In the first part of this series, we discussed the benefits and potential side effects of vaccinations, which provide an important means of protecting animals and people from clinically important infectious diseases. We also introduced the changing paradigm of vaccination in veterinary medicine. What impact has this change had over last decade on the way we approach this preventive health measure?

A recent survey of the profession indicated that some veterinarians were initially apprehensive about the recommendation to extend the timeframe for boosters from one to every three years or even less frequently. While clinicians wanted to offer what was best for the animals, they were concerned about the lost incentive for clients to visit them annually when vaccinations were not needed. However, partly because of the emerging documentation about adverse reactions associated with routine vaccinations, veterinarians accepted that they needed to address these concerns with their clients. Today, a decade later, the public has generally embraced the concept of having their animals seen annually for wellness examinations as well as during periods of illness.

But while this paradigm shift has been well accepted in small animal practice, relatively little emphasis has been directed towards horses. Hopefully, increased efforts will be made to incorporate the newer science and concepts about vaccination into equine medicine. But in the meantime, it’s more important than ever for us to become as informed as possible about our equine patients’ vaccine requirements. Let’s take a closer look at what clinical studies say about equine herpes, influenza and encephalitis viruses, keeping in mind that very little research has been done on the effectiveness of titers to help prevent over-vaccination.

1. Equine herpes viruses

Five distinct herpes viruses (EHV) are known to infect horses. Two of them, EHV-1 (also known as equine abortion virus) and EHV-4 (also known as equine rhinopneumonitis virus), are major causes of abortion and respiratory disease. Recent outbreaks of EHV-1 infection have caused neurological disease at raceways, horse shows, farms and clinics in several areas of North America, with many cases of illness and a few deaths.

In addition to abortion in infected mares, EHV-1 strains can cause respiratory disease, although many times horses incubating the virus will only exhibit fever. The virus can be shed from nasal secretions, and stress can precipitate illness. That’s why it’s not unusual to see outbreaks where horses feel stressed, such as at racetracks. Horses exhibiting neurological signs often harbor high viral loads in their blood and nasal secretions, and can transmit the disease to other exposed horses. To control the disease, it’s crucial to separate and isolate the sick animal from the rest of the herd. Diagnostic testing for EHV-1 using the PCR (polymerase chain reaction) is useful to establish exposure in the presence of clinically relevant disease.

The vaccines

Both activated (killed) and modified-live (MLV) herpes virus vaccines are available, as single or combination (EHV-1 + EHV-4) vaccines. As mentioned in Part 1 of this series, a recent comparison of killed and MLV EHV-1 vaccines found the latter offered superior protection when tested in an aerosol challenge.

Because of the severity and endemic spread of EHV, many equine veterinary groups recommend serial vaccination for EHV in pregnant mares, and booster vaccinations every six to 12 months. I would recommend weighing a mare’s risk of exposure and serious disease during a viral outbreak before following this course of action. However, many show horses and some racehorses are vaccinated every two to three months, even though there is no evidence that this prevents the disease. There is no immunologic reason to vaccinate this frequently and there is compelling anecdotal evidence that horses on this heavy a vaccine schedule have weakened immune systems.

The titers

Vaccine titers for EHV-1 and EHV-4 are available. Existing data indicates that serum antibody levels in vaccinated horses can last six to 12 months, especially if an MLV product is used. A serologic titer can assess levels of residual antibody if the horse recovered from a natural case of EHV infection.

2. Equine influenza virus

They are two types of equine influenza virus (EIV): H7N7 (subtype 1) and H3N8 (subtype 2). The horse influenza viruses evolved from avian influenza viruses, and the recent appearance of clinically significant canine influenza apparently resulted from a mutation of the equine H3N8 virus.

Equine influenza is endemic throughout North and South America and Europe, and is considered the most important viral respiratory disease of horses. Disease is characterized by fever, depression, coughing, and a nasal discharge, which develop one to five days after infection, and is occasionally complicated by secondary bacterial infections that may lead to pneumonia and death. Due to the short incubation period and resulting persistent cough, the disease can spread rapidly. In the past two decades, all major outbreaks of EIV have involved the H3N8 subtype.

The vaccines

Continually mutating influenza viruses in people and animals, including horses, means we need to regularly update the vaccines in order to maintain their efficacy. Researchers say this “antigenic drift” is partially responsible for the previous failures of equine influenza vaccines to protect horses. So what’s changed to improve the efficacy of the vaccine?

Until recently, influenza vaccines were the inactivated type. These produce relatively short-lived immunity and poor protection rarely lasting beyond six months. As well, a critical lag time (immunity gap) between the completion of the initial two doses of vaccine and the recommended booster at 12 months resulted in vaccine failure.

Newer approaches to vaccination were obviously needed and resulted in the development of the MLV intranasal and recombinant viral vectored and naked DNA vaccines. The advertised intranasal MLV vaccines state efficacy for up to six months or a year, with a decrease in the severity of disease after challenge at one year. Studies with recombinant canarypox vectored (rCP-EIV) vaccine showed that two boosters protected ponies from viral challenge and that a third booster dose provided immunity for at least one year thereafter. Thus, rCP-EIV vaccine effectively closes the immunity gap between the initial and one-year booster timeframe.

Once again, you need to consider the horse’s exposure to this serious disease. Performance horses will undoubtedly be more at risk, while “backyard” horses with strong immune systems will most likely be better equipped to recover naturally, especially if no secondary infection develops.

Influenza vaccines are available as single vaccines or in combination with herpes virus, encephalitis, and/or tetanus vaccines. While some manufacturers state that these polyvalent vaccines are safe for horses, ponies, pregnant mares, sucklings, weanlings and yearlings, vaccination of pregnant and very young animals should be done with caution and preferably only in the case of disease outbreaks. Clinically, the injectable vaccines produce significantly more immediate vaccine reactions than the intranasal, though some horses appear to become headshakers following the intranasal.

The prevalence and severity of equine influenza has prompted many vets to routinely vaccinate. Show and racehorses are frequently done on the same two- to three-month schedule as EHV. However, in older horses, vaccination has been associated with vasculitis and purpura (bruising), as well as dysbiosis (disruption of normal body functions leading to colic, laminitis and founder) so it’s important to carefully consider the pros and cons of vaccinating an older horse. Heavily vaccinated horses appear more likely to develop a chronic cough after an EIV infection (which can still occur in frequently vaccinated horses).

The titers

While serum titers for EIV are available, the short-lived duration of immunity from vaccination or natural disease makes their measurement of little use. Experience with rechecking titers over many years of practice indicates that the titer levels, though low, are persistent, which may indicate there is more residual duration of immunity than is shown by the limited available research

3. Equine encephalitis viruses

Eastern encephalitis virus (EEE), Western encephalitis virus (WEE), and Venezuelan encephalitis virus (VEE) are all mosquito-transmitted viruses that occur in North and South America. They spread rapidly and cause epidemics of neurological disease that require effective prevention and control strategies.

The vaccines

Available vaccines are of the inactivated killed virus type, as well as more recently developed MLV vectored vaccines. General recommendations are to give two doses of combination encephalitis vaccine 30 days apart followed by annual or biannual boosters. One of the newest vaccines uses recombinant canarypox-vectored vaccine technology. Dr. Madalyn Ward, a holistic equine veterinarian, recommends giving the two combination encephalitis/tetanus vaccinations at five to six months of age followed by a booster every three years, or sooner in the face of an outbreak. Dr. Ward also recommends using caution when vaccinating older horses, in particular those over 15 years of age. Dr. Joyce Harman recommends checking the prevalence of WNV in your state or area and making a decision based on need, as many states have very few or no cases, while certain localities continue to have cases each year.

The titers

Vaccine titers for all three types of equine encephalitis are available and offer a good indication of the immune status of the horse.

Being aware of the common infectious diseases that affect horses, and what your patients’ risk factors and options are, is your best line of defense against unnecessary over-vaccination.

What about Lyme disease?

Currently, there are no approved equine Lyme vaccines, although some veterinarians are using canine vaccines. The new Lyme multiplex blood test from Cornell may be able to differentiate between horses that have had a Lyme vaccine and ones that have been naturally exposed. However, Lyme tests are still open for interpretation and the vaccines may or may not work or be safe.

Factors that may affect vaccination

Ask yourself these questions before finalizing a vaccination program for a patient. He may need fewer vaccines than you think. 1. Is he a performance horse that attends a lot of shows/events, or more the stay-at-home type? 2. Does the client have her own acreage or is the horse boarded at a big facility where there’s more risk of exposure? 3. Does the client support the horse with a natural diet and supplements so he has a better chance of fighting off infection? 4. What is the horse’s age? 5. How is his overall health? 6. Which diseases are prevalent in your area?


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Dr. Jean Dodds, DVM, received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she moved to southern California to establish Hemopet, the first non-profit national blood bank program for animals. Dr. Dodds has been a member of many national and international committees on hematology, animal models of human disease, veterinary medicine and laboratory animal research. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1984.


Dr. Jean Dodds received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she established Hemopet, the first non-profit national blood bank program for animals. Today, Hemopet also runs Hemolife, an international veterinary specialty diagnostics service. Dr. Dodds has been a member of many committees on hematology, animal models of human disease and veterinary medicine. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1994, has served two terms on the AHVMA’s Board of Directors, chairs their Communications Committee, and currently serves on the Board of the AHVMF, as well as its Research Grant and Editorial Committees.