Mind-Altering Parasites

Parasites that cause skin rashes in people and infest ducks can commandeer DNA in the brains of snails and order the unwitting mollusks to give up sex. The findings, in the current issue of the Proceedings of the National Academy of Sciences, suggest that some parasites can enhance their ability to survive by hijacking the controls of specific host genes.

Wormlike creatures named Trichobilharzia ocellata spend part of their life cycle in the flesh of small freshwater snails (Lymnaea stagnalis). Previous studies had shown that snails with the parasites forgo sex, allowing them to grow more rapidly. At the same time, "within 1 1/2 hours of [the parasite] entering the host, gene expression is significantly and selectively affected," says Marijke de Jong-Brink, a neurobiologist at Vrije University in Amsterdam.

Hoping to link these two observations, Jong-Brink's team exposed snails to a parasite-filled bath. They removed the snails' brains after various durations to measure levels of mRNAs, which translate the DNA blueprint into a template for building proteins. In the parasite-ridden snails, mRNA levels shot up, with some mRNA types exceeding by three times the level of those in healthy snails. The parasite may be releasing a chemical, says Jong-Brink, "that stimulates or inhibits special genes in the host central nervous system, either directly or indirectly." These genes are involved in reproduction and growth regulation.

The finding reinforces the view that "parasites--somewhat like viruses--can force their hosts to act to their benefit," says Viktor Mutt, a medical biochemist at the Karolinska Institute in Stockholm. The findings could have implications for a similar species of parasite that causes schistosomiasis, a debilitating disease that threatens one out of six people in some developing countries.