Immune responses to common respiratory pathogens: problems and perspectives in equine immunology

Like other mammals, the horse has evolved a complex combination of non-specific and specific defence systems to combat a wide variety of parasites and infectious agents. Equine ontogeny reveals that the essential features of a mature immune system are established by about IS weeks while the foetus is maintained within its sterile environment (Penyman, McGuire and Torbeck 1980). The maturity of the equine foetal immune response has been shown by the production of antibody to antigenic challenge in uter-o with live attenuated Venezuelan equine encephalomyelitis vaccine or xenogeneic erythrocytes (Morgan, Bryans and Mock 1975; Mock, Morgan, Jochin and Lock 1978). Recent studies have shown that high levels of antibody and specific cellular immune responses can be generated to protein antigens by immunisation of the equine foetus (Hannant et ul 1991b). The potential of mature immune responses in the equine foetus could explain the occasional finding of pathogen-specific antibody in foetal circulation or in foetuses aborted for non-infectious reasons (Whitwell, Bowen and Hannant 1991). There is no opportunity for transfer of maternal immunoglobulins to the equine foetus pre-partum and the foal i s delivered immunologically naive at term (Mock er ul 1978) into a contaminated environment. Immediate protection from infectious organisms i s usually achieved by uptake of maternally-derived immunoglobulins in the colostrum. This source of passive immunity combined with physical barriers to infection and other non-specific (innate) immunological responses i s generally sufficient to allow the foal to survive and develop specific primary immunological responses during the first few weeks of life. However, passively acquired and actively achieved immunological mechanisms of the young foal are not always sufficient to prevent infectious agents becoming established in an otherwise healthy individual. The success of infectious agents in causing disease i s related to the immunological status of the host and to the capacity of the invading organism to avoid immunological attack. The purpose of this article is to summarise current knowledge about the equine immune response to infectious agents and highlight aspects that relate to protection and recovery from infection. Of necessity, much of the immunological work on equine infectious diseases has been focussed on the common respiratory pathogens. These ubiquitous agents within the horse population do not cause high mortality, but result in considerable economic loss to the performance and breeding industries and affect the recreational pursuits of horse and pony owners. Although there i s considerable expertise being applied to molecular biology, genetics and antigen analyses of some important agents of equine infectious diseases such as Equid Herpesvirus, arteritis virus, influenza virus. African horse sickness, infectious anaemia and Str-eptoc,oc,cx.s c q u i (Powell 1988), until recently, there has not been a similar input into fundamental immunobiology of the horse. Fortunately, recent advances in molecular and cellular biology have provided the tools for researchers to ask specific questions about the function of equine immune responses and target immunoprophylactic procedures to favour stimulation of responses that are protective.