Following up a scientific report, Timothy B. Wheeler of the Bay Journal reports a 6-year-old outbreak of food poisoning linked to eating raw Chesapeake Bay oysters has left behind a lingering mystery. Scientists seeking to identify the water-borne pathogen that sickened a pair of Baltimore restaurant patrons have tracked the culprit to Asia.

How a potent strain of Vibrio bacteria seemingly from so far away wound up in the Bay continues to puzzle Maryland health officials, who worked with researchers at the U.S. Food and Drug Administration to investigate the 2010 cases.

The microorganism could have gotten here in the ballast water of the many oceangoing ships that ply the Chesapeake every year, state and federal scientists suggested in a recently published journal article. Or, they added, perhaps it came via the introduction of non-native oysters or some exotic fish.

“It really is speculation,” acknowledged Dr. Clifford Mitchell, environmental health bureau director for the Maryland Department of Health and Mental Hygiene. “We didn’t sample ballast water. We didn’t take specimens that would lead us to know that we had fish coming over, or migration.”

But the case, published in the June issue of Applied and Environmental Microbiology, illustrates how disease-carrying organisms may travel around the world, researchers said. And while steps have been taken since 2010 to prevent the unintentional transport of pathogens, parasites and other potentially harmful organisms via ships’ ballast water, those safeguards still have significant gaps in them.

The bacterium involved in the 2010 food poisoning outbreak was Vibrio parahaemolyticus, strains of which are commonly found in coastal waters worldwide — including the Bay — though only some have been found to cause illness. When those are ingested, they can cause acute gastrointestinal distress, including diarrhea, stomach pain, nausea, fever and chills. It usually passes within a few days, but in rare cases can be more severe, especially in people with weakened immune systems.

There were 45 cases of Vibrio infections reported in Maryland in 2010, but it’s not that often, state health officials said, that they’re able to pinpoint the source of the bacteria that may have sickened a particular person. By the time laboratory tests identify Vibrio as the cause of someone’s GI distress and the information gets reported to the state, days or even weeks may have passed, and the food that person had eaten is long gone.

In this case, though, state health investigators got a lucky break. Two individuals who got sick said that shortly before they became ill that summer, they had eaten raw oysters at different Baltimore restaurants. They hadn’t traveled out of state or done anything else that likely could have exposed them.

When investigators visited the restaurants, they found the half-shells eaten by the two victims were from the Bay. And when they visited the Maryland aquaculture operation that supplied both eateries, investigators pulled some oysters from the water and discovered that they had Vibrio in them as well — 11 different potentially disease-causing strains, in fact. One of those appeared to match the Asian strain found in the two food poisoning victims.

The investigation ended there, for the time being. Even though the Vibrio involved were similar, researchers couldn’t positively identify them as the same, using the analytic techniques they had at the time. “The chromosome patterns matched, but we weren’t sure how common that pattern was in the environment,” explained Robert Myers, director of the state health department’s laboratory administration. “We hadn’t seen it before.”

A few years later, though, “whole genome sequencing” technology became available, Myers said, giving researchers the ability to draw a more detailed map of an organism’s genetic makeup.

With that new, more powerful analytical tool, FDA researchers re-examined the Vibrio strains involved in the 2010 outbreak and those from the oysters that state health investigators had sampled. They identified them as belonging to a family of strains known as “sequence type 8.” 

When researchers consulted a worldwide Vibrio database, they found that the Maryland strains were unlike any seen to date in the United States. Instead, they were closely related to strains reported only in Asia, most recently in Hong Kong about four years before the outbreak.

Changes were made to shellfish safety protocols after a larger outbreak in 2013of Vibrio parahaemolyticus illnesses associated with eating raw oysters harvested along the Atlantic Coast. More than 100 people in 13 states, including Maryland and Virginia, became ill.  According to a spokesman for the Maryland Department of the Environment, which regulates shellfish harvest waters in the state, the Interstate Shellfish Sanitation Conference, a joint state-federal body, tightened its requirements for investigating such cases, closing implicated harvest areas and ordering a product recall when more than 10 cases are traced to a given area. 

But officials caution that the protocols are not foolproof, and cases like this are a reminder of the risk people run in consuming raw seafood, Mitchell said, especially if they have underlying health conditions.

The number of reported Vibrio infections in the state varies from year to year, but has been trending upward since 2005, according to state health data. Concentrations of the bacteria increase in warmer weather, and climate change could be a factor as Bay water temperatures tick upward. But Mitchell cautioned that the bacteria are present year-round.

“Given the number of people who eat oysters, certainly it’s a relatively small number of infections, but it can be a very significant one,” Mitchell said.