Infectious Diseases in Recreational Waters

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In summer weather, it’s tempting to cool off at the lake, ocean, or the local public swimming pool. This water is sometimes contaminated with disease-causing organisms (also referred to as “pathogens“). It’s important to realize that even the clearest mountain stream may harbor bacteria and other organisms that can make you sick, often in the form of diarrheal disease.

Common human contaminants that can cause recreational water illness include fecal matter, vomit, and blood. Of these, fecal matter is, by far, the most dangerous. Just one person with diarrhea having an “oops” moment can contaminate an entire swimming area. Others in the water who swallow even a small amount of water become infected, and before you know it, you’ve got an outbreak.

Vomiting is often caused by swallowing too much water. Regurgitating a gullet full of pool water is not the most likely cause of spreading an infectious disease, but those who regurgitate stomach contents may be doing so as a result of a norovirus infection (sometimes called the “stomach flu”). For more information on this virus, see this article on Norovirus by Joe Alton MD.

Blood may contain viruses like Hepatitis B or HIV, but the concentration is small in open water and unlikely to spread disease. In properly chlorinated pool water, viral lifespans are even shorter. The CDC has no documented cases of HIV or Hepatitis B caused by exposure to pool water.

Given the significant risk caused by fecal contamination of recreational waters, let’s concentrate on some organisms common in recreational waters that can cause diarrheal illness:

· E. Coli (strain 0157:H7)
· Giardia
· Shigella
· Cryptosporidium

E. COLI (0157:H7)

Escherichia Coli is a common inhabitant of the intestinal tract but some strains, especially O157:H7, produce a toxin known as “Shiga” that causes bloody diarrhea, nausea and vomiting, dehydration, and other symptoms. Besides its presence in lakes, ponds, and pools, 0157:H7 is often the cause of “Traveler’s Diarrhea”.

After the organism enters the system, it usually takes several days for symptoms to appear. Unlike many infections, E. Coli tends not to cause high fevers; however, the abdominal pain, diarrhea, and vomiting can be severe. Dairy products or items with high fat content or fiber are thought to make your symptoms worse.
If untreated, the ensuing dehydration can cause decreased urine production, dark urine, weakness, and fatigue. Further neglect may lead to seizures, kidney failure, and worse.

Rehydration support will help support the victim for the 6-8 days it takes most to get over the infection. Antibiotics like Sulfa drugs may have some benefit, but are rarely indicated as the infection usually goes away by itself. It is now thought that Pepto-Bismol (bismuth subsalicylate) has some benefit as a preventative for those at risk. Using anti-diarrheals like loperamide (Imodium) is problematic, as it may prolong the illness.


The most common disease-causing parasite in the world is the protozoa Giardia Lamblia. It is not uncommon even in pristine backcountry waters in many national parks in the U.S. Symptoms commonly present one to two weeks after exposure to contaminated water. Patients complain of foul watery or greasy diarrhea, abdominal cramping, violent (“projectile”) vomiting, and gas.

Giardia lamblia exists in active and inactive forms. The active form is called a “trophozoite”; the inactive form is referred to as a “cyst”. Trophozoites attach to the lining of the small intestine and cause the symptoms.
Trophozoites can’t live long outside a host, however, so it is the hardier cysts they produce that spread the infection to other people. When as little as ten cysts are ingested due to contaminated water, your stomach acid turns them into trophozoites and the signs and symptoms of the illness take hold.

A course of metronidazole (veterinary equivalent: Fish-Zole) is a common drug treatment in conjunction with oral rehydration, although some newer drugs like Tinidazole have recently entered the marketplace; some are effective with just a single dose.


Shigella boydiiAlthough the diarrheal disease known as dysentery can be caused by a number of different microbes, the most common form is caused by the bacteria Shigella. It is seen in crowded unsanitary conditions, such as might be seen in community pools, as well as in poorly treated drinking water and food. Ciprofloxacin and Sulfa drugs, in conjunction with oral rehydration, are effective therapies.

Victims present with fever, abdominal pain, and bowel movements that are bloody mucus in nature. Symptoms usually begin one to three days after exposure and are associated with a sensation of incomplete evacuation. In severe cases, those with dysentery may pass more than one liter of fluid per hour.

Milder cases may resolve with rehydration. Ciprofloxacin and Sulfa drugs (Fish-Flox and Fish-Sulfa, respectively), in conjunction with oral rehydration, are effective therapies for severe cases.


An especially persistent cause of diarrheal disease from contaminated recreational waters is Cryptosporidium. Also known as “Crypto”, the organism survives for a longer time than other pathogens and is not uncommon in waterways. In high-density areas like pools, hot tubs, and water parks, Crypto is responsible for the grand majority of those made ill from contaminated water. The CDC has reported almost 500 outbreaks between 2000 and 2014, with 27,219 cases and eight deaths.

Crypto spreads in pool water when someone who is sick with the parasite goes swimming and has a bowel movement in the water. Diarrheal stools spread the pathogen faster than formed ones.

Those who swallow contaminated water can become sick for weeks with watery diarrhea, stomach cramps, and vomiting. This occurs even in properly chlorinated pools: Chlorine takes days to eliminate Cryptosporidium, as compared to minutes for organisms like E. Coli and others. The CDC has a formula for “hyperchlorination” which may speed up the process. Treatment of ongoing infections can include Nitazoxanide, Azithromycin (Aquatic Azithromycin), and anti-diarrheals like loperamide (Imodium).

An ounce of prevention is worth a pound of cure. The CDC recommends:

  • If your kids have diarrhea, keep them out of the water. If they have documented Crypto, wait 2 weeks after recovery before entering public waters.
  • Make special efforts to avoid swallowing pool water.
  • Take kids on bathroom breaks hourly; change diapers in designated areas away from the water.
  • Before swimming, know the quality of the water. Some use test strips to check pool water chlorine, pH, and bromine levels.

Nearby surfaces should be clean. An effective recipe for surfaces that might be contaminated with the above disease-causing microbes involves common household bleach. Nine parts cool water and one part bleach makes a reasonable cleaning solution. Batches should be made fresh as bleach loses its potency quickly over time.

There are more bacteria, viruses, and protozoa that can pollute natural and man-made recreational waters. Avoid swimming when you’re suffering from a diarrheal disease; you’ll be doing a public service and keep your community healthy.

Joe Alton MD

Joe Alton MD

Find out more about the above organisms and 150 other medical issues in times of trouble by getting a copy of the award-winning Third Edition of The Survival Medicine Handbook: The Essential Guide for When Medical Help is Not on the Way, available on Amazon and on this website.

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