Haematological changes and equine lymphocyte subpopulation kinetics during primary infection and attempted re‐infection of specific pathogen free foals with EHV‐1

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Abstract
Summary: Four specific pathogen‐ (Equid Herpesvirus type 1 [EHV‐1]) free foals aged 2 to 3 months were intranasally inoculated with 7 times 107 p.f.u. of EHV‐1. Virus isolation was attempted from peripheral blood and nasal mucus, and clinical signs of disease and haematological changes were monitored. Also, lymphocyte subpopulations were enumerated using flow cytometric analysis of lymphocytes identified using a polyclonal anti‐equine immunoglobulin reagent and monoclonal antibodies recognising the equine homologues of CD4, CD5 and CD8. The primary intranasal inoculation of virus resulted in an EHV‐1 infection demonstrated by viraemia, persistent nasal shedding of virus, seroconversion and clinical signs of disease. Associated with this infection there was a fall in lymphocyte and neutrophil counts, which subsequently rose to normal levels with alleviation of the clinical signs of disease. The decline in lymphocyte count resulted from a fall in T‐lymphocyte numbers only, and the subsequent increase was initially also a T‐lymphocyte‐restricted event with a later increase in the number of B lymphocytes. The fall in T lymphocyte count affected the CD4+ and CD8+ subpopulations equally; however, during the subsequent rise there was a proportionately greater increase in the CD8+ subpopulation. Following recovery from the primary infection, the foals were re‐challenged with a repeat inoculation two months after the primary infection. This second inoculation did not result in clinical disease, detectable viraemia, increased antibody titres or persistent nasal shedding of virus, suggesting an effective immune response or subclinical infection. The marked patterns of leucocyte and lymphocyte subsets of the primary infection were not evident; however, there was a definite decline in the numbers of CD8+ lymphocytes that lasted for several days. The implications of these findings are discussed.