A rash appeared the morning after a June lake swim. No fever, no bites, just an angry constellation of red bumps across every patch of skin that had touched the water. The doctor’s visit that followed turned into something of a small education in freshwater biology, and the findings were genuinely unsettling.
The short answer: your local lake in June is not a passive, neutral body of water. It is a living ecosystem that, at peak summer temperatures, can harbor at least two distinct classes of skin-reactive threats, and most people swimming in them have no idea.
Key takeaways
- Your local lake in June isn’t what you think it is—two distinct microscopic threats can trigger skin reactions you won’t see coming
- One parasite burrows into your skin and dies there; the other produces toxins that can damage your liver if ingested
- The water doesn’t need to look green or smell bad to be dangerous—contamination can be completely invisible
Two Culprits, Both Invisible
Swimmer’s itch, also called cercarial dermatitis, appears as a skin rash caused by an allergic reaction to certain microscopic parasites that infect some birds and mammals. These parasites are released from infected snails into fresh and salt water, such as lakes and ponds. The twist that most people find hard to believe: the parasite can burrow into the skin, causing allergic reactions and itchy rashes, but since the larvae cannot survive in the human body, they eventually die in the skin. The rash you’re experiencing isn’t an infection, technically, it’s your immune system reacting to a dead organism lodged in your dermis.
The itchy rash looks like reddish pimples or blisters, and it may appear within minutes or days after swimming or wading in infested water. Swimmer’s itch is generally found on exposed skin that has had direct contact with the water, and it rarely affects the areas covered by swimsuits. That last detail is actually one of the clearest diagnostic clues a doctor uses, if the rash stops exactly where your bathing suit started, cercarial dermatitis moves to the top of the list.
But there’s a second, more serious culprit: cyanobacteria. Cyanobacteria, also called blue-green algae, can produce toxins that make people and animals sick. The toxins produced by the bacteria, called cyanotoxins, can cause skin irritation such as rashes, hives, swelling or blisters. Unlike swimmer’s itch, which is purely allergic, cyanotoxin exposure carries broader health implications. Consuming raw water that contains these toxins can cause liver damage.
Why June Is the Danger Window
Cyanobacteria are microscopic bacteria, otherwise known as blue-green algae, that form harmful blooms during the summer, they flourish in warm, stagnant water filled with nutrients. June marks the opening of that window. Water temperatures climb, runoff from spring rains delivers a fresh load of agricultural and urban nutrients, and the conditions become ideal for blooms to explode in density within days.
This isn’t a niche regional issue. The median annual algal bloom frequency in freshwater lakes has increased by approximately 1.8% each year over the past two decades. Impacts of climate change like warmer water and sea level rise might lead to more intense harmful algal blooms occurring in more waterbodies. Many species of cyanobacteria thrive in warm, nutrient-rich environments and some can produce toxins that cause health problems for humans, pets, and wildlife, and climate change and ongoing nutrient pollution may cause more frequent and intense cyanobacterial blooms in the future.
Here’s where the counter-intuition comes in: the lake doesn’t have to look obviously green or scummy to be carrying a problem. People can have direct skin contact with water contaminated with cyanobacteria or their toxins during activities like swimming or boating, and cyanobacterial concentrations can vary dramatically across a single body of water, meaning one part of the lake where you wade in may look crystal clear while a cove 200 meters away is thick with bloom. You can swim right through a dispersed patch and never see it.
What the Doctor Actually Pulled Up
Harmful algal blooms can pose a significant health risk if the cyanobacteria or toxins are ingested, inhaled, or come into contact with skin. Inhaled bacteria or toxins could cause wheezing, coughing, chest tightness and shortness of breath. Skin contact could lead to rash, itching, blisters, and eye irritation. The scope of that list tends to surprise people who assumed a simple lake swim was benign.
Case reports dating from 1949 describe a range of illnesses associated with recreational exposure to cyanobacteria: hay fever-like symptoms, pruritic skin rashes, and gastrointestinal symptoms are most frequently reported. Some papers give convincing descriptions of allergic reactions while others describe more serious acute illnesses, with symptoms such as severe headache, pneumonia, fever, and blistering in the mouth. Most cases resolve on their own. But the severity spectrum is wide enough that it’s worth paying attention.
On the cercarial dermatitis side, management is fairly straightforward. Most of the time, swimmer’s itch will get better on its own, but applying anti-itch corticosteroid cream, cool compresses, Epsom salts, colloidal oatmeal, or baking soda pastes may help stop the itching. The rash usually gets better after a few days, but it can last for up to two weeks. Talk to your doctor if you have a rash after swimming that lasts more than three days, and if you notice pus at the rash site, consult your doctor. That last point matters: scratching is the enemy. Scratching the areas may result in secondary bacterial infections.
Before Your Next Swim
The practical takeaway is simpler than the biology. Before you head out, check online to find out if the swim area is currently monitored, is under advisory, or has been closed for health or safety reasons, this is especially important after a heavy rain. Most state environmental and health departments publish real-time advisories. Apps like Swim Guide aggregate local water testing data and flag contaminated areas with red markers before you drive out to the water. The EPA and state environmental agencies provide valuable information on water quality monitoring programs and public advisories, and local health departments can provide information on recent water testing results and any known health risks.
On-site, if the water looks cloudier than usual, is discolored, or smells bad, treat it as a warning, cloudy water can indicate more germs in the water than normal, and discolored or smelly water could mean there is a harmful algal bloom present. Unusual smells or discoloration should raise red flags — a rotten egg smell might indicate hydrogen sulfide, while green or blue-green scum could signal a harmful algae bloom. After any swim in natural water, shower after coming into contact with recreational water, rinsing off immediately after exiting the lake (rather than air-drying on the shore) is one of the most effective ways to reduce both cyanotoxin exposure and the cercarial parasites’ ability to burrow in.
One more thing the data shows that should reshape how you think about this: through 2024, the EPA’s beta forecasting model provided the public with weekly bloom forecasts for 2,192 lakes across the country. That infrastructure exists because the problem has scaled up enough to warrant it. The question is whether enough swimmers are actually checking before they get in the water.