Aug 13, 2002 (CIDRAP News) – When epidemiologists at Miami–Dade County Health Department noted an unusual rise in the incidence of typhoid fever during the winter of 1998-99, they faced a mystery. According to a report by Dolores J. Katz and associates in the Jul 15 issue of the Journal of Infectious Diseases, the clues to the source of the outbreak represented a new arena for public health surveillance.
Typhoid fever is spread by food and water contaminated by feces or urine of infected persons, with between 2% and 5% of those infected becoming carriers, the authors point out. Although about 17 million people around the world are infected each year and 600,000 die, clean water and safe sewage management have greatly reduced the risk in the United States. For more than 20 years, the annual US incidence has been less than one case per 100,000 population, and almost all of the 400 or so cases seen each year have been in people who traveled to areas where the disease is endemic.
In the Florida outbreak, at least 16 people were infected over a relatively short time. Other than the fact that they were all Hispanic, they seemed to have little in common. (About 57% of Miami-Dade County's 2.1 million people are Hispanic, according to census data.) These patients were not acquainted with one another, lived in several different counties, received drinking water from at least four different water systems, shared no common recreational or social activities, did not use the same grocery stores or restaurants, and with the exception of one had not recently been out of the country. Yet pulsed-field gel electrophoresis showed the genomic profiles of isolates from 15 patients with confirmed Salmonella serovar Typhi infection to be identical with each other but different from cases not epidemiologically associated with the outbreak.
Although answers to standard typhoid fever questionnaires did not point to any common risk factors, three patients mentioned that they had drunk fruit shakes made with a tropical fruit called mamey. This fruit, also called zapote, is commonly used in fruit shakes called batidos, where it is mixed with milk, ice, and sometimes sugar. When 11 other patients were asked open-ended questions about fruit and beverage use in a matched case-control analysis, 10 said they had drunk mamey batidos. The association between consumption of the fruit shakes and illness was significant (matched odds ratio, 7.6; 95% confidence interval, 1.4-81.4).
Food testing and tracebacks to manufacturers showed that mamey shakes consumed by the patients were made with commercially packaged frozen fruit from two manufacturing plants in Guatemala and one in Honduras. None of the samples obtained from 49 supermarkets and 12 distributors showed S Typhi, but all contained coliforms, including fecal coliforms and Escherichia coli. Although this did not prove a direct link with the typhoid outbreak, it did alert investigators to gross contamination and mishandling of the food product.
Within just over a month after the first case was recognized, the Florida Department of Agriculture and Consumer Services halted distribution and sale of all frozen mamey in Florida. No new cases were reported after the frozen mamey was pulled from supermarkets
The investigators point out that this is the first report of a typhoid fever outbreak in the United States caused by a commercially imported food. It demonstrates, say the authors, the importance of classic surveillance and epidemiologic techniques combined with modern molecular epidemiology in identifying outbreaks and their causes. It also highlights a need for new approaches to ensuring safety of imported foods, such as irradiating selected products or instituting hazard analysis and critical control point systems for processors, the authors say. The latter combines control of hazardous food processes with continuous monitoring to identify and correct dangerous conditions. In addition, more sensitive laboratory techniques may be needed to identify exotic organisms in foods.
The authors stress that the new concern for public health officials lies in the fact that future outbreaks of foodborne illness are likely as expanded trade brings in more products from all over the world. At present, federal agencies have limited authority and resources to inspect food imported from other countries. The report emphasizes that athough new procedures and various stop-gap measures may be helpful in the short run, "The solution that would provide the greatest public health benefit is the provision of clean water and adequate sewage treatment systems worldwide."
Katz DJ, Cruz MA, Trepka MJ, et al. An outbreak of typhoid fever in Florida associated with an imported frozen fruit. J Infect Dis 2002 Jul 15;186(2):234-9 [Abstract]
