Fly masks protect horses from fly-induced eye irritation and disease, with appropriate design and fit being essential for the welfare benefit to outweigh wearing discomfort.
Fly masks provide genuine welfare benefit by reducing the constant irritation of flies feeding on facial secretions - horses without protection in high fly pressure environments show persistent head-tossing, eye rubbing, and stress behaviors. The welfare trade-off is between fly protection and wearing comfort - properly fitted masks that are checked and cleaned daily provide net welfare benefit, while ill-fitting masks that cause rubs or trap flies inside are counterproductive. Daily inspection and prompt removal of problematic masks ensures the protection benefit is realized without adding mask-related suffering.