Norovirus, Salmonella top list of outbreak-causing pathogens

Jan 25, 2013 (CIDRAP News) – Norovirus and Salmonella continued to top the list of pathogens responsible for US outbreaks of foodborne disease, the Centers for Disease Control and Prevention (CDC) reported today.

Writing in today's Morbidity and Mortality Weekly Report (MMWR), CDC scientists said in their most current update that norovirus in 2009 and 2010 caused 42% of outbreaks and 37% of illnesses, with Salmonella causing 30% of outbreaks and 36% of illnesses.

In the previous update, published in MMWR in September 2011 and covering 2008, norovirus was responsible for 49% of outbreaks and 46% of illnesses, while Salmonella caused 23% of outbreaks and 31% of illnesses. The two pathogens were also No. 1 and No. 2, respectively, in 2006 and 2007, with the percentages varying somewhat.

During the 2009-10 reporting period, the CDC confirmed 1,527 foodborne disease outbreaks (675 in 2009 and 852 in 2010) that resulted in 29,444 confirmed illnesses, 1,184 hospitalizations, and 23 deaths.

Among the 790 outbreaks that had a single confirmed cause, norovirus accounted for 331 (42%), Salmonella caused 234 (30%), and Shiga toxin–producing Escherichia coli (STEC) caused 58 (7%), of which 53 were serogroup O157. Among the 225 confirmed Salmonella outbreaks with a serotype reported, Enteritidis was the most common serotype, accounting for 76 outbreaks (34%).

All told, bacteria caused 413 (52%) of the 790 outbreaks, viruses caused 336 (42%), chemicals and toxins caused 39 (5%), and parasites caused 2 (0.2%).

Among the 299 outbreaks that could be attributed to a food source from one of the CDC's 17 "predefined commodities," beef was most often implicated, in 13%, followed by dairy (12%), fish (12%), and poultry (11%). The foods associated with the most illnesses were eggs (27%), beef (11%), and poultry (10%).

Although Salmonella caused the largest number of cases requiring hospitalization (583), Listeria outbreaks accounted for the highest proportion of persons hospitalized (82%), followed by Clostridium botulinum (67%) and paralytic shellfish poisoning (67%).

Among the causes of the 23 deaths were Listeria monocytogenes (9 deaths), Salmonella (5), STEC O157 (4), C perfringens (3), Shigella (1), and norovirus (1).

Craig Hedberg, PhD, a food safety expert at the University of Minnesota's School of Public Health, complemented the CDC on its outbreak reporting, calling it "a great report and a timely update of this important set of public health data."

"CDC has improved the timeliness of analyzing and disseminating the results of its foodborne outbreak surveillance data," he told CIDRAP News.

"The importance of this is that it provides much more direct feedback on trends and implications of the outbreaks. It also provides strong reinforcement to state and local agencies to improve their reporting to CDC," he added.

He added one caution about interpreting the data, however, saying the definition of what constitutes an outbreak is not always clear.

As an example, Hedberg cited the large 2010 outbreak of Salmonella Enteritidis (SE) in eggs from two farms in Iowa that caused more than 1,900 illnesses in multiple states. He said it could be reported as either one large outbreak or several outbreaks in restaurants that received the contaminated eggs, and both ways would make sense.

"However, it is something that needs to be reported," he said, "so that the reader can have some perspective on the relationship between the outbreaks and progress toward meeting the Healthy People targets.

"If the 37 SE outbreaks in 2010 include multiple events that were linked to the multistate outbreak, it would have different implications than if only the large multistate outbreak was counted toward the yearly target of 22 SE outbreaks. It is not clear from the report how this situation was handled," said Hedberg.

CDC. Surveillance for foodborne disease outbreaks—United States, 2009-2010. MMWR 2013 Jan 25;62(3):41-7 [Full text]

See also:

Sep 9, 2011, previous CDC foodborne outbreak report

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