Toxoplasmosis is a parasitic infection caused by an intracellular protozoan parasite known as Toxoplasma gondii. T. gondii is one of the world’s most common parasites and infects most mammals, including humans, in addition to rodents and birds. In order to complete its life cycle, T. gondii requires both a definitive host in whom the parasite can sexually reproduce and an intermediate host who harbors the parasite for a short developmental period. Felines, in particular domestic cats, are the only definitive host of T. gondii and play a major role in the propagation of the disease. Cats primarily become infected when they ingest an encysted tissue cyst from intermediate hosts like rodents and birds. Infection in cats is frequently subclinical or asymptomatic and it is rare for them to develop evident clinical signs, which makes this disease harder to diagnose. Associated risk factors for infection in cats include feeding with raw meat instead of a commercial diet, being in a shelter, age, and sex and access to hunting.
Toxoplasma gondii is passed in the cat’s feces in a microscopic, non-infective oocyst “egg” form. For up to 3 weeks after initial infection, cats can shed millions of oocysts in their feces and this contaminates the environment. With exposure to appropriate air and moisture, oocysts sporulate in the environment over a period of 1 to 5 days and become an infective form called sporozoites. This new infectious form can survive in harsh environmental conditions. Intermediate hosts such as birds, wild game, dogs, rodents, farm stock, and even cats themselves become infected after ingesting contaminated soil, water or plant material. Once ingested, T. gondii will either multiply asexually and indiscriminately infect other cells of the body, including fetuses, or localize in neural and muscle tissue and develop into tissue cysts (see Figure 1). As the parasite multiplies within cells, it causes cell necrosis or death. Tissue cysts can be formed in the central nervous system (CNS), muscles, and visceral organs and unfortunately appear to persist for the life of the host. The life cycle is completed when cats ingest prey infected with tissue cysts. Tissue cysts may rupture at any moment triggering clinical signs in a mature animal.
Many factors affect the severity of T. gondii infection including age of the animal, concurrent infections, the degree and localization of tissue injury, medication therapy, and vaccination status. Severe cases are usually seen when the animal is immune-suppressed from disease or undergoing medication therapy. For example, cases of Toxoplasmosis have been noted in cats with FeLV and FIV and dogs with Distemper and Ehrlichiosis. Signs include weight loss, icterus, persistent or intermittent fever, anorexia, vomiting, diarrhea, abdominal effusion, hyperesthesia on muscle palpation, stiffness of gait, shifting leg lameness, neurologic deficits, and death. Neurologic signs depend on the site of lesion within the CNS; thus, seizures, cranial nerve deficits, tremors, ataxia, and paresis or paralysis may be seen. In general, postnatal acquired toxoplasmosis is less serious than prenatally acquired infection. Affected kittens may be stillborn, die before weaning, or have visual abnormalities.
There are several diagnostic methods for determining suspected infection with T. gondii. Primarily, T. gondii diagnosis is achieved by serology. The most widely used serological test is the indirect ELISA (IgM and IgG). However, there are complications with this method. As infected animals carry tissue cysts for their entire life, they will show a long-term immune response and elevated values of antibodies. Clinicians must interpret any noted antibody elevations with the overall clinical picture. In difficult cases, the polymerase chain reaction (PCR) test may be used. This molecular test is highly specific and sensitive and allows detection of parasite DNA. Routine laboratory blood tests are useful and abnormal parameters may include nonregenerative anemia, neutrophilic leukocytosis, lymphocytosis, monocytosis, and eosinophilia. Due to the limited shedding period of oocysts, fecal examination is of limited value. Radiographs, ultrasound, and MRI can be performed to search for abnormalities or lesions within organ systems and the CNS. Inflammatory changes are usually noted in body fluids; thus, CSF analysis can be useful in suspected cases of the nervous system. Biopsies can occasionally be performed to look for tissue cysts.
Clindamycin is the drug of choice for treating clinical toxoplasmosis in dogs and cats. It crosses the blood-brain barrier, has good intestinal absorption, and can be made into liquid form for easier administration. The dosages for treating toxoplasmosis are greater than those for treating anaerobic infections and may cause gastrointestinal upset. While most clinical signs begin to resolve within 24 to 48 hours after administration of the drug, some may take weeks to months. Neurologic deficits may not completely resolve due to permanent damage within the CNS.
In the United States, approximately 1% of cats shed oocysts on any given day. It is estimated that more than two billion people worldwide are infected with T. gondii according to the CDC. Common ways in which humans acquire the parasite include eating undercooked, contaminated meat, having contact with contaminated cat feces, being exposed to contaminated soil and water sources, or through the placenta. Although many people come into contact with T. gondii, people with healthy immune systems are typically unaffected by this parasite. As cats only shed oocysts for short periods after the first encounter with the parasite, it is not necessary to relinquish suspected pets. Similar to animals, symptoms of infection in humans vary and can be self-limiting or cause severe damage to the brain, eyes, or other organs. People with the most risk include the immunocompromised and pregnant women.
There are simple hygienic and preventative measures that can be employed to prevent infection of T. gondii. Cat owners should change the litter box every day, properly discard cat feces, disinfect litter boxes regularly with scalding water, and keep outdoor sandboxes covered when not in use. Those who are pregnant or immunocompromised should not change litter boxes if at all possible or wear gloves while handling contaminated material. Household pets should be kept indoors, fed commercial or fully cooked diets and restricted from hunting and eating potential intermediate hosts. Notably, dogs should be prevented from ingesting cat litter and any sporulated oocysts in it. They can serve as potential vectors for transmission to people because the oocysts are shed unaltered into the dog’s stool. Standard recommended hygienic practices for all people are to cook food thoroughly, wash fruits and vegetable, and wash hands after contact with uncooked meat or soil.
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