Cercarial dermatitis, more commonly known as swimmer's itch, is an itchy inflammatory reaction to non-human Schistosoma parasites that enter the skin. Generally, skin exposure to infected water in ponds, lakes or other water bodies causes the infection. This disease is also known by the names of clam digger's itch and sedge pool itch.

Prevalence

Cercarial dermatitis generally occurs in the warm, late summer weather. It is a common skin ailment. This is mainly because a large number of people have the regular habit of swimming in lakes and rivers. It is more common among children, who are obviously more avid swimmers.

Diagnosis and differential diagnosis

The standard diagnostic practice in cercarial dermatitis involves a detailed inspection of the monomorphic (presenting a single discrete unchanging form) maculopapular eruptions in parts of the human skin that comes in contact with the infecting water. This apart, it must also involve a record of the patient’s updated clinical history.

The differential diagnosis of cercarial dermatitis must exclude the following:

• Insect bites
• Human schistosomiasis (particularly in Africa, Asia and South America)
• Contact dermatitis from poison ivy
• Sea bather’s eruption

These syndromes can be differentiated from cercarial dermatitis if any of the following conditions occur:

• Rashes in the skin locations that are covered by swimming costumes
• Skin rashes after bathing in the salty sea water
• Presence of larval forms of crustaceans
• Presence of pieces of jelly fish tentacles

Clinical tests

Some of the clinical examinations that can help identify the disease and its causative factors are:

• Tests conducted with the infected patient’s serum. The serum is examined by means of various different serological procedures.
• The cercarial fluorescent antibody test (antigen Schistosoma mansoni) is also relevant.

Clinical features and pathology

Cercarial dermatitis displays the following clinical traits:

• Itchy monomorphic (occurring in a single form) inflammation triggered by an immunological response to the infiltration of cercariae into the skin
• Scattered eruptions, mainly in the bare areas of the skin
• Numerous reddened papules and papulopustules

Histological (microscopic tissue study) features include:

• An enlarged epithelium
• Subepidermal oedema
• Perivascular lymphohistiocytic penetration
• Presence of cercariae within the stratum corneum of the epidermal layer of the skin

Causative agents

The cercariae, which enters our skin and triggers cercarial dermatitis, is the larvae of trematodes belonging to species of Schistosoma and Trichobilharzia. The species T. ocellata is particularly widespread in central Europe.

The usual habitat of trematodes includes:

• Fresh water bodies such as lakes, ponds, streams, irrigation canals, rice paddies etc.
• All salt water bodies, besides Antarctica

The average cercariae life cycle has the following pattern:

Stage 1: Primary carriers such as water birds (ducks) pass parasite eggs in their feces.

Stage 2: The completely formed larvae (miracidiae) hatch from the eggs in the water and enter water snails, which act as transitional carriers.

Stage 3: Miracidiae reside in the digestive gland of the snail where they develop into sporocysts (a protective covering created by sporozoans in which sporozoites grow) in two months time.

Stage 4: When there is suitable light available in the upper 5 cm of the water and suitable temperature that should be greater than or equal to 17°C, the snails discharge innumerable cercariae.

Stage 5: These cercariae once again infect water birds. They cling to the carrier’s skin with the help of thermotactile stimulation and non-selective chemoattractants such as ceramide and cholesterol.

Stage 6: They enter their main target hosts - human beings - through the stratum corneum of the skin with the help of the production of proteolytic (that can breakdown proteins or peptides into less complex molecules) enzymes. They however die in the epidermis after a few hours.

Stage 7: The enzymes of the cercariae are immunogenic (creating immunity or an immune response) and cause allergic reaction only after two weeks after the initial exposure. Thereafter successive contact causes inflammation within a few hours. In certain patients, there has been no history of earlier skin rashes on exposure to cercariae. Often there have been only some skin lesions on certain body locations. The lesions progressed after the peak period of exposure.

Treatment

Therapy involves both prevention and symptomatic medications. Antihistamines and topical corticosteroids generally help check itching. After 1 to 2 weeks, the rashes reduce automatically. The best way to prevent the attack is to avoid long-term contact with shallow, stagnant, sluggish water in the hot summer weather.