Molecular pathogenesis of equine coital exanthema: temperature-sensitive function(s) in cells infected with equine herpesviruses.

The study investigates the growth of equine herpesvirus strains under various temperatures, focusing specifically on their role in the disease equine coital exanthema. The evidence suggests that a critical function in the viral lifecycle, responsible for enabling the virus to leave the host cell, does not occur at a temperature of 39 degrees Celsius.

Experiments on EHV Strains

• The research begins with exploratory tests on various lab and wild strains of equine herpesvirus (EHV). They are tested at a temperature of 39 degrees Celsius to determine their growth sensitivity to heat.
• The experiments showed that the efficiency of plating (EOP)—which is the measure of viral infectivity—was 10(-2) for EHV 1 and 2 strains, and 1 X 10(-6) for EHV 3 strains. This indicates that EHV 1 and 2 are more infectious than EHV 3 under these conditions.

Impact of Temperature on Viral Growth

• The EOPs were determined using plaque assays, a method used to measure the quantity of virus in a sample, comparing the titrations at 34 and 39 degrees Celsius on equine fetal dermal fibroblast cells. Notably, there were no differences in the yields when the infected cultures were placed at 34 and 37 degrees Celsius, underlining the specificity of the temperature sensitivity.
• Temperature shift experiments with EHV 3-infected cultures showed that a temperature-sensitive function, crucial for the reduction of virus amount, seemed to be a late-stage function in the virus lifecycle.

Observations and Hypothesis

• All the EHV strains incorporated H3-thymidine into the viral DNA even at the non-permissive temperature of 39 degrees Celsius. H3-thymidine is a radioactive tracer used to measure the synthesis of viral DNA.
• Under an electron microscope, the EHV 3-infected cell cultures exposed continuously to 39 degrees Celsius showed the accumulation of nucleocapsids (the virus’s genetic material enclosed in a protein shell) within the nucleus of the cells. This led to the hypothesis that in equine dermal cells infected with a specific EHV 3 strain, a function necessary for the exit of the nucleocapsids from the host cell’s nucleus is absent at 39 degrees Celsius.

Significance of Findings

• The significance of these findings lies in the context of the disease equine coital exanthema, which is caused by this virus. Understanding the specific behaviors and lifecycle of the virus can lend insight into its pathogenicity—the ability of the virus to cause disease—and potentially help in developing strategic ways to prevent or treat the disease.