One Step Ahead of Equine Infectious Disease

Spring is finally here, and flowers aren’t the only thing in bloom. Along with the nice weather, we see more equine infectious disease. Increased travel and training activities at this time of year mean our horses are more likely to encounter equine influenza, equine herpesvirus and other contagious diseases. At the same time, disease-carrying vectors such as mosquitoes are beginning to emerge, carrying deadly viruses such as West Nile virus (WNV) and eastern equine encephalomyelitis (EEE).

The last thing you want to worry about is your horse getting sick and interrupting your spring and summer riding adventures. By enlisting the help of your veterinarian, while reviewing some basic principles of biosecurity and disease prevention, you can reduce your horse’s chances of becoming ill.

“Core” for a reason

Some equine infectious disease threats are significant enough to justify vaccination for every horse, every year. These diseases, which are targeted by “core” vaccines, may have several characteristics including being highly infectious, endemic to a region, posing a risk of severe disease[1] or death, and/or presenting potential public health risks. The American Association of Equine Practitioners designates the following as core vaccines:

  • Eastern/Western Encephalomyelitis (EEE/WEE)
  • Rabies
  • Tetanus
  • West Nile Virus (WNV)

Signs of these diseases in the horse vary but most have a neurological component, which can be particularly devastating.

Although no longer at the forefront of equine health news, WNV remains an important concern. The availability and use of safe and effective vaccines has dramatically reduced the incidence of disease among American horses. However, in recent years, we have seen spikes in WNV cases – and almost all cases are in horses with unknown vaccination histories or horses that have not been vaccinated. We must stay vigilant in our WNV vaccination practices as the mosquito populations are not diminishing.

Risk-based protection

All horses need core vaccines, while the decision to give risk-based vaccinations is based on each individual’s specific needs and risk of exposure. Risk-based vaccination protocols should be directed by a veterinarian. The most common risk-based diseases we vaccinate for are equine influenza virus (EIV) and equine herpesvirus (EHV). Both are highly contagious and carry signs similar to cold or flu symptoms in people, such as fever, nasal discharge, lethargy and cough. Horses that travel and are in frequent contact with other horses should be vaccinated for the respiratory forms of EHV-1 and EHV-4, as well as EIV.

Equine influenza can be particularly challenging to manage because it is extremely contagious and is a unique virus that changes over time. And just like human flu vaccines, equine flu vaccines must be periodically updated to protect against the strains of influenza currently circulating and threatening horses.

We’ve been monitoring these changes in the equine influenza virus through Merck Animal Health’s ongoing 10-year respiratory disease biosurveillance program. Continued changes in circulating strains of equine influenza in the United States has resulted in numerous reports of influenza outbreaks over the past several years, even in well-vaccinated horses. During one of these influenza outbreaks in Florida we identified a new, highly infectious and relevant equine influenza strain. We added this strain – known as Florida ’13 – to our PRESTIGE line of vaccines to address that threat. To learn more about the most current influenza vaccines, visit

It has been an active year for EHV-1. The Equine Disease Communication Center (EDCC) has reported numerous cases from across the country. EHV-1 is monitored closely not only because it is so contagious – like flu – but also because it has the potential to develop into the neurological form (equine herpesvirus myeloencephalopathy – EHM), which can be fatal. Currently, there is not a vaccine to prevent the neurological form of EHV, but vaccination protocols decrease the severity of respiratory disease and reduce nasal shedding in infected horses.

The EDCC is a great resource for tracking disease threats and outbreaks in your area and across the country. Visit to sign up for alerts. The website also features a lot of very valuable information for horse owners and veterinarians.

Stay one step ahead with vaccination and biosecurity

Keeping your horse’s vaccinations up to date while implementing management practices that reduce your horse’s exposure to pathogens at home and away is the best way to protect your horse from infectious disease. Common sources of exposure to infectious disease include:

  • WHILE AWAY: Stalls, horses in adjacent stalls, trailers, shared equipment and water sources, vehicles coming and going, event officials and support personnel, human hands and clothing
  • AT HOME: Traveling horses returning home, visiting horses or new arrivals, shared trailers, buckets, equipment, horses in adjacent fields, professionals (veterinarian, farrier, feed delivery, etc.), hands, equipment, clothing, wildlife

The AAEP recently published updated biosecurity guidelines, which can be downloaded and saved to your phone or tablet for easy reference. In the meantime, keep in mind these basic principles of biosecurity:

  • Include all horses in your vaccination program 
  • Monitor your horse’s temperature daily. An elevated temperature is often the first sign of disease (normal is 99°F – 101°F) 
  • Practice good hand hygiene – wash hands after you touch one horse before touching another
  • Minimize nose-to-nose contact among horses and avoid use of communal equipment and water sources 
  • Separate and monitor horses post travel, as well as new arrivals, for signs of infectious disease

Remember, it is far easier and more economical to take preventative measures than to treat a potentially life-threatening disease. Every horse is unique, which means his disease risk is also unique. Your veterinarian will take into consideration many things when designing the right preventative program for your horse.

Ask your veterinarian about PRESTIGE vaccines from Merck Animal Health – covering your complete core and risk-based vaccine needs – and visit for more information.

Copyright © 2018 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.

[1] American Veterinary Medical Association  




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