Equine herpesvirus outbreak sheds light on nasal microbiota in horses


Research conducted on a horse farm in Ontario that was experiencing an outbreak of equine herpesvirus earlier this year found that the nasal microbiota of infected horses differed significantly from those of a healthy control group.

Ontario Veterinary College’s Dr Diego Gomez-Nieto said the team’s work on the Ontario farm revealed that nasal bacterial microbiota of healthy horses is richer and more diverse than that previously reported using culture-based methodology.

“We found that there is a myriad of different types of bacteria in the nasal cavity of the horse, and they are kept in a normal balance.”

But, he said, when there is a respiratory infection from a virus, such as equine herpesvirus, the normal balance of the nasal bacterial population is disrupted, allowing some pathogenic bacteria to proliferate and cause disease.

“One of those diseases is pneumonia. The results of our study help to explain why and how pneumonia develops in horses after a viral infection of the respiratory tract,” Gomez-Nieto said.

“Usually, the virus enters the respiratory system, produces inflammation and decreases the mechanisms of defense of the respiratory tract. When those mechanisms are not working anymore, pathogenic bacteria are able to colonize the respiratory tract.”

The study demonstrated how fast the virus spread on the farm, how many animals can get affected at the same time, and that this can have fatal consequences for the horses. The results have been published in the Canadian Journal of Veterinary Research.

Gomez-Nieto cautioned owners that if a horse develops a fever and is found to be shedding equine herpes virus, then the level of risk to other horses on the premises increases significantly, and it is important to take action early. Signs of EHV may include fever, limb edema, and nasal discharge. It can present as respiratory disease or neurological disease, and in some cases can cause abortion.

“If horse owners and caretakers detect any of those clinical signs, they should consult with their veterinarians and pursue testing,” he said.

“Sometimes the only sign will be fever. The result of our study and other studies shows that if a farm is experiencing an outbreak of equine herpes virus, owners should check temperatures for all horses twice a day because if they check only once a day, they can miss some fevers. Owners need to keep a log of these recordings.”

When a horse is shedding the virus it is highly contagious and quarantine protocols are of paramount importance.

Early intervention can help ameliorate the disease and speed up recovery time. Rigorous hygiene and infection control measures can help control the spread. This includes washing hands and changing clothing before handling other horses.

The disease can spread by direct contact from one horse to another or by contaminated nasal secretions, and indirectly through contact with contaminated objects.

The air around the horses can be contaminated with infectious viruses, so it is important to have ample distance between paddocks and separate new horses and those returning from events. Gomez recommends new horses or those returning from an event are quarantined for 21 days.

» Learn more about biosecurity with Equine Guelph‘s interactive Biosecurity Calculator.

Nasal bacterial microbiota during an outbreak of equine herpesvirus 1 at a farm in southern Ontario. Diego E. Gomez, corresponding author Luis G. Arroyo, Brandon Lillie, and J. Scott Weese. Can J Vet Res. 2021 Jan; 85(1): 3–11.

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