Toxoplasmosis is perhaps one of the most feared, yet least understood, examples of a zoonotic infection – one that can be passed from animals to people. This infamous disease is commonly associated with cats, but our feline friends are not the only – or even the most likely – route by which humans may be exposed to infection. A fuller understanding of what toxoplasmosis is and how it affects pets and people can help us keep ourselves and our feline companions healthy.
Toxoplasmosis is not caused by a virus or bacterium. Rather, it is caused by a single-celled protazoan parasite called Toxoplasma gondii. This parasite can infect many different host animals, including pigs, sheep, mice, deer, and in fact most warm-blooded animals, as well as humans. However, only one type of host animal allows the parasite to reproduce: the cat. When a cat eats meat from an infected animal – for instance, by catching an infected mouse or being given raw meat scraps – he or she allows the Toxoplasma gondii life cycle to begin. The organism will multiply in the small intestine and produce oocysts, or eggs. These oocysts will be shed in the cat's feces beginning 3-10 days after infection, and continuing for up to 3 weeks. Once the oocysts are in the environment for 24 hours, they become infectious to any animal that ingests them.
Toxoplasma gondii oocysts are very durable and can remain in the environment for a year or more, long after the fecal matter has decomposed. This means that animals like cows and sheep can be infected by grazing where there are oocysts in the soil. Humans are often infected by eating unwashed produce, or by failing to wash our hands before touching our mouths after gardening. Once ingested, the oocysts transform into tachyzoites and move into the animal's muscle and nerve tissues, where they form cysts and wait to be ingested by another animal. Humans can also become infected by eating raw or undercooked meat of an infected animal, or by drinking unpasteurized milk that is contaminated.
Fortunately, healthy cats, humans, and other animals are unlikely to have any disease symptoms. In fact, in the United States, about 22.5% of people over the age of 12 have contracted Toxoplasma gondii. However, the immune system is generally able to prevent any noticeable illness. At most, mild flu-like symptoms may appear for a few days or weeks. The cysts will remain dormant in the animal's tissues, however, and can still infect another animal if ingested. They may also lead to illness in the future if the immune system is weakened.
However, there are circumstances in which Toxoplasma gondii infection can cause significant disease, which is referred to by the name toxoplasmosis. Generally this occurs in individuals that have a compromised immune system that is not able to fight off the tachyzoites, such as cats with FIV (feline immunodeficiency virus) or FeLV (feline leukemia virus), or humans with HIV/AIDS or other conditions that interfere with normal immune system function. Symptoms in cats include fever, lethargy, and loss of appetite, along with other signs that may vary depending on which tissues are affected by the tachyzoites, such as pneumonia or respiratory distress (lungs); blindness or retinal inflammation (eyes); circling, seizures, loss of coordination, or other neurologic signs (brain or nervous system). A blood test can determine whether the cat has had exposure to Toxoplasma gondii, and treatment with antibiotics/antiprotazoals is generally successful. In humans with an active toxoplasmosis infection, common symptoms are fever, nausea, headache, confusion, loss of coordination, and seizures. Treatment again consists of a combination of an antibiotic (usually sulfadiazine) and an antiprotazoal drug (usually pyrimethamine).
Pregnant women are also warned about toxoplasmosis, because it can be transmitted to the fetus while it is in the womb. This mode of transmission is dangerous to the child and can cause miscarriage in the early stages of pregnancy, or serious complications if the pregnancy is able to carry to term. However, there are some important details that are often omitted from the discussion of toxoplasmosis and pregnancy. First, only a newly-contracted infection that occurs during the pregnancy poses a risk. Women who are exposed prior to becoming pregnant are not at risk for transmission to the fetus except in extremely rare cases. If you are pregnant or trying to become pregnant, your doctor can order a special test called a titer to determine whether you have been exposed to Toxoplasma gondii in the past. Second, there are several nasic precautions that can drastically reduce your risk of exposure to the parasite. It is not necessary to get rid of your cat if you are pregnant or want to become pregnant, according to the Center for Disease Control (CDC) and the National Institute of Health (NIH). The following precautions are a good idea whether you are pregnant, immunocompromised, or completely healthy!
Cook all meat thoroughly to the USDA-recommended internal temperatures.
Thoroughly wash all fruits and vegetables, even those that are organically grown or from your own garden, since oocysts can live in the soil for long periods.
Wash your hands carefully with hot water and soap after handling raw meat. Wash all utensils, such as cutting boards and knives, that have contacted raw meat before using them for other food preparation. Don't forget to clean your countertops as well!
Wear gloves when gardening and working outdoors, and wash hands thoroughly afterwards.
Keep your house cats indoors, and do not feed them raw or undercooked meat.
If you are pregnant or immunocompromised, have another family member clean the cat's litterbox. If you absolutely must clean the litterbox yourself, wear gloves and be sure to wash your hands well afterwards. You can even wear a painter's mask to help you remember not to absentmindedly touch your face until you've washed up.
Cleaning the litterbox every day can also reduce the risk of trasmission, since oocysts do not become infectious for at least 24 hours.
With these simple steps, the risk of becoming infected with Toxoplasma gondii is significantly reduced.
For more information, check out these great sources!
Center for Disease Control: http://www.cdc.gov/parasites/toxoplasmosis/
National Institute of Health: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001661/
National Toxicology Program, US Dept. of Health and Human Services: http://cerhr.niehs.nih.gov/common/toxoplasmosis.html
Cornell University College of Veterinary Medicine Feline Health Center: http://www.vet.cornell.edu/FHC/health_resources/toxoplasmosis.cfm
Eagle Animal Hospital serves Chester Springs, Downingtown, Exton, Glenmoore and the surrounding areas.
Written by Christina Gerling, technician