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EHV-1 specific nPCR performed on PBMC showed 14 thoroughbred mares and one saddlehorse mare to harbour EHV-1 genome after vaccination with Resequin" plus. By testing the antibody titres it could be shown that the mares reacted individually on the vaccination. In the saddlehorse mares no change in neutralising antibody titres could be detected. One mare showed a significant increase in the antibody titre tested by [FT. Eleven thoroughbred mares investigated during a period of six months showed no serological reaction on vaccination.Using nPCR EHV-1 genome could be found in PBMC at the day of foaling and three days p.p. and on the other hand seven days p.p. in a vaginal swab of one mare.These results indicate that abortion prognosis and the assessment of serological dates concerning he protection against EHV-11-4 infection is not possible. By detecting EHV-1 genome in PBMC after vaccination and after foaling and in a vaginal swab in healthy horses it could be shown that during this time reactivation of latent EHV-1 and virus shedding can take place.For the abortion diagnosis the impression smears proved to be suitable for rapid diagnosis of an EHV-1 induced abortion. In two cases there was a double infection with EHV-4 detected. Specifying the virus-DNA no hint to an involvement of the EHV1 strain RacH was given which is a component of live modified vaccines.The investigation of postmortem tissues from naturally infected horses indicated an acute EHV-1 respectively EHV-4 infection in two cases. The detection of EHV-4 genome in eight trigeminal ganglia using nPCR confirms studies describing the trigeminal ganglion as location of latency for EHV-4.In-situ hybridisation of tissues from experimentally infected mares could confirm the endotheliotropism of EHV-1. The detection of EHV-1 genome in endometrial glands revealed new aspects in the pathogenesis of EHV-1 induced abortion. Using immunhistochemistry no EHV-1 antigen could be detected in endometrial glands. In two cases of investigation of tissues from aborted fetuses out of experimentalinfected mares nPCR failed to detect EHV-1 genome. This fact confirms studies in which the abortion of virusnegative fetuses during an EHV-1 infection is described.In this thesis both results of other teams could be confirmed and new aspects of the infection with EHV-1 and -4 could be pointed out especially concerning vaccination, pregnancy and abortion. Some principles to improve stud management are layed down.These results should give reason to further investigations on prophylaxis, diagnostics and pathogenesis of EHV-11-4 infection.