The impact of herpesviruses on reproductive performance in horses

Abstract

The thesis addresses the largely-undefined influence of the equine herpesviruses (EHVs) and in particular EHV-1 and -4 on reproductive performance in horse-breeding systems. These pathogens cause significant losses to the international equine breeding industry primarily through infectious abortion and fatal neonatal disease. Keys to the success of EHVs are their ability to establish latent infections in asymptomatic horses which act via periodic reactivation as reservoirs of infection for susceptible horses. The primary aim was to define the effects of EHV-abortion on key outcomes determining reproductive performance in Thoroughbred broodmares. A predictive model developed to compare the relative influences and interactions of variables associated with EHV-1 and other causes of pregnancy attrition showed abortion, specifically EHV-1 abortion, did not influence either the subsequent establishment of pregnancy or foaling. A secondary aim examined the role of physiological stress in viral recrudescence. In-the-field data describing predisposing host and environmental risk factors are limited and this would provide more informed and locally-relevant prevention and interventional strategies to mitigate the impact and risks of EHV-1 and -4 infection. Formulating any appropriate preventative strategy for EHV-1 abortion must a priori consider the key epidemiological features. Retrospective data from two EHV-1 abortion epizootics affecting unvaccinated South African Thoroughbred broodmares identified differences in epidemiological features and interventions that influenced their markedly divergent outcomes. The data supported previously reported key risks associated with initiation of abortion epizootics, including large groups of unvaccinated, susceptible mares and inadequate post-arrival quarantine protocols. Other risks associated with viral recrudescence included social disruption subsequent to introducing late-gestation mares into similarly-susceptible resident populations. Quarantine itself may exacerbate risks and enhance spread of virus and supports isolation of newly-acquired pregnant mares until after foaling. Regular vaccination of all pregnant broodmares should be a feature of any preventative strategy for EHV-1 abortion. Interventional differences, most crucially qPCR testing, enabling rapid diagnosis and epidemiologically-targeted responses appeared to bias outcomes. Efforts aimed at reducing reactivation of latent virus and containment of infectious foci offer possibly the best chance of influencing outcomes.The caveat associated with vaccination during an epizootic was not supported and vaccination may be beneficial in reducing viral challenge and impact within an affected population. Establishing a definitive association between physiological stress and viral reactivation was problematic however a link was suggested via two studies. The first, in a population of pregnant broodmares consigned for sale established faecal glucocorticoid metabolite (FGM) measurements as a practical, non-invasive and reliable technique for monitoring medium to long-term stress. FGMs were monitored together with event-related viral reactivation via nasal swabbing to detect shedding of viral DNA via a duplex EHV-1/EHV-4 RT-qPCR assay. Results suggested social disruption of the mares was a key stressful event however stress-induced reactivation of latent EHV-1 infection was uncommon. This model was subsequently applied to a population of unvaccinated, young Thoroughbreds consigned for auction from their farms of origin. FGM monitoring showed that horses adapted to the novel environmental and management stimuli. Serology via type-specific EHV-1/EHV-4 ELISAs suggested a high prevalence of EHV-4 and only limited circulation of EHV-1 on South African farms. Nasal shedding of EHV-4 detected in several horses despite the presence of EHV-4 antibodies was most likely due to viral recrudescence and suggested that preventative isolation following their movement to a final destination may assist in limiting viral transmission. In conclusion, the findings of this thesis will enhance prevention strategies and presents a compelling argument for the value of prompt qPCR-based diagnostics and epidemiologically-informed intervention targeting reactivation in ameliorating EHV-abortion outbreaks.