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VETERINARY VIEWS
The first vaccine induces a primary response, and subsequent vaccines and future boosters then induce a secondary - or memory - immune response that then conveys immunity.
FREQUENCY OF VACCINES FOR HORSES
Vaccine guidelines are developed from scientific evidence that demonstrates reduced risk of contracting infectious disease following disease challenge post-vaccination. Vaccine recommendations for horses are based on available products licensed by the USDA (United States Department of Agriculture) and on AAEP guidelines. USDA licensing requires clear evidence that horses develop a protective immune response from vaccine constituents.
Immunologic memory refers to the immune response against an infectious pathogen that reacts quickly after the immune system has been primed with an initial “primary” series
of vaccines. Vaccine protocols are designed for young horses to begin receiving vaccines in the first year of life. The first vaccine induces a primary response. Subsequent vaccines in the primary series and future boosters then induce a secondary (anamnestic or memory) immune response that then conveys immunity.
Despite the fact that vaccines produce immunologic memory, circulating vaccine- specific antibodies and immune cells drop in concentration and numbers after a few months; annual boosters keep these numbers somewhat steady. For diseases with a high risk of infection, such as mosquito-borne viruses and high- prevalence respiratory diseases transmitted by direct or close contact, periodic vaccination is recommended. This is especially true for horses in training and competition that are in close contact with large numbers of other horses.
Tetanus is one pathogen that is ubiquitous in a horse’s environment due to equine fecal matter that contains a large degree of bacterial (Clostridium tetani) contamination. The potential for exposure in a wound is amplified in this kind of environment, thus necessitating annual boosters for horses against tetanus.
There are differences in frequency of recommended booster immunizations against respiratory and mosquito-borne viruses
depending on risk of pathogen exposure. Horses in a closed herd situation require
fewer respiratory vaccines than horses in training or competing in group activities. Some vaccines are inactivated (killed) and induce host protection, but protection may
not last more than 4-5 months. This supports the rationale for providing booster respiratory immunizations once or twice a year. Similarly, mosquito-virus vaccines are given in advance of mosquito season to confer the highest level of immunity, and in warmer climates horses may be boosted twice a year.
KILLED VERSUS MODIFIED LIVE VACCINES
A killed vaccine is composed of an inactivated pathogen or a component (subunit) of that pathogen, whereas a modified live vaccine (MLV) maintains just enough viability of the organism for it to replicate in the host yet not cause disease. The major difference is in how the immune system responds. Inactivated vaccines are effective at inducing antibody responses that provide the host with protection at the time of exposure or challenge, but this response may not last for an extended period of time. In contrast, modified live vaccines generally induce responses from multiple arms of the immune system.
Because MLV vaccines mimic natural infection, they are a better approach against pathogens with more complex pathophysiology and virulence in which cellular immunity is also required. Yet, some killed vaccines (EEE, EWE, tetanus) induce robust production of neutralizing antibodies that are effective in controlling these specific infections. Their proven effectiveness demonstrates that vaccinated horses do not get sick while horses that are not vaccinated do.
Tetanus is
in a horse’s environment
due to equine fecal matter that contains a large degree of bacterial contamination and makes the potential for exposure in a wound amplified, thus necessitating annual boosters.
Some vaccines are inactivated and induce host protection, but protection may only last 4-5 months which supports the rationale for providing booster immunizations once or twice a year.
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If contact with other horses is anticipated, then risk-based vaccines such as EHV-1/4 (herpes or rhinopneumonitis), EIV (influenza) and strangles, should be considered.
INTRAMUSCULAR VERSUS INTRANASAL VACCINES
The purpose of using intranasal vaccination in horses is to induce local immunity at the
site where the pathogen invades – in this case, the respiratory tract. For example, intranasal influenza vaccine has two advantages: It confers both local (mucosal) and rapid protection.
However, if the goal of vaccination is to induce a memory antibody response that can be transferred from host to another individual such as a broodmare providing protection to her foal through high quality colostrum, then an intramuscular vaccine is more desirable.
HERD IMMUNITY
The concept of herd immunity is important in groups of horses. If all individuals are immunized within a group, then it is less likely that an individual animal will succumb to infection. Overall risk of disease is lowered because there is far less chance for disease exposure to occur. Vaccination, particularly of the bulk of horses in a herd, decreases severity of disease and shedding of organisms, therefore decreasing transmission between individuals.
Here is a comparative scenario of how herd immunity works:
A group of 30 horses are not immunized against equine influenza virus (EIV).
Upon exposure, the horses develop variable severity and duration of disease; the virus spreads among the group until all have developed disease and recovered or had