Investigation of the frequency and selected prevalence factors of equid alphaherpesvirus 4 viremia in horses with acute onset of fever and respiratory signs

Equid alphaherpesvirus 4 (EqAHV4; <i>Orthoherpesviridae</i>, <i>Varicellovirus equidalpha4</i>; equine rhinopneumonitis virus) has seldom been associated with complications such as abortion and myeloencephalopathy, given the low tendency of this virus to induce viremia. We investigated the frequency of EqAHV4 viremia in horses with fever and respiratory signs. Case selection included all equids with EqAHV4 quantitative real-time PCR (qPCR)-positive nasal secretions (defined as EqAHV4 qPCR-positive cases) submitted to a diagnostic laboratory. Controls consisted of each case submitted before and after each EqAHV4 qPCR-positive case. Purified nucleic acid from blood samples collected from EqAHV4 qPCR-positive horses and control cases was tested for EqAHV4 by qPCR. We selected 183 EqAHV4 qPCR-positive horses and 376 EqAHV4 qPCR-negative horses. In general, EqAHV4 qPCR-positive horses were younger, and had a lower rate of anorexia and a higher rate of nasal discharge compared to the EqAHV4 qPCR-negative horses. A total of 25 of 183 (13.7%) horses with EqAHV4 qPCR-positive nasal secretions tested qPCR-positive for EqAHV4 in blood. EqAHV4 viremic horses were significantly younger (<i>p</i> < 0.015 for group <1-y-old) and had a significantly higher occurrence of distal limb edema (<i>p</i> < 0.05) than EqAHV4 non-viremic horses. Our data support the observation that EqAHV4 viremia is rarely detected in EqAHV4-infected horses, which explains the low level of reported complications, such as abortion and myeloencephalopathy.