Toxoplasmosis

Laurie Anne Walden, DVM

Toxoplasma gondii oocysts in fecal flotation solution. Image from CDC.

Have you ever heard that pregnant people shouldn’t handle a cat’s litter box? This is because of the risk of toxoplasmosis. World Zoonoses Day, July 6, is a great time to talk about how this disease passes from animals to people and whether pregnant people really do get a 9-month pass on cleaning the litter box.
 
Zoonoses are diseases that are transmitted from nonhuman animals to humans. Toxoplasmosis is a zoonotic disease caused by infection with the protozoan parasite Toxoplasma gondii. Toxoplasmosis is one of the most common foodborne illnesses in people.
 
The people who should be most cautious about T gondii infection are those with compromised immune systems and those who are pregnant.
 
Toxoplasmosis in People
People With Healthy Immune Systems
People with healthy immune systems usually have no signs of T gondii infection and don’t know they’ve been infected. For those who develop signs, most have swollen lymph nodes or flu-like symptoms that get better on their own within a few weeks. Much less commonly, the infection can affect the eyes and cause vision loss.
 
Whether or not the initial infection causes any signs of illness, the parasites remain in the body in a dormant state. The infected person develops immunity to T gondii. However, if the person’s immune system becomes compromised (because of chemotherapy, organ transplantation, or AIDS, for example), the T gondii organisms are reactivated and cause illness.
 
People With Weakened Immune Systems
In people with compromised immunity, toxoplasmosis affects the brain, eyes, lungs, and other organs. Either a new infection or a reactivated infection can cause seizures, confusion, and coordination problems.
 
Fetuses Infected Before Birth
A fetus won’t become infected if the mother was infected with T gondii before getting pregnant. In this case the mother already has immunity and won’t pass the infection to the unborn child (as long as she has a healthy immune system).
 
If a mother is infected for the first time during or immediately before pregnancy, the fetus can be infected too because the mother doesn’t yet have immunity. Most people don’t know if they’ve been infected with T gondii in the past, so it’s safest for all pregnant people to take precautions to avoid infection.
 
The consequences of infection in an unborn child can be severe. The pregnancy might end in miscarriage or stillbirth, or the fetus might develop anatomic abnormalities. More commonly, infected infants have no signs of infection when they’re born. These individuals can develop vision loss or neurologic problems years later.
 
How People Are Infected

• Foodborne transmission: eating undercooked meat or shellfish; drinking contaminated water or unpasteurized goat milk
• Zoonotic transmission from cat feces: swallowing the parasite after handling cat litter and not washing hands; consuming produce contaminated with cat feces (for example, eating unwashed vegetables from a garden that a cat has used as a latrine)
• Transmission from newly infected mother to fetus
• Blood or organ donation from infected donor (rare)

 
Toxoplasmosis in Cats
Domestic cats and other felines are the only species that spread T gondii into the environment. Cats become infected after eating birds or rodents that have T gondii cysts in their bodies. Infected cats then shed T gondii oocysts (eggs) in the stool. The oocysts are not infective to other animals right away. After at least 1 day, the oocysts mature to a stage that can infect other animals. Infective oocysts can survive in the environment for years.
 
Most infected cats don’t have any signs of illness. Cats with compromised immune systems can develop disease similar to that of humans with compromised immune systems.
 
Identifying cats infected with T gondii is tricky. Cats shed oocysts in the stool for only a short time, so fecal tests aren’t reliable for diagnosing infection in cats. However, if a fecal test happens to show oocysts that look similar to T gondii, the cat’s owners should take precautions with the cat’s stool for a few weeks. A blood test showing that a cat has T gondii antibodies doesn’t tell us whether that cat is likely to start shedding oocysts that could put the family at risk.
 
Precautions
The CDC recommends these precautions to avoid toxoplasmosis:

• Cook meat and fish to an internal temperature high enough to kill T gondii organisms.
• Rinse fruits and vegetables.
• Don’t eat raw or undercooked shellfish or drink unpasteurized goat milk.
• Follow hand and kitchen hygiene precautions (wash with soap and water).
• Wear gloves when gardening; wash hands after handling soil that a cat might have defecated in.
• Cover outdoor sandboxes so cats don’t use them as giant litter boxes.
• Don’t feed cats raw or undercooked meat.
• Scoop the cat litter box daily; it takes more than 1 day for oocysts in the stool to become infective.
• See this CDC page for more: https://www.cdc.gov/toxoplasmosis/prevention/index.html

 
Pregnant people should also take these precautions:

• Have someone else clean the litter box if possible. If you need to handle the litter, wear disposable gloves and wash your hands afterward.
• Keep cats indoors to prevent them from hunting animals that might be carrying T gondii cysts.
• Don’t handle stray cats or kittens while you’re pregnant.
• Don’t get a new cat while you’re pregnant.

 
For More Information

• Toxoplasma gondii, Companion Animal Parasite Council website: https://capcvet.org/guidelines/toxoplasma-gondii/
• Toxoplasmosis, Centers for Disease Control and Prevention website: https://www.cdc.gov/toxoplasmosis/about/index.html

 
Image source: https://www.cdc.gov/dpdx/toxoplasmosis/index.html

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