Equine Borreliosis (Lyme Disease) Study Guide

Overview and Clinical Importance

Equine borreliosis (Lyme disease) is a tick-borne infection caused by the spirochete bacterium Borrelia burgdorferi. While exposure to this organism is common in horses residing in endemic regions, clinical disease is considered rare. The distinction between exposure (seropositive) and clinical disease is crucial for the NAVLE. This condition represents a significant diagnostic challenge because clinical signs are vague and nonspecific, and positive serology only indicates exposure, not active disease.

The disease is primarily transmitted by Ixodes scapularis (blacklegged or deer tick) in the northeastern and midwestern United States and Ixodes pacificus on the West Coast. Understanding the documented clinical syndromes, appropriate diagnostic approach, and treatment protocols is essential for board examination success.

Etiology and Pathophysiology

Causative Agent

Borrelia burgdorferi is a Gram-negative spirochete bacterium characterized by its distinctive spiral or corkscrew shape. Key microbiological features include flagella enclosed between outer and inner membranes (endoflagella), a flexible cell wall measuring approximately 0.3 micrometers wide and 5-20 micrometers in length, and the ability to alter outer surface proteins (Osp) depending on host environment.