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

Abstract

REASONS 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.