Equine Anaplasmosis Study Guide

Overview and Clinical Importance

Equine Granulocytic Anaplasmosis (EGA) is a tick-borne infectious disease caused by the obligate intracellular bacterium Anaplasma phagocytophilum. This rickettsial organism infects granulocytes, primarily neutrophils, leading to a characteristic febrile illness. EGA is a seasonal, noncontagious disease transmitted by Ixodes species ticks and represents an important differential diagnosis for horses presenting with fever and hematologic abnormalities.

The disease was previously known as Equine Granulocytic Ehrlichiosis when the causative agent was classified as Ehrlichia equi. Taxonomic reclassification in 2001 unified this organism with the human granulocytic ehrlichiosis agent into a single species, A. phagocytophilum. This is clinically significant because equine and canine infections serve as sentinel indicators for human risk in endemic areas.

Etiology and Pathogenesis

Causative Agent

Anaplasma phagocytophilum is an obligate intracellular, Gram-negative, pleomorphic coccoid bacterium belonging to the order Rickettsiales and family Anaplasmataceae. The organism specifically targets granulocytes, particularly neutrophils and occasionally eosinophils, where it replicates within membrane-bound cytoplasmic vacuoles.