Toxoplasmosis In Cats, Dogs & Humans

Updated July 10th, 2020

Any disease that can jump from pets to people is famous, and toxoplasmosis is no exception. But that doesn’t mean everyone understands how it works.

Like how many cats get it and how they pass it on. Or how to stay safe, especially if you’re pregnant. And the disease in cats and dogs themselves. We’ll cover all these here.

The Toxoplasma Life Cycle

An understanding of the toxoplasma life cycle is essential to know how to prevent it in cats and people. The picture above, while a gross simplification, is a good place to start.

Toxoplasma gondii has basically two life cycles: one in cats that creates small egg-like oocysts, and one that creates tissue cysts in everyone else. It starts when an infected cat passes oocysts in their faeces into the environment.

These eggs can survive for months and end up being eaten by any variety of animals. In the new host, they hatch and multiply inside the cells until they form tissue cysts, especially in muscle and nervous tissue. There are three outcomes depending on the species involved:

  1. If the eggs were ingested directly by humans they will cause infection and tissue cysts
  2. If they were eaten by a human’s dinner (e.g. lamb) they might cause human infection too, depending on how the food is prepared
  3. If they were eaten by a cat’s prey, they will re-infect the cat and could start another round of oocyst production

How Common Is Toxo In Cats?

Thankfully, any individual cat is extremely unlikely to be passing oocysts or be capable of infecting humans. Large studies of German cats have shown that only around 0.25% of lab samples are positive for Toxoplasma, or one in 400.

Even infected cats are low risk due to their fastidious cleaning habits. In one study, eggs could not be detected on experimentally infected cats seven days after they were passing millions in their faeces. This may be why it has been shown repeatedly that having a cat is not a risk factor for getting toxoplasmosis.

How Humans Get Toxoplasma

There is strong evidence for two main modes of transmission:

  1. Eating tissue cysts from undercooked meat. This is probably the main way in developed countries.
  2. Ingesting eggs from the environment. This is thought to be the most common way in underdeveloped countries.

A 2019 study right here in Adelaide found that the chance of toxoplasma being found in a portion of supermarket lamb mince was 43%. While we don’t have data for beef, pork, poultry, rabbit and kangaroo, we expect them to also be positive, but lower. It’s a simple fact that any animals that eat foods or graze pastures contaminated by cat faeces will regularly be infected.

How To Prevent Toxoplasmosis

Most humans who get toxoplasma will have few or no symptoms. However, if you are pregnant or have a weak immune system, the consequences could be devastating (see your doctor for more details).

You should do all of the following:

  • Cook meat thoroughly to at least 67 °C (I understand this is considered ‘medium’)
  • Wash hands, utensils and surfaces with soap and water after handling raw meat
  • Wear garden gloves if digging, and wash hands well afterwards
  • Wash vegetables before eating

Additionally, the following are sensible guidelines for cat owners:

  • Wash hands after petting your cat before eating
  • Feed cats only dry, canned or cooked foods*
  • Avoid letting your cat hunt
  • Empty the litter box every day (the eggs aren’t infectious for the first 24 hours)
  • Get someone else to do it!

* As toxoplasma is killed by temperatures below minus 12 °C it is probably still safe to give chicken necks if they have been frozen first.

Toxoplasmosis In Dogs

Toxoplasma can’t create eggs in dogs so there’s a very low risk of their faeces being hazardous unless they eat cat poo. Probably an equally low risk is dogs rolling in cat poo and having the eggs on their coat.

Clinical toxoplasmosis as a disease of dogs is exceedingly rare. When it occurs, it usually affects the nervous system with vague signs such as behavioural changes, tremors, weak legs, or paralysis. However, in a young dog, especially with muscle wastage, it’s more likely to be caused by another parasite called Neospora.

Help! My Cat Has Toxoplasmosis

Toxoplasmosis in cats is a very different disease, with the following possible symptoms:

  • Severe and generalised illness
  • Trouble breathing
  • Neurological signs like wobbling or a head tilt
  • Eye infections
  • Liver infection and jaundice
  • Abdominal infection

Particularly at-risk groups are newborns, and cats who catch it after being immunosuppressed with cyclosporin or chemotherapy.

Diagnosis of Toxoplasmosis In Cats

The problem with such a range of possible symptoms is that almost any sick cat could have toxoplasmosis. This is made worse by the difficulty in getting an accurate diagnosis.

The only certain diagnosis is made by finding the actual organism in tissues or fluids. However, this is very difficult. Therefore, vets often make a tentative diagnosis if the cat has:

  • a high IgM titre
  • two IgG titres showing a four-fold increase
  • exclusion of other diseases
  • a positive response to treatment

Treatment of toxoplasmosis is via a range of unusual drugs such as clindamycin, pyrimethamine, toltrazuril or fluconazole. None work very well in severely unwell cats.

Vets will often use clindamycin when toxo is suspected, just to be sure. But most of the time, the problem is probably being caused by something else.

That’s because actual illness caused by toxoplasma is rare. Just like humans, most cats who get toxoplasma show no signs, and make a rapid and full recovery.

By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These blogs are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!
Have something to add? Comments are welcome below and will appear within 24 hours.

Andrew

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