A third of us are infected with Toxoplasma gondii, a parasite that can be transmitted to us by animals. Cats represent a major source of this organism, and are therefore a common route of infection for us since they continually shed the parasite. Consequently we can become infected with Toxoplasma gondii if we come into contact with anything that is contaminated with the organism as a result of a cat’s shedding. Some ways include:

  • Contaminated soil  (when gardening in an area where cats have defecated)
  • Cleaning the cat litter box
  • Contaminated water
  • Undercooked meat (lamb, pork, and venison especially – these animals are infected by cats in the same way as we are)
  • Cooking utensils coming into contact with undercooked, infected meats

Although many people are infected with this parasite, most are unaffected by it, and show no clinical symptoms since the immune system effectively prevents it from causing disease; however, clinical toxoplasmosis can be a real problem for people with weakened immune systems, such as:

  • HIV infected patients
  • Chemotherapy patients
  • Organ-transplant recipients

Additionally, it poses a particular risk for unborn babies. A  woman who comes into contact with the organism for the first time during her pregnancy may transmit it to the fetus in utero, resulting in birth defects or even infant fatalities. Transmission of the parasite to an unborn baby, however, is less likely if the woman has previously come into contact with the organism at least six months or more, prior to becoming pregnant.

Links With Brain Cancer?

Interestingly, a recent study reports a correlation between rates of infection of Toxoplasma gondii, and the incidence of brain cancer. Global data on brain cancer in people from 37 countries was collected and compared with the prevalence of Toxoplasma gondii infection in those regions. The research group reported that brain cancer rates increased in countries where the parasite was more prevalent.

This ecological study, however, merely points to a correlation between the two events – it does not imply that the parasite actually causes brain cancer in people. And certainly the opposite could be true – it’s not impossible that brain cancer could be the driving factor behind Toxoplasma gondii infection.

So as it stands, this report does not prove cause and association, but is predominantly hypothesis-generating, and does provoke scientific curiosity. In the words of one of the authors:“These were the best data available and we felt they were sufficient to take the first step. Working with actual brain cancer patients is an obvious next step, but it would be an expensive proposition.  It is a lot easier to justify the second, expensive step when you have some evidence for the hypothesis. We are hoping that our results motivate others in the field to do further studies.”

Thomas, Lafferty, Brodeur, Elguero, Gauthier-Clerc & Misse. 2011. Incidence of adult brain cancers is higher in countries where the protozoan parasite Toxoplasma gondii is common. Biology Letters



  1. Madeleine says:

    It is reports like these that do “… not prove cause and association, but is predominantly hypothesis-generating, and does provoke scientific curiosity” that will increase the abandonment of cats to the streets and shelters where they will most certainly die. And all this for an hypothesis. And surely this will be picked up by the media and shouted loud with the major risk being to the cat. This is irresponsible.

    • Madeleine, thanks for stopping by to comment. As a scientist and a veterinarian, I find this scientific hypothesis to be interesting, not irresponsible. The irresponsible ones are the owners who abandon their pets – whatever their reason. Toxoplasmosis has been known for a very long time to be a problem for people, and in particular a potentially fatal problem for unborn children – this issue alone is scientifically proven. So I’d find it bizarre if people would suddenly ignore “known” and potentially fatal health risks associated with cat ownership, and yet abandon their cat simply due to a scientific report like this. It’s always good to show concern for our pet population, but hopefully your fears will be unwarranted here.

    • I share your fear. However interesting this might be scientifically the reaction from the “ordinary” man in the street will most certainly be one of panic. People tend to not read the fact that nothing is proven. I am a veterinarian and have had to deal with many pregnant clients and the fear caused by their doctor asking them if they owned a cat and recommending to get rid of said cat because of Toxoplasmosis. The worse is that all these woman have been in cotact with cats from childhood and does not need to fear their feline friend. What happens when a pregnant cat owner does not consult a veterinarian on the subject after such advice from their doctor? It is our duty to be very careful with information given to the general public especially when their is no facts to prove the hypothesis.

      • Jeanette, I do agree with you about panic from the man on the street. I think this is where the internet has become a blessing and a curse. It’s great that people have access to so much information now, but some don’t necessarily use it in the best way. It sometimes creates a bit of a loose cannon effect. As a veterinarian myself, I also get to deal with peoples’ overreactions and panic. Thankfully many are good about taking information found in the media and asking for professional opinions about risk etc. Hopefully increasing numbers of people will do the same with time. Sadly we can’t do much about people who don’t seek professional opinions, but for me it’s even more troubling when physicians are poorly informed on the risks of toxoplasmosis and thus provide incorrect information to patients.

  2. Toxoplasmosis in the US is more likely to be obtained from eaten undercooked pork than from cats. At least that’s what the parasitologists in my 2004 vet school parasitology course said.
    Also, the data is from 37 countries. It would be interesting to see what the US and other developed countries data looks like alone.

    • Cats are the key animal in the parasite’s life cycle, so they represent the biggest risk to people, due to the fact that they shed the oocyst which is the environmentally resistant stage. But this doesn’t necessarily mean that they always represent the main direct source of confirmed cases – and certainly, different countries differ with respect to the incidence of cases that arise from poorly cooked pork too, & it’s definitely relatively high here in the US compared with other countries, like you were taught in parasitology. Although ultimately the pigs are picking up the parasite courtesy of shedding cats, so it all goes full circle! And I agree – I’d love to see breakdown of the data, country by country. Thanks for stopping by, Chris.

  3. Cat Parasite Linked To Brain Cancer In People, this is very scary for ordinary people when they read such scientific paper, if there is a chance , would love to see the break down data for those countries and the scientific factors and prove , did they found any traces of the parasite in those brain tumors to say its true positive infestation cancer relationship with Toxoplasma gondii infestation . many thanks.

  4. Hi Mourad – at the end of my post, I have included the citation for the article as published in Biology Letters, so if you have access to it, you’ll be able to check out the whole thing. It’s very interesting from a scientific point of view. It’s true that non-scientitst can become scared of information like this, but only when they use it inappropriately! It’s been known for a very long time that smoking kills people, or at the very least will cause tremendous pathology in the long term – yet those proven facts haven’t yet managed to stop all smokers from smoking! So hopefully if people read reports like this in the media, they will take it at face value – it’s an interesting hypothesis, but does not prove cause and effect. Thanks for commenting.


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