Equine External Parasites Study Guide

Overview and Clinical Importance

External parasites (ectoparasites) represent a significant cause of dermatologic disease in horses worldwide. These organisms live on or within the skin surface and cause pathology through direct tissue damage, hypersensitivity reactions, blood loss, and transmission of infectious agents. External parasites commonly affecting horses include lice, mites, ticks, and various biting flies. Understanding the identification, life cycles, clinical presentations, and treatment options for these parasites is essential for NAVLE success and clinical practice.

Recognition of ectoparasite infestations is critical because they cause significant welfare concerns including intense pruritus, self-trauma, secondary infections, anemia in severe cases, and transmission of vector-borne diseases such as equine piroplasmosis, Lyme disease, and equine infectious anemia.

Equine Lice (Pediculosis)

Pediculosis is infestation with lice and represents one of the most common causes of pruritus in horses. Two species of lice infest horses: Haematopinus asini (sucking louse) and Damalinia equi (also called Werneckiella equi or Bovicola equi, the biting/chewing louse). Lice are host-specific obligate parasites that complete their entire life cycle on the horse.