Nov 21, 2003 (CIDRAP News) An analysis of hepatitis A cases associated with a Chi Chi's restaurant near Pittsburgh points to green onions (scallions) from Mexico as the source of the virus, the Centers for Disease Control and Prevention (CDC) announced today.
"Preliminary analysis of a case-control study implicated green onions as the source of the outbreak," the CDC said in a special bulletin (MMWR Dispatch). "Preliminary traceback information indicates that green onions supplied to Restaurant A [elsewhere identified as the Chi Chi's in Monaca, Pa.] were grown in Mexico," the report says.
The announcement came a day after the Food and Drug Administration said the source of the outbreak had not yet been identified.
The Pennsylvania Department of Health said today that the outbreak has reached 575 cases. Three deaths have been reported. The CDC report said at least 13 employees of the restaurant have had the illness, but because they all became ill after Oct 26, none of them could have been the source of the outbreak.
For a case-control study, the CDC identified 181 people who had eaten at the restaurant between Oct 3 and 6 and became ill with hepatitis A between Oct 14 and Nov 12. The agency found another 83 people who had eaten at the restaurant in the same period but did not get sick.
By comparing what the case-patients and controls had eaten, the CDC found that the only menu items associated significantly with illness were mild salsa and chili con queso, both of which contained uncooked or minimally heated fresh green onions. Mild salsa was eaten by 94% of case-patients and 39% of controls, while chili con queso was eaten by 15% and 3%, respectively. Ninety-eight percent of case-patients reported eating a menu item containing green onions, versus 69% of controls. Hot salsa served at the restaurant did not contain green onions.
Eating an item containing white onions was also associated with illness, but 99% (174 of 176) case-patients who reported eating white onions also ate green onions, the report states. Also, white onions were not associated with illness in the four case-patients and 28 controls who had not eaten green onions.
"The green onions likely were contaminated with HAV [hepatitis A virus] in the distribution system or during growing, harvest, packing, or cooling," the report states. "Traceback investigations to date have determined that the green onion source is one or more farms in Mexico."
The CDC also said genetic analysis of the outbreak virus shows it is "very similar to viral sequences obtained from persons involved in hepatitis A outbreaks in Tennessee, Georgia, and North Carolina during September 2003 that were linked epidemiologically to green onions." The virus also appears very similar or identical to viral strains found in people living along the United States-Mexico border and travelers returning from Mexico, suggesting a source in Mexico, the report says.
No other hepatitis A outbreaks have been identified, the CDC said. The agency is working with state health departments to identify other outbreaks linked with green onions.
CDC. Hepatitis A outbreak associated with green onions at a restaurantMonaca, Pennsylvania, 2003. MMWR Dispatch 2003 Nov 21;52:1-3
Pennsylvania Department of Health outbreak information