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It’s a fluke: The scientific truth about swimmer’s itch

by Ian Withrow
| August 9, 2021 12:15 AM

Here in Montana, itchy skin has always been an unavoidable consequence when enjoying the great outdoors. From peeling sunburns to mosquito bites, many of the inflamed epidermal threats we experience are widely known and well understood by local residents and out-of-state visitors alike.

But there’s another cutaneous offender that exists more in the realm of local folklore than the rest — one that resides in the shoreline waters of Flathead Lake and other freshwater bodies, and starts to emerge when summer temperatures approach their peak. You probably know it as swimmer’s itch. Prepare yourselves, Leader readers, because this column is bound to get under your skin.

As someone who grew up in Montana, my childhood summers were peppered with mysterious swimmer’s itch tales. “Did you hear about so-and-so,” a fellow fifth-grader might say, in a tone reserved only for the scariest of ghost stories. “They got swimmer’s itch after cannonballing off of their family cabin dock.”

At the time I had no personal swimmer’s itch experiences to fall back on, so these tales often left me no choice but to imagine poor so-and-so in a hospital bed, pockmarked and malformed by their terrifying encounter with the infamous itch of the swimmer.

In reality, swimmer’s itch symptoms aren’t quite so dramatic. They typically manifest as mosquito bite-like bumps that emerge wherever lake water has air-dried on the skin. It’s an allergic reaction, in other words, not unlike the body’s response to poison ivy. For the longest time that’s actually what I assumed swimmer’s itch was — a simple reaction to some kind of irritant like algae or an underwater plant. It wasn’t until I joined the University of Montana’s Flathead Lake Biological Station (FLBS) that I learned the horrific reality festering inside those little red bumps.

Here’s the unflinching truth: swimmer’s itch, officially known as cercarial dematitis, is caused by an immune system response to the penetration of human skin by parasites known as blood flukes.

Still with me? Well hold on, because we haven’t even scratched the surface yet.

Less than a millimeter in length, blood flukes begin their life cycle as eggs that are released with the feces of infected waterfowl or mammals, which in Flathead Lake are often mergansers. Once these eggs reach the water, tiny larvae called miracidia hatch and immediately go on the hunt for the freshwater snails of the Flathead Lake ecosystem.

When the miracidia find their unsuspecting snail, they invade it and begin to asexually reproduce into their next life stage called cercariae. Over the next few months, over 250,000 blood fluke cercariae can be released from a single infected snail. They have only one aim: Find a merganser, mature into adulthood within the merganser’s blood, and start the cycle over again.

Meanwhile, you just happen to be perfecting your cannonball technique from your family cabin dock. Mistaking you for a giant waterfowl of some kind, the blood fluke cercariae attach themselves to your exposed skin. When you get out of the water and begin to air dry, the flukes sense this, penetrate your skin and burrow inside.

Fortunately, you’re not a giant waterfowl. The human body is inhospitable to these parasites, and they die seconds after boring into your skin. For some people, upon discovering this sudden arrival of blood fluke corpses, the immune system overreacts — which, I think we can all agree, is a completely understandable response. It isn’t long before red and itchy bumps start popping up on your body like tiny, irritated memorials commemorating where a tiny blood fluke has died.

It’s a lot to process, I know, so let me do my best to address any additional questions you may have.

Question: If the blood flukes aren’t a nuisance to humans until they’ve passed through a merganser and been released from a snail, can’t we just get rid of the mergansers and snails?

Answer: No. Waterfowl and snail eradication has been tried in other lakes. Not only did those efforts fail at reducing swimmer’s itch, but many of the lakes were polluted with toxic chemicals in the process. This is a far greater threat to Flathead Lake’s ecosystem and recreational cannonballers than swimmer’s itch could ever be.

Question: Is there an easy way to determine if blood flukes are currently in my favorite swimming area?

Answer: Not really. FLBS researchers have investigated swimmer’s itch over the years, and found that 60-80% of common mergansers on Flathead Lake suffer from blood fluke infections. According to a 1998 survey, the infection rate in Flathead Lake’s snail population is around 1%. This low percentage sounds encouraging until you remember that a single infected snail can transmit hundreds of thousands of blood flukes to birds (and humans) swimming nearby. To put it simply, if there’s a single snail somewhere in your favorite swimming area, you may be at risk of contracting the dreaded Itch.

Question: What can we do to safeguard ourselves from the dreaded swimmer’s itch?

Answer: Again, swimmer’s itch doesn’t affect everyone. There are a number of you who never realized you’ve had dead blood flukes in your skin (until now!). For those who are susceptible to stronger immune responses, here are a couple things you can do to decrease your risk of Swimmer’s Itch. First, swim offshore. Snails and parasites concentrate along the shoreline, so offshore swimming should keep you blood fluke-free. Second, after swimming be sure to use your towel. Researchers in Europe concluded that toweling off vigorously before the blood flukes have a chance to dig into your skin is the best way to avoid the Itch.

Once a person contracts swimmer’s itch, there’s little that can be done. Those itchy, red bumps will remain until the immune response runs its course, a process that generally takes a week or so. In the meantime, scientists recommend using hydrocortisone cream to ease the discomfort and getting back out on that dock. Those cannonballs won’t perfect themselves, after all, and there’s only so many days in Montana’s summer season.

Swimmer’s itch can be an uncomfortable experience, but it’s nothing compared to the feeling of missing any opportunity to recreate and relax in our incredible Flathead watershed.

Ian Withrow is the media and information program manager at the University of Montana’s Flathead Lake Biological Station, a world renowned freshwater research, monitoring, and education facility located on the shores of Yellow Bay. For more information about the Bio Station, visit flbs.umt.edu.