A woman developed an allergic skin reaction after a caterpillar landed on her back. The insect left behind toxin-coated hairs that penetrated her skin, triggering an immune response and causing hives.
Many caterpillars, especially larger and fuzzy ones, have urticating hairs or spines connected to venom sacs. When these hairs touch the skin and break off, they release toxins that can irritate or sensitize the area. The reaction often includes redness, itching, swelling, and raised welts or rashes, sometimes forming a distinctive pattern where the hairs made contact.
Treatment is usually straightforward if there are no severe symptoms: gently remove the hairs, wash the area with soap and water, apply cold compresses, and use over-the-counter antihistamine or mild hydrocortisone creams.
However, if symptoms worsen — such as swelling of the face, lips, or difficulty breathing — it’s crucial to seek medical attention immediately. Though rare, severe reactions can become dangerous, making prompt care essential for safety.

