How Do Parasites Control Their Hosts?

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Toxoplasma gondii is a fascinating parasite renowned for its chilling ability to modify the behavior of its host. Rodents infected with this organism lose their inbuilt fear of cat pee, making them an effortless dinner for hungry felines. But despite a great deal of interest in this parasite, how it achieves this remarkable mind manipulation has remained a mystery.

A few years back, scientists offered a piece to this intriguing puzzle with the discovery that the organism is able to alter the production of dopamine in the brain, a key chemical messenger involved in movement, cognition and behavior. Now, scientists may have found another clue after identifying previously unknown alterations occurring within a particular type of brain cell that could play a role in the parasites’ ability to affect behavior in rodents and humans. The study has been published in PLOS ONE.

T. gondii can infect any warm-blooded animal, although it is best known for its intriguing behavioral effects on rodents, which are believed to be an adaptation aimed at increasing its transmission to cats—the only hosts within which the organism can sexually reproduce. Studies in rats and mice have shown that individuals infected with T. gondii react slower, move around more and are impaired in their ability to learn new things. Furthermore, they lose their innate fear of cat pee and even become attracted it. All in all, this makes for a very easy prey subject.

This parasite is also extremely common in humans, infecting around one-third of those residing in developed nations. If our immune systems are not compromised by anything, such as HIV or cancer drugs, it is usually forced to retreat into a dormant state, encased in hard cysts that can’t be removed. But before this happens, it infects and activates a type of star-shaped brain cell called an astrocyte. These make up around 90% of tissue in some brain areas and are known to perform a variety of critical functions, such as assisting brain cell communication, repair and immune responses.

Since studies have suggested that infection with certain pathogens can alter astrocyte function, scientists from Indiana University wondered whether T. gondii could also trigger changes in these cells. To find out more, the scientists scrutinized proteins found in astrocytes obtained from rat brains in order to deduce which ones possessed a molecule called acetyl. Many proteins are modified by the addition of acetyl, a process known as acetylation, which can alter their function or location.

In uninfected cells, they found a total of 529 acetylation sites in 304 different proteins, which were found to be involved in things like metabolism. Within infected astrocytes, however, they found 34 proteins that exhibited significantly increased acetylation, and a further 24 with decreased acetylation when compared with uninfected cells.

Although scientists aren’t sure of the precise impacts of these changes, they believe the findings could help further our knowledge of how the parasites affect behavior in both rodents and humans. Some studies have suggested that T. gondii infection can alter human behavior, and these changes appear to vary between the sexes. For example, infected women were found to be less suspicious, but the opposite was seen in men. However, these studies are correlative, whereas studies in rodents are much more robust. 

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