Giardia duodenalis. Using DNA sequences from a number of different genes, there appear to be two or three genotypes of Giardia in people, as well as two distinct genetic groups isolated exclusively from dogs. However, whether these genotypes vary in biologic activity, including their zoonotic potential, is for the most part unknown. There have been varying results concerning cross-infection potential of Giardia spp. isolates. In one study, Giardia spp. from humans were inoculated into cats; the cats were relatively resistant to infection. In contrast, evaluation of human and feline Giardia spp. isolates by isoenzyme electrophoresis suggests that cats could serve as a reservoir for human infections.
Because it is impossible to determine zoonotic strains of Giardia spp. by microscopic examination, it seems prudent to assume feces from all dogs and cats infected with Giardia spp. to be a potential human health risk.
Fecal examination should be performed on all dogs and cats at least yearly, and treatment with anti-Giardia drugs like praziquantel-pyrantel-febantel, fenbendazole, or metronidazole should be administered if indicated. Albendazole is also effective for the treatment of giardiasis but has been associated with neutropenia. Vaccination against Giardia could be considered in animals with recurrent infection; this practice is being evaluated as a possible therapeutic approach.
People infected with the coccidian parasite Cryptosporidium parvum suffer severe gastrointestinal tract disease, including diarrhea and vomiting. Many infected individuals require hospitalization for the administration of intravenous fluid therapy. Infection of immunosuppressed individuals may be life threatening; people with AIDS may never be cured. Cryptosporidiosis has been documented in people and cats or dogs in the same environment, suggesting the potential for zoonotic transfer. C. parvum oocysts have been documented in feces of many domestic dogs and cats in the United States, Japan, Scotland, Australia, and Spain.
Until recently, no specific treatment was available. Although nitazoxanide is approved for treating cryptosporidiosis in immunocompetent patients 1 to 11 years of age, a review of (uncontrolled) data on nitazoxanide in treating cryptosporidiosis in patients with HIV argued that it might also be useful in this population.