Center for Infectious Disease Research & Policy (CIDRAP) reports that the latest complete data show that a total of 1,097 foodborne outbreaks representing 21,244 cases were reported for 2007, with 18 deaths, the CDC said in the 13 August issue of Morbidity and Mortality Weekly Report (MMWR). The numbers were significantly lower than the average of the preceding five years, it says.

There were 497 outbreaks clearly traced to a single pathogen, and norovirus was blamed for 193 (39 per cent) of those, including 47 per cent of the cases, the report says. Salmonella was the second-leading cause, with 136 (27 per cent) outbreaks, including 27 per cent of the cases.

The report shows that 235 outbreaks involving 4,119 illnesses were linked to a single food commodity. Poultry was the most common culprit, accounting for 691 cases (17 per cent), followed by beef, with 667 cases (16 per cent) and leafy greens, with 590 cases (14 per cent).

The pathogen-and-food combinations blamed for the most illnesses were norovirus in leafy vegetables, 315 cases; Escherichia coli O157:H7 in beef, 298 cases; and Clostridium perfringens in poultry, 281 cases.

The single biggest outbreak involved 802 illnesses that were blamed on hummus contaminated with Salmonella, according to the report. The second-largest outbreak consisted of 626 norovirus cases at a conference hotel, in which several food items were suspected. In third place was another Salmonella outbreak, involving 401 illnesses and three deaths tied to frozen pot pies.

The CDC also identified the largest outbreaks associated with a single food commodity. The leader was one involving 132 Cl. perfringens infections tied to a chicken dish, followed closely by a 128-case norovirus outbreak blamed on a leafy vegetable salad. Others included 125 C. perfringens cases blamed on chili beans and 124 E.coli O157:H7 cases linked to beef.

The total of 1,097 outbreaks reported in 2007 was actually eight per cent lower than the annual average of 1,193 for 2002 through 2006, and the 21,244 cases were 15 per cent lower than the average of 25,079 for those years, the report says. These decreases probably reflect growing population immunity to two new norovirus strains that emerged in 2006 and that likely increased norovirus cases that year, the CDC says.

"This pattern of emergence of new norovirus strains corresponding with a spike in norovirus outbreaks appears to occur worldwide approximately every two to three years," the article states. "The overall decrease in reported outbreaks in 2007 largely resulted from a reduction in the proportion caused by norovirus. The number of outbreaks caused by bacterial agents in 2007 was similar to the 2002-2006 average."

Dr Kirk Smith, foodborne disease supervisor with the Minnesota Department of Health in St Paul, said he agrees that there was a surge in norovirus outbreaks in 2006 but suggested that the numbers have to be taken with caution because many norovirus outbreaks are not investigated or reported.

State and local public health budgets have been shrinking in recent years, and "When resources get stripped, it's norovius investigations that are the first to go," Dr Smith told CIDRAP.

"Every state has all kinds of norovirus outbreaks, but a lot of states have one listed or none listed [in the MMWR article maps]," he added.

He commented that it is often possible to infer from illness characteristics and incubation periods that norovirus is the cause of a foodborne outbreak. "A lot of times if you think there's a norovirus problem, you can just send a sanitation worker to the restaurant to take care of the problem, without doing the investigation. That's the model that a lot of jurisdictions use," he said.

The MMWR report acknowledges that not all recognized clusters of illness are investigated or reported to the CDC, because of competing priorities in health departments.

Norovirus outbreaks are believed to result mainly from contamination of food by infected workers who don't properly wash their hands after using the toilet, the article notes. It says improved safety training for food service workers and the use of certified food protection managers might help reduce the numbers of outbreaks.

The article notes that foodborne outbreaks – two or more similar illnesses related to eating the same food– make up only a small fraction of all foodborne disease cases reported each year. For example, only 5.4 per cent of all Salmonella cases identified in the CDC's FoodNet surveillance system in 2007 were part of recognised outbreaks. And many foodborne disease cases are never reported at all. An oft-quoted CDC estimate is that 76 million such cases occur each year, with most going undocumented.

In other observations, the article says that norovirus accounted for 97 per cent of the virus-caused outbreaks, with the remainder attributed to hepatitis A and rotavirus.

Salmonella accounted for 53 per cent of all outbreaks traced to bacterial pathogens. The most common Salmonella serotype was Enteritidis, which caused 28 outbreaks with 555 cases, the report says.

Lower on the list of bacterial causes are Cl. perfringens (31 outbreaks, 1,304 cases), E. coli O157 and other Shiga-toxin-producing E. coli strains (40 outbreaks, 593 cases), Campylobacter (21 outbreaks, 346 cases), and Staphylococcus enterotoxin (11 outbreaks, 242 cases).