Transfer of naturally acquired specific passive immunity against Anaplasma phagocytophilum in foals in Southeastern Pennsylvania and Northern Maryland.
Authors: Rule Emily K, Boyle Ashley G, Stefanovski Darko, Anis Eman, Linton Jennifer, Lorello Olivia
Journal: Journal of veterinary internal medicine
Summary
# Anaplasma phagocytophilum Passive Immunity in Foals: What Farriers and Vets Need to Know Equine granulocytic anaplasmosis (EGA) is well-established in adult horses across endemic regions like Southeastern Pennsylvania and Northern Maryland, yet clinical disease in foals remains poorly documented—partly because the role of maternal antibody protection has been unclear. This prospective observational study tracked serum IgG concentrations specific to *Anaplasma phagocytophilum* in 22 healthy mare-foal pairs using immunofluorescent antibody testing, measuring antibody transfer at birth and monitoring foal titres at 3 and 6 months of age. The researchers found a moderate positive correlation between mare and newborn foal antibody titres (τ = 0.36–0.50, P = 0.009–0.01), with passive immunity successfully transferred to 80% of foals born to seropositive mares; however, the odds of detecting a positive foal titre dropped dramatically by 3 months of age (OR = 0.002), though three foals seroconverted between months 3 and 6, suggesting active infection. For practitioners, this means foals born to exposed mares inherit temporary protection that wanes by mid-summer, making EGA a genuine differential diagnosis in foals presenting with compatible clinical signs during this window—particularly in endemic areas where tick exposure remains a risk factor throughout the grazing season.
Read the full abstract on PubMed
Practical Takeaways
- •Foals born to seropositive mares in endemic regions are protected by maternal antibodies for approximately 3 months; clinical disease risk increases after this period
- •EGA should be included in differential diagnosis for foals aged 3+ months presenting with compatible clinical signs in Southeastern Pennsylvania and Northern Maryland
- •Foal testing for A. phagocytophilum antibodies before 3 months likely reflects maternal passive immunity rather than active infection
Key Findings
- •Positive correlation between newborn foal and mare antibody titers to A. phagocytophilum at pre-foaling (τ=0.38, P=0.009) and foaling timepoints (τ=0.36, P=0.01)
- •80% of foals born to seropositive mares acquired passive immunity to A. phagocytophilum
- •Passive antibodies declined significantly by 3 months of age (OR=0.002, P=0.02), making positive titers unlikely at this timepoint
- •Only 3 of 20 foals seroconverted between 3-6 months of age, indicating low active infection rate in this cohort