Theauthors of the study, Joseph J. Amon, PhD, MSPH, and colleagues at theCenters for Disease Control and Prevention (CDC), explained that thesemolecular epidemiologic methods had not previously been used in anongoing investigation of a hepatitis A virus outbreak. The methods,involving genetic sequencing analysis of virus found in blood samplesfrom infected individuals, have greatly improved understanding ofoutbreaks of other foodborne pathogens, but are time-consuming and notwidely available.
In September 2003, Tennessee, North Carolina,and Georgia reported a total of 422 cases of foodborne hepatitis Avirus infection to CDC. Preliminary investigations suggested clusteringof reported cases among patrons of three unrelated restaurants.Investigators identified green onions as the likely culprit in theoutbreak by interviewing infected and uninfected restaurant patrons. Inaddition to these standard techniques, the researchers also comparedviral RNA sequences from case patients and individuals concurrently illwith hepatitis A virus infection in non-outbreak settings in the UnitedStates and Mexico.
Viral RNA sequences from patients in the threestates, plus patients involved in a subsequent outbreak in Pennsylvaniain October 2003 (the latter recently described in the Sept. 1 issue ofthe New England Journal of Medicine), were slightly different from eachother. The viral sequence from each outbreak, however, was identical toone or more sequences isolated from northern Mexican residents infectedwith hepatitis A virus. The researchers concluded that the sources ofthe green onions served in restaurants in Tennessee and Georgia werethree farms in northern Mexico.
Dr. Amon and colleagues creditedthe viral sequencing techniques with helping them to identify therelationships between the outbreaks in four separate locations, and todefine the scope of the outbreaks quickly. The sequencing allowed themto determine if cases reported in other states were related to the fouroriginal outbreaks and provided reassurance that a larger outbreak wasnot occurring. The molecular epidemiologic methods also enabled publichealth officials to respond quickly to the later Pennsylvania outbreak.As a result, consumers were warned of the potential risk, and entry ofgreen onions from four Mexican farms into the state was banned.
"Thisresearch highlights the role of viral sequence analysis in improvingour overall understanding of the roughly 50 percent of hepatitis Acases in the U.S. that are from an unknown source," Dr. Amon said."Just as the E. coli-contaminated beef outbreaks in the early 1990sprompted changes in epidemiological surveillance that have increasedour knowledge of foodborne bacteria, the 2003 hepatitis A outbreaksdemonstrated the potential for integrated molecular surveillance toprovide a better understanding of the epidemiology of hepatitis A andfacilitate rapid responses to outbreaks."
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