Parasites dwell in us!

The uneasy relationship between parasites and human beings goes back a long way. Some of them have been with us since the appearance of Homo sapiens 150,000 years ago, while others have been part of our lives only since the domestication of animals.

Those parasites are found in different forms, whether the large group of the nematodes, including the Ascaris and other flatworms of every genus, or else the protozoans, those amoebas that are the cause of illnesses like la turista.¹

Each year, millions of people are affected by one sort of single-celled parasite. These, the protozoans, are moreover the primary cause of gastrointestinal infections on the planet, according to the World Health Organization.^2,3 Even though they’re found for the most part in developing countries, there’s also a very widespread protozoan we come across every day in North America and in Québec. This is Toxoplasma gondii.

The life cycle of T. gondii

But where does T. gondii, this protozoan parasite that causes toxoplasmosis, come from? The T. gondii life cycle usually needs a feline and a rodent. In fact, this charming little bug spends part of its life cycle inside domestic cats. The parasite will breed there, which is why we say about felines that they’re the definitive hosts of the parasite. The eggs will then be released in the cat feces. In nature, certain animals like small rodents (which are known as intermediate hosts) come into contact with those droppings and contract the parasite. This in turn will develop in the rodent’s muscles and brain, feeding on tissue within range. T. gondii will bring about a change in the rodent’s behavior, making it easier for the cat to catch.⁵ And the cycle is repeated.

Humans can also become intermediate hosts – either by coming into contact with the excrement of a contaminated cat, or by simply ingesting the parasite in its “oocyst” form, a state in which it can survive for a long time in the environment. Fortunately, T. gondii is not dangerous for healthy humans. However, it is recommended that pregnant women use gloves when changing kitty litter or that they let someone else do that chore to prevent any contagion to the fetus. People with autoimmune diseases should also avoid all contact with cat feces. That said, there are some solutions to reduce the odds of your cat contracting the parasite. It must never be offered raw meat, which may contain the parasite, and it should be kept from going off to hunt outdoors.⁶

Sources:

1. Cox, F. E. G. 2002. History of human parasitology. Clinical Microbiology Review 15:595-612
2. Fletcher, S. M., Stark, D., Harkness, J. and Ellis, J. 2012. Enteric protozoa in the developed world: a public health perspective. Clinical Microbiology Review 25:420-449.
3. Andrews, K. T., Fisher, G. and Skinner-Adams, T. S. 2014 Drug repurposing and human parasitic protozoan diseases. International Journal for Parasitology: Drugs and Drug Resistance 4:95-111.
4. Centers for Disease Control and Prevention (2015). Parasites – Toxoplasmosis (Toxoplasma infection), [online], U.S. Department of Health & Human Services.
   https://www.cdc.gov/parasites/toxoplasmosis/biology.html
5. Webster, J. P. 2007. The effect of Toxoplasma gondii on animal behavior: Playing cat and mouse. Schizophrenia Bulletin 33:752-756.
6. Pets and Parasites (2018). Cat Owners: Toxoplasmosis, [online], CAPC.
   http://www.petsandparasites.org/cat-owners/toxoplasmosis/