November 15, 2010
Parasites: Learning a Worm-Killer’s Modus Operandi
By DONALD G. McNEIL Jr.

Researchers have finally discovered how ivermectin, one of the most important worm-killing drugs used in both humans and animals for decades, actually works.

Ivermectin, extracted from a soil fungus in the 1970s, was originally sold under names like Heartgard to deworm pets.

Beginning in 1987, Merck began donating millions of doses to fight onchocerciasis, or river blindness, which is caused by microscopic worms that are spread by black fly bites and migrate into victims’ eyes. It is now also used against lymphatic filariasis, which is caused by worms spread by mosquitoes; worms nesting in the lymph nodes distort them and cause the grotesquely swollen legs and scrotums known as elephantiasis.

In a study posted online in The Proceedings of the National Academy of Sciences, researchers from McGill and Michigan State Universities showed that ivermectin does not kill the worms directly. It binds to proteins secreted by young worms to block the host’s immune system. With the worm’s defenses down, white blood cells move in for the kill. Luckily for humans, said Charles D. Mackenzie, a Michigan State professor of veterinary pathology and study author, the killing is slow, letting the worms leave the eyes. Older drugs, he said, killed so fast that dead worm bodies worsened the blindness.

Ivermectin “is an extraordinarily safe drug,” Dr. Mackenzie added. “We’ve given out 25 million doses in Tanzania and had only two minor side effects.”