How Vaccines Work

A vaccine is designed to mimic a natural infection. In a natural infection, the body’s immune cells recognize molecules on the surface of a pathogen (such as a virus, bacteria, or bacterial toxin) as foreign. The body’s protective immune response is triggered, resulting in the production of antibodies specific to the pathogen in question. Antibodies bind to the pathogen, inhibiting its function and triggering its destruction. Once an infection has been cleared, specialized cells remain in the body that are ready to produce the same antibodies to fight the same pathogen if it is encountered again.

Similarly, vaccines contain a weakened form or a component of a pathogen. The body’s immune system recognizes these particles as foreign and forms an immune response to neutralize it. When the real pathogen in question is encountered, the body is then prepared to mount a quicker and stronger immune response because the immune system has already been prepped by fighting off the vaccine’s simulated infection.

Components of a Vaccine

• Antigens - This may be a virus, bacteria, component of a virus/bacteria, or a toxin. This is what will be recognized as foreign by the immune system, and will trigger a protective immune response to prepare the body for fighting this infection in the future.

• Stabilizers - These ingredients help the vaccine maintain its efficacy during storage.

• Adjuvants - Not all antigens stimulate the same degree of immune response. Adjuvants are designed to enhance, accelerate and prolong the immune response to vaccine antigens.

• Antibiotics - Trace amounts are used during the manufacturing phase to prevent bacterial contamination.

• Preservatives - These are often added to multidose vaccine vials to prevent bacterial and fungal growth.

Several Common Types of Vaccines

• Live, attenuated vaccines (example: Strangles) - These contain a weakened form of the living microbe that can no longer cause disease. These vaccines do an excellent job of mimicking a natural infection, so they tend to cause a strong immune response and provide good immunity with fewer initial doses required. Because these vaccines contain a live microbe, they can rarely mutate and become more virulent than they were designed to be. For example, the modified live strangles vaccine given intranasally to very young foals can, very rarely, lead to the formation of mandibular or retropharyngeal abscesses.

• Inactivated vaccines (example: Rabies) - The disease-causing microbe is killed with chemicals, heat, or radiation. These vaccines often stimulate a weaker immune system response than that caused by live vaccines, and often require more initial doses, or the addition of an adjuvant. A variety of adjuvants are in use, and different horses’ immune systems may react to these different ingredients with varying intensity. Occasionally, an adjuvant may be a bit too effective with an individual’s immune system, triggering a strong enough immune response to result in adverse effects.

• Toxoid vaccines (example: Tetanus) - When a bacterial toxin is the main cause of illness, as is the case with Tetanus, a protective vaccine may contain an inactivated form of the toxin - a toxoid. As with any other vaccine, the immune system recognizes the toxoid as foreign and produces antibodies accordingly. When the actual toxin is encountered, these antibodies are ready to bind to the toxin and inactivate it.

Adverse Vaccine Reactions

If your horse has any type of vaccine reaction, it is important to communicate this to the veterinarian who administered the vaccine. They can make recommendations for managing the current vaccine reaction, as well as for planning future vaccinations. Your veterinarian will also want to report the reaction to the vaccine manufacturer. Manufacturers need this information so that they can continue to gather information to monitor the safety of their products.

The most common vaccine reaction encountered in horses is swelling and soreness at the injection site following an intramuscular vaccine. This typically resolves within several days with minimal, if any, treatment.

Occasionally a horse will experience a transient period of fever, decreased appetite, or lethargy following routine vaccinations. These reactions can often be managed easily at home with veterinary guidance.

As with any intramuscular injection, a small risk exists for abscess formation at the site of vaccine injection, and this would require additional attention from a veterinarian. Very rarely, severe reactions can occur such as urticaria (hives), purpura hemorrhagica (a serious type of vasculitis), colic, or anaphylaxis. Signs of anaphylaxis in horses might include dyspnea (difficulty breathing), or respiratory distress; sweating; colic; and diarrhea. These reactions warrant immediate veterinary treatment. 

Avoiding Vaccine Reactions

In general, the risk of adverse reactions is slightly higher when multiple vaccines (and therefore multiple antigens and adjuvants) are administered at the same time. Some horse owners and veterinarians prefer to vaccinate horses in a staggered schedule, with a 3-4 week interval between vaccines. In addition, it is important to discuss risks and benefits of the various vaccines available for any individual horse. Reducing the overall number of vaccines a horse is given will reduce the risk of that horse developing adverse reactions to vaccines. For example, a horse in a boarding stable that regularly travels to shows will likely be managed with a different vaccination regimen than a horse that lives alone and never leaves the farm.

Horses that have previously had adverse reactions to vaccines may benefit from making several management adjustments for future vaccinations. The brand used (and therefore vaccine ingredients), site of injection, or route of administration can be changed. For example, a horse that reacts adversely to an intramuscular strangles vaccine one year may tolerate an intranasal strangles vaccine very well the following year. Some veterinarians may also suggest administering an anti-inflammatory or antihistamine prior to vaccine administration. 

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