There's yet another good reason to keep flies off your food: Both houseflies and filth flies can transmit cryptosporidiosis, and better fly control is one key to decreasing the risk of this disease, Dr. Thaddeus Graczyk reported at the annual meeting of the American Society of Tropical Medicine and Hygiene. Cryptosporidiosis is a parasitic diarrheal disease, and children, travelers to foreign countries, and immunocompromised individuals can be at particular risk. Large waterborne outbreaks have occurred in several U.S. cities.
"Filth flies can acquire infectious cryptosporidium oocysts from unsanitary sites, deposit them on visited surfaces, and therefore may be involved in human or animal cryptosporidiosis," he said. Dr. Graczyk, of the Department of Molecular Microbiology and Immunology, Johns Hopkins University, Baltimore, described a study conducted over a 6-month period in which wild filth flies were collected from traps left for 7-10 days in a barn with infected calves shedding Cryptosporidium parvum oocytes in the feces and a control study with an uninfected calves.
The oocysts transported on the bodies of the flies and obtained from their droplets on visited surfaces produced cryptosporidiosis in mice. Molecular analysis of the oocysts shed by infected calves indicated that flies could carry them for at least 3 weeks and that these oocysts retained their infectivity. "This is the second study showing that flies can transmit cryptosporidiosis," Dr. Graczyk said. A previous study in his laboratory of houseflies hatched with contaminated bovine feces yielded similar results.
Cryptosporidiosis has been known predominantly a waterborne infection, he noted. However, a few outbreaks have been associated with foodstuffs such as apple cider, milk, and sausages. "The results of this study suggest that if these flies land on food, cryptosporidiosis could also be a significant foodborne infection," he said.
The American Society for Tropical Medicine and Hygiene (ASTMH) is the principal organization in the United States representing scientists, clinicians, and others with interests in the prevention and control of tropical diseases through research and education. Additional information on the meeting can be found at http://www.astmh.org/presskit.html.