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Toxoplasmosis in Cats and Humans

What is toxoplasmosis?

Toxoplasmosis is an infection caused by an intracellular parasite known as 'Toxoplasma gondii'. It infects multiple  warm-blooded animals including humans, livestock, and birds (all of whom act as intermediate hosts). Cats are the only definitive hosts to T. gondii, which means that the parasite is only able to sexually reproduce in felines (both wild and domesticated).

Most people have heard of toxoplasmosis due to the risks an infection poses to pregnant women. If infection occurs during pregnancy it can cause abortion or congenital defects to the fetus.

Life cycle of T. gondii:

Oocysts - The infected cat sheds thousands of oocysts in the feces. These are incredibly hardy and can survive in the environment for years. At this point, they are 'unsporulated' (and non-infective) but become sporulated between 1-5 days depending on environmental conditions. Once sporulated, they are infective. The next host becomes infected by ingesting water, food or soil contaminated with sporulated oocysts.

Tachyzoites - Once ingested, oocysts cell walls dissolve, and the parasites invade host cells. Once inside, they convert to tachyzoites which rapidly multiply. Eventually, the cell ruptures and dies, releasing the tachyzoites into the bloodstream, where they are spread to other organs and tissues in the body.

Bradyzoites - At this point, the host's own immune system mounts an attack in the infection. In response to this, tachyzoites convert to bradyzoites, becoming cysts within the tissue. Tissue cysts can remain in the host for the entire lifetime. The location of these cysts varies from species to species, but commonly affected parts include the brain, eyes, heart and skeletal muscles. At this point, if the host (for example a rat, bird, cow etc) consumes infected tissue containing cysts, they become infected.

Once a cat is infected, he begins to shed oocysts between 3-10 days and continues to do so for two weeks. Felines are the only host who sheds oocysts in their feces. After a cat eats infected prey or raw meat  containing cysts, they break open, multiplying in the small intestinal wall, producing oocysts, which are then shed in the feces. This is known as the intraintestinal cycle. Other tachyzoites in the intestinal wall move into other parts of the cat's body, eventually forming bradyzoites (cysts).

How do cats become infected with toxoplasmosis?

Cats become infected when they consume prey or meat containing bradyzoite tissue cysts.

They can be infected by ingesting sporulated oocysts shed in feces.

It is also possible for a mother cat to pass on the infection to her unborn kittens (congenital transmission).

What are the symptoms of toxoplasmosis in cats?

Most infected cats show no symptoms. Kittens, young cats, and cats who are immunosuppressed (for example, those infected with Feline Immunodeficiency Virus or Feline Leukemia Virus) are most likely to display symptoms, which may include:

  • Lethargy

  • Loss of appetite

  • Fever

More severe symptoms will depend on which part of the body is infected but can include inflammation in the eye (retina, iris or cornea), pneumonia, central nervous system disorders such as head pressing and circling, pancreatitis, hepatitis (inflammation of the liver),  jaundice, vomiting and diarrhea, enlarged lymph nodes.

How is toxoplasmosis diagnosed in cats?

Diagnosis usually involves a routine blood test to detect antibodies in the blood. Your veterinarian could also examine a stool sample, although this would only offer a diagnosis if the cat was shedding oocysts at the time.

How is toxoplasmosis treated in cats?

In healthy immunocompetent cats, treatment generally isn't necessary. Cats at risk will be prescribed antibiotics such as clindamycin, pyrimethamine, and sulfadiazine.

How do humans become infected with toxoplasmosis?

There are four modes of infection in humans.

  • Consuming water, undercooked or raw vegetables containing oocysts

  • Consuming raw or undercooked meat containing tissue cysts (bradyzoites)

  • Ingestion of oocysts via contaminated cat feces

  • Mother to baby transmission in utero. This usually occurs if the mother is infected for the first time during pregnancy, however re-activation of latent infection can occur in women who have suppressed immune systems due to medical conditions such as HIV, organ recipients who are on immunosuppressive drugs or those on high doses of corticosteroids.

The percentage of people who have had toxoplasmosis varies from country to country, but worldwide, approximately 1 in 3 people have antibodies to the parasite.  The most common mode of infection is via contaminated soil, undercooked meat or improperly washed fruit and vegetables.

What are the symptoms of toxoplasmosis in humans?

As with cats, symptoms are usually rare in immunocompetent and up to 80% of people infected are unaware they have the infection.  If symptoms do occur, they may include:

  • Localised lymph node tenderness

  • Flu-like symptoms such as a sore throat, fever, headache

  • Muscle aches

  • Fatigue

Severe toxoplasmosis in humans can sometimes occur and cause damage to the eyes, brain, heart, and liver. This is more likely to happen in immunocompromised individuals such as people with HIV, organ transplant recipients and patients undergoing chemotherapy.

Recently there has been suggestions that toxoplasmosis may trigger schizophrenia in humans.

What is the treatment for toxoplasmosis in humans?

Healthy, non-pregnant individuals usually require no treatment.

If you are pregnant or are immunocompromised due to HIV or medications you will be prescribed a course of antibiotics.

How does toxoplasmosis affect the unborn baby?

Transplacental infection occurs in cats and pregnant women who become infected for the first time during pregnancy. Tachyzoites cross the placenta and invade fetal cells, this is particularly dangerous during the first trimester when the organs are developing. Toxoplasmosis in the unborn baby can lead to miscarriage, low birth weight, vision problems, hearing loss, hepatosplenomegaly (enlarged liver and spleen), jaundice, learning disabilities and mental retardation. Between 80-90% of babies who acquired the infection in utero have retinal infection leading to inflammation and possibly a chorioretinal scar containing the inactivated cyst (bradyzoite).

In pregnant women, it is easier to pass toxoplasmosis on to the fetus during the second and third trimesters, however, disease is typically milder in the last 10 weeks of pregnancy than if the embryo/fetus is infected during the first and second trimesters.

What happens if a pregnant woman becomes infected with toxoplasmosis?

If you are worried you may have contracted toxoplasmosis during pregnancy, your doctor can perform a blood test to look for antibodies to T. gondii in the blood. Unfortunately, all a test can do is tell you if you have been exposed to toxoplasmosis at some time, it won't tell you if the infection is recent. If you think you may be at risk, being tested before pregnancy can tell you if you have antibodies, and if you don't, your doctor may want to perform several repeat tests during your pregnancy.

If you or your doctor suspect you have been recently exposed to toxoplasmosis, as a precaution, you will be put on a course of antibiotics (usually spiramycin) to reduce the chances of passing the infection onto your unborn baby.

It may be recommended that you also have an amniocentesis to determine if the baby has been infected with toxoplasmosis.

If it is determined that your baby has been infected, you will need to have a number of ultrasounds throughout your pregnancy to look for abnormalities in your baby.

Should a pregnant woman rehome her cat?

No, this isn't necessary. If you have a cat and become pregnant it is strongly recommended that you speak to your doctor, midwife or obstetrician about this, they may recommend a blood test which will check for antibodies to toxoplasmosis. If you have antibodies, you have been exposed to T. gondii at some stage in your life. This means your immune system has already built up a resistance, therefore you are very unlikely to become re-infected. My own obstetrician tested me multiple times during my two pregnancies as I was negative and he wanted to ensure I remained negative (I also had a cat who had tested positive).

What precautions can a pregnant woman take to avoid toxoplasmosis?

Cats aren't the only source of infection to humans, in fact, the greatest source of infection is undercooked meat or improperly washed vegetables. Drinking untreated water (from a stream or river for example) and gardening are also potential sources of infection.

  • Avoid cleaning the litter trays, if this is not possible wear gloves and a mask. During my pregnancy, I would also change and wash my clothes after cleaning litter trays.
  • Ensure litter trays are scooped at least once a day so that oocysts are removed before they can sporulate.
  • Ensure your meat is cooked thoroughly (it should have no pinkness inside). This means it is no longer pink in the middle, the juices run clear and it has been cooked at 160F. Freezing meat for at least 24 hours (-4F) also kills T. gondii.
  • Wash your hands after handling animals.
  • Wear gloves in while gardening.
  • Wash fruit and vegetables thoroughly before eating.
  • Wash your hands after handling raw meat, fruit and vegetables.
  • Wash your hands before eating.
  • Don't let your cat(s) hunt.
  • Cover sandboxes to prevent cats defecating in them.
  • Don't drink unpasteurised milk.
  • Thoroughly cleaning chopping boards and utensils. Use separate boards for fruit/vegetables and meat.
  • Keep the litter tray away from the kitchen and other eating areas.


Are there other ways to catch toxoplasmosis from my cat?

The only way you can catch toxoplasmosis from your cat is ingestion of the sporulated oocysts via the feces (either directly, or via contaminated soil, food etc). You can not catch toxoplasmosis from a bite, scratch, cat urine or petting and stroking your cat.

What about immunocompromised people?

Immunocompromised people can still enjoy living with their cat but must take precautions listed above.

If an immunocompromised person is looking to obtain a cat, it is worthwhile looking at an older (2 years or above).

Regularly treat your cat for parasitic infections such as worms and fleas.

Yearly physical exams for your cat are also important, including fecal tests and vaccinations.

Related articles:

Should a pregnant woman get rid of her cat?

Updated 14/8/14.