Some horse breeds may be more vulnerable to neurological problems arising from infection with equine herpesvirus-1 (EHV-1), the findings of German research suggest.
EHV-1 is a common cause of respiratory tract infections in horses worldwide. However, it can also cause abortions and neurological problems, known as Equid Herpesvirus-associated Myeloencephalopathy (EHM).
“EHM is of great concern, as permanent neurological gait anomalies can leave a horse unfit for future use,” Eva Klouth, Yury Zablotski and Lutz Goehring said in the journal Pathogens.
The authors noted that while all horses and ponies are susceptible to EHV-1-related respiratory tract infection, the risk of developing EHM is perceived to be unevenly distributed between different breeds or age groups, and between male and female animals.
EHM, they said, has been recognized as a multi-factorial disease.
“The virus and the magnitude and duration of viremia (the presence of the virus in the blood) are frequently discussed, but risk factors of breed, age, and sex are not always considered while analyzing final outbreak numbers.”
The study team, all with the Faculty of Veterinary Medicine at Ludwig-Maximilians University in Munich, assessed risk factors associated with EHM in a largely unmitigated outbreak in a large herd comprising multiple horse breeds.
A total of 141 adult horses and ponies of several distinct breeds were evaluated, exploring factors such as breed, age, and sex.
In total, 33 of the 141 horses showed signs of EHM, representing a 23% incidence rate.
Fjord horses and Warmblood horses were found to be over-represented among those developing EHM.
The pony breeds, Welsh and Shetland ponies, were under-represented.
The study team found that age and sex were not associated with the risk of developing EHM, and concluded that breed was a significant risk factor for developing EHM during this outbreak.
Their findings supported the conclusions of 2006 research in which nine EHM outbreaks were investigated involving a total of 195 horses and ponies on unvaccinated premises in The Netherlands. The team said those findings pointed to an increased risk for EHM in the draft horse, Standardbred, and Hispanic breeds (PRE and Lipizzaner horses). EHM numbers in the Dutch study had been extremely low for the small pony breeds (Shetland and Welsh).
Turning to vaccination, the German researchers commented: “There exists an opinion among some horse owners, and even among some veterinarians, that vaccination against EHV-1 puts a horse at a greater risk of developing EHM upon EHV-1 infection than if it were unvaccinated.
“In our study, with a 23% EHM fatality rate in the entire (unvaccinated) herd, it can be safely said that vaccination against EHV-1 has not contributed to the risk of developing EHM during this outbreak.”
However, in a study of an EHM outbreak among mainly (young) Paint horses in the US in 2011, horses that were vaccinated against EHV-1 (and other diseases) within a five-week period prior to EHV-1 exposure were, apparently, at an increased risk of developing EHM compared to those vaccinated more than five weeks ago.
“These findings could suggest a unique window of vulnerability for recently vaccinated horses, which needs further investigation.”
The study team said many factors could play a role in disease development in EHV-1 infections, resulting in different levels of disease severity: Virus strain variation, infectious dose and duration of exposure, host immunity, and other factors.
“The main findings of our study are that it seems likely that an increased risk for EHM exists in certain breeds over others, and within a high-risk breed, the influence of age seems less important.”
They said more work is required involving outbreak data sets to strengthen a risk factor hypothesis. “However, as each outbreak is uniquely different from the next in so many aspects, this will be a very difficult task.
“In the meantime, early detection of an index case with EHM, early identification of shedders through testing, and separating shedders from non-shedders will slow down disease spread and decrease EHM incidence regardless of ‘other factors’.”
The researchers said their study is the first to suggest an increased risk of developing EHM in Fjord horses.
Klouth, E.; Zablotski, Y.; Goehring, L.S. Apparent Breed Predilection for Equid Herpesvirus-1-Associated Myeloencephalopathy (EHM) in a Multiple-Breed Herd. Pathogens 2021, 10, 537. https://doi.org/10.3390/pathogens10050537