Cercarial dermatitis, also known as schistosome dermatitis, swimmer itch, swamp itch, or silt itch, is an acute inflammatory reaction characterized by local pruritic papules and caused by invasion of schistosomula.
Schistosomula in birds and mammals, as well as cercariae of schistosoma spindale, schistosoma bovis, and schistosoma matthei, can cause cercarial dermatitis.
The animal hosts most closely related to human infection are cattle and domestic ducks. The infected animals such as cattle and ducks discharge feces containing eggs into the paddy fields. The eggs are hatched into the miracidia, entering the body of lymnaea. In the snail bodies, miracidia develop into cercariae through mother sporocysts and daughter sporocysts. Cercariae are swimming in the water, enter their body if meeting cattle or ducks, and develop into imagoes and lay eggs in the portal vein system and liver. If meeting humans, cercariae invade the skin, causing dermatitis. However, cercariae cannot grow and develop in the human body, and die in about 2 hours, so they cannot cause visceral lesions.
Signs and Symptoms
Cercarial dermatitis is an allergic inflammation, and allergens are cercarial secretions and posthumous lysate. Slow onset and mild inflammation in the initial infection and rapid onset and severe inflammation in repeated infections are present. Due to the different number of cercariae and individual sensitivity, the clinical manifestations are different.
The affected areas are mostly the extremities, calves, and forearms that can contact contaminated water. Because the cuticles of the palms are thick and the feet often sink into the mud, the palms and feet are less involved. Pruritus occurs 15 - 30 minutes after exposure to the contaminated water, and can also occur within 5 minutes. Subsequently, pinhead sized, red papules occur, developing into pea sized, edematous, red papules or papulovesicles in few hours, surrounded by red halos. Bite marks or petechiae are visible at the top of the papules, and papules are scattered or densely clustered. Skin lesions usually reach the peak in 3 - 4 days and gradually subside in 1 - 2 weeks. In repeated infections, vesicles, wheals, severe pruritus, erosive exudate after scratches, secondary infections, pyodermia, and lymphadenopathy may occur, a small number of patients may have fever, and the illness can persist for 1 - 2 months.
Figure 1 cercarial dermatitis on the hands
Figure 2 cercarial dermatitis on the calves
There is histologically acute inflammation. Early dermal edema with infiltration of lymphocytes and neutrophils and late eosinophil infiltration can be seen. In natural infections, cercariae can be found within 24 hours after infection, but the optimum time to find cercariae is within 2 - 3 hours after infection, because cercariae may be quickly destroyed by secondary inflammation.
On the basis of exposure to paddy fields and clinical presentations mainly on the calves, the disease can be diagnosed.
Topical calamine lotion, 5% camphor spirit, or crotamiton ointment can be used. Anti-inflammatory treatment is needed when there is an infection. Granulomas in the late stage can be treated with electrocautery, carbon dioxide laser, or surgical resection.