Parasites of interest (Proceedings)

Small animal veterinarians prevent, diagnose, and treat parasitic infections every day, and most veterinarians are very comfortable managing these infections in their patients. However, when it comes to the zoonotic potential of parasitic organisms, it is challenging to keep up with new research, client questions can become tougher, and there becomes a fine line between educating a client about realistic risk and inducing unnecessary fear. By remaining up-to-date on the zoonotic potential and risk factors for transmission of parasites from pets to people, veterinarians can help determine who may be at greatest risk and be able to educate clients and staff and provide practical ways to minimize risk of these infections.

Toxoplasmosis

Toxoplasmosis is a well-established zoonotic protozoal parasite, but new information regarding epidemiology of this infection is important for understanding risk for our patients, clients, and staff. Cats remain the only definitive host of Toxoplasma gondii, but virtually all mammals and some birds can become infected. Transmission can occur through ingestion of mature oocysts (1-5 days after defecated) from the feces of a shedding cat, ingestion of undercooked meat with viable cysts from an infected intermediate host, or through congenital transmission. Infected cats typically shed for 2-3 weeks. In humans, toxoplasmosis is most often asymptomatic and self-limiting, but it may cause fever, lymphadenomegaly, and rarely chorioretinitis, uveitis, and myocarditis. Immunocompromised individuals can have severe systemic infections or recrudesce, which manifest most commonly as CNS, pulmonary, cardiac, and ocular infections. Toxoplasmosis is also of utmost concern if a pregnant woman becomes infected, especially during the first trimester, as the protozoa can pass to the fetus and can cause birth defects including a triad of hydrocephalus, intracranial calcifications, and chorioretinitis. Other complications can include prematurity, perinatal death, low birth weight.

Veterinary staff and cat owners have long been considered at increased risk for exposure and seropositivity to Toxoplasma gondii; however newer studies have investigated this risk. Jones et al found overall seropositivity in Americans over the age of 12yrs to be 22.5%, and found people with increasing age, foreign-birth, lower educational level, living in crowded conditions, and working in soil-related occupations to be at increased risk; cat ownership and amount of meat consumed were not associated with increased risk of seropositivity. Cook et al performed a multicenter case-control study in 6 major cities in Europe, investigating risk factors for pregnant women, and found that risk factors most predictive of acute infection in pregnant women were eating undercooked lamb, beef, or game, having contact with soil, and travel outside of Europe, the U.S., or Canada. Contact with cats, including those that hunt or are fed raw meat, and cleaning litter boxes were not found to be significant risk factors. Shuhaiber et al tested conference attendees at the 2002 Ontario Veterinary Medical Association Conference (126 veterinarians and 35 veterinary technicians, all considered to be healthy), and found 20/141 (14.2%) were seropositive for toxoplasma IgG antibodies. Based on perceived risk, greater than 50% of these participants anticipated they would have a positive titer. No information was collected in this study on exposure to undercooked meat or soil (gardening). While these studies help to clarify risk of toxoplasmosis, education of its zoonotic potential is still paramount to prevent human illness, with a focus not only on hygiene around cats and their feces, but also stressing the potential of transmission from soil and undercooked meats.

Roundworms

Infection with Toxocara canis is one of the most common diagnoses made by a small animal practitioner. While the CDC reports that in 1996, 30% of dogs less than 6 months old shed Toxocara eggs, recent prevalence studies found that roughly 5% of dogs (11% in puppies <6months) and 2.9% of cats (10% in kittens <6months) in the U.S. are infected with roundworms. Other factors affecting prevalence rates include breed, neuter status, and region of the country. This infection is easy to diagnose and treat in dogs and cats, but their hearty eggs are very challenging to eliminate from the environment, surviving for years in soil. In dogs, embryonated Toxocara eggs are ingested, hatch in the intestines, and then the larvae penetrate the intestinal mucosa to migrate through the liver and lungs and are coughed up and swallowed into the gastrointestinal tract where they mature into egg-producing adults. In addition to the fecal-oral route of ingesting eggs, transmission can occur in dogs across the placenta and during nursing or by ingestion of a paratenic host. Cats become infected by ingestion of eggs, paratenic hosts, or via transmammary transmission. Humans become infected by ingestion of eggs from contaminated soil. When humans ingest embryonated eggs, larvae migrate through the intestinal wall to the liver and other abdominal organs (visceral larval migrans), lungs, and eye (ocular larval migrans), and rarely brain. Human beings are considered a terminal host in which roundworms cannot complete their life cycle. Many infections in humans are asymptomatic or cause mild disease, but children less than 4-years-old are most at risk for visceral larva migrans while older children are at greatest risk for ocular larva migrans. Children with visceral larva migrans may present with signs of fever, pulmonary disease, hepatomegaly, and rash, and may be self-limiting. Older children may present with retinal damage, retinal granulomas/lesions, and blindness (often unilateral), which may develop up to 10 years post-infection. Rarely, humans can develop neurologic symptoms from Toxocara larvae migration into the brain.

The CDC reports that each year in the U.S. there are an estimated 10,000 people infected with toxocariasis, and more than 700 people/year experience partial but permanent loss of vision from Toxocara. Seroprevalence studies have found that 5-30% of children may be infected. Worley et al found that risk factors for seropositivity (but not clinical signs) in children age 5-7yrs include history of pica, puppy ownership, and African-American race; owning an older dog, playing in sandboxes, and living on a farm were not found to be risk factors in this study. Children are thought to be at increased risk because of contact with soil and pets, and an increased tendency to put their hands in their mouths. Geographia (eating dirt) is seen in up to 25% of children, and is a recognized risk factor for acquiring this infection. Produce grown in soil contaminated by animal feces can also be an opportunity for transmission. Strategies for minimizing risk of infection include responsible pet ownership, comprised of deworming and proper disposal of feces. The CAPC (Companion Animal Parasite Council) recommends puppies and kittens are routinely dewormed at 2 weeks of age, and then every 2 weeks until placed on a monthly intestinal control product for maintenance prevention. The importance of both prevention and prompt diagnosis and treatment for ill pets should be stressed with all pet owners. Feces should be disposed of as soon as possible by adult pet owners, children should be discouraged from having any contact with feces, and all family members should wash their hands after exposure to feces, soil, and pets. Children should be discouraged from eating soil. Pets and stray animals, including wildlife, should be prohibited from defecating in sandboxes (keep them covered when not in use) and should not be allowed in children's playgrounds. Fresh produce should be washed or cooked prior to consumption. Finally, minimizing hand-to-mouth contact and improving overall hand hygiene, especially before meals, are the most basic things parents can do to help prevent this infection in their families.

Baylisascaris procyonis is the common roundworm of raccoons and has a similar life-cycle as Toxocara, producing visceral, ocular, and neural larva migrans in non-definitive host species including humans. While raccoons and dogs are definitive hosts and do not typically show signs of illness from this infection, birds and many species of mammals can be intermediate hosts, and humans can be accidental hosts. Humans become infected by ingestion of eggs in contaminated soil. Baylisascaris larvae are larger than Toxocara, and tissue migration causes substantial mechanical damage and host inflammation; when this severe inflammation occurs within the central nervous system, prognosis remains poor despite treatment. Traditionally, zoonotic concern has been from raccoons nesting in yards and depositing feces contaminated with eggs in latrines near people's homes. Despite this concern, Baylisascaris infection in people is quite rare, with fewer than 25 human cases ever reported in the U.S. Recommendations for minimizing risk are similar to those mentioned above for Toxocara, with special focus on minimizing contact with raccoon feces. Recently, pet kinkajous, which are also in the procyonid family with raccoons, have been reported to be infected with Baylisascaris procyonis as definitive hosts. Exotic pets in this family, such as kinkajous, coatis, olingos, and ringtails, may all carry Baylisascaris, placing humans at risk of infection. Veterinarians caring for these species of pets should be aware of this potential so that preventative deworming can be performed and owners can be educated about minimizing risk at home.

Hookworms

Although there are many species of human hookworms that can cause disease in humans, such as Ancylostoma duodenale and Necator americanus, the zoonotic species responsible for causing cutaneous larva migrans in humans in the United States include A. braziliense and A. caninum. These are seen predominantly in the southern states, especially in the Southeastern and Gulf Coast states, but hookworm infection can also be acquired during travel to the Caribbean, Central and South America, and overseas. The normal life cycle of hookworms in puppies and kittens is similar to roundworms, with larvae migrating through the bloodstream and lymphatics to the lungs, and being coughed up and swallowed prior to maturity to adult worms. In adult dogs and cats, hookworm larvae are arrested in development and will not reach reproductive maturity to shed eggs. Rodents can be paratenic hosts. Eggs hatch in the environment, and development into infective larvae takes 2-7 days. Larvae can then survive for up to 3 weeks in a warm environment. Dogs and cats become infected by ingestion of or skin penetration by infective larvae; dogs can be infected when infective larvae passed in their mother's milk. Humans also become infected by ingestion of or direct penetration of their skin (more likely) by infective larvae. In humans, after skin penetration, the larvae migrate through the skin, causing very pruritic linear skin eruptions, called cutaneous larva migrans. Rarely hookworm larvae can migrate to visceral organs, including the intestines causing eosinophilic enteritis. Sexually immature A. caninum worms can become established in the small intestine of humans, causing acute-onset severe abdominal pain.

Risk factors for infection with cutaneous larva migrans include geographic location, recreational, and occupational risks. Walking barefoot on sand or soil that could be contaminated with feces from infected pets is a recognized risk factor; wearing shoes at these sites and prohibiting pets from beaches and playgrounds (including sandboxes) can help minimize this risk. Other recreational activities such as sunbathing on the beach and gardening can also increase risk in endemic areas. Occupations that involve close contact with soil (electricians, plumbers, workers who crawl under raised buildings) or rodents (pest exterminators) can also increase risk of exposure; rodent control can help to minimize risk, and washing hands after all contact with soil is recommended. As with roundworms, routine deworming of pets and prompt disposal of their feces is recommended to minimize environmental contamination. Prognosis for CLM is good, as the larvae tend to die after several months of migration, even without treatment.

Giardia

Dogs, cats, and humans acquire Giardia by ingestion of cysts directly from fecal-oral contact, or via contaminated fomites, water or food. Trophozoites are released in the duodenum, multiply by binary fission, and colonize the small intestine. These trophozoites adhere to enterocytes but do not invade. As they travel through the ileum and large intestine, trophozoites encyst. Both cysts and trophozoites are passed in feces, within 5-7 days of infection. Giardia cysts are very hearty and remain infective in the environment for months, preferring cold damp environments. Dogs, cats, and humans can have asymptomatic infections, or they can develop acute or chronic, non-bloody diarrhea, with variable abdominal pain, vomiting, weight loss, steatorrhea. The CDC reports that 2 million cases of human giardiasis occur each year in the U.S., with a bimodal age distribution in kids (<9 years old) and adults (ages 35-44). Infection in dogs and cats is highest in young animals and shelter populations.

Giardiasis in dogs, cats, and humans is caused by infection with Giardia duodenalis. This species is subdivided into 7 genetic assemblages or genotypes, A-G. While assemblages A and B are primarily responsible for human illness, assemblages C and D are responsible for canine infections, and F is responsible for feline illness. Although these genotypes are relatively host-specific, few dogs and cats have been reported to carry assemblages A or B, which may provide an opportunity for zoonotic transmission. Several studies (from Brazil, Argentina, Thailand, and India) have identified the same genotypes from dogs, cats, and humans, suggesting transmission is possible. However, the risk of zoonotic transmission of Giardia from dogs and cats to humans in the U.S. is considered to be a minor risk. Proper hygiene around the feces of animals is important, but minimizing risk of person-to-person fecal-oral transmission is most important to prevent human illness.

References

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