When is swimming in the Severn a health risk?

   Many people enjoy water sports in the Severn, and there are legitimate concerns about the health risks of swimming and other water contact sports. The risks can be broken down into several categories, as discussed below:
    Sea Nettles/Jellyfish are an annoyance rather than a health hazard. Their stings cause immediate pain and inflammation, but the effects diminish rapidly and are typically hardly noticeable after several hours. Tentacles should be removed from the skin to minimize the toxin injected into the skin.  More serious stings may be immediately treated by application of meat tenderizer, whose protease enzymes may cleave the nettle's protein toxin .  Serious reactions (anaphylaxis) are very rare.  Sea nettles enter the Severn in mid-late summer depending on the salinity (they are not found in salinities less than 5 parts per thousand).  Water skiers sometimes wear thin wet suits for protection against sea nettles as their stinging apparatus cannot penetrate deeply.
   Fecal microorganisms are a potential health hazard that should not be ignored. The Severn has been listed by the Maryland Department of the Environment as an impaired water body under the Clean Water act because of repeated findings of unsafe levels of fecal coliform bacteria at multiple times and locations. Although this impairment concerns shellfish harvesting, it indicates the presence of repeated sewage contamination in the Severn.  Such contamination is confirmed by measurements of Enterococcus bacteria at swimming beaches by both the Anne Arundel County Health Department and by Dr. Sally Horner's Operation Clearwater.  Enterococcus is an indicator group of bacteria chosen to reflect the chances of acquiring gastrointestinal illness as a result of sewage contamination. The monitoring results can give an indication of which beaches have a greater risk, and areas showing persistent elevated levels should be investigated to locate the source of sewage contamination.  The Anne Arundel County Health Department recommends that people avoid direct water contact for 48 hours after rainfall.  (The level of local rainfall triggering this caution is a judgment call.)  The reason for this recommendation is that runoff brings down sewage contamination from the watershed into tidewater, and monitoring often shows positive Enterococcus levels after rainfall.  It should be kept in mind that Enterococcus is found in the gut of most warm-blooded animals including birds, so dogs in the watershed or geese around beaches can be responsible for high counts shown by monitoring.
   Vibrio vulnificus is a pathogenic bacterium naturally found in estuaries and the ocean. When ingested it can cause gastrointestinal distress in healthy people, but it can be fatal in immunocompromised individuals. This bacteria can also gain access to the body via open cuts and wounds, resulting in serious infections.  Because of this risk, people with weakened immune systems or those with open wounds should not swim in the Severn or elsewhere in the Chesapeake. For more information on this bacterium, see the CDC factsheet.
   Mycobacterium marinum is another pathogenic bacterium associated with fish.  Infections can result from handling (typically cleaning) fish, but can also be acquired from water contact if open wounds exist.  Related bacteria cause tuberculosis and leprosy, and all these infections are difficult to treat with antibiotics.  This is another reason for avoiding water contact when you have open sores on the skin. For more information on this bacterium, see the MD Health Department's factsheet.
   Although the above health risks to swimming in the Severn are real, following the Health Department's recommendation allows most people to enjoy recreational water activities most of the time.