Poo Germs, Not Sharks, Make Swimming in the Ocean So Treacherous

By Ed Cara on at

It’s polluted, germ-filled sludge, not sharks, that will make going to the beach more dangerous than just staying home this summer — at least according to a cleverly titled review published this week in the International Journal of Epidemiology.

Researchers looked at 40 past studies that examined whether people who regularly swam in seawater experienced any increased risk of becoming ill. They then combined and analysed data from 19 of the studies that compared the swimmers to people who stayed dry. These studies collectively involved more than 120,000 people across highly industrialised countries, including the US, Norway, Denmark, and the UK, among others.

Compared to non-bathers, they found, seawater frolickers were on average 86 percent more likely to come down with any sort of illness soon after, typically the stomach flu or ear aches. The review is the first of its kind, according to the authors.

“Although most people enjoy the coastal waters without incident, this study shows for the first time that there is a significant increase in the risk of ear and gut ailments in those who are exposed to bathing waters,” co-author Andrew Singer, a pollution scientist at the Center for Ecology and Hydrology located in the UK, said in a statement.

The culprits behind the increased risk are poo-spread germs found in sewage, Singer and his team say. The majority of these germs end up in the ocean via treated sewage water, which is bad enough. But oftentimes, flooding can overwhelm already overworked systems, leading to tons of untreated sewage and lots more germs entering the environment. Some of the germs identified in the studies they reviewed included the bacteria Escherichia coli, the Hepatitis A virus, and the parasite Cryptosporidium.

“This release of untreated sewage during and following moderate to heavy rainfall is a hugely significant source of human pathogens and must represent a priority for mitigation—without which there is unlikely to be much progress in this area,” Singer said. In countries with poorer sanitation systems, the problem is even worse—Yemen, for example, is the middle of a cholera epidemic that has already killed 2,000 people and is expected to only escalate as their rainy season approaches.

Evidence from Singer’s lab and elsewhere has also shown that sewage acts as a potent catalyst for superbugs. Flushed away antimicrobials constantly encourage bacterial resistance, while widely different species of bacteria can lend each other their resistance genes through free-moving bits of DNA called plasmids.

Depressing as all this is, I do have to commend the authors on their study’s title—“Is it safe to go back into the water? A systematic review and meta-analysis of the risk of acquiring infections from recreational exposure to seawater”—which riffs on the tagline of the 1979 sequel to the classic 1975 film Jaws.

For their own sequel, Singer and his colleagues next plan to investigate whether freshwater also wants to secretly kill us. [International Journal of Epidemiology]