Differences in replication kinetics and cell tropism between neurovirulent and non-neurovirulent EHV1 strains during the acute phase of infection in horses.

Equine herpesvirus 1 (EHV1) replicates in the respiratory tract of horses, after which infected leukocytes transport virus throughout the body, resulting in abortion or nervous system disorders. Two EHV1 strains circulate in the field: neurovirulent and non-neurovirulent. To investigate differences in replication in the upper respiratory tract (URT), an experimental inoculation study in ponies was performed with both strains. Two groups of six ponies, were inoculated intranasally with 10(6.5) TCID(50) of either strain. Clinical signs, nasal shedding and viremia were evaluated. At early time points post-inoculation (pi), one pony of each group was euthanized. Tissues were collected for titration and immunostainings. Number and size of EHV1-induced plaques were calculated, and individual EHV1-infected cells were quantified and characterized. Inoculation with either strain resulted in nasal shedding and replication in several tissues of the URT. Both strains replicated in a plaquewise manner in epithelium of the nasal mucosa, but replication in epithelium of the nasopharynx was largely limited to non-neurovirulent EHV1. Plaques were never able to cross the basement membrane, but individual infected cells were noticed in the connective tissue of all examined tissues for both strains. The total number of these cells however, was 3-7 times lower with non-neurovirulent EHV1 compared to neurovirulent EHV1. CD172a(+) cells and CD5(+) lymphocytes were important target cells for both strains. Interestingly, in lymph nodes, B-lymphocytes were also important target cells for EHV1, irrespective of the strain. Viremia was detected very early pi and infected cells were mainly CD172a(+) for both strains. In summary, these results are valuable for understanding EHV1 pathogenesis at the port of entry, the URT.