In early December, 2000, Lake County Florida Health Department (LCHD) learned of seven hepatitis A cases, including five hospitalizations, in Lake and neighboring Sumter Counties in a two week span. During the previous two years, the total number of known hepatitis A cases in Lake County was twenty-two. Recognizing the possibility of an outbreak, LCHD notified the Florida Department of Health.

LCHD began its investigation immediately. Assisting in the investigation were individuals from the Florida Bureau of Epidemiology and the Florida Bureau of Environmental Epidemiology, Food and Waterborne Disease. The investigation commenced with a case definition:

A primary case of hepatitis A was defined as a positive hepatitis A IgM antibody test in a Lake or Sumter county resident who had elevated liver enzyme tests or an acute onset of jaundice or abdominal pain, onset of illness between November 10 and December 16, 2000, and no other explanation for the elevated liver tests or abdominal pain.

Known “cases” were interviewed using the CDC viral hepatitis case record form. Demographic and epidemiologic information, including onset dates and sources for virus acquisition or spreading, were collected. Also, hypotheses-generating interviews were conducted with several “cases,” and the information revealed in these interviews spawned a questionnaire that was eventually answered by all case-patients.

In an effort to locate unknown “cases,” the investigation team contacted area health care facilities and labs, known visitors to Lake County during the outbreak period, and known “case” family members and acquaintances who had experienced similar symptoms. Investigators also contacted the business acquaintances and contacts of those known cases who worked in the food and childcare industries.

After the first twenty-one cases were identified, two case-control studies were done. The first aimed to determine the source of the outbreak. For this study, “controls were defined as adults who did not report a history of hepatitis A in the past, or symptoms of hepatitis [in] November or December 2000, [and] who resided on the same street as a case.” Individuals who fit the definition were interviewed over the phone.

This first study revealed a strong association between cases and the Taco Bell Mexican restaurant in Fruitland Park. Consequently, serologic testing was done on all Taco Bell employees who had worked during the exposure period. Other than the individual who was a known case, the employees tested negative.

The second case-control study was conducted to identify the Taco Bell food item(s) that were, or had been, contaminated. Six meal items and eight ingredients were significantly associated with illness. Of the meal items, only two were eaten by a majority of cases. And of the eight ingredients, green onions bore the strongest statistical association. Further analysis revealed that the green onions were the most likely vehicle for transmission.

While the Lake County investigation was ongoing, the LCHD learned from the CDC that hepatitis A outbreak investigations were also underway in Russell County, Kentucky and Clark County, Nevada. Taco Bell green onions would soon be implicated in these outbreaks as well.

LCHD and other investigators ultimately identified twenty-three people who met the case definition.  In total, fifteen cases (65%) required hospitalization due to the severity of their symptoms.

Viral RNA was extracted from the sera of twelve cases for molecular sequencing by the CDC. Sequencing in eleven of the samples matched exactly, and sequencing of the twelfth varied by one base pair over a 250 base pair gene segment. The CDC then compared the matching Lake County samples to four serum samples from the Kentucky outbreak and one from the Nevada outbreak. Sequencing studies revealed a 100% sequence homology among all the samples, and, again, the twelfth Florida sample varied from all other samples by one base pair.

These studies, together with the epidemiological, environmental, and laboratory investigations, convinced the LCHD that the 2000 Florida hepatitis A outbreak occurred at the Fruitland Park Taco Bell.  The LCHD further concluded that “[a]lthough most foodborne outbreaks of hepatitis A are due to food contaminated by an infected food preparer, we believe the ingredients were contaminated prior to arrival at the outlet in this outbreak. . . . The most likely contaminated ingredient is green onion.”