Quantitative molecular viral loads in 7 horses with naturally occurring equine herpesvirus-1 infection.

The study examines the relationship between the viral load of equine herpesvirus-1 (EHM) in horses and its clinical outcomes, revealing that higher viral loads, particularly in horses with encephalopathy, tend to be associated with higher fatality rates.

Introduction

• This research was conducted on seven horses naturally infected with equine herpesvirus-1 myeloencephalopathy (EHM).
• The study aimed to observe and understand the relationship between the quantity of virus present in the horse (viral load) and factors such as the severity of symptoms, the clinical signs, and the horse’s survival rate.
• Previous to this study, no data had been reported associating the viral load of EHM with the severity of the condition and the horse’s recovery.

Methodology

• The sample of the study involved seven horses with EHM, presented from May to Sept 2011 at the University of California, Davis William R. Pritchard Veterinary Medical Teaching Hospital.
• Clinical signs in the horses were graded using a neurological scale. The researchers performed daily tests (quantitative PCR) on the horses’ nasal secretions and whole blood to determine the viral load.
• The treatment received by the horses, their survival, the outcomes and any relevant pathology were noted.

Results

• When presented, the severity of neurological impairment varied across the horses, with one horse scored at 5/5, three at 4/5 and three at 3/5 on the neurological grading scale.
• The treatment for the horses involved anti-inflammatory drugs, valacyclovir, and where necessary, management in a sling.
• From the tests performed, it was found that all horses were infected with a specific type (DNA polymerase D752 genotype) of equine herpesvirus-1.
• Among the five surviving horses, the peak viral load in nasal secretions and blood varied, with modest range approximately between 6.9 x 10(3) and 2.81 x 10(5) (median 5.11 x 10(4)) for secretions and between 143 and 4340 gB gene copies per million eukaryotic cells (median of 3146) for blood.
• The two horses that didn’t survive the infection had initially shown moderate neurological signs (graded 3/5) which later progressed to encephalopathy. Notably, these horses exhibited higher peak viral loads, with each showing figures of approximately 1.9 x 10(9) and 2.2 x 10(9) in nasal secretions and between 2.05 x 10(4) and 1.02 x 10(5) gB gene copies per million eukaryotic cells in blood.

Conclusion

• The study concluded that horses that did not survive the infection had significantly higher – almost by a factor of 1000 in secretions and tenfold in blood – viral loads compared to those that did survive.
• The study didn’t find any relationship between the severity of clinical signs at the presentation stage and the horses’ survival rate.
• These findings suggest a strong negative association between survival rates and both the onset of encephalopathy and the presence of a high viral load.
• The researchers recommend further research to validate the association between high viral loads, encephalopathy, and poor prognosis in EHM horses.