Outbreak study yields lessons about investigation techniques
BY GRETCHEN GOETZ | AUG 30, 2012
Salmonella Agbeni — unlike strains such as S. Enteriditis or S. Typhumurium — is rarely responsible for human illness in North America. But in May of last year, this serotype sickened 8 people in British Columbia, causing illnesses that were unusual enough to prompt a special investigation into the outbreak.
The resulting study was published in the September 2012 issue of Emerging Infectious Diseases.
While researchers were unable to pinpoint the source of the bacteria, they say that the unique characteristics of the outbreak — including an abnormally long incubation period and a high percentage of urinary tract infections — offer valuable lessons for future Salmonella investigations.
During this outbreak, victims began experiencing symptoms 5 -7 days after being exposed to the bacteria, marking an unusually long incubation period for Salmonella, which normally causes illness between 6 and 72 hours after ingestion. This fact suggests that investigators may need to reach farther back in time when looking for common exposures among victims, says the study.
Health officials currently question Salmonella patients about what they ate in the 3 days prior to illness, say the authors. But this outbreak suggests that this time frame may need to be expanded, they note.
In this case, that 3-day window didn’t include the event that turned out to be the likely source of infection for the majority of the B.C. patients — a wedding attended by 7 of the 8 victims.
“The 3-day time frame for exposures was not sufficient to identify appropriate exposures,” explained Marsha Taylor, epidemiologist at the British Columbia Centre for Disease Control and lead author of the report. “Expanding the period for collecting exposure information about Salmonella spp. infections and the reporting and investigation of persons with Salmonella spp. identified in urine to public health authorities might be needed to help identify and solve outbreaks,” Taylor said in an interview with Food Safety News.
The wedding was only identified as a potential common event because a victim happened to mention it in an interview, explains Taylor.
Officials are not sure why the time between exposure and onset – which averaged 5.5 days – was so long in this outbreak. While it may have been a result of the type of Salmonella involved, says the study, it may also have depended on the amount of bacteria ingested or on a variation in the temperature of the food, which was prepared a week in advance and served cold.
“The specific factor that led to it — whether it’s the stain, the host or the exposure — that piece isn’t that clear,” says Taylor.
High Number of Urinary Tract Infections
The second unusual characteristic of this outbreak was the high percentage of UTIs among victims, say the authors. Typically, 1 percent of non-typhoidal Salmonella infections are diagnosed through urine. However, 3 of the 8 outbreak victims (40 percent) were diagnosed through urine analysis after presenting with UTIs, according to the study.
“It was unusual to see that high a proportion of urine diagnosis within a cluster,” says Taylor.
In light of these findings, “the reporting and investigation of persons with Salmonella spp. identified in urine to public health authorities might be needed to help identify and solve outbreaks,” says the study.
Interestingly, while it’s unusual for Salmonella patients to experience gastrointestinal symptoms in addition to a UTI, 2 of the 3 victims who contracted UTIs in this outbreak experienced GI symptoms before the onset of the UTI.
U.S. Was Likely Source of Bacteria
While investigators never identified the reservoir of the Salmonella Agbeni that caused this outbreak, they say the bacteria likely originated in the United States.
One of the 7 victims who attended the wedding in B.C. had traveled to the province from the U.S. and had helped prepare food served at the event, according to the EID report. That person was also the first to experience symptoms, suggesting that he or she had been carrying the infection before spreading it to others.
The theory that the bacteria originated in the U.S. was bolstered by the fact that the one outbreak victim from B.C. who did not attend the wedding had recently traveled to the United States, and had visited the jurisdiction where the wedding attendee from the U.S. lived.
Furthermore, 5 illnesses of Salmonella Agbeni were reported among residents of that U.S. jurisdiction during the same time period, says the report.
The source of the bacteria in that U.S. jurisdiction was never located, says Taylor.
Before this outbreak, Salmonella Agbeni had rarely been recorded as a source of illness in Canada. Between 2000 and 2010, only 8 illnesses were reported in the country. None of these were in British Columbia. In the United States, 180 cases of Salmonella serotype Agbeni were reported between 1999 and 2009, according to surveillance data from the U.S. Centers for Disease Control and Prevention.