Monday, March 23, 2015

Love thy enemy at your peril!

Probably helped by Toxoplasma gondii.
Credit: The New York Times (NYT)
The predator-prey rivalry between the cat and rat is famous. It inspired a popular cartoon series, Tom and Jerry Show. One parasite, Toxoplasma gondii, can bring about changes in the rat so that it forgets its natural fear and apprehension for the cat. The parasite makes the rat to love its enemy, the cat.

The preferred site of the Toxoplasma gondii parasite is the brain. When it eventually gets there and encounters the lymphocytes, cells that protect the body, it retreats and encysts itself. The parasite sleeps when encysted, awaiting when the host’s immunity is compromised.

The same story for the rat can be told for man. Yet, the behavioral changes the parasite causes in the rat have not been demonstrated with certainty for humans.

When the cyst persists until favorable conditions, it attacks the nervous system, causing behavioral changes. The rat takes to liking the odor of the cat. Eventually, it gets attracted to the cat and does not take it as an enemy. The rat becomes easy prey and is then eaten.

Having the rat eaten is advantageous to the parasite because the cat’s intestine is where it undergoes sexual reproduction. Therefore, finding a cat host is more important than living inside a rat.

What about humans? Research has discovered that humans can have behavioral changes based on gender when infected with Toxoplasma gondii. Males have been found to be introverted, suspicious and rebellious while women could exhibit extroverted, trusting and obedient behavior. Pregnant women infected with the parasite could have a miscarriage or birth defect.

Lifecylcle of Toxoplasma gondii.
As you can see from the picture above, the life cycle of the parasite involves these three hosts. Humans are not prey for the cat, but they can make themselves susceptible to parasitic invasion through unhygienic practices – not washing hands carefully after handling cat litter, eating unwashed vegetables and undercooked meat.

Scientists have not yet decided if the parasite changes human behavior to docile obedience in face of danger like the rat. Such a discovery could help in treatment of schizophrenia and helping persons infected with the disease when their immune system is compromised.

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