The stinging hairs are responsible for the rash, which is rather similar to contact dermatitis. You will first notice discomfort 8-12 hours after you have been in contact with the gypsy moth caterpillar.
The rash becomes more pronounced a day or two later. Most of the time, the rash will clear up on its own in a few days (or it could linger for up to two weeks).
Some people could experience delayed hypersensitivity if airborne hairs come into contact with clothes hanging outside on a line.
A young caterpillar may even land or fall on you! Hydrocortisone may help with the itching, but if you experience symptoms such as fever or widespread rash you should see a doctor.
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