To explain this controversial issue of vaccines so you can make an informed decision for your horse, EW turned to one of the foremost authorities on vaccination in the veterinary world – researcher, lecturer and veterinarian Dr. W. Jean Dodds.

EW: What are the challenges with vaccines?

Dr. D: We all want the best for our animals. 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.

EW: How do vaccines work?

Dr. D: When an animal is vaccinated, its 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. feline immunodeficiency virus, feline leukemia virus, bovine leukemia virus, and equine infectious anemia).

EW: What’s the difference between “killed” and “modified live vaccines”?

Dr. D: Over the years, researchers have developed two types of vaccines – modified live virus (MLV) and killed or inactivated virus vaccines. Most single and combination vaccines available today for dogs and cats are of MLV origin, because  they’re less expensive to make and usually produce more sustained protection. By contrast, horses have traditionally been immunized with killed vaccines, although safe and effective 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?

EW: When is the safest time to vaccinate?

Dr. D: While veterinarians and vaccine manufacturers 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.

EW: What about adverse vaccine reactions?

Dr. D: 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). 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. 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, and laminitis.

EW: Is over-vaccination dangerous for horses?

Dr. D: 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 Not only is over-vaccination dangerous to your horse’s health, it also costs you time and money you don’t need to spend. Giving boosters annually or even more frequently as recommended for several equine diseases is likely to be of little benefit to your animal’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.

EW: Is titer testing a viable alternative?

Dr. D: By taking a blood sample, your vet 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. If you haven’t already done so, I would encourage you to get titer tests done to determine your horse’s status of immunity before vaccinating again. In this way, you may avoid the problems that arise from over-vaccination.

Reasons for vaccine titer testing:

  1. To determine if an animal is protected (suggested by a positive test result).
  2. To identify a susceptible animal (suggested by a negative test result).
  3. To determine whether an individual animal has responded to a vaccine.
  4. To determine whether an individual vaccine is effectively immunizing animals.

* Schultz R.D., Ford R.B., Olsen J., Scott F. “Titer testing and vaccination: a new look at traditional practices”. Vet Med, 97: 1-13, 2002 (insert). 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 Author: W. Jean Dodds, DVM, President, Hemopet, 938 Stanford Street, Santa Monica, CA 90403. References Dodds, W.J. “More bumps on the vaccine road”. Adv Vet Med 41: 715-732, 1999. Dodds W.J. “Vaccination protocols for dogs predisposed to vaccine reactions”. J Am An Hosp Assoc 38: 1-4, 2001. Dodds W.J. “Complementary and alternative veterinary medicine: the immune system”. Clin Tech Sm An Pract 17: 58-63, 2002. Goodman L.B., Wagner B., Flaminio M.J. 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. Paul M.A. (chair) et al. “Report of the AAHA Canine Vaccine Task Force: 2006 canine vaccine guidelines, recommendations, and supporting literature”. AAHA, March 2006, 28 pp. Schultz R.D. “Current and future canine and feline vaccination programs”. Vet Med 93: 233-254, 1998. Schultz R.D., Ford R.B., Olsen J., Scott F. “Titer testing and vaccination: a new look at traditional practices”. Vet Med, 97: 1-13, 2002 (insert). Seino K.K., Long M.T., Gibbs E.P. 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. Tizard I. “Risks associated with use of live vaccines”. J Am Vet Med Assoc 196: 1851-1858, 1990. Tizard I., Ni Y. “Use of serologic testing to assess immune status of companion animals”. J Am Vet Med Assoc 213: 54-60, 1998. Twark L., Dodds W.J. “Clinical application of serum parvovirus and distemper virus antibody titers for determining revaccination strategies in healthy dogs”. J Am Vet Med Assoc 217: 1021-1024, 2000.