by Cassie Bates
Now that summer is well underway, weekends in coastal Alabama are filled with offshore red snapper runs, kayaking trips, and swimming excursions. These attractions not only keep locals endlessly entertained, they also bring thousands of tourists to the Mobile Bay area. Of course, this season brings more than sunshine and fun as residents and visitors are often greeted by tropical storms and hurricanes. Another less understood issue which makes a seasonal appearance is exposure to and potential infection from Vibrio.
As waters heat up over the course of the summer, news reports about “flesh-eating bacteria” become more frequent along the Gulf Coast. Although Mobile Baykeeper does not directly monitor Vibrio in the environment, we do communicate the presence of other bacteria that are indicators of fecal pollution in our waterways. As a part of informing the public to make safe decisions on where to swim, fish, and play, we want to make sure you get to enjoy our waterways and are knowledgeable on how to do so with appropriate caution and confidence.
Vibrio species is a type of bacteria that naturally occurs in the environment and is not directly correlated with pollution. Comparatively, Enterococcus and E. coli are also naturally present in waterways, but are found in significantly greater concentrations when introduced through pollutant sources such as sewage spills, leaky septic tanks, and pet waste.
Every time you dive in, you are likely to come into contact with Vibrio species in some capacity, but don’t worry – there’s no need to panic. Infections from vibrios are very rare; in fact, the CDC estimates that less than 80,000 people in the United States become infected with vibriosis per year and only about 30,000 of those cases include infection via wound exposure (most Vibrio infections are caused by eating raw or undercooked shellfish). Further, only about 100 cases of vibriosis result in death each year. When you consider exposure opportunities along thousands of miles of shorelines, coastal lagoons, estuaries, bays, and beaches, it is reassuring to know the odds are on your side.
So, if vibrios are naturally occurring, and it is rare for people to become infected, why and how do people get sick? Out of nine potentially pathogenic species, two are of primary concern locally. Vibrio parahaemolyticus and Vibrio vulnificus can cause gastrointestinal infections, where exposure is primarily sourced from raw or undercooked seafood. The reason you are more likely to acquire vibriosis from eating an oyster versus jumping off a dock into the bay can be explained via bioaccumulation.
Shellfish are filter feeders, meaning that they constantly remove particulates from the water column as a means of acquiring food. Vibrio, along with other bacteria, are taken up and concentrated in the gut of an oyster; therefore, they typically contain higher amounts of Vibrio species relative to the surrounding water. Eating oysters does not automatically put you at risk for gastrointestinal complications, but there are individual health factors that can put you more at risk.
Remember to limit oyster consumption in warmer months if you have preexisting conditions as individual oysters are more likely to contain greater concentrations of Vibrio.
The more sensationalized form of vibriosis, described as “flesh-eating” or necrotizing, results from Vibrio vulnificus entering open wounds when exposed to water. An open wound could vary from a gash on your foot to a mosquito bite on your arm, so it is important to prevent exposure by limiting or avoiding submergence into water when you have wounds. Interestingly, the same factors that increase risk of vibrio infection via consumption also increases the likelihood of wound infection by V. vulnificus.
Understanding predisposed risks can help minimize the likelihood of vibrio exposure and infection at an individual level. However, it is also important to know that certain environmental conditions are conducive to vibrio presence and growth and can increase probabilities for exposure and infection.
Remember to avoid submersing wounds in waterways for extended periods of time and cover wounds with waterproof bandages. If you are shucking oysters during the summer, remember to cover wounds and sanitize your hands afterwards.
Like most bacterial species, vibrios thrive in warm environments; this is why cases are typically reported during the summertime. In cooler months, vibrios can settle into the sediment and enter a state of “hibernation” as a survival tactic. These two factors can provide insight as to where and when Vibrio species may be more prevalent. Studies indicate that there are relationships between vibrio abundance, warm water temperatures, and high turbidity in the water column. Anything that stirs up sediment such as wind, storm events, or dredging activities, especially during the summer, could reintroduce bacteria from the sediment into the water column. Salinity can also impact vibrio populations as freshwater input (i.e., stormwater runoff, outfalls, etc.) can create more favorable conditions in saline water for certain species.
Although vibrios are naturally occurring, there are man-made and climate change influences which may expand the length of time and spatial coverage in which vibrio populations remain dense. The most challenging aspect of anticipating trends for vibrio presence is understanding that localized conditions can impact numbers.
If you are worried about wound exposure, it is good practice to sanitize and wash the area with soap and water or alcohol. If you experience increasing pain, swelling, and/or redness (especially red radiating lines) from the site of the wound, you should seek further medical attention for early interception and minimization of infection.
There is an inherent risk that anyone on the Gulf Coast assumes when they fall into the water while tubing, pull a hook out of a speckled trout’s mouth, or simply wade into the surf at the beach. By knowing when you are most at risk to swim or eat oysters, what makes you personally at risk from a health standpoint, and how you can minimize exposure, you can play in the waterways of coastal Alabama and limit your risk of Vibrio species infection.
Special thanks to Blair Morrison, Science Policy Fellow at Florida Department of Agriculture and Consumer Services. Blair provided insight and expertise for the content of this blog. She studied Vibrio species populations in Fowl River Bay, Grand Bay and Mississippi Sound while obtaining her master’s degree in Marine Science at the University of South Alabama.