Vaccines and titer testing — an update

Which vaccines does your horse really need, and how often?

National guidelines for animal vaccines have been around for more than a decade. So why is a vocal cohort still reluctant to embrace this knowledge? After all, the national policies are all based on scientific fact. Do vaccine industry representatives fail to inform veterinarians about the duration of immunity, and the true regional and local needs for some types of vaccine? Are the risks as well as the benefits of today’s vaccines adequately discussed, given the fact that the newer guidelines reflect concerns about over-vaccination? This article summarizes the current issues about equine vaccines – and the alternative of titer testing.

Veterinarians may not be inclined to “fix” what is perceived to be unbroken when it comes to vaccines. And vaccination programs are viewed as “practice management tools” rather than medical procedures, so a “more is better” philosophy still prevails.

Benefits vs. risks

Modern vaccine technology has afforded effective protection of horses against serious infectious diseases. However, this advancement brings increased risk of adverse reactions (vaccinosis) – some are serious, chronically debilitating and even fatal. As caretakers of our companions we must balance this benefit/risk equation. In the words of Dr. Ron Schultz: “Be wise and immunize, but immunize wisely.”

Vaccines and herd health

In order to protect the population (herd), 70% of the horses should be immunized with “core” vaccines. However, horses receive more vaccinations on a more frequent schedule than any other domesticated animal. This practice has horse people from all walks of life asking questions. Vaccines are intended to protect against disease. So why are we causing disease by weakening the immune system with frequent use of combination vaccine products?

The core vaccines for horses include:

• Equine Herpes Virus

• Equine Influenza Virus

• Equine Encephalitis Virus (EEE /WEE)*

• Potomac Horse Fever

• Rabies Virus*

• Tetanus*

• West Nile Virus*

*Vaccines considered as “core” by AAEP, 2012

Vaccines and latent disease

Vaccine manufacturers seek to achieve minimal virulence (infectivity) while attaining maximum protection. This desired balance may be relatively easy to achieve in clinically normal, healthy animals, but what about those with compromised immune systems?

Horses harboring latent viral infections may not be able to withstand additional immunological challenges induced by vaccines. Further, stress associated with weaning, surgery, transportation, and subclinical illness can also compromise immune function. Reports of vaccine reactions and vaccinerelated diseases are on the rise throughout the animal world. Health issues in horses attributed to adverse vaccine reactions include fever and nasal discharge, temporary blindness, thrombocytopenia, muscle wasting or weakness, and laminitis.

Killed vs. modified live vaccines

In contrast to dogs and cats, horses have traditionally been immunized with killed vaccines. Safe and effective MLV (modified live) equine vaccines have more recently become available. MLV vaccines are less expensive to make and usually produce better and more sustained protection. A recent equine study compared killed and MLV equine herpes virus type 1 (EHV-1) vaccines, and found the MLV vaccine offered superior protection when tested by aerosol challenge. But another recent study comparing killed, MLV, and live-chimera West Nile Virus (WNV) vaccines found 100% protection with all three types following challenge with virulent WNV.

Risk-based guidelines (from AAEP, 2012)

The following vaccinations are included in a vaccine program after the performance of a risk-benefit analysis. The use of risk-based vaccinations may vary regionally, from population to population within an area, or between individual horses within a given population. Disease risk may not be readily identified by laypersons; it is important to consult a veterinarian when developing a vaccination program.

• Anthrax
• Botulism
• Equine Influenza
• Potomac Horse Fever
• Rotaviral Diarrhea
• Strangles

Why the resistance?

So why is there resistance to current vaccination guidelines? Possible reasons include:

• Failure of veterinarians to offer options (titers) to horse owners

• Poor horse owner awareness and education

• Lack of trust in veterinarians

• Perceived conflict of interest in veterinary settings (profit versus options)

• Failure to recognize or denial of adverse events

• No legal mandate standard for rabies vaccination

Practical solutions

Given this troublesome situation, and the lack of evidence for the need of yearly booster revaccination, what are the experts saying about these issues? In 1995, a landmark review commentary focused the attention of the veterinary profession on the advisability of current vaccine practices. Discussion of this provocative topic has generally led to other questions about the duration of immunity conferred by the currently licensed vaccine components.

More on serologic vaccine titer testing

Except where vaccination is required by law, all animals can have serum antibody titers measured triennially or more often, as needed, instead of revaccination. Reliable serologic vaccine titering is available from several university and commercial laboratories and the cost is reasonable.

When an adequate immune memory has already been established, there is little reason to introduce unnecessary antigen, adjuvant, and preservatives by administering booster vaccines. By titering triennially or more often, as needed, one can assess whether a given animal’s humoral immune response has fallen below levels of adequate immune memory. In that event, an appropriate vaccine booster should be considered.

But some veterinarians have challenged the validity of using vaccine titer testing to assess the protection status of animals against the clinically important infectious diseases. With all due respect, this represents a misunderstanding of what is called the “fallacy of titer testing”, because research has shown that once an animal’s titer stabilizes it is likely to remain constant for many years. You should avoid vaccinating horses that are already protected.

Available vaccine titers for horses

• Equine Herpes (EHV-1, and -4) (rhino)

• Potomac Horse Fever

• Equine Encephalitis (EEE, WEE, VEE)

• Equine Viral Arteritis

• Equine Influenza

• Rabies Virus (RFFIT: non export)

• West Nile Virus Antibody Titer

Furthermore, protection as indicated by a positive titer result is not likely to suddenly drop off unless an animal develops a medical problem such as cancer or receives high or prolonged doses of immunosuppressive drugs. Viral vaccines prompt an immune response that lasts much longer than that elicited by classic antigen. But, interpreting titers correctly depends on the disease in question. Some titers must reach a certain level to indicate immunity. A positive titer test result is fairly straightforward, but a negative titer test result is more difficult to interpret, because a negative titer is not the same thing as a zero titer and doesn’t necessarily mean the horse is unprotected.

Finally, what does more than a decade of experience with vaccine titer testing reveal? In general, serum antibody titers to the “core” vaccines along with any natural exposures last considerably longer than suggested by the vaccine label booster recommendations. This corresponds with what we see clinically as the number of cases, illnesses and deaths due to these diseases has decreased in the vaccinated population. So in contrast to the concerns of some practitioners, vaccine titer testing as a means to assess vaccine-induced protection will likely result in the horse avoiding needless and unwise booster vaccinations.


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AAEP, Vaccination Guidelines, 2012.

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Seino KK, Long MT, Gibbs EP et al. Investigation into the comparative efficacy of three West Nile Virus vaccines in experimentally induced West Nile Virus clinical disease in horses. AAEP Proceed 52:233-234, 2006.

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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.