Infection of the Ovine herpesvirus 2 in the reservoir host, sheep, and the susceptible host, cattle

Ovine Herpesvirus-2 (OvHV-2) is a gamma-herpesvirus that belongs to genus macavirus, is endemic in sheep worldwide. The virus infects sheep subclinically but when it is transmitted to cattle; it induces malignant catarrhal fever (MCF), a frequently fatal lymphoproliferative disease. The pathogenesis and site of OvHV-2 is unknown in both species. In this study, we tried to: first detect the virus presence and measure its DNA loads; secondly, localise the precise cellular location of the virus in the tissues of sheep and domestic cattle. For the first purpose, we optimised and validated a sensitive quantitative polymerase chain reaction (qPCR) technique using Taqman® probe system that can detect and measure the virus’s DNA as low as one viral DNA copy in a qPCR reaction. Secondly, we applied RNA in situ hybridisation (RNA-ISH) technique to detect viral transcripts (Ov2.5 a latent gene and ORF65 a lytic gene), and in addition we used immunohistology to stain the viral Ov8 antigen (glycoprotein) by specific polyclonal antibodies. For these purposes we have used a variety of organs and tissues, namely: respiratory tract, tongue, muzzle, lymphoid and reproductive organs as well as nasal swabs and peripheral blood leukocytes from randomly selected sheep (n=28), cows without MCF (n=50) and cattle with MCF (n=12). The results in sheep have shown that 88 % of them harbour viral DNA in most of their organs at very low amounts.The viral mRNA and antigen were also detected in a wide range of organs including epithelial cells of respiratory tract, tongue and muzzle, macrophages and lymphocytes (B cells) in bronchial associated lymphoid associated lymphocytes (BALT), lymph nodes in spleen as well as vascular endothelial cells of many of these tissues. Interestingly in cattle without MCF, results were very similar as in sheep i.e.; viral DNA was found in a large population of cattle (67 %); and viral transcripts and antigen detected in a large proportion of tested organs, similar as seen in sheep. In the MCF-affected cattle, similar types of cells were found infected as in cattle without MCF, but with significantly higher viral loads (more than three logs). This study shows for the first time OvHV-2 location and cell types they infect in sheep, and in cattle that do not show any evidence of MCF. The new question is what triggers inducing of MCF in the subclinically infected healthy cattle?. That can be addressed by further investigation