Chapter 25

Introduction

To investigate a food that has been implicated as the causative vehicle in an outbreak of illness, the microbiologist should make certain observations and perform certain tests as a matter of course; further analysis depends on the circumstances of the particular case. It is always crucial to note the general condition of the food sample, such as its consistency, color, and odor. As much information as possible should be obtained about its pre- and post-collection history (see Chapter l). Microscopic examination and Gram staining must be carried out, as described in Chapter 2.

To decide what treatments, enrichments, or other tests are needed, the microbiologist should evaluate the data in relation to two types of information: l) the causes epidemiologically associated with the type and condition of the implicated food, and 2) the clinical signs and symptoms observed in afflicted individuals. If possible, clinical microbial isolates (usually from stool specimens) and blood serum samples for serological and biochemical testing should be obtained from patients by way of their physicians.

Table l lists the major microbial or chemical agents of foodborne disease and their commonly associated food sources. Recently reported causative agents of foodborne outbreaks, cases, and deaths are given in Table 2. Clinical symptoms most often associated with specific microbial or chemical agents and their duration are listed in Table 3. Analysts should use these tables as an aid in deciding the most probable, less probable, and least likely associations. The tables should not be used to assume a single cause or to eliminate possibilities entirely. The information in Tables 1-3 concerns mostly those infections designated as "reportable" in the United States by the centers for Disease Control and Prevention (CDC). This agency, which is the principal source of epidemiologic data on reported foodborne disease outbreaks in the United States, periodically publishes summary surveillance reports of foodborne diseases in the Morbidity and Mortality Weekly Report series.

Most reports of foodborne illness are submitted to CDC by state health departments. CDC defines a foodborne disease outbreak as an incident in which at least two (or more) persons experience a similar illness after ingestion of a common food, and epidemiologic analysis implicates the food as the source of the illness. A few exceptions exist; for example, one case of botulism or chemical poisoning constitutes an outbreak. Although CDC's foodborne disease surveillance system has limitations (i.e., except for illnesses linked to chemicals or toxins, sporadic cases of foodborne illness are not reported), the system does provide helpful epidemiologic insights. The etiologic agent was confirmed in 909 (38%) of the 2397 outbreaks of foodborne disease reported to CDC from 1983 through 1987.

With new pathogens there is an inevitable lag before methods are installed and reporting by clinical and food laboratories becomes routine. Changes in food production or processing may make a food the vehicle or growth medium for microorganisms not previously associated with that product. For example, new varieties of tomatoes that are less acidic than the traditional types might support the growth and toxin production of Clostridium botulinum; freezing procedures improved to preserve taste may also preserve microorganisms that are killed in blast freezing. The food microbiologist should be aware that the clinical symptoms and diagnosis of the patient's illness, available when analysis of the food sample must begin, may be preliminary or incomplete. To proceed from the generalities given in the tables to an analytical course of action, the microbiologist must use reason, imagination, and caution.

See attached for tables.

Supplementary Data