EHV infection: the immunologist’s viewpoint

Equine herpesvirus-1 (EHV-1) is highly prevalent in most equine populations worldwide and has a high medical impact through respiratory disease, abortion, and myeloencephalopathy. A number of EHV-1 vaccines, including inactivated and modifiedlive virus vaccines, are available. Through the use of these vaccines and improved management practices, the incidence of abortion storms has decreased. However, outbreaks of EHV-1 continue to occur in the face of widespread vaccination. Currently available EHV-1 vaccines induce good antibody responses but only weak T cell immunity. Different studies also documented differences in antibody isotype responses after vaccination compared to EHV-1 infection. None of the available vaccines are completely effective at eliminating nasopharyngeal virus shedding or cellassociated viraemia. Improvement of vaccines, however, requires further elucidation of the protective immune response to the virus. EHV-1 is an immunosuppressive virus and interferes with host immunity at various levels. Recent research has shown that EHV-1 down-regulates innate immune responses by suppressing the production of chemokines, cytokines and type I interferon (IFN). EHV-1 also established mechanisms to down-regulate adaptive T cell responses and IFN-γ production in both T-helper and cytotoxic T cell subsets. The ability of EHV-1 to interfere with host immunity is likely essential for its potential to circumvent immune recognition and viral clearance. This results in the establishment of latency for the life-span of the host and sometimes causes severe disease outcomes after infection or re-activation of the virus. New vaccination strategies must be adjusted to address the challenge that this virus poses to the host immune system.