October 25, 2016

Maryland Vibrio Outbreak in 2010 Traced to Asia

A study published in the Journal of Applied and Environmental Microbiology, of the American Society for Microbiology, has discovered that a Vibrio parahaemolyticus outbreak in Maryland in 2010 was caused by raw oysters harvested from the Chesapeake Bay, but the bacteria in those oysters came from Asia. The outbreak sickened two people who ate raw oysters at two different restaurants in Baltimore.

Raw OystersNeither patient had graveled outside of the state in the week before they got sick, and neither had other risk factors for Vibrio infections. The outbreak strain of Vibrio, called “sequence type B” was identified because it was the only strain present in both the patients’ stools and in the oysters. And that particular strain had previously only been found in Asia.

So how did bacteria from a continent away cause an outbreak in shellfish harvested in the United States? The Asian strain of the bacteria could have arrived in the Chesapeake Bay through ballast water released from ships coming from Asia, or from exotic oysters or fish carrying the strain introduced into the Bay. That Bay gets a lot of marine traffic and ballast water release. Ocean currents could also have moved the bacteria into the Bay.

The study was authored by scientists at the Department of Health and Mental Hygiene of Baltimore, the Division of Seafood Science and Technology, the Gulf Coast Seafood Laboratory, and the FDA, among others. The scientists conducted whole genome sequencing (WGS), which identifies bacterial DNA in a way that is much more thorough and sophisticated than pulsed field-gel electrophoresis (PFGE).

The two strains in the patients and the oysters differed by only two bands, so they were considered part of the same outbreak. But the oysters also was harboring Vibrio strains that were not related to the outbreak. Those bacteria did not cause illness because their concentration was low enough that people didn’t get sick, but the strains were detectable with scientific methods.  The PFGE pattern of the bacteria had not been detected in any Vibrio outbreaks in Maryland before.

Whole genome sequencing let scientists get a detailed look at the outbreak strain of Vibrio that caused illness and confirmed that the bacteria came from another country. Doctors and scientists should be aware that non-native bacterial species can be introduced into a new environment and cause illness.

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