The Equine Immune System and Vaccine Basics

To understand a bit more about how vaccines work we must first understand a bit about the immune system and immunity, so bear with me.

There are two basic components of the immune system, these being innate and acquired or adaptive immunity.

Innate immunity is what your body has in place when you are born. These are physical barriers to pathogens (viruses, bacteria, etc.) such as your skin, mucous membranes and the lining of your respiratory and gastrointestinal tract. The body also utilizes enzymes, tears, skin oils and stomach acid to rid the body of pathogens. This part of your immune system acts quickly but has no “memory” meaning when it sees a pathogen once it doesn’t remember it when encountered again.

Acquired immunity occurs in a few ways: active, passive, humoral and cell mediated. An example of active acquired immunity would be when you are exposed to a cold and your body mounts an immune response, then when you encounter that same cold again you don’t get sick (or don’t get as sick) because your immune system has already encountered this pathogen and knows how to get rid of it. Vaccination is another type of acquired immunity, this simulates natural infection but usually with a modified or killed version of the pathogen. Passive immunity is gained through maternal antibodies from the placenta while in the womb or through breast milk. Plasma created from horses that are vaccinated providing foals with ready-made antibodies is another example of acquired immunity. The humoral and cell mediated are the more complex “sciency” ways that the body mounts an immune response by creating different types of white blood cells (T and B lymphocytes) tasked with destroying specific pathogens.

 

What are we looking for protection wise from vaccines?

Individual protection

Herd immunity

Decreased illness or shedding of the virus to decrease the risk to horses around them

 

What is important for you to understand about vaccines?

Vaccination does not mean complete immunity. Vaccination aids in the prevention of disease and decreases the severity of the disease. Protective immunity from vaccination can be overwhelmed in circumstances such as overcrowding, stress, massive amounts of virus or bacteria, horse is exposed to pathogen too soon after vaccination or in cases where the immune response to the vaccine for that particular animal is not appropriate to provide protection.

 

Vaccine reactions or adverse events

These are unfavorable or unintended events following a vaccine. These include muscle soreness that is prolonged for more than 2-3 days, or a systemic reaction such as colic or seizure (these complications are very rare and unable to be predicted).

It is important to note that mild soreness and low-grade fever/lethargy isn’t necessarily a “vaccine reaction,” this is the body’s immune system mounting an appropriate response to a pathogen.

There is an increased risk of muscle soreness and lethargy in horses that are getting multiple vaccine injections in one visit. This occurs because of several things; there is quite a lot of volume being injected into the horse’s muscles when multiple vaccines are administered in one visit and inherently this is uncomfortable. An example is if the horse is getting its Rabies, EWT/WN, Botulism and Flu/Rhino vaccines all in one appointment that is 7ml of fluid that is being put into muscles which can be quite uncomfortable! Think of how your arm feels after a vaccine, it’s the same for horses. Multiple vaccines mean multiple adjuvants, this is the part of the vaccine that sets the immune system up to create a response and prevent future infection, more adjuvant = more immune response.

 

What can we do to decrease these “vaccine reactions” or adverse events? First, if possible split vaccines up – meaning only give one or two at a time if that is feasible for your situation. Ideally if we are going to split vaccines, we will give them 3-4 weeks apart. Second, we can use combination vaccines meaning that in one vaccination, or with only one needle, we can vaccinate for more than one disease (for example EWT/WN combo vaccine), less adjuvant and more disease protection in one injection.  This isn’t the answer for each horse but this is where we can help you decide what is best for your horse in your situation.

 

What about anti-inflammatories such as bute/banamine before and/or after vaccinations? Studies have shown that the use of NSAIDs or steroids (bute/banamine/dex) decrease the body’s immune response. It is best to not administer these 24 hours prior to or for 24 hours following the vaccination to allow the horses immune system to do its job, though we understand there are cases when these drugs to need to be utilized and again that is something that we can discuss with you. The use of local anti-inflammatories such as surpass has not been studied.

 

How often do I need to vaccinate my horse? We get this question a lot and the answer is: It depends. Most equine vaccines are boostered annually while others require more frequent boosters. For example, the rabies vaccine is boostered annually while Flu/Rhino and EWT should be boosted twice yearly in most cases.

 

            How do I decide what vaccines my horse needs? The American Association of Equine Practitioners has vaccines broken down into categories, Core vaccines and Risk-based vaccines. It is important that ALL horses receive the core vaccines at the recommended intervals. Risk based vaccines are more individualized and are given based on individual lifestyle and need.

The list of core vaccines may seem extensive but these diseases have high morbidity and many of them can be fatal especially to horses that are not vaccinated against them. Prevention is easy with vaccines and luckily many of these vaccines are offered in combinations which reduce the number of needle sticks, and adjuvants that your horse will have to receive at their vaccine appointment. We are happy to offer a core combo vaccine that encompasses all of the core vaccines into one, so with one injection we can vaccinate your horse for Rabies, Eastern/Western Encephalitis, West Nile Virus and Tetanus!

 

Below is the AAEP list of core and risk-based vaccinations:

Please don’t hesitate to contact VMC if you have any questions.

Dr. Caitlin Hutcheson,  BVMS