New Data Can Assist Outbreak Investigations

When a consumer becomes ill from what is suspected to be foodborne illness, the first thing most do is blame the last thing they ate. In some cases, this is accurate, but in most cases, signs and symptoms of the disease do not appear for more than 10 hours after exposure to the germ, and many take days, or even weeks, for impact.

For example, as reported in a new publication from CDC, Incubation Periods of Enteric Illnesses in Foodborne Outbreaks, United States, 1998–2013 (and shown in the table below), the median incubation period of Salmonella contamination is 32 hours; for E. coli, it is 87 hours (more than 3 ½ days); and Hepatitis A, which has been all the news lately, can take a month or more from the time of infection until symptoms appear. However, the infected person can transmit the disease prior to that.

What this means. During an outbreak, knowing a disease’s incubation period can help guide the diagnostic and investigative strategies, as it can help direct case interviews and definitions, focus clinical and environmental evaluations, and predict an etiology.

But because the data on incubation periods have been limited, CDC conducted the study to examine foodborne disease outbreaks from laboratory-confirmed, single etiology, enteric bacterial, and viral pathogens that were reported to U.S. Foodborne Disease Outbreak Surveillance from 1998–2013.

While textbook descriptions of incubation periods have existed, CDC noted most as being backed by little scientific documentation based on a small series of outbreaks. An example of this was CDC’s finding that in a systematic review of incubation periods among enteric viruses, only 50% of evaluated studies cited actual data, and of those, most of the data were traced back to a small number of original studies.

Why accurate data are important. Accurate incubation data can mean the difference between your product being implicated or not in an outbreak, and/or how much product must be recalled. Thus, once the pathogenic source of an outbreak is determined, epidemiologists can use the incubation period to estimate a likely exposure period on which to focus interviews of cases and controls, make a case definition more precise, and then determine foods or products on which to begin evaluations.

Alternatively, if the pathogen has not been determined, but the specific exposure is known, investigators can use the measured incubation period, along with other clinical information, to hypothesize likely etiologies and focus diagnostic testing of clinical and food specimens on these.

It is important to realize, however, that the data must be used with caution, because possible exposures outside the noted incubation ranges cannot be absolutley excluded. For each pathogen, outbreaks occur with incubation periods outside that range, and the variation can be quite real. Similarly, individual cases within an outbreak can vary, so the data cannot “prove” that an individual case is or isn’t part of a given outbreak. However, the analysis does suggest possible etiology and can help focus the time window for potential exposures. These timelines also can help you determine if your product is more likely than not in the incubation period window.