Arch Virol (2001) 146: 633–652 Comparative pathogenesis of acute and latent infections of calves with bovine herpesvirus types 1 and 5 G. Meyer 1,* , M. Lemaire 1,** , C. Ros 2 , K. Belak 2 , A. Gabriel 3 , D. Cassart 4 , F. Coignoul 4 , S. Belak 2 , and E. Thiry 1 1 Department of Infectious and Parasitic Diseases, Virology, Faculty of Veterinary Medicine, University of Liège, Liège, Belgium 2 Department of Virology, The National Veterinary Institute, Biomedical Center, Uppsala, Sweden 3 Anatomy, Faculty of Veterinary Medicine, University of Liège, Liège, Belgium 4 General Pathology and Autopsy, Faculty of Veterinary Medicine, University of Liège, Liège, Belgium Accepted November 20, 2000 Summary. This study was conducted to compare the pathogenesis of acute and latent infections with closely related bovine herpesvirus types 1 (BHV-1) and 5 (BHV-5) in their natural host. Two groups of eight calves were inoculated in- tranasally with BHV-1 or BHV-5. Although BHV-1 and BHV-5 similarly replicate in the nasal mucosa after inoculation, both viruses differ markedly in their ability to cause disease, BHV-5 being responsible of some fatal encephalitis while BHV- 1 inducing rhinotracheitis. Virus isolation and immunohistochemistry demon- strated that BHV-5 replicates extensively in neurons of the central nervous sys- tem (CNS) and in respiratory cells of lungs, tracheal and nasal mucosae. Invasion of the CNS likely occurs through the trigeminal and olfactory pathways. Both groups developed cross-neutralising antibodies during this experiment suggest- ing partial clinical cross-protection afforded by the two infections. Three months after primary infection, experimental reactivation showed that BHV-5 was able to establish latency in the trigeminal ganglia but also the CNS of surviving calves. Moreover, laboratory findings suggested that BHV-5 could also persist in the tracheal and nasal mucosae. These results indicate that, after primary infection, BHV-1 and BHV-5 displayed similar biological features and consequently need to be considered together for the control of BHV-1 infection. * Present address: UMR 959 Physiopathologie Infectieuse et Parasitaire des Ruminants, Ecole Nationale V´ et´ erinaire de Toulouse, 23 chemin des Capelles 31076 Toulouse cedex 3, France. ** Present address: UMR 960 Microbiologie Mol´ eculaire, Ecole Nationale V´ et´ erinaire de Toulouse, 23 chemin des Capelles 31076 Toulouse cedex 3, France.