A 30-something year old male comes to the emergency department one evening complaining of an itchy rash for the last 3 days. He returned from a beach vacation in Curaçoa three days ago and since developed a pruritic, papular rash on his truck, legs, and extremities.
What is your differential diagnosis? What other historical questions would you ask?
Scroll down for the Case Conclusion.
Final diagnosis: Cercarial Dermatitis aka “Swimmer’s Itch":
Case Conclusion: On further questioning, the patient is asked whether he came into contact with any snails. He states, “Funny you should ask!" and pulls out his cell phone which is full of snail pictures. The patient admits to collecting on them on the beach because they were so beautiful.
Learning Points: Cercarial dermatitis is an allergic skin response which develops in reaction to the larvae (cercariae) of avian schistosomes from the genus Trichobilharzia. It manifests as a maculopapular skin eruption accompanied by intense itching. The differential diagnosis includes insect bites, contact dermatitis, and sea bather’s itch, a hypersensitivity reaction cause by the immature nematocysts of larval cnidarians such as thimble jellyfish or sea anemones . In contrast to swimmer’s itch, seabather’s itch is characterized by development of smaller red papules (1 to 1.5 mm in diameter) in skin under swimwear and between hairs.
Schistosome flukes are characterized by a two-host life cycle. Parasites mature and produce eggs in their definitive hosts. The eggs are then shed into water, hatch into larvae (miracidia) and actively search for snails which function as an intermediate host. The larva mature within the snails and are released as cercariae back into the water .
After leaving the snail, avian schistosome cercariae cling to the water and wait for their definitive host. They respond to shadow stimuli and swim from the water surface towards the prospective host. Occasionally, humans swim by instead of water fowl and the cercariae invade the wrong host by penetrating skin wrinkles and hair follicles submerged in the water. For the most part, avian schistosomes invading mammalian hosts are trapped in the skin by an allergic reaction that eliminates them. This inflammatory reaction is responsible for the development of cercarial dermatitis, which becomes more severe with repeated exposure. The treatment is the same as any allergic dermatitis - anti-histamines, sometimes steroids, and time.
Rarely, in massive infection with avian schistosomes, patients can develop fever, nausea and diarrhea. Some species of avian schistosomes have been shown to invade past the skin and to the CNS in vertebrate hosts, but only in experimental models . In contrast, mammals can serve as the definitive hosts of mammalian schistosomes, which cause severe disease including portal hypertension, cirrhosis or bladder cancer depending on the visceral organ they tend to settle in, leading to a significant disease burden in endemic areas .
Case Conclusion by Maia Dorsett (@maiadorsett)
- Kolářová, L., Horák, P., Skírnisson, K., Marečková, H., & Doenhoff, M. (2013). Cercarial dermatitis, a neglected allergic disease. Clinical reviews in allergy & immunology, 45(1), 63-74.
- Horák, P., Mikeš, L., Lichtenbergová, L., Skála, V., Soldánová, M., & Brant, S. V. (2015). Avian schistosomes and outbreaks of cercarial dermatitis. Clinical microbiology reviews, 28(1), 165-190
- Lucie, L., & Petr, H. (2012). Pathogenicity of Trichobilharzia spp. for Vertebrates. Journal of parasitology research, 2012.
- King, C. H. (2009). Toward the elimination of schistosomiasis. New England Journal of Medicine, 360(2), 106-109.