Equine Influenza Study Guide

Overview and Clinical Importance

Equine influenza (EI) is one of the most economically significant and highly contagious respiratory diseases affecting horses worldwide. Caused by Influenza A virus subtypes H3N8 (the H7N7 subtype is considered extinct since no isolation since late 1970s), this disease is characterized by rapid spread, high morbidity (approaching 100% in naive populations), and low mortality. EI remains endemic in most countries except New Zealand and Iceland, which are considered disease-free.

Understanding equine influenza is essential for the NAVLE because it represents a high-yield topic that integrates virology, pathophysiology, clinical medicine, and preventive medicine concepts. Questions frequently focus on clinical recognition, differentiation from other respiratory pathogens, appropriate diagnostic testing, and vaccination protocols.

Etiology

Viral Classification

Equine influenza virus belongs to the family Orthomyxoviridae, genus Alphainfluenzavirus. It is an enveloped, negative-sense, single-stranded RNA virus with a segmented genome (8 segments). Two key surface glycoproteins determine subtype classification: