Marine, oyster experts weigh in on Vibrio bacteria, warm waters and safe, sustainable consumption

That little asterisk on a menu at restaurants serving fresh oysters on the half shell indicates the health risk for potential foodborne illness when consuming raw or undercooked shellfish.

While that warning may not deter some oyster lovers, recent headlines about sickness and death related to Vibrio -- a potentially deadly bacterial species found in raw shellfish and seawater that can cause symptoms in humans ranging from diarrhea to life-threatening infections -- have prompted questions about how climate could potentially cause environmental changes in waters where the briny bivalves are bred, in turn making them potentially more risky to consume.

"The shellfish industry in the U.S. is managed very well as far as food safety goes," aquaculture and oyster expert Pete Malinowski told "Good Morning America." "It's much more about the people involved -- the fresh, well-prepared, well-cared-for oysters are going to be almost entirely safe to eat all the time. It's always been possible to get sick from eating raw oysters, it's just incredibly rare. If the oysters are handled properly throughout the chain of custody, then it's near zero percent chance of getting sick."

Malinowski, executive director of the Billion Oyster Project, grew up farming oysters with his family on the Fishers Island Oyster Farm, long before he launched his aquaculture education and oyster restoration efforts at the New York Harbor School.

"In order to open an area up to shellfishing there's a series of water quality tests that happen over a long period of time, and what they're looking for primarily are fecal coliform. Any concerning levels of fecal coliform at any point during the testing, then those waters won't be open to shellfishing," Malinowski explained. "That's how we can all rely on the oysters that get to market in the U.S. being safe for consumption."

Malinowski, who also works with dozens of top culinary restaurant partners to collect and recycle oyster shells for reef restoration, noted that negative side effects from eating shellfish stem from the absence of two factors: "a healthy environment for oysters, and then a healthy environment for people who are eating the oysters to be food."

How water temperature affects oysters

As cases of the more dangerous Vibrio vulnificus in particular have increased, health authorities, including the National Centers for Environmental Information, have seen the prevalence of the bacterial species increased by rising water temperatures.

"There's definitely a link between water temperature and the bacteria we're talking about. Vibrio vulnificus is present everywhere, all the time, but it has the ability to multiply at higher temperatures," Malinowski explained. Those higher temperatures can come not just from warmer water but also from the food-handling and processing environment.

"I wouldn't say that it's dramatically worse now than it was, but it is linked to water temperatures and so as water temperatures continue to rise that could be a slow signal," Malinowski further noted. "It happens pretty gradually and a degree or two probably isn't going to make that big of a difference."

"Water temperature directly affects an oyster's metabolism, growth rate, and reproduction cycle," international oyster expert Julie Qiu, co-founder of Oyster Master Guild, told "Good Morning America." Her organization describes itself as "an international network of oyster enthusiasts" who are "committed to advancing oyster appreciation and service standards through education, training, and community."

"In North America," Qiu said, "the Eastern oyster that's native to the Atlantic coast has a fairly wide temperature range. When water temperature rises, the oyster is able to grow faster if there's sufficient food -- algae -- in the water. When the water temperature drops below 40 degrees Fahrenheit, the oyster stops eating and begins to hibernate."

Understanding and ensuring biosecurity of oyster environments

From a biosecurity standpoint, Malinowski explained that "strains of Vibrio that are prevalent on the West Coast -- that have learned to survive and reproduce and thrive in a much colder environment -- are different than the strains that are prevalent on the East Coast."

When it comes to Billion Oyster Project's shell collecting and recycling, for example, Malinowski said "there's a lot of rules in place to prevent us from introducing any soft tissue or organisms off the shell that's going into the harbor in New York City. And that's a hard and fast rule biosecurity-wise nationwide that keeps those systems separate, because the strains of bacteria are different and specific to different temperatures, the risk isn't as connected to water temperature region to region."

Billion Oyster Project adds oyster the shells back into their natural habitat not only for reef restoration but also to create to sustain new beds for the mollusks to grow and, in turn, naturally filter and remove pollutants from the sea. Malinowski and the team keep the recycled shells out of the water and exposed to the elements for at least six months while the temperature is over 50 degrees to ensure they are pristine for restoration efforts.

"None of the shells we collect from restaurants will touch the water ... within a year of them coming from the restaurant," he said. "In that way we can be very confident that nothing that was alive on those shells when they got to the restaurant is still alive when we're putting [them] in the water."

Vibrio bacteria and potential risks of vibriosis

"The risk of contracting a life-threatening vibriosis infection from eating oysters for a healthy adult is extremely rare," Qiu said, echoing Malinowski's sentiments. "Immunocompromised or immune-suppressed individuals must know that they are more at risk of getting vibriosis and experiencing complications from vibriosis than the average healthy person. For those individuals, I would recommend simply eating cooked oysters to minimize risk."

Typically U.S. deaths related to vibriosis are more common in Gulf Coast states, but rare in East Coast states. Individuals who are more at risk of serious infection include people with liver disease, people with hemochromatosis (an inherited order causing iron overload in the blood), and immunocompromised individuals. In response, the Centers for Disease Control and Prevention issued a health advisory to warn doctors, laboratories and public health departments to be on the lookout for these infections.

Vibrio bacteria cause an estimated 80,000 illnesses each year in the U.S., according to the CDC. In particular, Vibrio vulnificus can lead to life-threatening infections. Between 150 and 200 such infections are reported to the CDC every year, with about one in five infected people dying, often within a day or two of becoming ill, the agency said.

While this information can serve as a useful warning for more vulnerable diners, Qiu reiterated that oysters offer "tremendous benefits to our health and the environment." Even the Cleveland Clinic has noted the benefits of key nutrients found in the shellfish.

Tips to enjoy all the benefits of eating oysters

While Qiu reiterated that "no raw food can be guaranteed to be bacteria-free, including oysters," she said that "there are many things to do to minimize risk."

"In a restaurant, do not order raw oysters that have been pre-shucked," she suggests. "Ideally, they should be shucked on demand. The key sign to look for is front-of-restaurant oyster shucking with visible display of oysters on ice."

"Know which oysters you order. Keep track of the names and production locations. Be wary of any restaurant that cannot tell you where the oysters come from," Qiu continued, encouraging diners to "ask to see the shellfish tag, which is tagged on every bag or box of oysters, that tells you harvest date and location."

"You often get the best value and quality when you buy oysters directly from the farmer either at your farmers market or online," Qiu said, adding that "many oyster farms now ship oysters via FedEx and UPS."

Ultimately, Qiu reminded consumers to "use common sense: do not order oysters in places that have no business serving live shellfish, particularly if they do not appear to have any cold storage or refrigeration."

Many oyster connoisseurs suggest enjoying oysters in months ending with "r" because, as Qiu said, "fall to spring would be considered safer as water temperatures are cooler, as well as oysters grown more north."

Malinowski hailed oysters as "the most sustainable form of protein production that exists" -- along with other bivalves including mussels, scallops and clams -- because they provide major ecological benefits such as water filtration and habitats for other animal species. These benefits have also been touted by organizations such as One Earth, a nonprofit organization working to accelerate collective action to solve the climate crisis.

Malinowski added, "The only reason those oyster farms exist is because people are eating the oysters. So by consuming shellfish, you're actually having a net-positive impact on coastal environments."

So the next time there's an opportunity to enjoy an oyster happy hour or a raw seafood tower, Malinowski reminded that it's more about how those oysters have been handled versus where they came from.

"Make sure you get oysters from a place that sells a lot of oysters and knows what they're doing," he declared.

ABC News' Mary Kekatos contributed to this report.