Horses receive more vaccinations on a more frequent schedule than any other domesticated animal. Based on a protocol of fear, not fact, this practice has horse people from all walks of life asking questions. To explain this controversial issue so you can make informed decisions for your patients, we turned to one of the foremost authorities on vaccination in the veterinary world – researcher, lecturer and veterinarian Dr. W. Jean Dodds. In this, the first of a two-part series, Dr. Dodds provides an overview of vaccination and raises some interesting points about this complex topic.

We all want the best for our patients. That includes providing them with proper nutrition and good health care. It also includes protecting them against disease, which is why researchers first developed vaccines. 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?

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? Animals harboring latent viral infections may not be able to withstand the additional immunological challenge induced by vaccines. In addition, the stress associated with weaning, transportation, surgery, and subclinical illness can also compromise immune function. It’s no surprise, then, that reports of vaccine reactions and vaccine-related diseases are on the rise throughout the animal world.

Overview of the immune system

When an animal is vaccinated, his immune system responds by producing two types of specialized white blood cells called lymphocytes. As the name suggests, lymphocytes are produced by the lymphatic organs (bone marrow, thymus, lymph nodes and spleen). You’ll find them throughout the body — in circulating blood and body fluids as well as in the tissues.

These lymphocytes, which are descendants of the bone marrow’s pool of “mother” stem cells, form a cooperative interaction between the circulating (humoral) immune system and the cellular (cell-mediated) immune system. Think of them as a tag-team working together to provide short and long-term protection. The team is made up of two types of cells:

1. B-Cell immunity (humoral) These antibodies provide an important defense mechanism against disease in healthy individuals but can become hyperactive or hypoactive in a variety of acute and chronic disease states, or in the rare genetically based immunodeficiency status.

2. T-Cell immunity (cellular) These lymphocytes act as coordinators and effectors of the immune system (the lymph nodes, thymus, spleen and intestine are also involved). Hyperactive cellular immune responses produce autoimmune and other immune-mediated diseases (e.g. autoimmune or immune-mediated hemolytic anemia, immune-mediated thrombocytopenia, pemphigus, rheumatoid arthritis, and systemic lupus erythematosis) while hypoactive cell-mediated immunity causes immune suppression and incompetence. Classical examples of this latter situation occur with retroviral infection such as human AIDS or the animal equivalents (e.g. equine infectious anemia).

Killed versus modified live vaccines

Over the years, researchers have developed two types of vaccines – modified live virus (MLV) and killed or inactivated virus vaccines. Horses have traditionally been immunized with killed vaccines, although MLV equine vaccines have more recently become available. A long-standing question remains, however, about the comparative safety and efficacy of MLV versus killed virus vaccines, especially when a properly constituted killed vaccine is safer.

A published study of the risks posed by MLV vaccines concluded that they are intrinsically more hazardous than inactivated products. The residual virulence (infectivity) and environmental contamination resulting from the shedding of vaccine virus is of concern not only for domestic animal populations but also for wildlife. So why would anyone use them?

The answer is simple: they appear to provide better and longer protection. Giving single (mononvalent) or combined (polyvalent) viral antigens of MLV type elicits a stronger antigenic challenge to the animal. This is often viewed as desirable because a more potent immunogen presumably mounts a more effective and sustained immune response. For instance, a recent equine study comparing killed and MLV equine herpesvirus type 1 (EHV-1) vaccines found the MLV vaccine offered superior protection when tested in an aerosol challenge.

But every rule has its exception. A 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. In this instance, they all provided adequate protection, so wouldn’t it make sense to use the safest vaccine available?

In the next part of this series, I’ll be reporting on the specific vaccines available and issues of concern regarding each one. It’s important you know the pros and cons of each so you can make the best choice for your patients’ individual needs. It’s equally important that you know when to vaccinate.

When is the safest time to vaccinate?

While we are aware of the general rule not to vaccinate animals during any period of illness, relatively little attention has been paid to the hormonal status of the patient. The same principle that applies for illness (don’t vaccinate when a horse is sick) should apply to times of physiological hormonal change. This is particularly important because the combination of hormonal change along with infectious agents can trigger an autoimmune disease.

Regardless of what you hear, vaccinating animals at the beginning, during or immediately after an estrous cycle is unwise, as is vaccinating animals during pregnancy or lactation. In horses, the WNV vaccine is stated to be safe for pregnant mares, although in 2005 the American Association of Equine Practitioners recommended vaccinating mares before breeding when possible.

Research in cattle shows the MLV herpes virus vaccine induces necrotic changes in the ovaries of heifers that were vaccinated during estrus. Even heifers that were not vaccinated but shared the same pasture were affected. In addition, vaccine strains of these viral agents are known to be causes of abortion and infertility. If one extrapolates these findings from cattle to other species, including horses, the implications are obvious.

Adverse reactions

When we refer to vaccine reactions, we’re talking about more than just immediate hypersensitivity reactions such as redness and inflammation. Clinical signs associated with reactions typically include fever, stiffness, sore joints and abdominal tenderness, susceptibility to infections, neurological disorders including seizures and encephalitis, collapse with autoagglutinated red blood cells and jaundice (autoimmune hemolytic anemia, AIHA), or generalized pin point or blotchy hemorrhages (immune-mediated thrombocytopenia, ITP) and lamintis. Liver and kidney laboratory values may be significantly elevated, and liver or kidney failure may occur by itself or accompany bone marrow suppression.

Regardless of species, acute events tend to occur 24 to 72 hours after vaccination, or seven to 45 days later in a delayed immunological response. Even more delayed adverse effects include death in infants from high-titered measles, joint diseases in dogs from canine distemper antibodies, and feline injection-site fibrosarcomas. Though not as clearly documented, laminitis can occur soon after vaccination, but can also be delayed.

Viral disease and recent vaccination with single or combination vaccines are increasingly recognized contributors to immune-mediated diseases of blood and other tissues, bone marrow failure, and organ dysfunction. We know that potent adjuvanted killed vaccines like those for rabies virus can trigger immediate and delayed (vaccinosis) adverse vaccine reactions. It’s likely that the genetic predisposition to these disorders in humans has parallel associations in domestic animals, including horses.

Health issues in horses attributed to adverse vaccine reactions have included fever and nasal discharge, temporary blindness, thrombocytopenia, muscle wasting or weakness, anasarca or purpura hemorrhagica, lymphangitis and laminitis.


Curiously, while concerns about over-vaccination have been raised for years in dogs and cats, little has been said about the fact that horses routinely receive more vaccines more frequently than other species. For example, many horses are vaccinated annually for rabies, even though this vaccine is known to confer a longer duration of immunity — at least three and likely more years! Perhaps this just reflects the lack of awareness that the vaccine issues pertaining to dogs and cats also apply in principle to other species such as horses

Giving boosters annually or even more frequently as recommended for several equine diseases is likely to be of little benefit to a horse’s existing level of protection against these infectious diseases. It also increases the risk of adverse reactions from the repeated exposure to foreign substances.

The accumulated evidence indicates that vaccination protocols should no longer be considered as a “one size fits all” program.

In the next issue, I’ll discuss some of the vaccines used routinely in horses, their benefits and potential side effects, and available information on results of titer testing for these vaccines.

Vaccine titer testing

So how do you know if your patients are protected from disease? By taking a blood sample, you can use serum vaccine titer testing to assess the immunologic status of the animal against the common, clinically important infectious diseases. Research has shown that once an animal’s titer stabilizes, it is likely to remain constant for many years. “It is often said that the antibody level detected is only a snapshot in time,” states eminent expert Dr. Ronald Schultz, referring to the value of titer testing. “That’s simply not true; it is more a motion picture that plays for years”.

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. So once you have an acceptable titer, you shouldn’t have to repeat the test — and more importantly, re-vaccinate — for years to come.

Available vaccine titers for horses:

Equine herpes III (rhino) Potomac horse fever Equine encephalitis (EEE, WEE, VEE) Equine viral arteritis Equine influenza Rabies titer (RFFIT: non export) West Nile virus antibody titer Strangles Lyme disease


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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.