Low virulent respiratory viruses in standardbred trotters

Respiratory viral infections are known to affect equine health worldwide and are commonly associated with clinical signs such as fever, cough, and nasal discharge. While clinical respiratory signs will prevent horses from training and racing, it is also known that horses can be infected by viruses and yet not exhibit any clinical signs (i.e. subclinically infected). However, it is not clear whether these subclinical infections may have any effect on the horse´s performance and health. In this thesis well-known equine respiratory viruses such as equine influenza (EIV) and herpesvirus (EHV) type 1 and 4, as well as the less explored equine rhinitis A and B virus (ERAV and ERBV) and EHV type 2 and 5, were investigated on monthly basis in a longitudinal study over a year in actively racing Standardbred trotters. The presence and amount of the viruses and their antibodies were identified by diagnostic methods such as serology and PCR assays. Genetic information was obtained by sequencing. The health of the horses was also monitored and their performance determined by both objective and subjective methods. A high seroprevalence of ERAV and ERBV in the samples suggested that these viruses were widespread in the study cohort of well-managed horses. EHV-5 and EHV-2 were detected in about three-quarters and almost one third of the nasal secretions samples respectively. We could however not find any associations between these subclinical infections and poor performance. This work also includes a case report of equine multinodular pulmonary fibrosis (EMPF) in a horse, which was identified with co-infection by EHV-5 and the asinine herpesvirus type 5. Genetic analysis of a segment of the gB gene of EHV-5 in healthy horses with two samples taken one year apart and from the EMPF case study horse pre and post mortem allowed to classify EHV-5 strains into four different genotypes. While viral strains appeared stable over time, horses could be infected with multiple strains that varied in detection qualitatively and quantitatively over time. The single strain detected in the EMPF case was also found in the clinically normal horses. The work in this thesis provides knowledge of subclinical presence of respiratory viruses in elite Standardbred trotters and further genetic information of EHV-5 showed a range of interactions between EHV-5, the host and the environment over time