There are few summer pastimes that compare to the bliss of a cannonball into a cool lake. But if you think the dangers of swimming are limited to drowning, you’re ignoring the sometimes invisible hazards that can lurk in the depths.
The Centers for Disease Control and Prevention recently issued reports on disease outbreaks in both treated and untreated bodies of water from 2000-2014. Although Americans swim hundreds of millions of times each year without incident, we also see dozens of self-reported outbreaks caused by bacteria, viruses, and parasites, and warming waters — over time — can bring about new risks. Taking precautions before taking a dip can prevent the spread of disease for everyone.
The parasite Cryptosporidium is a leading cause of waterborne illness in the U.S. It accounts for 79 percent of cases of illnesses in the CDC’s report of chlorinated waters. The danger of this bug is that it is encased by a hard outer shell that allows it to survive in chlorine-treated pools for up to 10 days. The best practice is to keep it out of the water in the first place, according to Michele Hlavsa, an epidemiologist and Chief of the CDC’s Healthy Swimming Program. That means refraining from swimming if you or your child have recently had diarrhea. The report shows the most frequent settings for outbreaks are hotels, possibly because pool maintenance is undertaken by staff with many other responsibilities, Hlavsa says. You can check with pool staff to find out their inspection score from the CDC. Hlavsa recommends buying test strips to measure chlorine and pH levels as an added pool precaution.
Untreated waters, like lakes, rivers, and beaches, present a much wider variety of risks. Water that is crystal clear isn’t necessarily safe for swimming, Hlavsa says. Norovirus, E. coli, and Shigella outbreaks peak in July. A growing risk is the presence of harmful algal blooms: “In recent years, harmful algal blooms have been observed with increasing frequency and in more locations in the United States, possibly because of increasing nutrient pollution and warming water or improved surveillance.” Evidence suggests that harmful algal blooms are happening more often and for longer periods of time in a variety of American waters due to field runoff and climate change. Hlavsa recommends steering clear of discolored, smelly, foamy, or scummy water and obeying all posted advisories when using untreated recreational waters. She also says to avoid swimming wherever discharge pipes can be found and after a heavy rainfall that can contaminate waters.
The most terrifying parasite in the water is also among the rarest: Naegleria fowleri. Known as the “brain-eating amoeba” that enters through the nose in warm freshwater, Naegleria fowleri kills individuals within two weeks of infection. Each year brings only 0-8 cases of the parasite, but Hlavsa says the locations of recent cases have been alarming: “It used to be found only in southern states, but recently we’ve seen cases in Virginia, where we haven’t seen it for decades, and Minnesota and Indiana, where we’ve never seen it before.” The reasons for the amoeba’s northward migration aren’t clear. In fact, little is known about why some become infected and others do not, since cases are so rare. Since the amoeba can only enter the body through the nose, wearing nose clips is a good precaution against this killer bug.
The statistics in the CDC’s reports are at the low end of realistic numbers for recreational water disease outbreaks. Because of differing capacities around the country to properly identify and report illness, exact numbers would be impossible to obtain. The tendency for swimmers to be travelling during their use of recreational waters also makes it difficult to pin down an outbreak to its source.
Like anything else, swimming has its risks, but that doesn’t mean you should completely eschew the water. Only ten deaths from disease outbreaks in recreational waters were reported from 2000-2014. When compared with the more than 100 fatalities from fireworks in the same time span, the perils of taking a plunge might not seem so bad.