Mites, Sheep, and Mad Cows

Although endemic in Europe for centuries, the sheep disease known as scrapie achieved notoriety only during the 1980s, when it was apparently transmitted to cows in Britain via infected sheep remains in cattle feed, thereby causing mad cow disease. Both scrapie and mad cow disease are thought to be caused by brain-destroying proteins called prions. But while the source of mad cow disease is fairly well established, no one knows how scrapie infects sheep. Now neuropathologist Henryk Wisniewski and his colleagues at the New York State Institute for Basic Research in Developmental Disabilities have found evidence that hay mites may be the culprits. Wisniewski’s group, working with researchers at the University of Iceland in Reykjavík, collected samples of six mite species from five scrapie-plagued Icelandic farms. They injected ground-up mites into the brains and abdomens of 71 four-week-old white mice. After about a year had passed, 14 of the mice developed scrapie’s symptoms: a wobbly walk, difficulty grasping, and a rigid tail. And antibody tests showed that prions were in their brains. If sheep do indeed catch scrapie by eating mite-infested hay, Wisniewski says, there may be a simple way to control the disease. Wrapping bales of hay in plastic, he says, kills the mites. They are very sensitive to temperature and humidity.