Study confirms role of food handling in spread of noroviruses

Nov 14, 2003 (CIDRAP News) – A study from the Netherlands confirms that food-handling practices play an important role in the spread of noroviruses (Norwalk viruses), which have been blamed for many outbreaks of intestinal illness on cruise ships and in institutions in recent years.

The study, which focused on the spread of noroviruses in the general population rather than on specific outbreaks, indicates that the main risk factor for contracting a norovirus infection is contact with someone with gastroenteritis. But the investigators also found that in households, food-handling hygiene was an important factor in transmission.

"This association indicates that in a household setting these viruses do not necessarily transmit directly from one person to another but by means of food," states the report, an early online release from Emerging Infectious Diseases. "Hygienic food-handling procedures can therefore prevent the infection spreading from one person to another." The report was authored by Matty A. S. de Wit and colleagues from the National Institute of Public Health and the Environment in Bilthoven, Netherlands.

Viral infections, especially noroviruses, are the most common cause of gastroenteritis, the report notes. In a prospective cohort study of people registered with a network of general practice physicians in the Netherlands, the authors found an overall incidence of gastrotenteritis of 283 cases per 1,000 people per year, with noroviruses detected in 11% of cases. The authors used the same cohort to examine causes and risk factors for gastroenteritis. When gastroenteritis cases occurred in the cohort, the patients were matched with healthy people from the same cohort, who served as controls.

Case-patients and controls provided stool samples and completed questionnaires concerning risk factors in the week before the onset of symptoms. The participants were asked about food-handling practices, such as frequency of shopping, duration of keeping eggs, and washing of cutting boards between use for raw meat and other products. The investigators used polymerase chain reaction and other techniques to test the stool samples for noroviruses, Sapporo-like virus, and rotavirus group A.

Confirmed norovirus cases totaled 152, most of which were in children (median age for case-patients, 2 years), the report says. Norovirus gastroenteritis was independently associated with having more than one household member with gastroenteritis (population-attributable risk fraction [PAR], 17%), contact with a person with gastroenteritis outside the household (PAR, 56%), and poor food-handling hygiene (PAR, 47%).

The researchers concluded that the main risk factor for all three viral illnesses is contact with others who have gastroenteritis, but that food-handling practices play an important supporting role in the case of noroviruses.

"The impact of food-handling hygiene can be partly explained by food contamination that occurs when a sick household member prepares meals," the report says. "However, food contaminated at an earlier step in the food chain may also be a source. On the basis of our data, an estimated 12% to 16% of NV gastroenteritis and 4% of rotavirus gastroenteritis cases are caused by introduction of contaminated food or water." Norovirus cases were not linked with any specific food product, which is not surprising, because noroviruses can probably survive on almost all foods that are not cooked before eating, the article says.

The investigators say their findings show that using standard food hygiene could prevent "a substantial portion" of sporadic norovirus cases. They add that several foodborne outbreaks have been reported in which the food worker who most likely contaminated the food was not yet symptomatic. Thus, "making professional food handlers aware of their higher probability of being infected when living with a household member with gastroenteritis might be useful," they state.

Craig Hedberg, PhD, a University of Minnesota food safety expert, called the Dutch study "really the first population-based study of Norwalk viruses in the community—illnesses that aren't occurring specifically in the context of outbreaks, which is really what we've always been reduced to looking at in the United States." Hedberg is an associate profession of environmental and occupational health in the School of Public Health in Minneapolis.

Hedberg said norovirus infections are the most common foodborne illness in the United States and possibly worldwide, but a lack of effective tests has held back understanding of the virus's transmission pathways until recently. He commented that the Dutch study "gives us some insights into why we see the types of outbreaks we see in restaurants, cruise ships, and nursing homes."

He agreed with the suggestion that food workers should be made aware of the risk of transmission in households. "What the study says is that if you're a food worker and you live in a house where people are sick with vomiting and diarrhea, you need to understand that you may be at greater risk of spreading that yourself. So you have to be extra careful with how fastidious you are with handwashing."

De Wit MAS, Koopmans MPG, van Duynhoven YTHP. Risk factors for norovirus, Sappporo-like virus, and group A rotavirus gastroenteritis. Emerg Infect Dis 2003 Dec(early online release) [Full text]

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