AAEP Convention 2005: Vaccine Efficacy and Controversies

"There are a huge number of (vaccine) choices out there and it's hard sometimes to see our way through the forest," said Julia Wilson, DVM, Dipl. ACVIM, associate professor in Veterinary Population Medicine at the University of Minnesota. "What we should be doing is thinking about what the risk is of that disease, not just for the individual horse, but the group of horses it lives with. Think good and hard about how good the vaccine is, its safety, and whether that horse is going to be at risk for adverse effects."

Wilson encouraged veterinarians to think critically about vaccine recommendations during the In-Depth: Vaccination session at the 2005 American Association of Equine Practitioners' Convention, held Dec. 3-7 in Seattle, Wash. "Think of where the horse is going to be going over the next year," she said. "We have had really bad strangles in the Midwest, for example. So if you don't have strangles in your area, and you have a client that wants to move to the Midwest, that's one I want you to think about."

Remember that seasonal risks are longer for arboviral diseases (those spread by mosquitoes) in some areas such as Florida, so horse owners should have their horses boostered with vaccines against encephalitis (Eastern equine encephalitis, EEE, and West Nile virus, WNV) if they're planning to travel to events such as the Florida winter show circuit. Some diseases tend to be more regional; for example, mares going to the breeding shed in Kentucky should be vaccinated for rotavirus and botulism. Wilson included a "Risk Assessment for Vaccination Decisions" form in the convention proceedings, for horse owners to complete before consulting with their veterinarian about vaccination.

Wilson encourages veterinarians to look at not only the individual's risk of exposure to diseases, but the entire farm's risk. She said, "I would like to advocate that, whenever possible, we convince facilities to have a policy where they don't bring unvaccinated horses or horses with unknown vaccination status to the facilities." She also promotes open lines of communication. "We need to share when we have a contagious disease (with the community) for the best interest of the horses. There has to be some way around the client confidentiality where we can let each other know about outbreaks without violating that."

Vaccine Efficacy

"The type of vaccination we select influences the type, strength, and duration of immunity," Wilson said. How can true efficacy of vaccines be determined? Experimental models of disease vary--some closely mimic naturally occurring disease and others remain controversial--and all results should be interpreted with caution.

Along with and sometimes in spite of what efficacy studies show, Wilson stresses the importance of veterinarians explaining to their clients that vaccines are never 100% efficacious--there will always be some upsets.

"Vaccines most often reduce the severity of clinical signs and/or duration of the illness, which translates to lower morbidity (illness rates) and lower mortality (death rates)," she explained. "These important points must be stressed to clients, who are quick to blame vaccine manufacturers when a vaccinated animal develops clinical disease or seems to have brought strangles, influenza, or herpes back to the farm after an excursion or competition."

While it's not common, it's important to note that vaccinated animals can carry infections home to the herd. "Vaccination is not a substitute for quarantine, but something we can use in addition to it," Wilson said. "I think we not only need to push vaccinating, but understand the practices to keep from bringing other diseases in."

If an infectious disease hits a farm on which the majority of horses are vaccinated against that disease, Wilson said that horses will shed less virus and can return to use more quickly after the disease runs its course. She recommends vaccinating horses well before they experience the stress of change (such as from sales and shipping), and she recommends quarantine of new arrivals for at least four weeks before allowing them to mingle with resident animals.

Modified live vaccines produce a stronger immunologic response than killed vaccines that use adjuvants to enhance signaling of the immune system. This means modified live products can prevent disease better, but they also have a greater risk of side effects, including shedding the vaccine strain. Killed vaccines typically have a shorter duration of immunity, so it's important that you schedule administration of killed vaccines for seasonal diseases (such as encephalitides and Potomac horse fever, or PHF) shortly before the risk is anticipated. Wilson recommended that some killed vaccines be given every two to three months in high-risk environments (influenza vaccines in a herd of young horses, for example). "Even then, not all horses will have high titers that are predictably protective," she added.

Adverse Effects

"Every vaccine we put into our animals has the potential to cause an adverse effect including anaphylaxis (A rapidly developing, exaggerated, and sometimes life-threatening allergic reaction) and death," Wilson said. Sometimes you can prevent problems simply by following the instructions. "What I didn't realize until I started putting this talk together is that there are some vaccine companies that don't want us putting their vaccine in the neck (Potomacguard and Pneumabort, for example). We should pay attention to those recommendations."
She made the following important points on avoiding adverse reactions:

  • Do not vaccinate through dirt or mud on the horse's hair--clean up the horse before injecting vaccine. This can help avoid clostridial infections at the injection site.
  • Do not allow your vaccines to freeze; this increases the adverse reaction rate.
  • Watch for purpura (an immune reaction that typically causes hives and edema in the abdomen, limbs, head, and scrotum) or immune-mediated thrombocytopenia (low blood platelet count) after strangles vaccination. "It's a very rare thing to happen, but we need to remember before we make blanket strangles vaccination recommendations that these risks are present," she said. 
  • Store the vaccines in the dark at the recommended temperatures. Check the color of the vaccine before using it; many vaccines have color indicators that signal microbial growth. If the vaccine has a color indicator and the product has turned red, don't use it. 
  • Never mix vaccines in the same syringe with other products. There is potential for cross-reactivity and adverse reactions. 
  • Again, check the directions--those of at least two products (Calvenza and Cephalovac) tell you not to put the booster at the same place you put the first vaccine.
  • Faithfully report reactions at www.aphis.usda.gov/vs/cvb/html/adverseeventreport.html.
  • Use silicone-free needles on horses that have reactions to the coating on disposable needles.
  • Consider non-adjuvanted vaccines in the future, perhaps intranasal ones.
  • Consider using concurrent antihistamines or an immunomodulator (http://www.TheHorse.com/emag.aspx?id=6582) to vaccinate the horse that tends to develop hives.

Specific Vaccine Controversies

Strangles--In my opinion, we have three choices out there: Strepgard (M-protein inactivated intramuscular, or IM, vaccine that requires two initial vaccinations), Strepvax (inactivated IM vaccine that requires three initial vaccinations), and Pinnacle I.N. (a modified, attenuated live bacterial intranasal vaccine)," she said. "We must remember these vaccines do not always prevent the disease, they just diminish clinical signs."

Label recommendations suggest that only yearly boosters are needed with these vaccines, but Wilson reports many farms with endemic strangles found that semi-annual vaccination was more effective in reducing morbidity when using the IM vaccines.

However, she added, "My experience in the Midwest is that Pinnacle provides much stronger protection than the M-protein vaccines and it has been successfully used in outbreaks."


In high-risk environments, Wilson recommends administration of Flu Avert I.N., a cold-adapted, modified-live intranasal vaccine, every six months. This is in contrast to every two to three months for the older killed-virus IM flu vaccines. The exception is in pregnant mares, in which the intranasal vaccine doesn't produce high-circulating serum titers, so foals would be protected only minimally by the antibodies provided in the colostrum. Wilson suggests veterinarians incorporate an intramuscular product such as Calvenza (which can be used intranasally as well) into the pregnant mare's regimen.

Vaccinating in the Face of Exposure

"With West Nile virus (WNV) in 2002, vaccination in the face of exposure seemed to work quite well, and later use of Recombitek (the Merial canarypox-vectored vaccine) appeared to thwart that disease in California," said Wilson. In Minnesota's dealings with WNV, Wilson has recommended that vaccination in the face of the illness is acceptable so long as clients realize that the vaccine will not have time to become fully protective. However, "Equine herpesvirus type-1 (EHV-1) is one disease where you don't go vaccinating in the face of exposure in the neurologic form," she stresses, reviewing the neurologic EHV-1 outbreak at the University of Findlay in Ohio (www.TheHorse.com/emag.aspx?id=4104). "At the Findlay outbreak in 2003, they had 118 horses, 40 of which developed neurologic signs. Then it got into Ohio State through some of the transferred patients…it was a big deal."

Wilson said in the Findlay population, horses that had been vaccinated with a modified-live vaccine (Pfizer's Rhinomune), appeared to be better protected than those vaccinated with some of the killed-virus EHV vaccine products. She referenced the study by Cornell researcher Klaus Osterreider, DVM, DVM Habilitation (German equivalent to a PhD), that examined the modified-live vaccine's efficacy against neurologic EHV-1 (www.TheHorse.com/emag.aspx?ID=5809). "Rhinomune perhaps should be reconsidered in horses that are at a high risk of exposure," she suggested. 

Vaccinating after Natural Infection

Little information is available on this topic, so Wilson could only make general recommendations. A horse that has recovered from a clinical case of strangles seems to gain long-lasting strangles immunity, with many horses never developing clinical disease later in their lives. She suggests you wait a year after infection and then resume vaccination with strangles. Horses that have had purpura may again develop signs following vaccination, hence checking titers first and avoiding vaccination if the titer is high has been suggested. The exception to long-lasting immunity would be in horses that have Cushing's disease; these horses tend to be a bit more susceptible to infectious diseases because their bodies' high cortisol levels can induce immunosuppression.

Veterinarians need to better understand the pathogenesis of EHV and PHF before making solid recommendations on revaccination following natural infection with these diseases, she said.

Overvaccination vs. Titer Levels

"Are we overvaccinating?" Wilson asked. "It depends on the horse and the circumstance. Some horses get vaccines they don't need, but many more horses aren't getting the protection we can offer them." She offered a list of her vaccine recommendations:

Non-negotiables/Core Vaccines:

Arboviruses (EEE, WEE, and WNV)

Strongly Recommended:


Recommended if Exposure is Likely:

EHV-1 and/or EHV-4

Farm- or Horse-Specific Risks Only:

Potomac horse fever
Equine viral arteritis
Leptospirosis (extralabel)
Lyme disease (extralabel)
Clostridial enteritis (extralabel or custom-produced)

"In small animals, now, veterinarians are using titers to make vaccine decisions," she said. "But we cannot send blood to Cornell and get a kitty cat panel turned into a horse panel. There is little data for interpretation, and titers vary from lab to lab (because of variations in testing methods)." She said that there needs to be some standardization of titer data before a clear call can be made on whether titers show useful information in horses. When in doubt on whether a horse has protective titers against a disease, increase the frequency of vaccination. "We're realizing there's a subset of horses that are not good responders, and those might end up being my vaccine failures," she opined.

Vaccination and Pregnancy

Wilson says there aren't any long-term, comprehensive safety studies of potential adverse effects of vaccine use during pregnancy equivalent to those available for humans. However, there has been no scientific basis for any concerns regarding possible abortogenic (abortion-causing) effects of equine vaccines used in the first trimester. "The recommendation to avoid all unnecessary vaccine use during this time period seems to stem from precautions advised by a subset of physicians for pregnant women," Wilson said.

She is curious to find out if mercury, which has been shown to cause abortion and cancer in pregnant women, could cause similar effects in horses. Mercury is found in the vaccine preservative thimerosal, which is found in more than 50% of approved vaccines for horses.

Claims that the killed WNV vaccine caused foal losses have not been substantiated, and there have been no shown detrimental effects of that vaccine on live foal rates, in a study performed on a large number of broodmares at Texas A & M.

In a nutshell, the majority of equine vaccine labels (with exception of a vaccine for equine arteritis called Arvac, which shouldn't be used in the final two months of gestation) don't discourage use in pregnant mares, but most do not include safety claims for this practice. Wilson says mares should be vaccinated for encephalitides post-foaling and before breeding to protect the mare during the riskiest times for WNV infection and to give maternal antibodies to the foal through her colostrum. This "prompts the question of whether the mare that receives an EEE, WEE, or WNV booster pre-foaling may diminish her ‘reserve' of circulating (humoral) antibody by transferring significant levels to the colostrum and thereby increasing her susceptibility," she said, adding that further studies should be conducted to better understand this.

Extralabel Use of Vaccines

Finally, Wilson touched on the extralabel use of vaccines, or using vaccines differently than their label instructs. "Lacking approved products for use in horses, veterinarians may consider using vaccines manufactured for other species or custom-manufactured vaccines when an infectious agent causes morbidity in a group of horses," Wilson said. Lyme disease, leptospirosis (which causes uveitis and abortion), and clostridial enteritis all lack vaccines labeled for horses.

Anecdotal reports suggest using a canine Lyme disease vaccine could be protective for horses. Additionally, equine veterinarians have successfully used cattle and swine leptospiral vaccines, and a recent study showed the swine vaccine promoted a longer time interval between uveitis flare-ups in horses. Cattle vaccines or custom-made bacterins (vaccine made by a private company from the strain of bacteria isolated on that farm)  have been applied on farms where management factors have not helped control cases of clostridial enteritis.

In all cases of extralabel use, it's important for the owner to understand the risks of "unproven" biologicals in case reactions occur or the vaccine is not protective. "I would advise you to get written consent (from the owner before using some of these vaccines)," Wilson said to the veterinarians.


"I think as veterinarians, we need to use the available information that we have to guide our vaccination choices," Wilson said. "I think we need to look at the individual and farm risks and optimize on what's best for the horse, and certainly do a better job at educating our clients. Owners must understand that vaccines are tools, not panaceas, and they must be combined with other preventive measures to reduce the risk of infectious diseases in their horses."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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