Article: Shaw, Kristi S., et al. “Recreational swimmers’ exposure to Vibrio vulnificus and Vibrio parahaemolyticus in the Chesapeake Bay, Maryland, USA.” Environment international 74 (2015): 99-105: doi:10.1016/j.envint.2014.09.016
Vibrio: Natural born, and sometimes killers
Both Vibrio vulnificus and Vibrio parahaemolyticus are naturally occurring, estuarine microrgansms. These bacterioplankton are common inhabitants of Chesapeake Bay waters during the summertime, but are a cause of fear among beach-goers in the region. Exposure to Vibrio sp. has the potential to cause severe infections and even can lead to death in serious cases.
Illness caused by Vibrio sp. infections has increased in Chesapeake Bay. In fact, as of 2012, Vibrio-associated illnesses have increased by 43% at 10 sites throughout the United States since a 2006-2008 baseline. Even scarier, 93 life-threatening cases are reported annually….that is 93 recreational swimmers a year who required a hospital visit.
There are two main ways a person can get infected by Vibrio: ingestion (usually of contaminated fish or shellfish) or through the skin (usually due to an open wound). It is likely that these two exposure pathways are roughly equal in causing infections. However, a huge problem is that little is actually know about the magnitude and risk for Vibrio infections by skin contact. This type of information could be very important to recreational swimmers in Chesapeake Bay, especially for those with compromised immune systems.
The goals of this study by Shaw et al. was (1) to measure the presence (and amount) of Vibrio sp. in hand-wash samples, (2) to determine the total skin exposure of Vibrio in recreational places, as well as to investigate a link to temperature and salinity, and (3) to assess the efficiency of common hand-washing practices in remove Vibrio from the skin.
Four recreational swimming locations were selected for this study. As Figure 2 shows, all were within rivers in Chesapeake Bay so that different salinity regimes could be sampled. Swimmers (the test subjects) were recruited from a local academic institute (oh what graduate students do in the name of science!). This study first used a total of 19 swimmers in 2009 to establish the minimum number of swimmers needed to get a proper result. Shaw et al., determined that at least 3 swimmers per location would be needed to test a significant sample size.
Swims were timed in intervals from 2 minutes to 20 minutes. During this test, swimmers were instructed to keep their hands completely submerged in the water. It was determined that after 8 minutes of swimming the Vibrio concentrations stabilized.
After the timed swims, the test subjects washed their hands in a bag containing a sterile solution. They washed their hands twice (in separate bags) to determine the efficiency of hand-washing. In other words, if Vibrio is detected in the second hand-wash, then hand-washing does not completely remove Vibrio.
Vibrio spp. presence was measured by extracting its DNA from the hand-washing bags, allowing for both species (Vibrio vulnificus and Vibrio parahaemolyticus) to be separately measured and identified. A common DNA technique called polymerase chain reaction (or PCR) was used to measure the presence and concentration of Vibrio in the units of colony forming units (CFU). The surface area of the individual swimmers’ palms were measured and used to determine the exposure and concentration of Vibrio to a swimmer.
Lastly, surface water samples were also collected and measured for Vibrio, in addition to temperature, salinity, and concentration of fecal indicators (in general, fecal presence from waste water can promote Vibrio’s presence in beach waters.)
What did they find?
In a nutshell, Shaw et al. determined that all the swimmers used in this study were exposed to Vibrio. It must be noted that the surface water concentration of Vibrio was much lower than the “close-the-beach” threshold of 104 CFU per 100 mL water, so the beach sites were open during the exposure. This confirms that you could still be at risk for a Vibrio infection, even after a beach passes a bacterial inspection. In other words, swimmers are most likely safe to swim, but there is still that slight chance of infection, especially if you have an open wound!
Figure 3 is a dream-like log-linear relationship to any modeler. As you can see, there was a very strong relationship between the amount of Vibrio in the water and the amount of Vibrio extracted during a hand-wash. This type of log-linear relationship could allow for future predictions of Vibrio skin contact if the surface water Vibrio concentration is measured. Past case studies of Vibrio infections suggest that swimmers with open wounds even in low Vibrio concentration could be in danger of infection; thus these relationships could be important for future exposure predictions.
On average, hand-washing reduced the Vibrio vulnificus skin concentration by 89.4% and the Vibrio parahaemolyticus by 93.9%. With all this data, Shaw et al. suggest that risk for infection could be estimated using hand exposure (time in water with Vibrio present) and the size of an open wound.
Skin exposure studies like this one could help establish guidelines to limit infection risk. For example, it could help determine how long someone could swim in Vibrio-present waters before an infection could occur. This is especially important for children and people with compromised immune systems (such as someone with cancer). Presently, scientists are working on predictive models to determine where Vibrio is likely to be present in Chesapeake Bay. Combined with an exposure study like this one (the first of its kind, by the way!), we could be one step closer to preventing serious health infections from this naturally-occurring bacteria.
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!