Gammaherpesviruses and their potential role in equine disease

Viral infectious upper respiratory tract disease in horses is an important problem. Recognized causes of viral respiratory disease in the horse include equine influenza viruses, rhinitis viruses, adenoviruses, arteritis virus, and herpesviruses, although the pathogenic significance of each of these agents differs substantially. Influenza and equid herpesvirus (EHV)-1 and 4, which are alphaherpesviruses, are well-known causes of epidemic outbreaks of respiratory disease in susceptible horses; viruses such as adenoviruses and gammaherpesviruses are more ubiquitous in equine populations, and their association with disease is less well defined. Screening of the nasal secretions of apparently healthy young racehorses for viruses associated with respiratory disease resulted primarily in the detection of the gammaherpesviruses EHV-2 and -5. Similarly, adenovirus was detected in healthy foals at a frequency comparable to that in foals with respiratory disease.;Studies documented in this thesis were undertaken to better characterize gammaherpesvirus infections in equid populations. An isolate of EHV-5 was reported for the first time in the United States, and a gammaherpesvirus was isolated from the nasal secretions (NS) of a mule. Further studies undertaken on populations of mares and foals revealed that EHV-2 and -5 infections were ubiquitous, foals were infected early in life, and simple detection of a gammaherpesvirus typically was not associated with any overt disease. Phylogenetic analysis on EHV-2 detected in serial NS collected from a cohort of foals demonstrated considerable genetic heterogeneity.;The gammaherpesvirus Epstein Barr virus (EBV) has been shown to cause febrile illness in humans associated with viral immunopathologic effects. Due to the similarity between EBV and EHV-2, the ontogeny of the immune response of foals to natural infection with EHV-2 was examined in conjunction with clinical evaluation of the foals. The occurrence of fever in foals was significantly associated with increased numbers of antigen-specific peripheral blood mononuclear cells, suggesting that cellular immune responses to EHV-2 may lead to an immunologically mediated disease of foals analogous to infectious mononucleosis caused by EBV infection in humans.;In summary, the studies described in this thesis have better characterized virus infections in equids in California, particularly the potential role of gammaherpesvirus infections in disease.