Equine herpesvirus-1 is a cause of significant morbidity and mortality in horses, and its most important disease manifestations are abortion and myeloencephalopathy. The pathogenesis of both these diseases depends on establishment of viremia, and control of viremia is currently regarded as a likely critical goal for immunoprophylaxis. An experiment was performed to determine whether two current commercially available vaccines were capable of reducing EHV-1 viremia and nasal shedding.
The study design was a blinded, randomized challenge trial. Three groups of 8 yearling colt ponies, with no history of EHV-1 infection, were established. Each pony group received one of three treatments: vaccination with a modified live vaccine (MLV) against EHV-1 (RhinomuneTM, Pfizer); vaccination with an inactivated EHV-1 vaccine (Pneumabort-KTM, Fort Dodge); or injection with a saline placebo. Each treatment was administered 3 times, at intervals of 1 mth between the first two treatments, and 3 mths between the second and third treatments. All ponies were challenged by nasal instillation of 5 x 107 pfu of EHV-1 (Findlay, Ohio 2003) one month after the third treatment. Clinical signs of disease including rectal temperature, nasal discharge, anorexia, coughing, and depression, were recorded daily for 2 days prior to challenge infection, and 21 days post-challenge. Nasal shedding of virus and viremia were measured on the same days, using a real-time PCR test procedure. Differences in viremia and viral shedding between groups were analyzed using a generalized linear model with ANOVA (SASTM), and differences were declared significant when P < 0.05.
All ponies demonstrated clinical signs of disease consistent with EHV-1 infection post-challenge infection, including pyrexia, nasal discharge, inappetance and partial anorexia. The duration and severity of these signs appeared reduced in the vaccinated ponies but statistical analysis has not been completed. In the control group, 6/8 ponies were viremic on one or more days (total of 13 pony viremic days), in the Rhinomune group there were 4/8 viremic ponies (total of 5 pony viremic days), and in the Pneumabort-K group there were 2/8 viremic ponies (total of 3 viremic days). Statistical analysis demonstrated a significant effect of vaccination on viremia on Day 5 (P = 0.0015). For nasal shedding, there was a reduction in shedding in both groups, which was significant on Days 1-Day 11 post-challenge.
This study demonstrated that both a commercial MLV, and an inactivated vaccine containing a high antigen titer, were capable of significantly suppressing EHV-1 shedding after a highly pathogenic viral challenge. This is the first report of any suppression of viremia by commercial vaccines, which may be important in controlling the most important clinical manifestations of EHV-1 infection.