LONDON -- Like munitions experts defusing a bomb, American scientists have disabled a common human parasite and prevented it from causing disease.

By inactivating a single enzyme, researchers at Dartmouth Medical School in New Hampshire changed a deadly parasite into a harmless tummy bug in a finding that could lead to better treatments and vaccines for parasitic diseases such as malaria, which kills two million people worldwide each year.

Barbara Fox and David Bzik disarmed the parasite toxoplasma gondii, which is found in undercooked meat. When they injected the mutant parasite into laboratory mice they found it was not only harmless but it protected the animals from the normal parasite.

"The findings have important implications for developing chemotherapy against parasitic infections and they may offer a new approach for vaccination against infections caused by parasites," Reuters quoted Bzik as saying in a telephone interview.

The mutated parasite caused a strong immune response in the animals which the scientists believe could be used to fight other diseases that work in the same way.

---- One Enzyme Makes All the Difference ----

Parasites are ancient organisms that cause many of the world's worse diseases. They live in and draw nourishment from their animal or human hosts.

Toxoplasma gondii, the parasite that Fox and Bzik altered, can be deadly for people with a depressed immune system and can cause birth defects if women are infected during pregnancy.

Mosquito-born malaria is caused by another parasite called plasmodium falciparum. A female mosquito transmits the disease by passing on the parasites to everyone she bites. The parasites travel to the liver where they multiply by the thousand and are released into the bloodstream.

In a report in the science journal **** Nature ****, Fox and Bzik described how they knocked out a single enzyme, destroying the parasite's ability to replicate and survive in its host.

The scientists believe the same technique might work against other parasitic diseases and could provide strategies for better drugs that can target the parasite enzymes.

The insights into parasite-host interactions may also lead to a better understanding of how to develop vaccines for parasitic diseases.

"We were expecting to observe a modest difference between the virulence of the mutant compared to its highly virulent parent. Instead, the mutant did not cause any disease in a mouse model," Bzik explained.

One dose of the normal parasite is enough to kill a mouse with a suppressed immune system. But when the scientists injected millions of the altered parasites into weakened mice it did not harm the animals and protected them from infection with the virulent strain.

"The mice were completely protected from a lethal challenge infection. These mutants offer great potential as a strategy for vaccine development," Fox said in a statement.