Cryptosporidium parvum — an insidious, one-celled, waterborne parasite that lodges in the intestines of infected people and animals and for which there is currently no effective treatment — is missing key structures normally found in similar parasites, say researchers supported by the National Institute of Allergy and Infectious Diseases (NIAID), one of the National Institutes of Health. The results of their genome sequencing project, now available in the online issue of Science, could help scientists home in on new drug targets that may lead to therapies for the disease.

C. parvum is an extremely hardy parasite found in water supplies throughout the world, including the United States. In persons with healthy immune systems, symptoms of infection include diarrhea, stomach cramps, upset stomach and fever. For persons with weakened immune systems, however, such as individuals with HIV/AIDS, symptoms may be more severe and can lead to serious or life-threatening illness. Because C. parvum could potentially be used as a bioterrorist agent, the NIAID has classified it as a Category B priority pathogen.

After reconstructing the predicted genes and resulting proteins of one form of C. parvum, researcher Mitchell S. Abrahamsen, Ph.D., University of Minnesota, St. Paul, MN, and his team discovered that Cryptosporidium is missing two organelles commonly found in related protozoan parasites. Gone is the apicoplast, a cellular component that provides essential metabolic functions in related parasites, including those that cause malaria and toxoplasmosis, respectively. Also absent is the mitochondrion, the so-called "energy factory" found in the cells of most plants, animals, fungi and one-celled organisms. In addition, the researchers found that Cryptosporidium has significantly fewer genes than related parasites, and, as a result, can carry out fewer metabolic functions on its own.

Because Cryptosporidium has been so difficult to study up until now — presumably because its demands for energy and nutrients have made it virtually impossible to grow in the laboratory — the decoding of the genome sequence provides valuable opportunities to inform and study the organism's biology. And with an understanding of its biology, researchers are better positioned to find treatments that zero in on unique biological processes essential for the organism's survival.

NIAID is a component of the National Institutes of Health, an agency of the U.S. Department of Health and Human Services. NIAID supports basic and applied research to prevent, diagnose and treat infectious diseases such as HIV/AIDS and other sexually transmitted infections, influenza, tuberculosis, malaria and illness from potential agents of bioterrorism. NIAID also supports research on transplantation and immune-related illnesses, including autoimmune disorders, asthma and allergies.

Press releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at http://www.niaid.nih.gov.

Reference: MS Abrahamsen et al. Complete genome sequence of the apicomplexan, Cryptosporidium parvum. Science DOI: 10.1126/science.1094786 (published online March 25, 2004).