Transfer of naturally acquired specific passive immunity against Anaplasma phagocytophilum in foals in Southeastern Pennsylvania and Northern Maryland

BACKGROUND: Equine granulocytic anaplasmosis (EGA) is a common disease in adult horses, but clinical disease in foals is rarely reported. The relationship between equine maternal and neonatal antibodies to Anaplasma phagocytophilum is unclear. HYPOTHESIS/OBJECTIVES: That mares in an endemic region would be seropositive for A. phagocytophilum and that mare and foal serum IgG concentrations for A. phagocytophilum would correlate. Additionally, we hypothesized that foal IgG concentrations for A. phagocytophilum acquired by passive immunity would decline by 6 months of age. ANIMALS: Twenty‐two healthy mare‐foal pairs. METHODS: This prospective observational study investigated serum IgG concentrations specific for A. phagocytophilum in mares and foals using an immunofluorescent antibody test (IFA). The association between foal titer (as a binary variable) and age in months was assessed using a mixed‐effects logistic regression. RESULTS: A positive correlation between newborn foal antibody titers and mare titers was identified at both the pre‐foaling (τ(a) = 0.38, τ(b) = 0.50, P = .009) and foaling timepoints (τ(a) = 0.36, τ(b) = 0.47, P = .01). In A. phagocytophilum seropositive neonates, it was unlikely that a positive titer would be detected by 3 months of age (OR = 0.002, P = .02, 95% CI: 0.00001‐0.38). Three out of 20 foals seroconverted between 3 and 6 months of age. CONCLUSIONS AND CLINICAL IMPORTANCE: Transfer of specific passive immunity to A. phagocytophilum occurred in 80% of foals born to seropositive mares and declined by 3 months of age. A. phagocytophilum infection should be considered in foals displaying clinical signs consistent with EGA.