Some parasites require two animal hosts to complete their life cycle. One such parasite is called Toxoplasma gondii. The mature form of Toxoplasma gondii (life cycle) lives in the intestines of cats. The parasite reproduces in the cat's intestines and the eggs (really oocysts) of the parasites are eliminated from the cat when the cat defecates. After 3-4 days the oocysts have matured enough to go to their next animal host. Out in the wild that animal is usually a rodent (rat or mouse). They ingest the oocysts in the dust or from cat droppings. After ingesting the oocysts the parasite then goes into the bloodstream and takes up housekeeping in various places in the rodent's body. While in the rodent the parasite develops into an immature form that will allow it to infect another cat when it eats the rodent.
The problem is that rodents are naturally afraid of cats and will avoid them at all costs. The poor parasite is now stuck in this immature form with no means of reproducing itself. At first this seems like a very stupid way to make a living. However, Toxoplasma gondii has a few tricks in its DNA. Recent studies by David Macdonald of Oxford University indicate that when he infected rats with the parasite they were less afraid of cats. When he placed cat scent in the cages of uninfected and infected rats he noticed the uninfected rats avoided the cat scent. However, the infected rats did not avoid the scent at all. They seem to have lost the fear of cats that they once had. The rats were also much more hyperactive. No other changes were seen in the infected rats behavior.
Unfortunately, this lack of cat fear also makes these rats more likely to be eaten by a cat. Thus the life cycle continues. Most of you are probably saying we need more Toxoplasma gondii. The fewer rats in the world the happier we will be. Unfortunately, rats and cats are not the only animals this parasite can live in. Humans, mammals (cattle, sheep, pigs), and birds can also be infected with this parasite.
There are two ways in which humans can become infected with Toxoplasma gondii: contact with cat droppings and eating undercooked. Most times we don't have any problems if we are infected with this parasite. We don't become less afraid of lions nor do we become hyperactive. However, if a women is pregnant and becomes infected by this parasite the fetus can be severely damaged or die. People that are severely immunosuppressed (e.g. cancer patients receiving chemotherapy, transplant recipients, and
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