by Alan Oglesby, MPH

The primary goal of any outbreak investigation is to identify causative factors in order to correct them and prevent additional cases in the future. Thorough outbreak investigations can also improve our understanding of the epidemiology of diseases, help to identify emerging trends, and can improve the public’s confidence in the public health system. For these reasons, outbreaks or clusters of unusual disease incidence are reportable to the Indiana State Department of Health (ISDH) according to the Communicable Disease Reporting Rule [IAC 410 1-2.1]. Outbreak investigations, ideally, should be a collaborative effort between the local health departments and ISDH. Generally, it is the local health department’s responsibility to notify ISDH of the outbreak and to perform the majority of investigative procedures, while the ISDH’s role is primarily one of coordination, consultation, and occasionally, direct assistance.

The ISDH Communicable Disease Division (CDD) participated in 15 outbreak investigations in 1997 (See Table 1). In February, a waterborne outbreak occurred when forty-two people developed dermatitis and/or otitis media after swimming in a Tippecanoe County hotel swimming pool. Pseudomonas aeruginosa was cultured from both pool water samples and from the ear exudate of one affected case. In the fall, an outbreak of pertussis occurred in Clark County and was investigated by an ISDH Immunization Representative. From September through November, eight cases, all confirmed by PCR analysis, were identified. All eight cases occurred in children under two years of age, four of whom attended the same day care center. Erythromycin prophylaxis was mandated by the Clark County Health Department for the remaining 150 children and staff in the day care center. No new cases were reported following administration of erythromycin. Pertussis is a highly communicable disease that is most commonly transmitted through airborne contact with respiratory droplets. Immunization with pertussis vaccine is the most effective means to prevent illness. The remaining 13 outbreaks were all related to infection by enteric pathogens. Most of these were attributed to a food source and are described in more detail below.

In May, an outbreak caused by Clostridium perfringens toxin was linked to peel-n-eat shrimp consumed from a lunch buffet at an Allen County hotel restaurant. At least 22 people were affected. Food poisoning caused by this toxin can usually be attributed to temperature abuse.

The ISDH investigated two suspected E. coli O157:H7 outbreaks in 1997. In May, six cases of E. coli O157:H7 were confirmed in St. Joseph County. Two of the cases developed hemolytic uremic syndrome (HUS) but eventually recovered. Pulsed field gel electrophoresis (PFGE) was performed on the positive isolates, and the results indicated that three different strains were responsible for this cluster of cases. This finding coupled with varied exposure histories and onset times suggested that these cases were sporadic and that a single point source outbreak had not occurred. In October, six Vanderburgh County children were infected with E. coli O157:H7 following a field trip to a local apple orchard and cider pressing operation. None of the children developed HUS. All six cases shared the same PFGE pattern which indicated a common point source of exposure. While it was determined that exposure likely occurred at the apple orchard, an exact source was unable to be identified. A thorough inspection of the cider pressing facility failed to identify any critical food safety violations and samples of cider produced by the facility were negative for the presence of E. coli O157:H7. No other cases in the community were identified. E. coli O157:H7 can be transmitted through raw or undercooked foods of animal origin (particularly ground beef) or through foods cross contaminated by animal products. It can also be transmitted from unpasteurized milk and fruit juices and from person to person.

Three hepatitis A outbreaks were investigated by the CDD in 1997. In March, two cases of hepatitis A were identified in Jay County. It was determined that both cases became exposed while visiting Russia as part of a short-term missionary team. Follow-up with remaining team members identified two additional cases in northeast Indiana. In May, eight elementary school students were diagnosed with hepatitis A in Knox County. A previously unrecognized case who was infectious during the previous month was identified as the likely index case. Transmission was determined to be from person to person. In June, seventeen cases were identified in Elkhart County. A case-control study was conducted which implicated an Elkhart County restaurant as the source of exposure. Further investigation identified the index case to be a former restaurant employee who had worked at the establishment one month earlier while infectious. Hepatitis A is most commonly transmitted from person to person via fecally contaminated hands, food, water, or other inanimate objects.

In November, a suspected Salmonella outbreak occurred in Boone County. At least thirty-nine people became ill following a catered Thanksgiving dinner. Three of the 39 ill people submitted stool cultures which were positive for Salmonella Group B. A case-control study identified turkey as the potential vehicle of transmission. However, leftover foods were unavailable for sampling which prevented confirmation of the suspected source. Salmonellosis can be prevented by thoroughly cooking foods of animal origin; thorough handwashing after using the bathroom and before preparing foods; and by washing cooking utensils after contact with raw foods of animal origin and before contact with other foods.

In December, an outbreak attributed to Staphylococcus aureus toxin occurred among staff members of a Marion County high school following a carry-in holiday meal. At least 37 people became ill, 24 of whom sought medical attention due to the severity of their symptoms. A case control study implicated cooked ham as the vehicle responsible for causing illness. Leftover ham was sampled, and results indicated high levels of S. aureus present as well as high aerobic plate counts, which indicated temperature abuse and mishandling. Food poisoning caused by S. aureus toxin can usually be attributed to time and temperature abuse.

The 5 suspected viral outbreaks occurred all over the state. In most cases, they were linked to foods served raw (vegetable trays, lettuce, etc.); foods which were handled extensively after cooking (sliced meat, etc.); or by person to person transmission. Food-borne viral illness usually occurs when an infected person handles food without thoroughly washing their hands after using the bathroom.

This summary described only those outbreak investigations in which the CDD participated in 1997. The ISDH Food Protection Program (FPP), in addition to lending their valuable expertise and experience to CDD during a majority of these investigations, also investigates a substantial number of other food-related outbreaks and complaints for which the CDD never becomes involved.