The CDC again amended its case-count in the Fairbank Farms ground beef E. coli O157:H7 outbreak. Secondary DNA tests (surely MLVA) have helped the CDC whittle the number of cases down from 28 in 12 states on November 2, to 26 in 11 states on November 3, to 25 in 10 states today. These changing case-counts got me thinking about an important aspect of every outbreak of foodborne disease: that the number of "confirmed cases" is rarely, if ever, an accurate count of the number of actual victims in any outbreak situation.
The reality of these outbreaks (whether E. coli O157, Salmonella, or anything else) is that the number of people who are actually ill, as opposed to the number who have a stool sample that tests positive, is much bigger than the reports would indicate. In fact, one of the leading studies 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.
There are several primary reasons why this is the case. First, as noted by Mead et al. in the classic epidemiological study on the subject, underreporting of foodborne disease is common. “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.” See Mead et al., Emerging Infectious Disease, Vol. 5, No. 5, September-October 1999. 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 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 at 609.
Also, frequently in the setting of cantagious foodborne diseases, there is a significant likelihood of secondary transmission of disease. 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 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.
Thus, for many reasons, publicized case-counts in outbreaks are only indicators of how many people were actually affected. Turning back to the Fairbank Farms outbreak and recall, its a little frightening to apply the Mead formula: 38 X 25 = A lot of sick people, and way too many kids laying in hospital beds connected to dialysis machines.