Earlier this week, we gave you a hypothetical patient with the following case:
This week's hypothetical patient has an itchy rash on her ankles and the lower parts of both of her legs. Last night, she has been swimming in a pond on her family's property. On the way back home after swimming, she first felt the itch and noticed the rash, but it went away after an hour or so. This morning, both the itch and the rash had returned and the rash looked (and felt) much worse than it had last night. After speaking with her physician, she went home with a corticosteroid cream and planned to avoid swimming in the pond again.
Today, we reveal that our hypothetical patient was diagnosed as having:
Cercarial Dermatitis ("Swimmer's Itch")
Cercarial dermatitis, a.k.a. "swimmer's itch", is caused by the cercarial stages of parasitic worms burrowing into accidental human hosts and dying within the skin rather than being able to effectively infect human hosts. As these little worms die, they cause the characteristic itchy rash that we saw in our hypothetical patient. This rash can last up to several days, so it is important to help patients avoid scratching to prevent secondary skin infections. The worms that cause cercarial dermatitis are searching for water fowl or other aquatic vertebrate hosts when they accidentally infect humans. The typical life cycle for these parasites begins with eggs being deposited into the water along with feces of a definitive host, such as a duck. The eggs hatch into miracidia and swim through the water until they can infect a suitable intermediate host, such as a snail. After growing and changing inside of the snail, the worms exit their snail host as cercariae, which then seek out definitive hosts, like our duck, to complete the life cycle. It is in this last stage that the cercariae might accidentally make their way into a human host swimming or wading in contaminated water.
While there is no specific diagnostic test for cercarial dermatitis, knowing a little about a patient's history can help distinguish this from other rashes. For example, knowing that our patient's rash began shortly after swimming in a pond, disappeared, and reappeared later, put our patient's actions and symptoms into context that would allow for a reasonable diagnosis of cercarial dermatitis. This condition typically clears itself over time, but again it is important to help a patient avoid scratching to prevent secondary skin infections. Anti-itch lotions, topical corticosteroid creams, and baths with colloidal oatmeal, baking soda, or Epsom salts can be helpful to that end.
Patients can avoid swimmer's itch by swimming in well-maintained, chlorinated pools. Patients who have had swimmer's itch before may experience worse reactions if they get it a second time. A 2015 review article of cercarial dermatitis calls it an emerging disease and asserts that it is responsible for the majority of dermatitis outbreaks worldwide (you can read the article here). While it is possible to treat waterfowl with medication to reduce parasite populations, this requires capturing, treating, and releasing birds individually, which isn't always practical. Control efforts have also targeted killing the snail intermediate hosts, but those efforts often have detrimental effects on other animals. The best way to prevent swimmer's itch is for people to know where cases of swimmer's itch have been reported to originate and to avoid those areas for the season.
While there is no specific diagnostic test for cercarial dermatitis, knowing a little about a patient's history can help distinguish this from other rashes. For example, knowing that our patient's rash began shortly after swimming in a pond, disappeared, and reappeared later, put our patient's actions and symptoms into context that would allow for a reasonable diagnosis of cercarial dermatitis. This condition typically clears itself over time, but again it is important to help a patient avoid scratching to prevent secondary skin infections. Anti-itch lotions, topical corticosteroid creams, and baths with colloidal oatmeal, baking soda, or Epsom salts can be helpful to that end.
Patients can avoid swimmer's itch by swimming in well-maintained, chlorinated pools. Patients who have had swimmer's itch before may experience worse reactions if they get it a second time. A 2015 review article of cercarial dermatitis calls it an emerging disease and asserts that it is responsible for the majority of dermatitis outbreaks worldwide (you can read the article here). While it is possible to treat waterfowl with medication to reduce parasite populations, this requires capturing, treating, and releasing birds individually, which isn't always practical. Control efforts have also targeted killing the snail intermediate hosts, but those efforts often have detrimental effects on other animals. The best way to prevent swimmer's itch is for people to know where cases of swimmer's itch have been reported to originate and to avoid those areas for the season.
Thanks for joining us for this week's Mystery Case and we hope to see you in two weeks!
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