Norovirus – a bug that causes gastrointestinal illness – is responsible for 12 percent of all diarrheal disease worldwide and is estimated to cause 218,000 deaths among children under 5 each year.
Norovirus is derived from fecal matter, and can infect people via 4 routes: contaminated water, contaminated food, contaminated environmental surfaces or contact with an infected person.
Researchers from Emory University and the University of Michigan looked at 902 Norovirus (NoV) outbreaks documented between 1993 and 2011, finding that most were associated with transmission through food or in a food service setting. The fewest number of outbreaks were linked to contact with contaminated surfaces and to school or daycare settings.
Another factor the study examined was the “attack rate,” of an outbreak – how many people of those exposed to the virus became infected.
Outbreaks from food and foodservice settings again topped the charts with the highest attack rates, followed by waterborne outbreaks.
This may be due to the fact that people exposed to NoV through these routes consume higher infectious doses of the virus, say researchers. These outbreaks may also have higher attack rates because the pool of at-risk people is more easily identifiable (i.e. anyone who ate the contaminated food), whereas in a person-to-person epidemic, the number of people exposed is harder to quantify.
Healthcare facilities had the lowest attack rates. This may be due to stringent sanitation measures in hospitals and long-term care facilities, say the authors.
These settings were also associated with fewer primary cases – cases in which the patient is sickened by the original source of the outbreak, i.e. the contaminated surface, rather than someone who is already sick.
Foodborne and foodservice outbreaks, on the other hand, had fewer primary cases on average, as did Schools and daycares.
“Our finding that food-related outbreaks were associated with the fewest primary cases and persons at risk suggests that viral transmission within a confined space, such as a restaurant, may limit the primary cases and persons at risk to only…persons sharing that space.”
On the other hand, researchers noted, waterborne outbreaks may have more primary cases because the source can reach a whole community, just as person-to-person outbreaks can “when an infected person becomes a mobile source of exposure.”
The study also found a link between Norovirus epidemics and season. Among outbreaks with single route of transmission, higher attack rates were noted in winter. Among outbreaks with multiple modes of transmission, highest attack rates were recorded in sprng.
Over time, though, attack rates seem to be on the decline. More recent outbreaks had a lower average of attack rates, according to the study.