The Truth About Flesh-Eating Bacteria

If you were reading the news in the summer of 2019 you probably noticed a flood of articles about “flesh-eating bacteria.” This is just the sort of click-bait headline that is guaranteed to attract attention and boost ad revenue. Unfortunately, the lazy journalists who posted these articles were also scaring people away from shellfish consumption and marine-related recreational activities. I spent much of the summer trying to respond to these reports and to persuade authors to retract or amend their articles to reflect the facts. But good news is boring and as the saying goes in journalism circles, “If it bleeds, it leads.”

So why did we see this explosion of deceptive reporting? One answer can be traced to a report out of Rutgers in spring 2019 that documented a handful of Vibrio vulnificus wound infections that were traced to Delaware Bay. The author repeated the popular theme that Vibrios are spreading because of climate change. Graphic descriptions of disfiguring skin blisters, amputations and life-threatening illnesses were enough to get dozens of news outlets to pick up the story and run with it.

In doing so journalists have sown confusion and caused significant harm to those who make a living from seafood and the businesses surrounding marine recreation. One business owner in New Jersey who rents boats and gear for folks to fish for blue crabs claimed that his business was off by over 50 percent that summer due to fears driven by reports of flesh-eating bacteria. Resource managers in the mid-Atlantic complained that they were deluged with questions about whether it was safe to go in the water or eat crabs.

What are the facts about Vibrio vulnificus?

Vibrio vulnificus (Vv) is a naturally occurring bacteria that is ubiquitous in warm, brackish waters. We find it pretty much everywhere on the East and Gulf Coasts when waters are warm, but it has no relation to water pollution. Thankfully Vv illnesses are extremely rare. The Centers for Disease Control (CDC) reported a total of 119 cases of Vv in the U.S. in 2014 (the most recent data), meaning that most doctors will never see a case in their entire career.

The CDC reports that 60 percent of cases are related to wound infections, while 16 percent are related to eating raw or undercooked seafood (most often oysters). Most of the infections occur in the immune-compromised population (such as people with liver damage; or who are taking medications that suppress their immune response to treat HIV or cancer, or to prevent transplant rejection). For these individuals the infections can quickly invade the blood (sepsis), and the mortality rate can be as high as 20-40 percent.

To be accurate, Vv is rarely described by the medical community as “flesh-eating.” True flesh-eating wound infections are mostly associated with Type A Streptococcus or Staphylococcus aureus bacteria. Although septic Vv infections can cause disfiguring skin blisters requiring surgery or amputation, it is not accurate to lump Vv in with the much more common staph and strep bacterial infections.

If the articles had stopped there it might not have been so bad. Most people would see the low rates of infection and conclude that their own risk is minuscule. Hopefully, the immune-compromised population would take appropriate precautions by protecting wounds and scrapes from brackish water and by avoiding raw shellfish, while healthy folks would go about their lives with little to fear. Unfortunately, these irresponsible journalists did not stop there. Without putting in the clutch, they shifted gears and reported that the CDC estimates there are 80,000 cases of Vibriosis a year across the country, effectively lumping a very rare and potentially life-threatening disease in with all the other types of Vibrios that are more common, but far less dangerous.

What is vibriosis?

The broad category of vibriosis includes illnesses caused by dozens of pathogenic Vibrio species, such as V. cholera and V. parahaemolyticus. The CDC collects data on all lab-confirmed Vibrio illnesses. In 2014 a total of 1,252 lab-confirmed cases occurred nationwide, but because of under-reporting and misdiagnosis the CDC estimated that 80,000 vibriosis cases occur annually.

What about Vibrio parahaemolyticus?

The most common of these pathogens is V. parahaemolyticus (Vp), which causes an average of 130 to 287  confirmed cases a year. The CDC estimates that for every confirmed case 142 cases go unreported or misdiagnosed, so that brings the total estimate to 44,950 Vp cases annually (about 34,664 foodborne).

Vp prefers seawater (while Vv is more common in brackish water), but again Vp is ubiquitous in warm marine waters around the globe. There are hundreds (possibly thousands?) of strains that are benign, but several strains have caused significant outbreaks, often sickening dozens at a time.

Most Vp illnesses are associated with the consumption of raw or undercooked seafood. Since many Vibrios are chitinolytic (they consume the shells of crabs, shrimp, lobster and copepods) we see many Vp illnesses from handling or consuming raw or undercooked crustaceans. But most cases are associated with the consumption of raw shellfish, and oysters are the chief culprit because they filter the water quite efficiently and can concentrate bacteria as they feed. Oysters are also implicated frequently because consumers like to eat them raw.

Most Vp illnesses cause nasty skin infections or gastroenteritis, but again, for the immune-compromised population Vp can cause life-threatening septic infections. The CDC estimates that Vp illnesses have a 0.9 percent mortality rate (~4 mortalities from foodborne Vp annually nationwide).

How should we interpret the CDC’s numbers?

First of all, we need to recognize that this is not an exact science. The Cholera and Other Vibrio Illness Surveillance (COVIS) system was initiated by the CDC, FDA, and four Gulf Coast states as a national database for Vibrio in 1989. Most states were voluntarily reporting Vibrio cases in the early 2000s; in 2007 the CDC mandated that all states report Vibriosis illnesses.

Vibrios are not easy to culture from fecal samples, so many cases go undiagnosed, and most people don’t bother to go to the hospital for a mild case of gastroenteritis.

Is Vibriosis increasing?

The CDC believes the prevalence of Vibriosis has been increasing over the past decade, and so is pushing for tighter regulations. With most foodborne illness on the decline, what could be the cause of this perceived increase? Several explanations are possible:

  1. States are clearly getting better at complying with reporting requirements that have been mandated for only the past 12 years;
  2. Recently we have seen widespread adoption by hospitals of a new diagnostic tool called Culture Independent Diagnostic Testing (CIDT). This is a PCR device that detects snippets of DNA or RNA from the most common pathogens in fecal samples. The device is inexpensive and generates results in hours instead of days, meaning that detection is far more common than it was just a few years ago. It is safe to say that the multiplier used by the CDC to estimate total cases based on lab-confirmed cases should be revised downward to reflect this new reality;
  3. In New England we know that the frequency of Vibriosis cases shot up after the introduction of a new hyper-virulent strain of Vp in 2012. But we have also seen a doubling of oyster production in the past five years, leading to the likelihood that more raw oysters are being consumed, and more often in summer, than ever before. Unfortunately, harvest data is pretty thin so it is tough to make this case; and
  4. Many reporters are linking the spread of Vibriosis to climate change. Certainly, warming will increase the pathogen’s range and lengthen the season, but the modest amount of warming we have seen in the past decade hardly explains the CDC’s numbers. If temperature were the main driver it would be hard to explain why there are more Vp illnesses in Massachusetts than in Virginia.

And so the question remains: Is Vibriosis becoming more common or are we just getting better at detection and reporting? With the advent of CIDT there is little doubt that detection is improving. Unfortunately, we have no way of determining whether illness rates per meal are increasing or decreasing. One thing is certain: since the CDC believes illnesses are on the rise, and our industry lies squarely in the cross-hairs, we will see a slew of proposals to tighten regulations at the ISSC meeting in October 2019.

What should we do?

  • Keep ‘em cold. Vibrios love heat, so the single most important thing we can do is to promptly cool down our harvest to stop bacterial growth. We can’t eliminate Vibrios in live shellfish, but if we can keep our shellfish below 50 degrees we can keep safe shellfish from becoming unsafe.
  • Fight back with facts. When you see one of those click-bait articles in the press, push back with facts in the comment section. I can’t do this all by myself!
  • Educate the immune-compromised population about the risk of consuming raw shellfish. They should enjoy their shellfish well cooked!
  • Support your state and regional shellfish associations. They are coordinating the industry response to the CDC’s regulatory push at this year’s ISSC meeting, and they need your support.