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Toxoplasmosis in humans, cats and sheep

Written by  Dr. Joan Kleynhans-Jordaan
| in Veearts
| April 7, 2016

Many of us have heard that we should avoid gardening and cats while pregnant. When I was younger, medical doctors sometimes advised expectant mothers to have their cats put down to avoid contracting toxoplasmosis.


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We are now discovering a lot more about this complex and interesting parasite. Although cats are now at least partially off the hook as the main source of human infections - as they only shed oocysts in their faeces for a week or two when first becoming infected - the facts are no less scary. In recent research, infected humans and mice have been observed to be less aversive to cat urine than non infected humans and mice . Latent human infections by Toxoplasma gondii have also recently been associated with a number of neurological disorders, including schizophrenia.

Toxoplasmosis is caused by the protozoan parasite Toxoplasma gondii. It occurs worldwide and is a common parasite of many animal and bird species, as well as humans. Studies done in Southern Africa have found that between 29 and 47% of humans have antibodies to Toxoplasma, indicating that they have been exposed. Various tests are available to confirm whether infection is recent or historical. Immunoglobulin G (IgM) antibodies appear sooner after infection than IgG and also disappear sooner. Paired serum samples taken with a two to four week interval are more meaningful than a single sample in distinguishing between acute and chronic infection.

Humans very rarely show clinical symptoms, although fever, enlarged lymph nodes, muscle pain, headache and fatigue may be seen. People who suffer from AIDS, or are undergoing immunosuppressive therapy may become severely ill and show signs of confusion, poor coordination, seizures, lung problems and blurred vision due to inflammation of their retinas. In these individuals, a previous infection with Toxoplasma may reactivate and cause illness. These symptoms require immediate medical care.

If you're generally healthy you won't need any treatment for toxoplasmosis. If you are pregnant or have lowered immunity certain medications can help reduce the infection's severity. The best approach though, is prevention.

If you become infected for the first time just before - or during - your pregnancy you may not have symptoms, but can still pass the infection to your baby (congenital toxoplasmosis).

Many early pregnancy infections result in stillbirths or miscarriages. Children who survive may be born with serious problems, including seizures, an enlarged liver and spleen, jaundice and severe eye infections.

“Only a small number of babies who have toxoplasmosis show signs of the disease at birth”.

Only a small number of babies who have toxoplasmosis show signs of the disease at birth. Often, infected children don't develop signs and symptoms until their teens or later. These symptoms include hearing loss, mental disability and serious eye infections.

If you are immuno-compromised e.g. living with HIV or AIDS, or on immunosuppressive treatment, or considering becoming pregnant, it is advisable to be tested. If you are healthy and have antibodies, it indicates an earlier infection and you should be immune to acquiring a new infection during pregnancy, therefore your unborn baby will not be at risk of contracting toxoplasmosis.

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Toxoplasma in cats

Toxoplasma gondii is a parasite with domestic cats and wild felids as its definitive hosts. Intermediate hosts are other warm blooded species, including almost all mammals and birds.

Cats generally become infected after ingesting tissue cysts in infected intermediate hosts - often rodents or birds. They can also be exposed to oocysts in soil contaminated by other infected cats. Recent research has shown that Toxoplasma infected rodents display greatly decreased predator evasion, increasing their chances of being preyed upon by cats. This is thought to be an evolutionary manipulation that increases the parasite's reproductive success, as the parasite can only reproduce sexually in felids. Cats mostly become infected within the first six months of their lives. Cats will catch an infected rodent or bird with Toxoplasma cysts - containing bradyzoites - in its tissues. The organisms then multiply in the wall of the small intestine and produce oocysts, during what is known as the intraintestinal infection cycle. After infection, it takes from three to nineteen days before they start shedding oocysts in their faeces. Nearly all cats will shed oocysts after first ingesting meat containing tissue cysts. They may less frequently ingest oocysts from soil, or another life stage, tachyzoites, from meat. In these cases, less than 50% of cats shed oocysts. Cats also get cysts in their body tissues after the intestinal stage. They are therefore intermediate hosts as well as definitive hosts.

Freshly shed oocysts are not infectious. They need to sporulate first. This takes one to five days. Therefore if you have a pet cat which uses a sandbox, it is most important to clean the sandbox daily. Do not empty the sandbox into the garden, but dispose of it in garbage. A cat will shed millions of oocysts for two to three weeks after becoming infected. They will then usually develop a solid immunity to shedding of oocysts after the initial infection. They may shed oocysts again after a re-infection, but numbers will be much smaller. Large numbers of oocysts are shed in this period and will then infect soil, where they remain infective for up to a year. Sporulated oocysts are resistant to drying and freezing. Because of the short period of oocyst shedding, keeping a cat does not significantly increase one’s risk of acquiring Toxoplasmosis, although raising a litter of kittens may do so. Oocysts are not present on the cat’s fur. An indoor cat that does not hunt is protected from acquiring infection.

“The most common way humans become infected is by eating raw or undercooked meat”.

The most common way humans become infected is by eating raw or undercooked meat.

More than 60 million people in the USA carry the Toxoplasma parasite. Infected people cannot infect other people, with the exception of a newly acquired infection during pregnancy infecting the unborn child. Tissue cysts have been isolated from mutton (sheep and goat), pork, and chicken. Eating wild game may also infect people. Cattle are resistant to Toxoplasma gondii infection and are not regarded as a significant risk to people. Chicken meat is generally well cooked, and is also not important in the spread. People can also become infected by handling infected cat faeces,being exposed to oocysts in soil during gardening, or by eating unwashed contaminated fruit and vegetables that have been in contact with soil. Handling raw meat also increases the risk of infection. Implements and hands must be thoroughly washed after handling raw meat.

In countries with humid tropical climates, the incidence of serological positive humans increases from one year of age to adolescence. This is believed to be due to children playing in oocyst contaminated soil.

Toxoplasmosis in sheep

Herbivorous animals become infected as a result of faecal contamination of pastures, water or stored feed, by cats or wild felids. Due to their small size, sporulated oocysts are easily dispersed by wind and rain. Pigs may also become infected by swill, which is raw or undercooked, by eating infected rodents, or by cannibalism of other pigs.

In countries such as the USA, Australia, New Zealand, the UK and Norway, Toxoplasma gondii may be the cause of abortion or stillbirths in 10 to 20% of abortion problems in sheep flocks. It may also affect goat herds. In susceptible pregnant animals, abortion is a result of placentitis and/or foetal infection, which occurs during the second week after oocyst ingestion by pregnant sheep and goats.

Toxoplasmosis occurs in almost all warm blooded species. Some are more susceptible to getting ill from it. New World monkeys, Australian marsupials and Pallas cats are most susceptible, with Old World monkeys, rats, cattle, buffaloes and horses most resistant. Stress and other infections, e.g. ehrlichiosis and distemper in dogs, may make animals more susceptible to becoming ill. Acute systemic toxoplasmosis may cause different symptoms depending on the organs involved. Encephalitis, pneumonia and retinitis are most commonly seen.

When a person becomes infected with T. gondii, the parasite forms cysts that can affect almost any part of the body - often your brain and muscles, including the heart. If you're generally healthy, your immune system keeps the parasites in check. They remain in your body in an inactive state, providing you with lifelong immunity so that you can't become infected with the parasite again. But if your resistance is weakened by disease or certain medications, the infection can be reactivated, leading to serious complications. Although treatment is available, diagnosis may be difficult as the signs are non-specific and depends on the organs affected.

Prevention of toxoplasmosis in humans is the best option:

  • Do not eat raw or under cooked meat. Meat should be cooked to a temperature of at least 71°C for 20 minutes.

  • Do not drink unpasteurized milk.

  • Do not eat unwashed fruits and vegetables.

  • Wash hands and food preparation surfaces with warm soapy water after handling raw meat.

  • Wear gloves when gardening. Wash hands after gardening.

  • Wash hands before eating (especially for children).

  • Keep children's sandboxes covered.

  • Do not drink un-boiled water from the environment.

  • Do not feed raw meat or under cooked meat to cats. Also, do not give them unpasteurized milk.

  • Do not allow cats to hunt or roam.

  • Do not allow cats to use a garden or children's play area as their litter box.

  • Remove faeces from the litter box daily and clean with boiling or scalding water.

  • Pregnant women, and persons with suppressed immune systems, should not clean the litter box.

  • Control rodent populations and other potential intermediate hosts.

Further reading:

Torrey, E.; Simmons, Wendy; Yolken, Robert (June 2015). "Is childhood cat ownership a risk factor for schizophrenia later in life?". Schizophrenia Research 165 (1): 1–2. doi:10.1016/j.schres.2015.03.036

lFlegr Jaroslav. "Effects of Toxoplasma on Human Behavior". Schizophrenia Bulletin 33 (3): 757–760. doi:10.1093/schbul/sbl074. PMC 2526142.PMID 17218612.