Toxoplasmosis is caused by a common parasite or one-celled organism (protozoa) called Toxoplasma gondii. Blood tests in the United States suggest that about 30-40% of humans and 15-65% of cats have been exposed to this parasite and have developed a reaction (or antibody).
The life cycle of Toxoplasma gondii consists of what are termed a sexual phase and an asexual phase. Cats are the definitive host and the only species in which the parasite can complete the sexual phase of its life cycle. The sexual phase results in the passage of oocysts (similar to eggs) in the feces. In approximately 3 days from being passed in the stool, the oocysts matures and develops spores (sporulate) within the oocyst capsule before they become infective to other animals (intermediate hosts). Intermediate hosts include most species of mammals (including humans), birds, reptiles and fish. These intermediate hosts become infected by ingesting sporulated oocysts in contaminated food or water. The oocyst capsule ruptures and spores (sporozoites) infect cells lining the wall of the intestinal tract. The sporozoites burst through the intestinal lining and then spread to various parts of the body of the intermediate host where they eventually lodge. The immune system of the intermediate host forms a wall around the parasite (encyst). If a female animal is pregnant, sporozoites can spread to the placenta during this phase of infection and infect the fetus. The asexual phase of the parasite's life cycle occurs when an animal ingests cyst infected meat from other intermediate hosts (fish, chickens, beef and pork). The parasite's life cycle is completed when a cat eats meat infected with cysts and the life-cycle starts all over again.
The cat is the only species in which the parasite can complete both the sexual and the asexual phases of its life cycle. Most cats get infected as intermediate hosts soon after weaning either by being fed raw infected meat or by catching and eating infected rodents. The incidence is higher in feral (wild) and outdoor cats than cats who always stay indoors. During the sexual phase of toxoplasmosis, infected cats shed oocysts in the feces for 1-3 weeks after the initial infection. Once a cat has been infected and shed oocysts once, it rarely will shed them again. The intestinal phase of infection does not cause significant clinical problems in the cat. It is the asexual phase that can cause disease especially in immunodeficient cats including young kittens and cats infected with the Feline Leukemia Virus (FeLV) or the Feline Immunodeficiency Virus (FIV or feline AIDS virus). The signs of toxoplasmosis include weight loss, fever, lethargy, neurologic or respiratory problems and eye problems. The eye problems include inflammation (uveitis), glaucoma (high pressure), retinal detachment and blindness.
Humans and dogs can become infected with Toxoplasma gondii by ingesting sporulated oocysts or tissue cysts, or through the placenta (transplacentally). In normal adults, the infection is generally a flu-like condition. Transplacental infection occurs when a pregnant woman is exposed and infected during the first 2 trimesters. Approximately 50% of the fetuses may be infected and severe clinical disease may develop in some of these (about 10%) newborn children. Clinical signs include abortion, stillbirth, hydrocephalus (water on the brain), eye problems and neurologic disease. These problems occur because the infection occurs prior to the development of the immune system. As the immune system develops, the body sends out "messengers" to sample all the parts of the body that it can. All these things are then considered "SELF" and, unless abnormal conditions occur, the individual doesn¹t try to fight off those "self" things. The baby's immune system then considers the parasite as part of the baby and the immune system doesn't try to wall off or kill the toxoplasma organism. Immunocompromised individuals (organ transplant recipients and AIDS patients) are also at risk of serious disease from Toxoplasma gondii for the fact that they can't fight off the parasite. An immunocompromised person may have the tissue cysts from prior infection reactivated. Handling cats is not considered a major risk even in immunodeficient people. People are probably at greater risk of contracting the disease while working in their gardens or preparing dinner than when employed in a kennel situation. Dogs who are on high doses of corticosteroids for any number of problems are at the same level of risk as the immunocompromised human. Dogs can also acquire the infection by eating cat feces found in the yard, parks or the family litter pan. About 30-40% of women of child-bearing age have antibodies to toxoplasmosis indicating previous infection. These women are not at risk of transmitting the infection to their fetuses. That leaves 60-70% of women at risk. It is estimated that there is about 1 infected child born per 1000 births or somewhat over 3000 babies per year in the US. Oocysts can live in the environment for long periods and most human and dog infections probably result from contact with contaminated soil or water. Gardens and sand boxes are frequently used by cats for defecating and may be contaminated. Oocysts can be transported by accidental hosts such as cockroaches and flies which in turn can contaminate surfaces and food. Raw or undercooked contaminated meat is a major source of infection for humans and their pets. Pork products in the US have a high incidence of tissue cysts, but fish, chicken and other types of meat may be infected. Milk can also be contaminated. Most cow's milk is pasteurized, but goat's milk is frequently consumed raw. Vegetables may be contaminated by soil or accidental hosts. Cooking for 20 minutes to an internal temperature of 150 F will kill tissue cysts and freezing to -200 F for several days will reduce the number of cysts. Rubber gloves and regular hand washing will minimize the chance of infection. The sporulated oocysts in the environment and tissue cysts are infective immediately and protective measures are much more important here than when handling and caring for cats.
Infection from direct contact with cats shedding oocysts is not very likely. Cats only shed oocysts for a few days to weeks in their lives after primary infection. Cat feces are generally firm and cats do not soil their coats or they groom away any feces quickly. As stated above, it takes several days for the oocyst to sporulate and become infective, thus fresh feces are not contagious. Wearing rubber gloves and washing hands after cleaning litter boxes will prevent infection. Clothes or coveralls worn when cleaning cat pans and cages should be washed after each use. Cat feces and litter dust are not likely to be a source of human infection in the context of a kennel situation if the litter is changed daily and the pans are disinfected by steam or hot water.
Numerous tests for antibody (body's reaction) to toxoplasma can be used to detect infection in humans and other animals (dogs and cats). Many of these measure blood antibodies (Immunoglobulins [Ig] G [IgG] and M [IgM]. The IgG titer may remain high for months to years in cats and is not a good test for recent infection but does tell of past infection. The IgM titer is a better indicator of recent infection. The IgG and IgM titers do not indicate the likelihood that the cat is currently passing oocysts because they indicate infection during the asexual and not the sexual phase. (Remember, oocysts are only passed in the sexual phase and the cat doesn¹t get ill from the sexual phase.) Fecal exams for the detection of oocysts can be done, but the oocysts may be missed and it is a rather laborious procedure.
When a cat or dog is systemically ill with toxoplasmosis, there are several medications that can be used. Clindamycin (Antirobe®) or trimethaprim/sulfa drugs (Tribrissen® or DiTrim®) are the best choices. These have to be given for 6 weeks. These drugs can cause an upset intestinal tract and may cause vomiting or diarrhea.
Thanks to Dr. Lenny Southam for input for this handout.
Have any questions on this subject? Contact
Dennis Hacker via E-mail:
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