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The present work was performed to study clinical relevance, particularly for conjunctivitis, keratitis and keratoconjunctivitis, tissue tropism and latency of EHV-2 in the horse and mouse, respectively. Blood samples and swabs were aken from 94 naturally infected horses from Germany and tested by serological, virological and microbiological methods for EHV-2. From the standard diagnostic work of the institute of Virology, 44 cases came from horses with different clinical preliminary reports and 50 horses were investigated at the Horse Clinic in Hannover. Additionally, postmortem tissues from 76 naturally infected animals were examined. Furthermore, double infections with the other equine herpesviruses, especially EHV-1 and EHV-4, were considered. For experimental infections the mouse was used as an animal model.One third of the swabs from horses with eye problems, as well as samples from horses with different medical histories, were shown to be positive for EHV-2-DNA by nPCR. EHV-2-DNA or antibodies were found in blood, swabs and cornea samples of healthy horses and horses showing clinical signs of keratitis and/or conjunctivitis examined in the Horse Clinic in Hannover. In this animal group, horses with keratoconjunctivitis showed a statistically significant higher detection rate of EHV-2 in eye swabs than horses without clinical signs. There was no association between the detection of EHV-2 in PBMC by nPCR or serological data and clinical findings. By cocultivation, PBMC from 21 horses were found to harbour EHV-2. None of the swabs yielded virus, neither by cocultivation nor by virus isolation.Fragments amplified from PBMC, swabs or virus coculitivated in the EHV-2 specific nPCR showed a striking size polymorphism and restriction enzyme patterns of isolated strains varied. In vitro there were differences in growth kinetics and plaque
morphology as well. However between these characteristics in culture and clinical findings in the horse no correlation could be defined.EHV-1 and EHV-4 were detected by nPCR in a few samples as well, but always in presence of EHV-2.In summary, nPCR was shown to be a rapid and safe method for diagnosis of EHV-2 in cases of keratoconjunctivitis. The PCR results facilitate the indication for treatment with antiviral drugs.Postmortem tissues investigations confirmed the results of other workers. EHV-2DNA was detectable in neural, lymphoid and respiratory tissues. Additionally, eye samples from healthy horses were positive for EHV-2 by nPCR.Investigations in the mouse model gave esults different from available studies. Infection with EHV-2 produced no specific clinical signs and neither infectious nor latent virus was detectable. After intranasal infection the lung and nose, after intraperitoneal infection the spleen, were all positive for EHV-2-DNA for approximately 30 days. Therefore these issues should be favoured as a place of latency. EHV-2-DNA was found in PBMC only after intraperinoneal infection when also a follicular hyperplasia of the spleen could be recognised.Particularly for the question of latency mechanism and factors triggering reactivation from latency, more work must be done. Also, the strain specific differences in pathogenicity and tissue tropism need further investigation.