Immunity to equine herpesvirus 1 infection in foals during the first year of life.

This study investigates how foals (young horses) develop immunity to equine herpesvirus 1 (EHV-1) during their first year of life, revealing that while antibodies are not transferred in utero from mare to foal, there is subsequent colostrum-mediated immunity that decays over time and determines susceptibility to EHV-1 infection.

Methodology

• The researchers studied a group of 23 pregnant mares (female horses) on a Thoroughbred breeding farm.
• All mares had been identified to carry serum virus-neutralizing antibody titres to EHV-1.
• Tracking the presence and levels of these antibodies in their foals’ bloodstream was the primary tool in the research.

Findings

• Despite the mares carrying EHV-1 antibodies, this immunity was not transferred to the foals during pregnancy (in utero).
• All foals, however, received EHV-1 antibodies through their mothers’ first milk, called colostrum, after birth.
• On receiving this colostrum, the foals developed antibody titres – measures of the concentration of antibodies – similar to that of their mothers.
• In a sample of 10 foals, researchers observed that these serum virus-neutralizing antibody titres for EHV-1 did not remain constant. The immunity provided by these passively received antibodies decreased at a rate of 3.25 two-fold dilutions every 100 days, reaching zero by approximately 180 days (roughly six months) in average.

Implications

• The foals in the study were exposed to the EHV-1 virus twice during their first year.
• The researchers found that foals with a geometric mean titre of 1:25 experienced infection and their antibody titres rose in response, presumably due to the body’s immune response.
• However, foals with a higher geometric mean titre of 1:76 were able to resist infection, suggesting that a higher concentration of antibodies provided a level of immunity against EHV-1.
• This study provides insight into how foals develop immunity to EHV-1 in real-world settings and offers potential strategies for managing herd health in breeding contexts, potentially by timing exposure based on this decay of immunity.