Common and atypical presentations of Anaplasma phagocytophilum infection in equids with emphasis on neurologic and muscle disease.
Authors: Aleman Monica, Vedavally Ujwala, Pusterla Nicola, Wensley Fiona, Berryhill Emily, Madigan John E
Journal: Journal of veterinary internal medicine
Summary
# Anaplasma phagocytophilum infection in horses: neurologic and muscle disease presentations Anaplasma phagocytophilum (the causative agent of granulocytic anaplasmosis) is well-recognised as a cause of fever and thrombocytopenia in equids, yet presentations involving neurologic or muscle disease remain poorly characterised in the published literature. This retrospective case series examined 38 horses and 1 donkey diagnosed with EGA between 2000 and 2022, with particular focus on atypical clinical manifestations beyond the typical febrile syndrome. Neurologic disease occurred in 41% of cases, predominantly presenting as diffuse symmetrical proprioceptive ataxia, whilst brain involvement (obtundation and cranial nerve deficits) was less frequent; muscle disease was uncommon overall but notably severe in Quarter Horses with the myosin heavy chain myopathy (MYHM) variant, who exhibited markedly elevated muscle enzyme activities. The cohort was predominantly older (median 14 years) and male-biased (77%), with importantly, all cases responding favourably to prompt tetracycline therapy, with no deaths or long-term sequelae reported. For practitioners, this work broadens recognition of EGA's neuromotor manifestations as differential diagnoses in ataxic presentations, and crucially, suggests that Quarter Horse breeds presenting with concurrent EGA and muscle disease warrant MYHM genetic testing to guide prognosis and inform breeding decisions.
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Practical Takeaways
- •Consider EGA as a differential diagnosis in horses ≥4 years old presenting with unexplained proprioceptive ataxia or neurologic signs; tetracycline treatment has excellent prognosis if initiated promptly.
- •Quarter Horses with muscle disease and EGA should be genotyped for MYHM, as this variant is associated with severe myopathy requiring careful monitoring.
- •Early recognition and treatment of EGA prevents progression to severe complications like DIC and cardiomyopathy—contact your veterinarian immediately if acute neurologic or muscle signs develop.
Key Findings
- •Neurologic disease was the most common atypical presentation (41%) of EGA, primarily manifesting as diffuse symmetrical proprioceptive ataxia in horses with median age of 14 years and 77% male overrepresentation.
- •Muscle enzyme activities were significantly elevated in horses with muscle disease, with Quarter Horse breeds carrying the MYHM variant showing severe myopathy.
- •Prompt tetracycline treatment resulted in favorable outcomes with no reported deaths or long-term sequelae across all 39 equids studied.
- •Brain disease, cavitary effusion, cardiomyopathy, and DIC were uncommon presentations of EGA in this cohort.