Toxoplasmosis is caused by an organism called Toxoplasma gondii which is a type of protozoan. This particular protozoan is an intracellular parasite of many organs and tissues of birds and mammals, including man. The only known definitive or complete hosts are wild and domestic Felidae or cats. This means that only in infected cats will the entire life cycle of this parasite be completed. The cat is the only animal which will shed and thereby spread oocysts (eggs) in their feces.

Infection can be obtained in one of three ways:

1. Oocysts that have sporulated in cat feces (sporozoites)
2. Tissue cysts in meat (bradyzoites)
3. Transplacental transfer of trachyzoites.

Essentially all warm blooded animals may serve as an intermediate host for this parasite. Oocysts are passed in the stool of infected cats. These eggs take one to five days in the environment to become infective through a process called sporulation. The length of the sporulation process depends on the temperature and humidity in the environment. The sporozoites may remain viable and infective for several months depending on the environmental conditions that are present. The sporulated oocysts or infective eggs rupture in the intestinal tract releasing sporozoites (a further developed state of the protozoa). The sporozoites multiply in the host’s intestines and lymph nodes to form an additional stage of the parasite called a trachyzoite. The trachyzoites then spread to all tissues of the body where they invade cells and continue to multiply in number. The trachyzoites may be forced into an inactive state by the host’s immune system causing an inactive tissue cyst called a bradyzoite. A bradyzoite can form in the brain, striated muscle, or liver. These tissue cysts or bradyzoites will remain infective or viable for the life of the host.

Sporozoites, trachyzoites, and bradyzoites are all stages of toxoplasmosis that can become infective to a new host upon ingestion. Virtually all warm-blooded animals may become infected. Animals and humans can be infected through ingestion of oocysts passed in the stool of infected cats. This may be accomplished by direct ingestion, contamination to the hands that are then placed in the mouth, or by the ingestion of vegetables or grasses upon which the oocysts have been deposited. Humans and cats are often infected by eating undercooked beef or pork through encysted bradyzoites. Cats are more commonly infected by hunting mice, rats, and birds containing encysted bradyzoites. Cows are infected by sporozoites deposited by infected cats while grazing. Other farm animals can be infected by contaminated soil or foods.

Clinical signs are caused by tissue damage created by the trachyzoite stage. The trachyzoites rapidly multiply thereby continuing to infect more and more cells, eventually leading to the destruction of the cells that are infected. Thankfully, older animals mount an immune response to the trachyzoites which is able to control infection by driving the trachyzoites into the tissue cyst or bradyzoite stage (inactive tissue cyst). In adult humans with an intact immune system, the only clinical signs of infection may be flu-like symptoms and a sore throat. Rarely is the true etiology of these symptoms diagnosed and therefore toxoplasmosis is often misdiagnosed as the flu. Problems arise when a fetus or an immune-compromised individual such as a person who is infected with the AIDS virus becomes infected. These individuals cannot control the trachyzoite stage and more severe clinical signs of infection will result. Clinical signs in immune-compromised individuals include fever, diarrhea, cough, dyspnea (difficult breathing), icterus (jaundice), seizures, muscle soreness and even death. The trachyzoite stage, while rapidly multiplying, can cause pneumonia, myocarditis (inflammation of the heart muscle), hepatic necrosis, meningoencephalomyelitis (inflammation of the central nervous system), chorioretinitis (inflammation of the eye which can lead to blindness), lymphadenopathy (swollen lymph nodes), and myositis (muscle inflammation).

A major concern in human medicine is that transplacental (through the placenta) transfer of trachyzoites may occur resulting in congenital toxoplasmosis in an unborn child. Congenital toxoplasmosis may result in blindness or worse for the unborn baby. Immune competent women previously exposed to toxoplasmosis do not need to worry about exposing their unborn baby because they have already mounted an immune response that should have forced most of the toxoplasmosis organisms into the inactive bradyzoite stage. Most human toxoplasmosis infections occur through the consumption of undercooked meats via consumption of bradyzoites and raw vegetables which have been contaminated with sporozoites. Nearly 30% of women in the general population may have circulating antibodies to toxoplasmosis and these antibody levels are considered to be protective against congenital toxoplasmosis. The remaining 70% of pregnant women should avoid undercooked meat and cat feces. If you are concerned about prior exposure to the Toxoplasmosis organism, a titer run from a blood sample will be diagnostic. Steps to prevent exposure to toxoplasmosis include the following:

1. Have another family member who is not pregnant or immune compromised clean the litter box.

2. Clean litter pans on a daily basis. This prevents toxoplasmosis from having time to sporulate and therefore it is not able to develop to the infective stage.

3. Wear disposable gloves to clean the litter box.

4. Eat only adequately cooked meat.

5. Avoid gardening or wear gloves while gardening.

6. Wash raw vegetables and fruits carefully or just eat cooked vegetables.

If you are still concerned about exposure to toxoplasmosis from the family cat, collect a stool sample in a plastic bag or container. Bring the sample to your local veterinarian. A simple test conducted by your vet will determine if your cat is shedding oocysts. The cat can also be tested for any previous exposure by running a toxoplasmosis titer. You can limit exposure of your cat to toxoplasmosis by not feeding raw meat and by keeping your cat from hunting birds and small rodents.