Characterisation of immune responses in healthy foals when a multivalent vaccine protocol was initiated at age 90 or 180 days

SummaryReasons for performing study Protection from infectious disease requires antigen-specific immunity. In foals, most vaccine protocols are delayed until 6 months to avoid maternal antibody interference. Susceptibility to disease may exist prior to administration of vaccination at age 4–6 months. Objectives The aim of this investigation was to characterise immune activation among healthy foals in response to a multivalent vaccine protocol and compare immune responses when foals were vaccinated at age either 90 or 180 days. Study design Randomised block design. Methods Twelve healthy foals with colostral transfer were blocked for age and randomly assigned to vaccination at age 90 days (treatment) or at age 180 days (control). Vaccination protocols included a 3-dose series and booster vaccine administered at age 11 months. Results Immune response following vaccination at age 90 or 180 days was comparable for several measures of cellular immunity. Antigen specific CD4+ and CD8+ expression of interleukin-4, interferon-γ and granzyme B to eastern equine encephalomyelitis, western equine encephalomyelitis, West Nile virus, tetanus toxoid, equine influenza and equine herpesvirus-1/4 antigens were evident for both groups 30 days after initial vaccine and at age 344 days. Both groups showed a significant increase in antigen-specific immunoglobulin G expression following booster vaccine at age 11 months, thereby indicating memory immune responses. Conclusions The data presented in this report demonstrate that young foals are capable of immune activation following a 3-dose series with a multivalent vaccine, despite presence of maternal antibodies. Although immune activation does not automatically confer protection, several of the immune indicators measured showed comparable expression in foals vaccinated at 3 months relative to control foals vaccinated at age 6 months. In high-risk situations where immunity may be required earlier than following a conventional vaccine series, our data provide evidence that foals respond to immunisation initiated at 3 months in a comparable manner to foals initiated at an older age.