Why Norovirus Out-Pathogens Salmonella and E. Coli

A recent report in the Journal of Infectious Diseases underscores why norovirus represents “the perfect human pathogen,” according to Aron Hall, DVM, MSPH, an epidemiologist with the CDC’s Viral Gastroenteritis Team.

The case report (J Infect Dis. 2012;205(11):1639-1641) involved eight members of a girls’ soccer team. The index patient, a teenage girl, developed vomiting and diarrhea while staying in a hotel for an out-of-town tournament and went home the next day without any additional contact with her team. Three days later, seven other team members became ill with the same symptoms, which were ultimately traced to a norovirus infection associated with packaged snacks kept in a reusable grocery bag in a team hotel room.

“This scenario highlights a number of factors that make noroviruses so difficult to control,” said Dr. Hall. “It’s easily spread by a number of different means; it’s shed in prolific numbers by infected persons, but it only takes a small amount of the virus to make someone else sick. People don’t seem to develop lasting immunities, so they can get infected multiple times throughout their lifetime. It also tends to cause only mild to moderate illness, which, from a pathogenic standpoint, is advantageous. If you wipe out your host, you have nobody left to infect.”

In March, a CDC report with Dr. Hall as lead author found that deaths from gastroenteritis had doubled between 1999 and 2007. Although Clostridium difficile was the leading cause of gastroenteritis-associated deaths by far, this study showed, for the first time, that norovirus is likely the second leading infectious cause, Dr. Hall noted. Norovirus causes about 800 deaths annually in the U.S.

“We didn’t really begin to understand norovirus until the 1990s,” explained Dr. Hall. “The food safety industry is still focused much more on identifying bacteria, and there are major challenges associated with identifying norovirus in foods.”

A vaccine for norovirus appears to be almost ready for primetime. In December 2011, the New England Journal of Medicine published the results of an 84-person study of a nasal vaccine for the pathogen. Only 37% of those in the vaccine group who were later exposed to norovirus developed gastroenteritis symptoms, compared to 69% of those given a placebo.

“It’s reasonably far along,” said Dr. Hall. “We have the targets and we have the tools, and now it’s a question of refining the formulation and determining the groups it would be most beneficial for.”

But even a vaccine, which would most likely be used in high-risk settings like nursing homes and daycare centers, won’t completely eliminate norovirus. “These are very common causes of foodborne disease and are also spread directly by person-to-person contact, contaminated water, and contaminated surfaces,” said Dr. Hall. “Any attempt to control norovirus needs to employ a multi-pronged approach.”

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