A Closer Look At: The Whats And Whys Of Equine Vaccines

In this series, we ask some of the equine health questions you’ve wondered about but were too afraid to ask. Today, Dr. Earl Gaughan, Senior Equine Professional Services Veterinarian with Merck Animal Health, tackles a few common questions about respiratory illness in horses.

We’ve all heard a lot in the last year about the way vaccines work. What’s the mechanism of action for most equine vaccines? (Killed virus vs mRNA, etc.)

Dr. Earl Gaughan: The simple way a vaccine works is to mimic a natural exposure to an infectious organism that can cause disease, and to stimulate an animal or human to develop an immune response such that infection and disease are prevented. The normal mammalian immune system has several ways it responds to these kinds of exposures and threats of disease. The immune system’s “goals” are to rid the body of the infectious organism and/or to reduce any potential life-threatening results such that the body can survive.

Among the responses of the immune system are humoral (antibodies) and innate (chemical and cellular) mechanisms. Infectious organisms present antigens which are usually proteins that stimulate these immune responses. Antibodies are the immune system proteins that bind or block these outside antigens to reduce the number and threat these organisms can present. This response takes the body hours to days to reach effective levels. Cellular and chemical production to fight infectious organisms develop rapidly and can be the quick, first responses after exposure to these threats.

“Killed virus” vaccines are just that – a product that contains killed organisms or proteins that typically stimulate humoral antibody responses. “Modified-live” vaccines perform similarly to killed vaccines but utilize a similar organism to the targeted infection that has been altered such that it will stimulate an immune response but not disease. These vaccine products are most commonly administered to a horse by injection deep into muscle tissue. At least one modified-live vaccine can be administered up the nose of a horse (intra-nasal) such that the vaccine itself is placed on the lining of the nasal passage. This stimulates what is known as innate or mucosal immune responses. This is a rapid (minutes to hours) response based on release of chemical and cellular mediators of the immune defense.

Currently, we do not have mRNA vaccines for horses. These vaccines work by entering body cells and stimulating protein production that can reduce invading viral reproduction and entry into host body cells. Long term repercussions of these products are unknown at this time.

Do equine vaccines vary year to year like the human flu shot?

Equine vaccines do not vary year to year. The AAEP-recommended core vaccines against rabies, tetanus, Eastern Equine Encephalomyelitis/Western Equine Encephalomyelitis (EEE/WEE) and West Nile virus remain very effective and should be administered to every horse in every location in North America. These diseases are typically lethal and the vaccines are successful at preventing disease with very few negative effects.

The reason human flu vaccines change each year is the rapid ability of influenza to undergo what is called “antigenic drift” or mutation of the virus to make it more able to infect a host (human). Antigenic drift in horses occurs, but at a much slower pace than what is seen in people. Equine flu vaccines are changed to keep up with evolving strain development, but again, we are fortunate this does not happen at the rate seen in humans. The rate of equine flu strain and therefore vaccine change is more in a five to ten-year window vs. annual. To date, influenza appears to be the only viral pathogen where annual change is monitored in horses. The strains of equine herpesvirus appear to be stable or more slowly changing, although more strains of herpesvirus are being discovered with more sophisticated diagnostic methods, so additional vaccines may be on the horizon for horses. The same can probably be said for some of the other, less commonly seen, risk-based vaccines.

Some horse owners are skeptical of the efficacy of equine vaccines; how well do they really work? 

Skepticism is rational and informed questions are welcomed by equine veterinarians. The vaccines available for horse diseases like rabies, tetanus, Eastern and Western Equine Encephalomyelitis (EEE/WEE) and West Nile virus work very well to prevent these diseases in horses. These vaccines are considered core or essential for a horse’s well-being, as they help prevent diseases that are typically lethal. While there is never an absolute guarantee, it is interesting to note that when these diseases are diagnosed in horses, they are most commonly seen in horses that have not been vaccinated or are far past the appropriate date for booster vaccines.

Other disease-causing organisms do not lend themselves to as complete vaccine protection as the vaccines for the infectious neurologic disorders mentioned above. However, these are also far less lethal infections cause by influenza, herpesviruses, and others. Vaccinating for these diseases is considered “risk-based” and is encouraged for horses that may encounter infectious agents at home or during travel. Vaccinating for these concerns is performed to help reduce infections, reduce severity of clinical disease, and reduce spread of the virus, or other organisms. These vaccines can be very helpful and are recommended for most horses in consultation with a veterinarian. 

Why is it important for a horse who doesn’t leave home to still get regular vaccinations?

While a horse at home would appear unlikely to encounter disease-causing organisms, these infectious agents can often come to the horse. For instance, rabies is usually the result of a bite from a skunk, raccoon or other carrier; EEE, WEE, and West Nile can be delivered by mosquitos and the tetanus organism is found in soil everywhere. Vaccinating the horse at home is important to prevent these highly lethal diseases.

Vaccinating horses that do not travel for things like influenza and herpes should be done in consultation with a veterinarian. If other horses do travel and come home, or if horses from varied places mix at all, these infections can be transmitted and are typically very contagious. Other, less common, disease entities may not require vaccination unless presence of the diseases occurs locally or regionally, putting the horse at home at risk. Even trail rides with neighbors can expose horses to infectious organisms not necessarily present at home. Again, discussion with your veterinarian is a great place to get the best answers. 

What are some common reasons horse owners give for not vaccinating their horses? 

Sound medical reasons for not vaccinating:

  1. The horse has reacted severely with past vaccine administration
  2. The horse has recently recovered from the targeted disease
  3. Known or potential adverse events/reactions to a vaccine product

The post A Closer Look At: The Whats And Whys Of Equine Vaccines appeared first on Horse Racing News | Paulick Report.

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