Current Advice on Parasite Control:
Parasites of Other Systems - Toxoplasma
Toxoplasmosis is one of the most important zoonotic parasites dealt with in veterinary medicine. It is caused by a two-host coccidian, Toxoplasma gondii, for which cats are the only known definitive hosts.
- Oocysts (resistant stages for environmental transmission; only develop in and shed by cats)
- Tachyzoites (rapidly dividing tissue stages)
- Bradyzoites (slow dividing, encysted tissue stages)
- Toxoplasmosis is a multisystemic infection characterized by granulomatous inflammation associated with tachyzoite proliferation in the tissues.
- Organs and tissues commonly affected include lymph nodes, liver, lung, brain/spinal cord, and eye.
- Toxoplasmosis may cause focal or generalized lymphadenitis, encephalitis, pneumonitis, myocarditis, and retinochoroiditis.
- There may be associated fever, weight loss, and lethargy.
- Congenital infection with Toxoplasma is associated with neurologic disease, birth defects, stillbirth, and ocular disease in humans and some other animals.
- Toxoplasmosis is a common, worldwide infection affecting a large variety of mammals and birds.
- Seroprevalence of human infection in the United States is approximately 11% in people between the ages of 6 and 49 years.
Prevalence of oocysts (fecal stage) in cats in the United States is quite low. At any point in time, approximately 1% of cats have intestinal infection and will be shedding oocysts.
- In six surveys from different states, with n > 10 cats in all studies, oocyst shedding ranged from 0.0 to 6.6% (mean of 0.7%).
- Much higher prevalence of oocyst shedding has been cited from other countries, (e.g., 17% in Czechoslovakia, 20% in Brazil, 23% in Costa Rica, 40% in Turkey, 41% in Egypt).
Positive antibody titers in cats, which indicate previous exposure, are much higher.
- In 21 studies in the United States, involving 855 cats, the mean seroprevalence was 40.7% (range 14 to 100%).1
- Surveys in other countries report comparable prevalences in the higher ranges.
1Dubey JP, Beattie CP, 1988. Toxoplasmosis of Animals and Man. CRC Press, 1988. Table 1, p118.
- Ingestion of tachyzoites or bradyzoites in mammalian or avian tissues
- Ingestion of sporulated oocysts from areas or articles contaminated by feline feces (e.g., soil, water, vegetation)
- Transplacental or transmammary transfer of tachyzoites
Prepatent Period and Environmental Factors
Cats shed oocysts:
- in 3 to 10 days following ingestion of bradyzoites in raw meat
- in 19 to 48 days following ingestion of oocysts
- Oocysts shed by cats sporulate (become infective) in 1 to 5 days and survive for months to years in the environment.
- Cats shed oocysts:
Host Associations, Sites of Infection, Pathogenesis
- Toxoplasma gondii is a two-host coccidial organism.
- Sexual stages develop only in the intestine of felids (wild and domestic).
- Extraintestinal, asexual stages (tachyzoites, bradyzoites) are found in all mammalian and avian species.
- Tachyzoites divide rapidly within various cells, leading to cell death, tissue necrosis, and granulomatous inflammation.
- Following acute infection, the organism survives for extended periods as slow dividing bradyzoites in tissue cysts. These may be reactivated by immunosuppression.
- Clinical disease is more severe in immunocompromised individuals, developing fetuses, and the elderly.
Unsporulated oocysts (10 µm by 12 µm) are shed in feline feces.
- Fecal examination (e.g., flotation) can identify active shedding, which only occurs over a short period (i.e., 1 to 3 weeks).
- Finding oocysts has limited clinical relevance because most cats shedding oocysts are subclinically infected.
- If a cat is shedding oocysts identified as Toxoplasma, the cat could be quarantined until shedding ceases (i.e., 1 to 3 weeks) if any household members are pregnant or immunocompromised. Owners should be given this option or at least warned of the risk of transmission and the need for meticulous hygiene when cleaning litter boxes or other articles contaminated with cat feces during the period of shedding.
- Cat feces containing oocysts should be disposed of in a manner that prevents exposure to humans or other animals (e.g., landfill, incinerator).
- Serologic testing cannot accurately predict oocyst shedding status in cats and no serologic assay accurately indicates when a cat has shed T. gondii oocysts in the past.
- Antibody testing may be useful for determining the approximate time of acquired infection (i.e., recent or in the past). Antibody testing is also useful in determining susceptibility to acquiring toxoplasmosis (seronegative cats are susceptible) or if toxoplasmosis could be a possible cause of current, clinically compatible illness.
- Most cats shedding oocysts are seronegative.
- Most seropositive (IgM or IgG) cats have completed oocyst shedding and are unlikely to repeat shedding or be a source of human infection.
- Unsporulated oocysts (10 µm by 12 µm) are shed in feline feces.
- There is no approved treatment for toxoplasmosis in cats or dogs
The following medications and regimens have been used successfully in dogs and cats.
- Clindamycin hydrochloride (10 to 12 mg/kg orally twice daily for 2 to 4 weeks) can be used to treat disseminated toxoplasmosis.
- Pyrimethamine (0.25 to 0.5 mg/kg) plus a sulfonamide (30 mg/kg twice daily for 2 to 4 weeks) also can be used to treat disseminated toxoplasmosis and to reduce oocyst shedding.
- Trimethoprim-sulphonamide combination can also be used at the rate of 15 mg/kg orally every 12 hours for 4 weeks.
- Supportive care should be provided as needed.
- Prevention is only accomplished by eliminating exposure.
- Prevent hunting activity by cats (e.g., keep cats indoors).
- Avoid feeding raw or undercooked meat or viscera to cats.
- Feed cats only commercially prepared diets.
- Because oocysts require at least 24 hours to become infective, remove fecal material from litter boxes daily. This is especially important when introducing a new cat to a household with other cats. Using disposable litter pans can help.
- Disinfecting litter boxes is very difficult, since sporulated Toxoplasma oocysts are resistant to most household disinfectants. Cleaning with scalding water or steam is most effective, but must be done carefully to avoid burns in human caretakers.
Public Health Considerations
- Follow the recommendations for prevention of infection in cats to avoid human transmission.
- People are unlikely to contract T. gondii infection from direct contact with their cats and, assuming the guidelines are followed, should feel comfortable keeping them in the household.
In addition, the Centers for Disease Control and Prevention (CDC) recommend the following to prevent contracting toxoplasmosis:
- Wash hands with soap and water after exposure to soil, sand, raw meat or unwashed vegetables.
- Cook meat completely to an internal temperature of 160˚F.
- Do not sample meat until it is cooked.
- Freeze meat for several days before cooking to greatly reduce the chance of Toxoplasma infection.
- Wash all cutting boards and knives thoroughly with hot soapy water after each use.
- Wash and/or peel all fruits and vegetables before eating them.
- Wear gloves when gardening or handling sand from a sandbox. Wash hands well afterward. Keep sandboxes covered when not in use, to avoid contamination by cats.
- Avoid drinking untreated water, particularly when traveling in less developed countries.
Since illness is likely to be more severe in fetuses and immunosuppressed individuals, additional precautions include:
- Allow only immunocompetent, nonpregnant persons to perform daily litter box cleaning.
- Adopt only mature, healthy cats.