Swimmer’s itch (cercarial dermatitis)

Are You Confident of the Diagnosis?

What you should be alert for in the history

Patients may present with tingling, burning, or itching of the skin occurring hours to days after water exposure; itching may be mild initially with progression of worsening intensity. Patients may have participated in shallow, fresh water activities during warm months. Friends and relatives participating in water activities may be symptomatic. Additionally, patients may have experienced similar episodes in the past. It is important to note that the initial reaction may be mild and go unnoticed by the patient. Subsequent reactions develop more rapidly and may be more severe.

Characteristic findings on physical examination

Patients may present in the first hours post-exposure with erythematous macules, which can progress to papules, with or without urticaria. Days to weeks post-exposure, the patient may exhibit purpuric lesions, vesicles, or pustules. The distribution of the rash only manifests where the patient has had direct contact with infested water.

The rash may appear diffusely (patient fully submerged) or may be localized to specific parts of body (patient wading legs in water). Of note, the rash is observed on areas not covered or protected by clothing (versus seabather’s eruption). Patient excoriation may lead to secondary skin infections and/or scarring. Systemic symptoms may include fever and local lymphadenopathy.

Expected results of diagnostic studies

Diagnosis of swimmer’s itch is based on clinical suspicion. Serologic tests are not commonly done, but the leukocyte differential may show peripheral eosinophilia. Histopathology may show spongiosis, dermal edema, and mixed inflammatory infiltrate. Rarely, cercariae can be observed.

Diagnosis confirmation

Several conditions should be considered in the differential diagnosis of swimmer’s itch. Seabather’s eruption usually occurs after saltwater exposure and tends to affect areas of skin covered by clothing or hair (opposite to swimmer’s itch). Hot tub folliculitis generally occurs following recent exposure to hot tub, whirlpool, swimming pool, or waterslide water. The patient will have diffusely distributed follicular lesions.

Insect bites (mosquitoes, bed bugs, chiggers, scabies) present with patient history consistent with arthropod exposure. For contact dermatitis (poison ivy), patient reports history consistent with poison ivy or other similar exposure. Lesions are limited to areas which have been in contact with foliage. Skin manifestations of viral causes such as viral exanthems or more defined entities such as varicella/zoster, rubeola, rubella, herpes simplex, should be suspected if a patient is immunocompromised or has not received routine immunizations.

Who is at Risk for Developing this Disease?

Frequency and duration of open-water activity increases the incidence of swimmer’s itch. One study reports 6.8 episodes per 100 water exposure days. Swimmer’s itch occurs worldwide and is commonly observed in bodies of water with on-shore winds. Cases primarily occur in shallow freshwater; however, cases associated with saltwater exposure have also been reported.

Time of year also affects risk. Warm months may increase intermediate host snail populations leading to increased transmission. Presence of hosts is also an important factor. Resident or migratory birds (definitive hosts), snails (intermediate host), and humans (incidental host) must all be present in order for a person to contract swimmer’s itch. Children may be at a higher risk due to more exposure to shallow water and they may be less likely to towel dry.

What is the Cause of the Disease?

Etiology

Swimmer’s itch is most often caused by non-human schistosomes (over 20 species), particularly avian schistosomes. The Trichobilharzia genus is commonly implicated. Cases are less often due to human schistosomes (e.g., Schistosoma haematobium, Schistosoma japonicum, Schistosoma mansoni, Schistosoma mekongi, Schistosoma intercalatum). Human schistosomes may additionally cause systemic schistosomiasis.

Pathophysiology

Swimmer’s itch occurs when free-swimming cercariae penetrate the skin of humans (incidental host) and cause an allergic skin reaction (types I and IV hypersensitivity). The life cycle begins as cercariae infect birds (definitive host), maturing into adult worms within blood vessels. Adult worms then produce eggs that are passed in avian feces.

The eggs hatch and liberate ciliated miracidia into the water, which infect snails (intermediate host). Miracidia mature within snails and produce free-swimming cercariae, which either reinfect birds or infect humans. Cercarie die upon penetration into human skin, thus halting the schistosome life cycle.

Systemic Implications and Complications

Possible complications include secondary bacterial skin infections. Patients are at an increased risk when lesions are excoriated or manipulated. Standard treatment is topical or oral antibiotics.

Treatment Options

Treatment options are summarized in the Table I.