The CDC reported yesterday that there are 203 confirmed "cases" (i.e. victims who’s stool sample has tested positive for the outbreak strain of bacteria) in the recent Salmonella outbreak linked to recalled salami (Daniele, Inc.) and the ingredient pepper (Wholesome Spice). The outbreak is truly nationwide, having sickened people in at least 42 states and the District of Columbia. But as in any outbreak situation, particularly one involving a relatively common product distributed over a wide geographic region over a seven month time frame, there are many more people ill across the country than the CDC’s official count would suggest.
A leading study on the subject suggests that the number of actual victims in a given outbreak, as opposed to merely those with positive stool samples, is as much as 38 times the number of stool sample confirmed individuals. See Paul Mead et al. http://www.cdc.gov/ncidod/EID/vol5no5/mead.htm.
There are several reasons why. First, it is highly unlikely that, considering the widespread distribution and availability of the potentially contaminated products, only the salami and pepper consumed by official "cases" was contaminated with Salmonella. More likely is that many more of the individual packages of salami were contaminated. After all, Daniele Inc. has now recalled 1,257,000 pounds of Salami. The 203 confirmed victims in this outbreak have certainly not purchased, much less consumed, that much salami.
Second, as noted by Mead et al., underreporting of foodborne disease is common. See Mead Article at 607. “Surveillance of foodborne illness is complicated by several factors. The first is underreporting. Although foodborne illnesses can be severe or even fatal, milder cases are often not detected through routine surveillance.” It is frequently the case that only the more severe illnesses come to the attention of health department officials. The less severe illnesses in any given outbreak often require less medical treatment, and the possibility that the causative agent—e.g. Salmonella bacteria—will be identified decreases as well.
Many cases of foodborne illness are not reported because the ill person does not seek medical care, the health-care provider does not obtain a specimen for diagnosis, the laboratory does not perform the necessary diagnostic test, or the illness or laboratory findings are not communicated to public health officials.
See Mead Article at 609.
Additionally, the likelihood that secondary transmission of Salmonella-bacteria occurred must be considered. As stated in the Mead Article, “many pathogens transmitted through food are also spread through water or from person to person, thus obscuring the role of foodborne transmission.” See Mead Article at 607. In other words, it is common in outbreak situations for people who became infected by contaminated food to thereafter sicken co-workers or family members by unwitting transmission of a small amount of bacteria.
Finally, in this particular outbreak, it must also be considered that the CDC’s official "case" count is only for Salmonella Montevideo, which is the only strain that the CDC has officially linked to the recalled, contaminated product. But the CDC has itself stated that testing by at least one State Health Department has detected the presence of other strains of Salmonella in or on the recalled product. It is difficult to accurately estimate the number of people who have suffered illness, confirmed by stool test or not, due to infection by a strain of Salmonella that has not even been made public.
These circumstances constellate to make it highly likely that the salami outbreak is much larger than just 203 sick people. As Mead states:
To calculate the total number of illnesses caused by each pathogen, it is necessary to account for underreporting, i.e., the difference between the number of reported cases and the number of cases that actually occur in the community. For Salmonella, a pathogen that typically causes nonbloody diarrhea, the degree of underreporting has been estimated at ~38 fold . . . [W]e used a factor of 38 for pathogens that cause primarily nonbloody diarrhea (e.g. Salmonella, Campylobacter).
See Mead Article at 607.
203 multiplied by 38 is a big number = 7,714.