Toxoplasmosis is a parasitic infection of cats. The parasite can only complete its life cycle in the cat, but other animals — including people and, most worryingly, unborn babies in pregnant women — can accidentally become infected (though the risk is quite low, and here’s why).
Mice infected with toxoplasma are the source of infection for cats. When a hunting cat catches and eats an infected mouse, the parasite breeds in their guts and sheds an egg-like form into the feline’s feces.
It takes a few days for this to develop into an infective stage, but afterward, the feces do pose a risk to those coming into contact with them.
People can avoid infection by:
- Cleaning litter boxes the same day the feces is produced
- Avoiding contact with soil and undercooked meat. (As an aside, even unwashed salad can pose a risk to people — it is always best to wash fruits and vegetables before eating them.)
Fit, healthy cats that catch toxoplasmosis often get rid of the infection without event, but some cats, especially those with weak immune systems, can become ill. In some cases, the infection can be life-changing.
The immune system polices the cat’s body and fights off infection. For the majority of cats who eat an infected mouse, this is exactly what happens. They either show no signs of ill health or mild, nonspecific symptoms such as lack of appetite and tummy upsets.
However, a small number of cats develop a more serious infection whereby the toxoplasma settles in the organs, such as the liver, lungs and brain, causing jaundice, pneumonia and neurological signs.
I remember a huge tabby cat, Mozart, who was as soft as they come but a hobby hunter. He caught toxoplasmosis, was a little unwell, but soon started having seizures. The parasite had invaded his brain where it caused damage, leading to fits.
Mozart was started on a course of antibiotics to kill the toxoplasma and anticonvulsants to control the seizures. He did well for many years, and his main gripe was that his mom wouldn’t let him go hunting anymore.
The culprit is a coccidial parasite called Toxoplasma gondii. It has a complex life cycle only completed in the cat. Other mammals, such as mice, act as an intermediate host (a kind of “vacation home” for toxoplasma), and cats become infected by eating these mice.
Once inside the cat, toxoplasma do 2 things:
- They multiply and produce an egg-like form that passes out in the cat’s feces
- And they invade the cat’s bloodstream and set up cysts in tissue and organs.
The risk to people is from the egg-like form, which the cat sheds for several weeks after initial infection. However, the good news is that it takes 48 hours for the shed “eggs” to become infectious, thus if cat feces are removed within 24 hours of being passed, the risk is very low.
Once infected, a cat develops immunity to further infections. However, the cysts already in the muscle, liver or lung can hatch out over the course of several years and cause ongoing damage.
A variety of blood tests can detect toxoplasma in a cat. However, these tests cannot distinguish between active infection and previous infection that the cat has under control.
This is where the science gets a little complicated. To work out if the cat has an active infection, it takes 2 blood samples taken 2 to 3 weeks apart. The levels of antibodies are compared; a 4-fold rise of the second sample over the first indicates active infection (i.e., something that could be making the cat ill.)
An antibiotic called clindamycin is effective at killing toxoplasma. It needs to be given twice a day for 4 weeks (quite a challenge). However, antibiotics kill only the active form and do not harm the encysted parasite, so some cats do suffer ongoing tissue damage over time.
For cats with pneumonia, liver damage or brain damage, giving them medicines to combat these secondary problems can improve their health.
Where possible, stop cats from hunting and feed them only cooked meat (raw meat sometimes contains cysts).
With regard to preventing human infection, pregnant women should avoid contact with soil, cat feces and raw meat. Removing cat droppings within 24 hours of being passed also reduces the risk as it takes a couple of days for the egg-like form to become infective.
Finally, if you take one message away from this article, it is to wash your hands after petting a cat.
In my next article on toxoplasmosis, I talk about why cats are not the enemy.
- “Toxoplasma gondii infection in cats: ABCD guidelines on prevention and management.” Hartmann. J Feline Med Surg, 15: 631–637.
- “Toxoplasmosis.” Davidson. Vet Clin North Am Small Anim Pract, 30(5): 1051–1062.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed July 1, 2015.