CONICET | Buscador de Institutos y Recursos Humanos

Bovine herpesvirus type 1 (BoHV-1) is an alpha-herpesvirus responsible for respiratory and genital disease in cattle. Encephalitis by BoHV-1 is only occasionally reported. This condition is mainly attributed to the closely related alpha-herpesvirus, BoHV-5. However, it has been recently demonstrated that the participation of BoHV-1 in neurological disease is not a rare event, as previously thought. In this study, the distribution and the pathological alterations caused by two BoHV-1 strains in the nervous system of experimentally infected calves during primary (acute) infection are described. For this study, 6 crossbred, 6 months to 1 year old calves were used. Calves were intranasally inoculated with BoHV-1 Los Angeles (LA) strain or Cooper strain. Animals were grouped as follows: Group 1 (4 calves). Calves in this group were inoculated with 106.3 tissue culture infective doses (TCID50) of LA (2 calves, 25ml inoculum) or Cooper (2 calves, 10 ml inoculum) strains; Group 2 consisted of two mock-infected calves, which were inoculated with culture medium as control. Euthanasia was performed at 6 (2 Cooper strain-inoculated calves and 1uninfected calf) and 7 days post-inoculation (dpi) (2 LA-inoculated calves and 1uninfected calf). Nasal and ocular secretions were collected on 0, 4, 5 and 6 dpi. At necropsy, 1-2 cm-thick sections of the brain were collected aseptically and individually for virus isolation, PCR and histopathology. The following areas were evaluated: anterior cerebral cortex, including the olfactory cortex (sample 1), frontal cortex (samples 2 and 3) and dorso-lateral cortex (sample 4); posterior cerebral cortex, including the marginal groove area (sample 5) and the ectomarginal groove area (sample 6); cerebellum (sample 7); cervical medulla (sample 8); medulla oblongata (sample 9), pons (sample 10) and diencephalon (sample 11). Trigeminal ganglia (TG) were also collected. During primary acute infection with LA strain, the presence of nasal secretions was observed in one calf at 4 dpi and in both inoculated calves at 6 dpi. Ocular secretions were intermittently detected. For calves inoculated with Cooper strain, nasal secretions were detected at 1 and 3-5 dpi. Ocular secretions were also evident during the period evaluated. Differences in rectal temperatures were recorded at 4 to 6 dpi for animals inoculated with LA strain, when compared with mock-infected calves (P 0.05). Neurological signs were not observed in animals from any group. However, BoHV-1 strains were isolated from all neural tissue sections (1-11), except from TG of Cooper-infected calves. In some samples from LA-infected animals, the amount of infectious virus was too low to determine the viral titer. In comparison with the findings for Cooper-infected calves, virus titers in neural tissues for LA-inoculated animals were 3 to 4 log lower. As expected, virus was not isolated from control animals. In calves inoculated with Cooper strain, at 6 dpi, viral DNA was detected in frontal and posterior cortex, cervical medulla, medulla oblongata and TG. Similar distribution of virus DNA was observed for LA-infected calves. LA or Cooper DNA was not detected in pons or diencephalon and it was not detected in samples from uninfected calves, as expected. Macroscopic lesions were not evident in calves from any group. Microscopic observation of neural tissue from LA-infected calves revealed focal hemorrhages in all brain areas. Furthermore, mild non-suppurative meningitis was evident in these calves. Mononuclear perivascular cuffings and focal gliosis were present in anterior and posterior cortex and medulla oblongata. Diffuse gliosis was present in anterior and posterior cortex and areas of neuronophagia and satellitosis were detected in the cortex, base of the brain and diencephalon. Microscopic lesions were not observed in TG. In Cooper-infected calves microscopic lesions were similar but not as remarkable as in LA-infected cattle. An important finding from these experiments is the fact that, like BoHV-5, BoHV-1 is able to replicate and disseminate within the nervous tissue. It is also noticeable that there are differences in the ability of the strains to replicate and induce microscopic lesions in the brain.

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