Some parasites that thrive in animals find their human hosts rather hostile. For instance, a protozoan called Trypanosoma brucei brucei (T b brucei), which infects both humans and cattle, quickly degenerates in the former's blood. But in cattle, the bug causes a disease resembling African sleeping sick- ness in humans. Now, a group of American researchers may have the came only in 1978, when biologist Mary Rifkin, now in Mount Sinai Medical School atjjie New York City, discovered that the killing factor is localised in a high-density lipoprotein (HDL) - known for its role in removing blood cholesterol.

Taking a cue from Rifkin's finding, Hajduk's group zeroed down to a small HDL sub-fraction - which they called answer to what in the human blood wipes out the parasite (Science, Vol 268, No 5208).

Molecular biologist Stephen Hajduk and his colleagues at the University of Alabama, Birmingham, claim to have isolated and purified in laboratory cultures, a protein in the human blood that destroys the parasite. Because this disease is widespread among livestock in Africa, the researchers say it may be possible to use the gene encoding the protein to produce trypanosome-resistant strains of cattle for die continent.

For decades, parasitologists had no idea of what in the human blood proved fatal for trypanosome. The first clue the trypanosome lytic factor or TLF.

From this material, researchers finally succeeded in isolating what they believe is the killing factor - a protein related to the blood protein called haptoglobin, which binds haemoglobin released from dying red blood cells and helps prevent loss of iron from the body.

George Cross, a molecular parasitologist at the Rockefeller University in New York City, says the finding "makes a convincing caw that the haptoglobin-related protein is a key element in killing of T b brucei." What they appear to have done is solved one of the mysteries of the African trypanosomiasis, he adds.