PROTOZOA INFECTIONS
Toxoplasma gondii
Toxoplasma gondii is a coccidian parasite widely dispersed in nature. Cats are the definitive hosts for this protozoan, which they acquire
when they eat infected intermediate hosts (rodents and many other mammals) or ingest oocysts excreted in the stools of other
infected cats. Infected cats are important in the epidemiology and public health importance of toxoplasmosis because they
excrete and widely disperse the environmentally resistant oocysts.22 Numerous herbivorous and omnivorous animals become infected when they ingest infective oocysts in soil or contaminated food.
Humans become infected by ingesting food and water contaminated with oocysts shed in the feces of infected cats, by eating
undercooked meat from infected animals, or in utero (by congenital transmission from infected mothers). Rarely, humans become
infected through blood transfusion or organ transplantation. Recent serosurveys of the U.S. population have documented antibodies
(evidence of current or past infection) in about 23% of the U.S. population.23 Ingestion of oocysts shed in the feces of infected cats is believed to directly account for up to 50% of human cases in
the United States. Clinical disease caused by toxoplasmosis is generally mild following primary infection of immunocompetent
people. Self-limiting fever, malaise, and lymphadenopathy are the most common clinical abnormalities, and most infected people
never realize when their first T. gondii infection occurred. However, acute infections acquired by pregnant women can be transmitted to the fetus and cause severe
illness (e.g. mental retardation, blindness, epilepsy) and death. According to a 1999 report by the CDC, an estimated 400 to 4,000 cases
of congenital toxoplasmosis occur each year in the United States.24,25 Another permanent manifestation of toxoplasmosis is ocular disease, which is estimated to occur in up to 12,000 people per
year in the United States. Toxoplasmosis can cause more severe or fatal illness in people who are immunosuppressed (people
with human immunodeficiency virus [HIV], transplant recipients).
Giardia species
Giardia duodenalis (synonyms Giardia
lamblia, Giardia intestinalis) is a protozoan parasite that infects the intestinal tract of many animal species including humans. Motile trophozoite stages
occur in the intestines, and environmentally resistant cysts are passed in the feces of infected animals, which are immediately
infective if ingested by other susceptible hosts. In all hosts, G. duodenalis can cause acute gastrointestinal signs as well as chronic disease, including chronic malabsorptive and allergic manifestations
and childhood failure to thrive.26 Transmission of infection occurs by fecal-oral routes either by direct contact or by ingestion of contaminated food or water.
Giardia species infections are common in dogs and cats throughout North America; however, prevalences are often underestimated because
the parasite detection methods commonly used in practice have low sensitivity.27,28
Giardia species have long been considered zoonotic because morphologically similar organisms infect humans and a variety of mammals
and birds.29 However, evidence of giardiasis being directly transmitted from one host species to an immunocompetent host of another species
is limited. Although variants of Giardia species in human, canine, and feline hosts lack differentiating morphologic characters, the application of molecular tools
(e.g. PCR) has revealed genetic differences in isolates from different hosts such that it has become clear that the genotypes commonly
infecting dogs and cats are not those commonly infecting humans.29,30 Most confirmed infections in humans with Giardia species acquired from dogs or cats have been reported in individuals with recognized immunodeficiency disease (e.g. HIV infection).29
Cryptosporidium species
Protozoan parasites belonging to the genus Cryptosporidium are ubiquitous and among the most common nonbacterial causes of diarrhea in a wide range of vertebrates, including humans.
Cryptosporidium species are transmitted via the fecal-oral route by environmentally resistant cysts that are shed in the feces, contaminating
soil and water, and, thus, providing multiple routes into the food chain.30 In an immunocompetent host, cryptosporidiosis of the intestinal tract may be asymptomatic or lead to self-limiting diarrhea,
but in an immunocompromised host, it can be life-threatening.31
Cryptosporidium species have been reported in numerous mammals and, like Giardia species, appear to have evolved with their respective hosts such that they do not readily cross-infect and develop in hosts
of other species. The application of molecular tools has revealed that Cryptosporidium species are a phenotypically and genotypically heterogeneous assemblage of species and genotypes that are morphologically
similar.30,31 In humans, the most commonly detected species are the anthroponotic Cryptosporidium
hominis and the zoonotic Cryptosporidium
parvum (cattle). Both Cryptosporidium
canis and Cryptosporidium
felis, whose natural hosts are dogs and cats, respectively, have also been demonstrated in infected humans suffering diarrhea.32 Young children and immunocompromised individuals are at greatest risk. Information regarding the role of pets in zoonotic
transmission of Cryptosporidium species in immunocompetent humans is insufficient. While it is clear that most outbreaks and individual cases of cryptosporidiosis
in humans are related to the contamination of water, food, or fomites with organisms of human or cattle origin, it is also
clear that inter-species transmission from dogs or cats to humans can occur in certain situations, especially among very young
children or immunodeficient individuals.30
CONCLUSION
Dogs and cats are infected with a number of helminths and protozoa that can infect and sometimes cause life-threatening illness
in humans. Awareness of these infections and their zoonotic potential is essential for practicing veterinarians in order to
diagnose and treat the infections in pets as well as to provide preventive advice to pet owners.
*None of the zoonotic parasitic infections acquired from dogs and cats are reportable diseases in the United States; consequently,
no systematically collected data on the frequency of these zoonotic parasitic infections exist.
Peter M. Schantz, VMD, PhD
Division of Parasitic Diseases
National Center for Zoonotic, Vectorborne and Enteric Diseases
Centers for Disease Control and Prevention
Atlanta, GA 30333
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