A multistate, foodborne outbreak of hepatitis A

Background We investigated a large, foodborne outbreak of hepatitis A that occurred in February and March 1997 in Michigan and then extended the inves tigation to determine whether it was related to sporadic cases reported in other states among persons who had consumed frozen strawberries, the food s uspected of causing the outbreak. Methods The cases of hepatitis A were serologically confirmed. Epidemiologi c studies were conducted in the two states with sufficient numbers of cases , Michigan and Maine. Hepatitis A virus RNA detected in clinical specimens was sequenced to determine the relatedness of the virus from outbreak-relat ed cases and other cases. Results A total of 213 cases of hepatitis A were reported from 23 schools i n Michigan and 29 cases from 13 schools in Maine, with the median rate of a ttack ranging from 0.2 to 14 percent. Hepatitis A was associated with the c onsumption of frozen strawberries in a case-control study (odds ratio for t he disease, 8.3; 95 percent confidence interval, 2.1 to 33) and a cohort st udy (relative risk of infection, 7.5; 95 percent confidence interval, 1.1 t o 53) in Michigan and in a case-control study in Maine (odds ratio for infe ction, 3.4; 95 percent confidence interval, 1.0 to 14). The genetic sequenc es of viruses from 126 patients in Michigan and Maine were identical to one another and to those from 5 patients in Wisconsin and 7 patients in Arizon a, all of whom attended schools where frozen strawberries from the same pro cessor had been served, and to those in 2 patients from Louisiana, both of whom had consumed commercially prepared products containing frozen strawber ries from the same processor. Conclusions We describe a large outbreak of hepatitis A in Michigan that wa s associated with the consumption of frozen strawberries. We found apparent ly sporadic cases in other states that could be linked to the same source b y viral genetic analysis. (N Engl J Med 1999;340:595-602.) (C)1999, Massach usetts Medical Society.