Human and animal health
The Oak Processionary Moth caterpillar's defence mechanism is the production of a toxin (thaumetioieun) that is held in the hairs on it's body. The small brown hairs that cover their bodies can cause a number of problems, most commonly severe skin rashes but also respiratory problems and conjunctivitis. Health problems can occur even where there is no direct contact with the caterpillars as their hairs break off easily and can be easily transported in the wind. Spent nests should be treated with extreme care as they are often full with hairs from shed skins.
Animals are subject to the same risks as humans but consequences are often exacerbated as hairs can easily become trapped in their fur. Horses are particularly sensitive too as they may try to sniff or eat the caterpillars or their nests. Signs of this include excess salivation, swollen tongues, conjunctivitis, gagging, respiratory distress and mouth inflammation.