Differential diagnosis and aspects on epidemiology and pathogenesis of equine herpesviruses

Herpesviruses constitute a family of DNA viruses found in virtually all animal species. In horse five herpesviruses have been identified namely Equine herpesvirus (EHV) type 1, 3 and 4 belonging to the subfamily alphaherpesvirinae and Equine herpesvirus type 2 and 5 belonging to the gammaherpesvirinae. EHV-1 and 4 are the major causes of abortion and respiratory disease respectively and both cause large economical losses. EHV-2 has been associated with respiratory disease, immunosuppression and is a predisposing factor for more severe secondary infections. EHV-5 is a new virus first isolated in Australia from horses suffering from upper respiratory disease. This thesis is dealing with the diagnosis, epidemiology and pathogenesis ofEHV-1,2,4 and 5.EHV-2 is a predisposing factor for bacterial infection particularly with Rhodococcus equi and this role was studied by vaccination of foals with an EHV-2 subunit (ISCOM) vaccine. Foals vaccinated twice developed neutralising antibodies and were protected against the pneumonia while the non-vaccinated control group developed respiratory disease including pneumonia with abscesses containing R.equi which in some cases was fatal. Natural infection induce neutralising EHV-2 antibody as detected by a blocking ELISA. This ELISA recorded a much higher number of seropositive horses in a stable with annual EHV-2/R.equi infections compared to a stable with no such problems. Thus, the blocking ELISA was found to be a reliable tool for detecting recent EHV-2 infections. Likewise antibodies induced by a newly acquired EHV-2 infection in foals were also detected by the blocking ELISA and all foals tested were found to have experienced an EHV-2 infection by the age of 4 to 6 months.The prevalence of EHV-2 and EHV-5 were studied by type-specific Polymerase chain reaction (PCR). Up to 68% of tested horses in Sweden were EHV-2 positive, i.e. had virus DNA in peripheral blood leukocytes, while 100% ofthe tested foals in Sweden and Hungary were positive before 2 to 8 months of age. No EHV-5 positive horses were found in Sweden while 4 out of 27 tested foals in Hungary had EHV-5 specific DNA by the age of 13 to 23 weeks showing that EHV-5 cause infections later in life.Convalescent sera from horses contain EHV-l/EHV-4 cross-reactive antibody which up to recently made differentiation virtually impossible. A new indirect ELISA that differentiate between the two virus types showed that the epidemiology between EHV-1 and EHV-4 differ considerably. While almost 100% of the horses were seropositive to EHV-4, the number of EHV-1 seropositive horses varied between 9 and 56 % in different stables. Most foals had maternally-derived antibodies to EHV-4 until 4 to 6 months of age, after which they all became infected with EHV-4 and seroconverted. Only 2 out of 48 foals had maternal antibodies to EHV-1 and very few became infected.