Viruses and vaccines: avoiding over-vaccination

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

A recent survey of the profession indicated that some veterinarians were initially apprehensive about the recommendation to extend the time frame for boosters from one year 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, especially in dogs and cats, 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. The availability and growth of veterinary pet insurance has also helped bring people into the clinics on a regular basis.

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 caretakers to become as informed as possible about their horses’ vaccine requirements. Let’s take a closer look at what the 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.

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 is 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.

EHV 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 that 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 your mare’s risk of exposure and serious disease during a viral outbreak before following this course of action.

EHV vaccine 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. You can also arrange for a serologic titer to assess levels of residual antibody if your horse recovered from a natural case of EHV infection.

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 to be the most important viral respiratory disease of horses. Disease is characterized by fever, depression, coughing, and a nasal discharge, which develops one to five days after infection, and is sometimes 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.Picture 1

EIV vaccines

The continually mutating influenza viruses in people and animals, including horses, means we regularly need to 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 that rarely lasts 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, with a decrease in the severity of disease after challenge at one year. Recent 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 time frame.

Once again, you need to consider your horse’s exposure to this potentially fatal 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.

The prevalence and severity of equine influenza has prompted many vets to routinely vaccinate. However, in older horses, vaccination has been associated with producing a vasculitis and purpura (bruising), as well as dysbiosis (disruption of normal body functions leading to colic, laminitis, and founder) so it is important to carefully consider the pros and cons of vaccinating an older horse.

EIV vaccine 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.

Equine encephalitis viruses

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

Equine encephalitis virus 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. 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.

Equine encephalitis virus vaccine titers

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

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


Boone TJ. Respecting equine herpes virus-1. Calif Vet 61(2):18-19, 2007. Desmettre P. Diagnoses and prevention of equine infectious diseases: present status, potential, and challenges for the future. Adv Vet Med 41:359-375, 1999. Goodman LB, Wagner B, Flaminio MJ et al. Comparison of the efficacy of inactivated combination and modified-live virus vaccines against challenge infection with neuropathogenic equine herpesvirus type 1 (EHV-1). Vaccine 24:3636-3645, 2006. Minke JM, Toulemonde CE, Coupier H, et al. Efficacy of a canarypox-vectored recombinant vaccine expressing the hemagglutinin gene of equine influenza H3N8 virus in the protection of ponies from viral challenge. Am J Vet Res 68:213-219, 2007. Rosenthal M. Practitioners concerned about safety, embracing new vaccine recommendations. Product Forum & Market News, Spring 2007 Tizard I, Ni Y. Use of serologic testing to assess immune status of companion animals. J Am Vet Med Assoc 213: 54-60, 1998. Townsend HG, Penner SJ, Watts TC, et al. Efficacy of a cold-adapted, intranasal, equine influenza vaccine: challenge trials. Equine Vet J 33:637-643, 2001.

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W. Jean Dodds, DVM received her veterinary degree from the Ontario Veterinary College. After working for several decades in upstate New York doing non-invasive studies of animal models of inherited bleeding diseases, she moved to southern California in 1986 to start Hemopet, the first non-profit national animal blood bank. Today, Hemopet’s range of nonprofit services and educational activities include: Providing canine blood components, blood bank supplies, and related services; adopting retired Greyhound blood donors as companions through Pet Life-Line; contributing to the social needs of the less fortunate in our society by volunteer and interactive programs with the Greyhounds; specialized diagnostic testing using all “green” patented technology and consulting in clinical pathology through Hemolife, Hemopet’s diagnostic division; teaching animal health care professionals, companion animal fanciers, and pet owners on hematology and blood banking, immunology, endocrinology, nutrition and holistic medicine.