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Deadly Human Parasite Sequenced

By Kate Ruder

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The finger-like cells of C. parvum rupture from their oocyte—ready to attach themselves to the cells of our intestines.
Scientists have sequenced the genome of a parasite that infects people’s intestines and can be deadly for individuals, such as AIDS patients, who have weak immune systems. There are no effective drugs to treat the infection despite decades of research.

Now, the genome sequence reveals that the parasite, Cryptosporidium parvum, lacks some of the key structures that similar parasites carry—which could explain why many drugs have failed.

“One of the most important conclusions of the research is that most drugs don’t work because the targets of the drugs are missing,” says Mitchell Abrahamsen of the University of Minnesota in Minneapolis, who led the sequencing project.

Researchers did find several proteins that show potential as new targets for drugs. Because the parasite cannot be grown outside its animal hosts, it has been incredibly difficult to study until now.

The National Institute of Allergy and Infectious Diseases classifies the parasite as a level B pathogen because of its threat as a bioterrorism weapon.

The waterborne parasites live in most lakes and rivers in the United States, and they can get into public drinking water supplies. They are exceptionally hardy, with tough cell walls that are resistant to chemicals such as chlorine, which is often used to treat drinking water.

In 1993, 400,000 people in Milwaukee, Wisconsin, became sick from the parasites, which were living in the public drinking water. The most common symptom is diarrhea. A number of AIDS patients are also thought to have died during the Milwaukee outbreak.

Scientists identified the parasite in the 1970s, but it wasn’t until AIDS became widespread that people realized the seriousness of the disease. The parasites also infect animals, and up until that time it had been viewed largely as a disease in livestock.

Abrahamsen, M.S. et al. Complete genome sequence of the apicomplexan, Cryptosporidium parvum. Published online in Science Express March 25, 2004.

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