New EHV-1 variant found in outbreak in horses in Pennsylvania in March – study

Ten horses at a 31-horse show barn in rural Pennsylvania showed an elevated temperature, but all appeared bright and alert. Eight were positive for EHV-1.
Ten horses at a 31-horse show barn in rural Pennsylvania showed an elevated temperature, but all appeared bright and alert. Eight were positive for EHV-1. File image by TheOtherKev

An outbreak of equine herpesvirus-1 (EHV-1) at a Pennsylvania horse barn in March was caused by a new variant of the H752 genotype, researchers report.

The H752 genotype has the potential to cause neurological disease. This new variant has a particular mutation in the ORF 30 gene, similar to that reported in a recent outbreak from France, the study team reported.

The mutation is at a pivotal nucleotide position which has previously been shown to discriminate between neuropathogenic (D752) and non-neuropathogenic (N752) forms.

Nicola Pusterla and his fellow researchers, writing in the journal Pathogens, described EHV-1 as an important, widespread viral pathogen that can induce respiratory disease in young horses, as well as abortions in mares, early death in foals, and occasionally neurological disease.

Although neurological problems are a relatively uncommon consequence, they can cause devastating losses and severely impact the equine industry.

On March 9, two of the authors, Julia Miller and Sarah Varnell, visited a 31-horse show barn in rural Pennsylvania to perform routine dental care and vaccination.

As part of the health assessments, 10 horses were unexpectedly showing an elevated rectal temperature, even though they all appeared bright and alert.

Blood and nasal secretions were collected from the horses with a fever to check for common respiratory viruses, including EHV-1.

Eight out of the 10 horses tested positive for EHV-1. Over the following 35 days, each of the 31 horses was monitored daily for abnormal physiological parameters. Blood and nasal secretions were collected weekly for molecular-based testing for EHV-1, and blood was collected 35 days apart to checked for antibodies to EHV-1.

The horses comprised 14 geldings and 17 mares, aged 1 to 27. Various breeds were represented, including 24 warmbloods, four Welsh ponies, two thoroughbreds and a thoroughbred/draft horse cross.

The virus caused ill health in 84% of the animals, with 26 clinically infected horses displaying fever and, less frequently, a loss of appetite and swelling in the lower legs. Four horses showed mild short-term neurological problems.

Five horses did not develop any abnormal clinical signs during the entire observation period.

The diseased horses experienced a high viral load of EHV-1 in their blood and/or nasal secretions, while subclinically infected horses had detectable EHV-1 mainly in nasal secretions.

All 31 horses were treated with valacyclovir, while clinically infected horses further received flunixin meglumine and sodium heparin.

Discussing their findings, the authors said one of the features of this EHV-1 outbreak was that the investigation started when most horses did not display apparent clinical signs.

“One could argue that without early detection of EHV-1 and medical intervention, more horses might have developed myeloencephalopathy (brain inflammation), considering the neuropathogenic potential of the H752 genotype strain,” they said.

It is likely, they added, that early medical intervention with valacyclovir in all the performance horses may have prevented severe neurological consequences.

The fact that the attending veterinarians noticed the elevated rectal temperature without additional clinical signs emphasizes the importance of regularly assessing the rectal temperature of at-risk horses.

They said the lack of respiratory signs seen in the horses in the study might be explained by the recent vaccination of the horses against the respiratory form of EHV-1 as part of their vaccination schedule.

Both the H752 EHV-1 strain from France and Pennsylvania showed the ability to cause neurological problems.

The authors said the discovery of a new H752 genotype adds to the already complex pathophysiology and diagnostic challenges of EHV-1.

“The French and Pennsylvanian strains of EHV-1, while mostly causing a self-limiting disease characterized by fever, lethargy, anorexia, respiratory signs and distal limb edema, has also shown neuropathogenic potential.

“The distribution of the new H752 genotype in horse populations is yet unknown,” they added. The study also showed the highly contagious nature of the new H752 variant.

“While the spread of equine alpha herpesviruses is almost impossible to prevent at large equine operations, the daily monitoring of at-risk horses and use of proper biosecurity protocols around horses undergoing transportation may reduce the risk of EHV-1 spread,” they said.

The study team comprised Pusterla and Samantha Barnum, with the veterinary school at the University of California, Davis; Miller and Varnell, with Alliance Equine Health Care in Glenmoore, Pennsylvania; Barbara Dallap-Schaer and Helen Aceto, with the New Bolton Center at the University of Pennsylvania; and Aliza Simeone, with the Pennsylvania Department of Agriculture.

Pusterla, N.; Barnum, S.; Miller, J.; Varnell, S.; Dallap-Schaer, B.; Aceto, H.; Simeone, A. Investigation of an EHV-1 Outbreak in the United States Caused by a New H752 Genotype. Pathogens 2021, 10, 747.

The study, published under a Creative Commons License, can be read here.

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