Toxoplasmosis is a zoonotic infection (acquired from animals) caused by the protozoan parasite Toxoplasma gondii (T. gondii). It is thought to be one of the most common human infections throughout the world, but is usually asymptomatic (rarely causes clinical disease). Infection is usually only clinically significant in immunocompromised individuals or in congenital infection.
Cats, both wild and domestic, are the only definitive hosts for T. gondii. When a cat ingests prey (that is infected with T. gondii) the parasite is released into the cat’s digestive tract. The organisms then multiply in the wall of the cat’s small intestine and produce oocysts. These oocysts are then excreted in the cat’s faeces. Cats previously unexposed to T. gondii will usually begin shedding oocysts between three and 10 days after ingestion of infected tissue, and continue shedding for around 10 to 14 days, during which time many millions of oocysts may be produced. Following this, the cat’s immune system will usually prevent excretion of the organism. Oocysts passed in a cat’s faeces are not immediately infectious to other animals. They must first go through a process called sporulation, which takes one to five days depending on environmental conditions. Oocysts are very resistant and may survive in the environment for well over a year.
Other animals, including humans, are intermediate hosts of T.gondii. These hosts can become infected but do not produce oocysts. Intermediate hosts become infected through ingestion of sporulated oocysts, and this infection results in formation of tissue cysts in various tissues of the body. Tissue cysts remain in the intermediate host for life and are infectious to cats, people and other intermediate hosts if the cyst-containing tissue is eaten.
How are people infected with Toxoplasma gondii
Adults most commonly acquire toxoplasmosis by eating raw or undercooked meat infected with tissue cysts. Tissue cysts in meat remain infective for as long as the meat is edible and under-cooked.
Consumption of contaminated, unpasteurised milk has also been implicated.
Contact with oocyst-contaminated soil is another major way humans are exposed to T. gondii, for example, gardening without gloves/not washing hands after gardening.
People can also become infected by ingestion of the oocysts via eating unwashed (faeces contaminated) fruits and vegetables or drinking contaminated water.
Transplacental transmission may occur when a woman has a primary infection during pregnancy.
Humans may also acquire infection via organ donation or blood transfusion.
Can I “catch” toxoplasmosis from my cat?
Owning a cat does not mean you will become infected with the disease.
It is unlikely that exposure to the parasite would occur by touching an infected cat, because cats usually do not carry the parasite on their fur. It is also unlikely that infection would occur through cat bites or scratches. In addition, cats kept indoors that do not hunt prey are not likely to be infected with T. gondii.
If a cat becomes infected they generally will only shed the organism for a short period in their entire life, so the chance of human exposure is small. Cats are generally infected as kittens and only excrete the oocysts for two weeks after their original infection. Also, oocysts passed in a cat’s faeces are not immediately infectious to other animals. They must first go through sporulation, which generally takes one to five days, therefore emptying litter trays twice daily (before the oocysts sporulate and become infective) is advisable to help prevent transmission.
A study published in the Australian Family Physician (2001) stated that direct contact with a cat is often quite irrelevant. Transmission of oocysts almost always occurs without the knowledge of the patient (such as via contaminated vegetables or water). Indeed a history of soil contact/gardening (especially without gloves) is often more significant.
What can I do to prevent toxoplasmosis?
There are several general safety steps you can take to reduce your chances of becoming infected with Toxoplasma:
- Do not eat raw or undercooked meat. Cook food to safe temperatures (please see links below for more details).
- Do not drink unpasteurised milk or its products.
- Wash all fruit and vegetables carefully before eating.
- Wash hands before eating (including children)
- Wash hands and food preparation surfaces after handling raw meat.
- Wear gloves when gardening. Wash hands after gardening or touching soil/sand.
- Sandpits should be covered when not in use to stop cats defecating in the pit.
- Do not allow cats to hunt or roam. RSPCA Australia encourages the containment of cats in an enclosed area, at a minimum from dusk until dawn (see the article below ‘Is it okay to keep my cat confined to my property all of the time?’ and RSPCA policy A8).
- Remove faeces from the litter box twice daily and safely disinfect with boiling water – the Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s faeces so by emptying twice daily the oocysts should be able to be disposed of before they are infectious (generally infectious from 24 hours). If you are pregnant or immune-compromised avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and warm water afterwards.
Please consult your doctor directly for more safety information. Your vet can also provide advice about Toxoplasmosis infection in animals.
For more information, please see: