The pine processionary caterpillar (Thaumetopoea pityocampa), is the larval form of a night moth. The moth is perfectly innocuous but unfortunately its offspring are extremely dangerous for people and animals. Adult moths lay their eggs on the top of pine  or cedar trees which they infest during the summer. The caterpillars then hatch and spend the fall and winter on the tree, building silk nests that look like clusters of cotton candy on the ends of the tree branches. During the night the caterpillars leave the nest and feed on the leaves of the trees, returning at dawn, always in procession.
At this time of the year, December to April depending on the temperature, the caterpillars may descend from the tree in procession to bury themselves in the ground until they become the adult moth that then emerges from the earth in the summer.
It is at this moment, when they descend from the trees and walk on the earth that they present the greatest risk for animals and humans.
The caterpillars are covered in fine highly irritant hairs -  contact with these hairs can provoke rashes (urticaria) on the skin or mucous membranes. Children and animals are especially vulnerable because of their curiosity in observing these creatures with such strange behavior. This curiosity easily leads to a medical emergency due to the intense itching caused by the slightest contact or, in the case of a more prolonged contact there can be severe inflammation and ulceration of the affected area - this type of reaction is serious and requires urgent medical treatment.
Most of the dogs that we attend appear with the following symptoms, inflamed eyes or lips, with discharge which eventually leads to ulceration and non-healing wounds. Unfortunately we have also seen much more serious cases in which dogs have licked or even ingested a caterpillar. These cases may be fatal to patients in the short term due to anaphylactic shock or in the days following the contact due to the necrosis of the area, secondary to the  inflammation caused by the poison on the caterpillars' hair.
If you identify nests or the caterpillars themselves, keep a safe distance and contact the local 'Junta de Freguesia' to warn about the problem or ask a professional exterminator for advice. There are different ways of controlling this pest depending on the timing of the caterpillars' life cycle, which aim not only to limit its spread but especially to limit its risk to people and animals. These caterpillars are still poisonous after they are dead, burned or trampled.