The effect of EHV-1 infection upon circulating leucocyte populations in the natural equine host.

It has been suggested that EHV-1 infection may perturb immune responsiveness in the natural equine host. The mechanism underlying this phenomenon is not clear, but disturbances of circulating leucocyte populations could contribute. In order to objectively assess the nature of the haematological changes provoked by EHV-1 infection, two groups of conventionally-maintained Welsh mountain ponies were challenge-infected intra-nasally with the Ab4 isolate of EHV-1. These groups were controlled by similarly-sized groups of non-infected ponies. All data generated was subjected to rigorous statistical analysis. Whole leucocyte count, neutrophil count, lymphocyte count, pan T cell count (RVC1 + cells-putative CD5 homologue), T cell subset count (RVC3 + cell-putative CD8 homologue), RVC2 + cells (putatively class II MHC+) and B cell count were recorded in experimental and control subjects at frequent intervals post-infection via flow cytometry. The principal abnormalities post-infection were T cell lymphopaenia, neutropaenia and the appearance of blastic cells of undetermined lineage. This study underlined the variability of EHV-1 infection in the natural, outbred equine host.