By Jackie Bellamy-Zions, Communications, Equine Guelph.

“Most horses have been exposed to the equine herpesvirus,” says Dr. Diego Gomez-Nieto, researcher at the Ontario Veterinary College. 

Gomez has been part of a research study on the equine herpesvirus (EHV) which discovered the nasal microbiota of infected horses differed significantly from those of a healthy control group. The study came together quickly and was conducted on a horse farm in Ontario that was experiencing an outbreak. The January 2021 research paper explains nasal bacterial microbiota of healthy horses is richer and more diverse than 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,” says Gomez. “However, when there is a respiratory infection from a virus (like EHV), 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.” 

Dr. Gomez discussed the findings of this study and what horse owners need to know to protect their horses from EHV, opening with the statement: “Most horses have already been infected with equine herpesvirus.” But this fact need not cause alarm, as for most equines this does not cause any serious problems.

When a horse is showing signs of illness, then it is important to pick up on these signs and take action early on. For EHV these signs may include fever, limb edema, and nasal discharge. When a horse is shedding the virus it is highly contagious, and quarantine protocols are of paramount importance. 

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

Related: Equestrian Stable Hygiene - The "New Normal"

EHV can present as respiratory disease or neurological disease, and in some cases can cause abortion. Early intervention can help mitigate 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. Communication is vital to alert other horse owners with whom the horse has had contact.

“We are learning more about the interaction between virus and bacteria,” says Gomez. “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 demonstrates how fast the virus spread in the farm, how many animals can get affected at the same time, and how this can have fatal consequences for the horses.

A biosecurity plan is essential to practice infection control. The disease can spread by direct contact from one horse to another or by contaminated nasal secretions. EHV can also spread indirectly through contact with physical objects that are contaminated with the virus. Sharing of equipment between horses is discouraged. The air around the horses can be contaminated with infectious viruses; therefore, it is important to have ample distance between paddocks and separate new horses and those returning from events. Gomez recommends a 21-day quarantine for new horses or those returning from an event. 

In conclusion, Gomez cautions owners that if a horse develops a fever and is found to be shedding equine herpesvirus, then the level of risk to other horses on the premises increases significantly.

“If equine herpesvirus is suspected the owner needs to immediately notify a veterinarian and [must] not move the horse or take the horse to another farm or to another event. The veterinarian can make a plan for testing and institute bio-safety measures in the farm.”

Isolation Plan

Horses in isolation should be housed away from other horses in a separate barn or a paddock with a shelter. Photo: Shutterstock/Jeff R Clow  

Your plan to isolate sick, new, and travelling horses involves both physical structures and your management practises.

• A sign stating that horses are in isolation is located at the entrance of the farm/barn area.
• Access is restricted to official visitors.
• A sign-in and sign-out sheet is provided for personnel.
• A stall that has been cleaned, disinfected, and filled with fresh bedding is ready.
• A separate barn or paddock with shelter is used (best practice).
• If the stall at the end of an aisle in the main barn is used (not ideal), several empty stalls separate the isolated horse from other horses.
• All equipment, tack, buckets, grooming tools, medication, and other tools are labelled (e.g., with red tape) and kept in a separate area.
• The isolated horse is worked with last each day, and the handler washes his/her hands before and after.
• The manure pile for the isolation area is separate, and manure is not spread until it is well-composted.
• All personnel are educated on biosecurity protocols, and visitors are made aware of protocols. Only necessary visitors are allowed, such as the isolated horse’s owner and veterinarian.
• Disposable examination gloves, disposable shoe covers, and protective clothing are available for personnel and necessary visitors.
• Pets and wild animals are prevented from entering the isolation area.
• New and travelling horses are isolated for three weeks.

Related: What Are You Vaccinating Your Horse Against?

Main Photo: Shutterstock/Vprotastchik