Vaccines: What does your horse need?

The purpose of a vaccine is to reduce the severity of or prevent disease. The CDC states “vaccines help the body learn how to defend itself from disease without the dangers of a full-blown infection.” In almost all instances a vaccine will be more cost effective than treatment of a disease and will also result in fewer side effects or long term effects that may be present following disease. In some cases vaccines don't prevent disease completely, but they do provide the immune system with the appropriate tools needed to fight disease and ultimately prevent death as well as reduce recovery time. 

How does a vaccine work? We are injecting a part of a bacteria or virus that has been modified or killed in order for the immune system to recognize it and create antibodies against it while avoiding infection with the live bacteria or virus. There are two primary categories of immunity. These include active immunity and passive immunity. Active immunity is what the vaccines are affecting. This is the natural immunity that the body forms against disease over time as we are exposed to infective material in the environment.  This is the immunity you were developing when your parents told you to play in the dirt.  Passive immunity is what our foals receive from their mother through colostrum within the first 12 to 24 hours after birth and also includes blood products such as plasma or immunoglobulin (Ig). These are used for particular treatments in cases where antibodies are needed immediately to prevent infection and include things like tetanus antitoxin that is often given to foals to aid in the resistance of tetanus, especially after birth from mares that have not received a tetanus vaccine.  

The efficacy of vaccines is highly variable and dependent on a number of factors such as biosecurity, timing of administration, and proper administration. Always ensure you are washing your hands and not using the same tools/equipment between horses, especially those horses which may be ill. This will aid in preventing direct transfer of infectious material between horses. We should always be cognitive of our timing and route of administration when vaccines are given. Most vaccines are given in the muscle. Some vaccines are given intranasally. Improperly giving an intranasal vaccine in the muscle can result in serious complications such as local abscesses or more severe reactions requiring emergency medical treatment.  

Vaccinated horses are not all protected equally. Not all horses will have the same innate immune response to a vaccine meaning they will have variable levels of immunity against active infection. This is why it is vital for all horses in a herd to be vaccinated. This aids in development of herd immunity.  Meaning that when a horse is added to the herd or a horse leaves the farm and is around another horse that has not been vaccinated it is less likely to expose a horse in the herd that has not had a complete response to a vaccine. An important note on how vaccines provide immunity: the minimum amount of time required for minimum protection from a vaccine is 2 weeks and the maximum response to a vaccine requires 3 to 4 weeks.  When you are planning to go to shows/sales, this should be accounted for so you can ensure your horse is protected. 

Core vaccinations: what makes a vaccine a core vaccine? Core vaccines are vaccines against the diseases endemic to a region. We know they are here. We know that we have them. We know they pose a risk of severe disease aka they are highly virulent or highly contagious. These also include vaccines with a public health significance, such as rabies, or in some cases, are required by law. In most cases, our core vaccines are those which have clearly demonstrated efficacy and safety. The core vaccines have a high enough level of patient benefit and low enough level of risk to justify their use in all equids, essentially meaning it is way safer to give these vaccines than it is to risk the disease coming into our population. Core vaccines are recommended for every horse, and are usually given as part of a combination vaccine. The core vaccines include Eastern/Western Equine Encephalitis, Rabies, Tetanus, and West Nile Virus. 

Non-core vaccines are risk based vaccines. These are dependent on the location of the patient and circumstances such as travel and going to sales or shows. There are many non-core vaccines which include: anthrax, botulism, Equine herpes virus (1 and 4), equine viral arteritis, Equine influenza, leptospirosis, potomac horse fever, rotavirus, snake bite, and strangles. A few of these that we see locally and recommend giving are  Equine Herpes Virus (EHV 1&4) and Equine Influenza. These are going to be more important for shows, sales, or trail riding in areas where many other horses may be present. EHV is going to be especially important for brood mares during pregnancy to prevent the occurrence of abortion storms in a herd on a farm. Potomac Horse Fever is recommended for our patients that are used on trails or in a location with a lot of May flies, especially horses traveling to Beaver Creek as several cases can be attributed to trail riding in that area. Strangles is seen a lot in our area. Horses coming from local sale barns are frequently exposed to strangles while at the sale and may carry active infection or infectious material home with them from these sales. 

When to vaccinate my horse with core vaccines is a common question we get and is different for each horse depending on their age, history, and/or their use or stage of gestation (broodmares). Adult horses that have been vaccinated before should be vaccinated annually at the onset of the vector season (spring). Vectors are things such as mosquitos and mayflies which carry and transmit disease. Adult horses that have no vaccine history should get a 2 dose series, where the second dose is given 3 to 4 weeks following the first dose. Then the horse should continue to get vaccines annually. Broodmares should be vaccinated annually then 4 to 6 weeks pre-partum. This aids in the development of good quality colostrum. Foals get a 3 dose series starting the 1st dose at 4 to 6 months of age, 2nd dose at 3 to 4 weeks after the 1st dose, then the 3rd dose at 10 to 12 months of age followed by annual re-vaccination. The only exception is tetanus, all foals should receive tetanus anti-toxin at or near birth, especially if the mare was not vaccinated for tetanus.  The reason we begin vaccinations of foals at 4 to 6 months is due to antibodies from the mare being present up until that period of age. Around this time period, antibodies the foal received via colostrum will begin to wane, and our foals’ active immunity is beginning to develop. Giving a vaccine while antibodies from colostrum are still present may result in those antibodies from colostrum attaching to our vaccine particles and preventing the foals' own immune system from responding to the vaccine. 

When should I be vaccinating my horse with the non-core vaccines? This is dependent on the factors previously discussed. These guidelines should help your decision making process, and, if you have questions, we can help you assess your risk during an appointment. Let’s discuss EHV 1 and 4 as well as Equine Influenza. The efficacy of these, per the manufacturer, is that they should be protectice for about 1 year. We know that after about 2 to 3 months the efficacy is greatly reduced. Therefore we recommend any horses that are in breeding barns get vaccinated more often, and broodmares should be getting vaccinated at 5, 7, and 9 months of gestation as well as 4 to 6 weeks pre-partum with the core vaccines. The goal is to prevent abortion storms. Those horses that are going to be sold at a sale or going to a show should be given the vaccine at least 2 weeks prior to aid in developing their immunity prior to the stressful event. Some practitioners suggest vaccinating show horses every 2 to 3 months during show season to improve overall immunity against respiratory disease due to the high risk of exposure. Potomac Horse Fever should be given along with the core vaccines in the spring and the same protocol as core vaccines for horses that are vaccinated historically or not applies. The one exception is broodmares who should be vaccinated with the 1st dose at 7 to 9 weeks pre-partum, then 2nd dose 4 -6 weeks pre-partum if not previously vaccinated. If the broodmare was previously vaccinated then 1st dose annually then 2nd dose 4 -6 weeks pre-partum. Strangles in adult horses and broodmares should be given as a 2 dose series with a 2 to 3 week interval. Foals should be given a 2 dose series beginning at 9 months of age. If a horse's strangles history is unknown it is recommended to get a titer (SeM Antibody ELISA) to ensure that the horse is not at an increased risk of developing complications from the strangles vaccine. If the horse is in a high risk situation of contracting the disease, you should consult your veterinarian to see what the vaccine manufacturer’s recommendations are.

That was a lot of information, and I hope you have learned some useful information and enjoyed this short article. Let’s finish up with some information about vaccine reactions. The first thing you should know is that anaphylactic reactions to vaccines that result in symptoms such as severe hives, colic, laminitis, and severe swelling of the face and legs (purpura hemorrhagica) are quite rare. These reactions are estimated to occur in approximately 1 in 10,000 horses. More often what you will see is a local reaction to a vaccine that will result in some mild pain and swelling at the injection site, a low grade fever, or anorexia that does not persist beyond 24 hours. This latter, low grade reaction is often fairly easy to manage with some anti-inflammatory medications and in many cases, no medication is necessary. If you are unlucky enough to have a horse that is prone to more severe reactions, the risk of vaccination can be mitigated in multiple ways. This includes pre-treating your horse with an anti-inflammatory or antihistaminergic medication to reduce inflammation while still allowing your horse to develop an immune response to the vaccine or trying a different product from a different manufacturer which oftentimes will not contain the same adjuvant and may result in a less severe reaction in your horse. As always, if your horse is having a severe reaction, the best practice is to call your veterinarian to talk through the symptoms and assess the need for treatment. We here at Carrollton Equine are happy to talk to you about your horse and help guide your treatment and supportive care vaccine reactions as well as any of your equine medical needs in Carroll county and the surrounding communities.  

References 

https://www.cdc.gov/vaccines/basics/explaining-how-vaccines-work.html#:~:text=how%20they%20work.-,Vaccines%

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etime.

https://aaep.org/guidelines-resources/vaccination-guidelines/principles-of-vaccination/https://aaep.org/wp-content/uploads/2024/02/Adult_Horse_Vaccine_Chart_2023_SUB.pdf

https://www.merckvetmanual.com/nervous-system/equine-arboviral-encephalomyelitis/equine-arboviral-encephalom

yelitis

https://www.merckvetmanual.com/nervous-system/rabies/rabies-in-animals#Epidemiology_v83306256

https://aaep.org/wp-content/uploads/2024/02/Foal_Vaccine_Chart_2023_SUB.pdf

https://www.merckvetmanual.com/nervous-system/equine-arboviral-encephalomyelitis/equine-arboviral-encephalom

yelitis?query=west%20nile%20virus%20in%20equines#Etiology-and-Epidemiology_v3289143

https://www.merckvetmanual.com/respiratory-system/respiratory-diseases-of-horses/equine-herpesvirus-infection?q

uery=ehv-1#Etiology-and-Epidemiology_v3293806

https://www.merckvetmanual.com/digestive-system/infectious-diarrheal-diseases-in-horses/potomac-horse-fever?ga

d_source=1&gclid=CjwKCAiAzba9BhBhEiwA7glbasF_1b2YBSbwK4IIVHElsBTrI5tuum-0OkKK8dlSXaijuvfQPs4Noh

oC2DIQAvD_BwE&gclsrc=aw.ds

https://aaep.org/guidelines-resources/vaccination-guidelines/adverse-reactions/#:~:text=After%20receiving%20a%20

vaccine(s,requiring%20prolonged%20treatment%20and%20convalescence