What Is Swimmer’s Itch

Introduction

Outdoor swimming is a popular pastime, though some bathers complain of itchy skin rashes after swimming in certain areas. This is often caused by an allergic reaction to the larvae of microscopic parasites.

Swimmer’s itch, scientifically known as cercarial dermatitis, is an allergic rash that occurs when the larvae of parasitic flatworms (also known as ‘trematodes’ or ‘flukes’) mistakenly enter the skin of humans.¹ The trematodes that cause swimmer’s itch are specifically a group of species called ‘schistosomes’, or ‘blood flukes’. The schistosomes that cause Swimmer’s itch are normally parasites of birds or small mammals and they can’t survive for long in humans. However, skin contact with their larvae can cause an allergic response, resulting in a rash and itching.

Although it is not a serious condition, Swimmer’s itch is one of the more common water-borne illnesses. Understanding this condition can help you recognise, treat, and avoid it during future swimming trips.

How swimmer’s itch spreads

Schistosome life cycle

Schistosomes, like all trematodes, have a complex life cycle involving two separate hosts. After hatching from eggs, the larvae of these schistosomes spend the first stage of their life - a ‘miracidium’ - searching for a water snail host. Once they have found and infected a snail host, the schistosome reproduces asexually, producing many copies of itself. These copies look different and can swim freely, known as ‘cercariae’. The cercariae emerge from the snail host once conditions are ideal for them to infect their next host.

At this stage cercariae swim to the water surface. They then penetrate the skin, entering the bloodstream to continue their life cycle.² Most Schistosomes tend to infect birds to complete their life cycle, although some target mammals like dogs or raccoons. Once in the final host's circulatory system, the schistosomes mature and migrate to the intestinal lining to lay eggs, with the next generation of schistosomes returning to the water in the animal's droppings.

Human infection

Cercariae navigate by responding to simple environmental cues such as differences in light or temperature, and cannot always tell their host apart from humans. Schistosomes require specific hosts to complete their life cycle, and so will die after infecting a person. However, the parasitic infection of the skin signals the immune system to respond by causing inflammation and irritation, resulting in the characteristic skin rash of swimmer’s itch.²

Symptoms and diagnosis

Swimmer’s itch is generally diagnosed from observation of the skin rash symptoms and patient history - i.e. whether the patient has been exposed to contaminated water.¹

Swimmer’s itch often begins as a prickling or tingling sensation in the affected areas shortly after exposure to cercariae. After about 12 hours, the itching may intensify. Raised pimples called ‘papules’ will appear, sometimes developing into blisters, usually persisting for 1-2 weeks.

If someone has had swimmer's itch before, the reaction may be stronger and happen more quickly.^1,2 There are no long-term effects associated with swimmer’s itch, however, scratching at the rash can expose healing skin. This can cause a secondary infection, worsening symptoms and delaying recovery, possibly requiring antibiotic treatment.¹ 

A clinician is unlikely to take a sample for diagnosis in the lab from individual patients. If an outbreak is suspected, public health authorities may collect snail specimens from the contaminated water body for further testing and identification.

Similar illnesses

The rash from Swimmer’s itch can sometimes resemble other rashes associated with water-borne illness. Sea bather’s eruption only occurs in saltwater, but causes similar red papules to appear but is caused by the stings of jellyfish or sea anemone larvae as they get trapped underneath bathing suits due to their small size.⁴ Other water-associated rashes may be caused by bacteria in poorly maintained hot tubs, toxic algae, or chemicals used to treat pools.^5,6,7 Before assuming a rash is swimmer's itch, consider the type of water the person has been exposed to.

Treatment and management

Swimmer’s itch is very rarely serious and does not require medical attention. Treatment aims to relieve the symptoms until the rash and itching resolve themselves. At home, using things like oatmeal, Epsom salts, baking soda, and cool compresses at home can help stop the itching. Avoid scratching, as this can cause bacterial infections and require further treatment.

In more severe cases, treatment options such as topical corticosteroid creams or antihistamines may be recommended by a clinician.¹

Risk factors and preventing swimmer’s itch

Areas to Avoid

The best way to prevent swimmer’s itch is to avoid the areas where the parasite and its hosts are found. Swimmer’s itch occurs globally. It is most likely to occur wherever the initial snail host is found, in slow-moving rivers, lakes, ponds, and coastal areas. Heed signs from health authorities that caution about areas prone to swimmer’s itch.

Shallow water is where most snails live, especially near reedy areas or thick vegetation. Avoid swimming too close to the shore in marshy areas or where there's a lot of vegetation present. Care should also be taken to avoid bodies of water where birds such as gulls or ducks congregate regularly, as their droppings may introduce schistosomes to the water. Also, avoid feeding birds in areas where people regularly swim.^1,2

When is swimmer’s itch most likely?

In cooler climates, such as Europe and North America, Swimmer’s itch can occur seasonally. This is typically in the summer months when aquatic snails are active, and migratory birds are present and shedding schistosome eggs into the water. Unfortunately, cercariae tend to emerge when conditions are also ideal for recreational activities - when the water is warm and still. These warm, calm days are associated with higher levels of cercariae, making it more crucial to take precautions during such conditions. In warmer climates, swimmer’s itch may occur year-round.

How can I protect myself?

Another factor influencing the risk of swimmer’s itch is the duration and number of times spent in the water - if you are concerned about swimmer’s itch, try to minimise the number of trips and time spent submerged in the water.⁴ Wearing bathing suits or wetsuits can also help prevent cercariae from entering the skin.² 

After swimming, promptly towelling your skin dry and rinsing again in clean water are effective ways of reducing the chances of developing swimmer’s itch by removing any cercariae from your skin.^1,2

Who’s most at risk?

Anyone participating in recreational water activities is liable to contract swimmer’s itch, though activities that don’t involve being immersed in the water (e.g. kayaking or paddle-boarding) are lower-risk. Swimmer’s itch is more commonly observed in children, though this likely is due to a tendency to spend longer swimming in shallower waters and not dry themselves after leaving the water.^1,4

FAQs

Is Swimmer’s Itch contagious?

No, swimmer’s itch cannot be transferred between people. It also cannot spread from one part of the body to the other.¹

Can Swimmer’s Itch be serious?

Itching typically is worse 1 - 3 days after exposure to the parasite and can be irritating but not dangerous. If a skin infection occurs due to scratching, contact your healthcare provider.^{1, 2}

Can I get Swimmer’s Itch from swimming pools?

Swimmer’s itch only occurs in bodies of water where the snail hosts required for the schistosome life cycle are present. Treated and maintained pools and public baths are safe to swim in.¹

Is it safe to swim in water again after an outbreak of Swimmer’s Itch?

Swimmer’s itch can be a temporary phenomenon and, outside of warmer climates, is seasonal. Cercariae emerge under ideal conditions and only survive a short time after leaving the snail host (around 24 hours). This means cercariae can be rapidly cleared from the water when conditions change again.^{1, 2}

How do I know if I’m at risk of Swimmer’s Itch?

Public health bodies often monitor popular swimming locations for various outbreaks and will signpost any dangers to bathers. Always research where you are going before entering the water to ensure it is safe. If you suspect you or someone else has contracted swimmer’s itch after swimming in open water, notify your public health organisation as this will help them monitor and track any potential outbreaks or high-risk locations.¹

Can the water be treated to make it safe?

Some research into chemically or biologically controlling the population of parasites or their snail hosts has been performed. However, environmental concerns about the impact of these treatments on other aquatic life mean most public health bodies do not attempt to remove the parasite from the water. In most cases, the emergence of the parasites is temporary, and educating and warning the public is a better control strategy.²

Are all schistosomes harmless?

Several species of schistosome parasitise humans and may cause serious illness without diagnosis and treatment. However, these species are generally found in tropical and subtropical regions, particularly in developing countries with limited access to clean water. The symptoms of infection with these schistosomes (schistosomiasis) are different, affecting the gut, liver, and kidneys, and take longer to appear. Schistosomiasis is extremely rare in temperate climates.⁸

Summary

Swimmer’s itch is a common parasitic infection caused by swimming in waters contaminated with schistosome larvae. Whilst not dangerous, due to improved monitoring and awareness, more bathers are aware of the disease. It can be avoided by taking precautions such as checking for warning signs, avoiding high-risk areas for parasites, and rinsing and towelling off soon after leaving the water.