From a Google search:
The Puss Moth Caterpillar, aka the "Asp"
This is one of the most toxic caterpillars in North America. This caterpillar may infest shade trees and shrubbery around homes, schools, and in parks. They are of little importance as enemies of shade trees, but they can cause a severe sting. When a puss moth caterpillar rubs or is pressed against skin, venomous hairs stick into the skin, causing a severe burning sensation and rash.
Puss moth caterpillars can pose a genuine health hazard. Intense, throbbing pain develops immediately or within five minutes of contact with the caterpillar. Stings on the arm may also result in pain in the axillary (armpit) region. Erythematous (blood-colored) spots may appear at the site of the sting. Other symptoms can include headaches, nausea, vomiting, intense abdominal distress, lymphadenopathy, lymphadenitis, and sometimes shock or respiratory stress.