Shigella is found in fecal matter and is transmitted through the fecal-oral route. Shigella is commonly found in developing countries where sanitation techniques are still somewhat primitive. Four species (of more than 40 serotypes) have been identified. S. sonnei currently accounts for more than half of the cases in the United States and S. flexneri accounts for a large percentage of the remainder. S. dysenteriae type 1 (the Shiga bacillus) is rare in the United States but widespread in rural Africa and the Indian subcontinents. S. boydii is also uncommon in this country. Pulsed-field gel electrophoresis (PFGE) determined that the patterns of the parsley-related outbreaks were all the same, indicating to epidemiologists that the tainted vegetables came from a common source.
"The restaurant voluntarily closed when it was suspected there was a problem," said Carol Mordhorst, PhD, public health administrator for Mendocino County health department. "They had all their employees tested and have worked hard to throw away any food that may have been contaminated."
Ironically, the taco restaurant is directly next door to the Mendocino County Health Department and one of the first people to exhibit Shigella bacterial infection was the director of the department, who along with many of his employees, were sometime visitors.
Following the closure, the health department's environmental health division inspected the facility and collected various samples of food from the restaurant. A number of food samples were found to be harboring the bacteria. Previous inspection reports yielded excellent ratings for the restaurant. Fifty-four people tested positive for Shigella while 300 exhibited Shigella like symptoms, said county health inspectors.
"Normally when food is prepared it is recommended that workers use utensils and gloves and not their hands," said Bela Matyas, MD, state epidemiologist with the Massachusetts Department of Health's Bureau of Communicable Disease Control. "[The restaurant owners] told us that this is how they did it and they never had problems before; well, yeah, but you don't always have someone with Shigella contaminated feces on their hands preparing it, either. Ideally you want to use a mixture of gloves and utensils to do the job."
To avoid the self-reporting bias that often occurs during outbreaks, Matyas and his team used credit card receipts from the restaurant to construct a group of 22 people for a case-control study. They discovered that the attack rate among the group was 64%, which meant that two of every three patrons had gotten sick - an enormously high percentage, Matyas said.
The last Shigella outbreak to occur in Massachusetts happened in 1995 when person-to-person transmission was detected in an Orthodox Jewish community. That outbreak was linked to Shigella outbreaks in the same religious group in several parts of the world. Matyas said it appeared to be travelling from one part of the world to another through migration of infected individuals.
Complaints poured in from more than 100 unrelated victims who had eaten at the restaurants and were infected; 40 people tested positive for the S. sonnei strain. Many of the diners had been in to see their doctors after getting sick or hearing of the outbreak on the local news, so state epidemiologists were able to retrieve their stool cultures and conduct tests. Several people were hospitalized but no deaths have been reported, according to Minnesota health officials.
PFGE confirmed the two outbreaks had a common pattern, so state health inspectors, with help from county health teams, began their search for a common bacterial source.
Both restaurants voluntarily closed their doors while epidemiologists conducted their investigation. No common water source infections were found, but several employees who tested positive for Shigella may have inadvertently spread the bacteria by handling the tainted parsley, epidemiologists speculated. In both cases, the restaurants reported using chopped parsley on their dinner platters.
"We've been working with the Food and Drug Administration to try and trace the parsley," said Craig Hedberg, MD, chief of the food, vector and zoonotic disease investigation unit at the Minnesota Department of Health. "One of the problems in tracing fresh produce back is that you rarely get totally unambiguous tracebacks. But in this case there does appear to be a common supplier from Mexico."
"Despite all our efforts to discover what the vehicle for the outbreak was, the results of our investigation are inconclusive," Hammond said. "We cannot say for certain that it is related to the Mexican parsley."
In all, about 37 people exhibited symptoms similar to the Shigella confirmed case. The cases were linked to a local family-owned restaurant in the county, but Hammond said she could not confirm whether tainted parsley had indeed been served to the unsuspecting patrons. However, county sanitation officials said they were continuing the investigation.