Equine Core Vaccines: Why Are They Important?

First off, what are equine core vaccines? Well, to answer that, you must first ask, what is a vaccine?

A vaccine is a preparation of living attenuated organisms, dead (killed) microorganisms, or living, fully virulent organisms. This preparation is administered to an animal to produce or artificially increase immunity to a specific disease.

In simpler terms, a small amount of organisms are injected to allow the immune system to learn how to fight them on a smaller scale.

The main goal of vaccinating is to develop individual and herd immunity against infectious diseases.

We use four main types of vaccines:

Modified-Live Virus: presents a weakened form of the virus
Killed (inactivated): presents small pieces taken from the virus or bacteria
Toxoid: presents an inactivated form of the toxin
Vector: uses a specific piece of DNA from the pathogen that is carried into the cells by a vector

Additionally, we administer vaccines to horses in one of two ways:

Intramuscular (IM): administered into the muscle (most common)
Intranasal (IN): administered through the nose

Diagram showing the acceptable intramuscular vaccination locations

Equine Core Vaccines

Equine (and all other) vaccines are easily separated into two categories. These categories reflect the susceptibility to the disease: core and risk-based vaccines. This week, I thought we would discuss the equine core vaccines.

Equine core vaccines protect against diseases that:

are endemic to a region
have a potential public health risk
the law requires
are virulent or highly infectious
are capable of causing severe disease

Gloved hand preparing for equine core vaccines — Photo by Mufid Majnun on Unsplash

The equine core vaccines include:

Rabies (inactivated/intramuscular)
Tetanus (toxoid/intramuscular)
Eastern/Western/Venezuelan Equine Encephalomyelitis (EEE/WEE/VEE) (killed/intramuscular)
West Nile Virus (killed/muscular)

Now that we’ve got all that general information out of the way, let’s discuss each vaccine and the terrible diseases they protect against individually.

Note: All vaccination schedule recommendations are based on normal, adult horses.

Rabies

First, let’s talk about rabies. Rabies is a neurological disease that is invariably fatal to horses and has considerable public health significance. However, this disease is rarely observed in horses.

As many may know, rabies is highly contagious and transmitted when infected saliva comes in contact with mucous membranes or open cuts. More commonly, horses become infected through the bite of an infected, rabid animal, such as raccoons, foxes, bats, and skunks.

Symptoms of rabies include:

Depression
Abrupt behavioral changes
Recurrent twitching
Lameness or recumbency (laying down)
Circling
Head pressing
Trouble urinating
Extreme sensitivity to light and sound

Unfortunately, all horses are at risk and clinical signs will appear in 2 to 6 weeks after infection. Once symptoms are observed, the disease progresses rapidly with death occurring within 5 days.

As far as treatment goes, if a horse is bitten and rabies exposure is suspected, the animal that bit the horse should be located and tested (this is of course an ideal situation).

If the animal tests positive for rabies, the horse must be euthanized if unvaccinated. If the potentially rabid animal cannot be found, the horse must be quarantined and checked for symptoms for 6 months.

According to Merck Veterinary Manual, “a vaccinated horse that is bitten by or exposed to a rabid animal should be given a rabies booster immediately and observed for 45 days for any signs of rabies.”

The lack of treatment options blatantly illustrates the importance of this vaccine. A normal adult horse requires revaccination annually by a veterinarian.

Tetanus

Now, let’s talk about tetanus! Tetanus is a disease that results from exposure to Clostridium tetani, a spore forming bacteria. This bacteria is present naturally in the intestinal tract and manure of horses.

Interestingly, spores are also everywhere in the environment, including soil, dust, and manure. Consequently, all horses are at risk for tetanus (which is often fatal).

Symptoms of tetanus include:

Colic
Stiffness progressing to generalized muscle spasticity
‘Sawhorse’ stance
Third eyelid prolapse
Eventual diaphragm paralysis/death

My crude representation of “sawhorse” stance

Tetanus is not contagious from animal to animal. Transmission occurs in a couple different ways with clinical signs appearing in 1 to 3 weeks.

Tetanus transmits through:

Puncture wounds
Open lacerations
Surgical incisions
Exposed tissues (such as the umbilicus of foals and the reproductive tract of the postpartum mare)

If diagnosed early, treatments aim to destroy the bacteria to ensure no more toxin production. The other goal is to diminish the effects of already released toxins.

Typically, we use large doses of antibiotics along with the tetanus antitoxin. The horse will need to be in a darkened, quiet stall to reduce muscular spasms and anxiety. If the horse is capable of eating, offer food at an accessible height.

In severe cases, a sling is used to support the horse’s weight and intravenous fluids/urinary catheters may be required. Once this point is reached, the prognosis for survival is not great.

This vaccine is a simple measure that helps prevent your horse from unnecessary suffering and possible early death. Annual spring revaccination is necessary for all horses. Previously unvaccinated horses should be given a 2-dose series with a 4-6 week interval between doses.

Eastern/Western/Venezuelan Equine Encephalomyelitis (EEE/WEE/VEE)

Now, we have a bit of a mouthful: equine encephalomyelitis (EE). These diseases are arboviruses. An arbovirus transmits via blood-feeding insects or ticks. The virus causes fatal neurological disease.

Horses become infected through the bite of a mosquito. EE is not contagious and clinical signs appear in 3-14 days. More specifically, 5-10 days for EEE and WEE and 1-6 days for VEE. After the onset of symptoms, death can occur in 2-3 days.

Horses at risk for EEE include those east of South Dakota and Texas. Horses west of the Mississippi are at risk of WEE. South American horses are at risk for VEE.

Symptoms of EE include:

Moderate to high fever
Cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing, etc.)
Depression
Loss of coordination
Behavioral changes (self-mutilation/drowsiness)
Severe central nervous system signs (circling, blindness, seizures)

There is a 75-95% fatality rate for EEE, 20-40% for WEE, and 40-90% for VEE. Scary stuff! Unfortunately, there is no specific treatment for EE. The main focus is to decrease the inflammatory response caused by the virus. We use NSAIDS and DMSO to accomplish this the most often.

EEE/WEE/VEE’s mortality rates are reason enough to get this core vaccine. Annual spring revaccination is necessary for all horses. Previously unvaccinated horses should be given a 2-dose series with a 4-6 week interval between doses.

West Nile Virus (WNV)

Finally, we have a fairly well-known disease, West Nile Virus (WNV). This disease is the leading cause of arbovirus encephalitis in both horses and humans in the United States. WNV is very widespread and documented in all of the continental U.S., most of Mexico, and Canada.

WNV transmits through infected mosquito bites. Although all horses are at risk, juvenile (less than 5 years old) and geriatric (over 15 years old) seem to be more susceptible to WNV.

Although not very contagious, WNV transmission to humans was once observed after contact with the neural tissue of an infected animal during a necropsy. Clinical signs will appear in 3-15 days. While there is only a 33% fatality rate, up to 40% of horses that do recover may exhibit neurological signs for a minimum of six months after diagnosis.

Symptoms of WNV include:

Loss of appetite
Depression
Loss of coordination
Muscle trembling

Just like the EEs, there is no specific treatment for WNV other than supportive care. This care focuses on decreasing the inflammatory process caused by the viral agent via NSAIDS and DMSO. Additionally, recumbent horses require a sling, catheterization for incontinent horses, and/or IV fluids for horses that are unable to eat or drink.

Although this is one of the least severe diseases that we’ve discussed here, it is still a key vaccination to get in order to prevent unneeded suffering. Annual spring revaccination is necessary for all horses.

Final Considerations

Obviously, I and many others in the equine field highly recommend that any and all horses get these vaccinations. Scientists developed vaccines to create and maintain immunity against specific diseases. Also, vaccines reduce an animal’s risk of getting sick. Moreover, if they get sick after vaccination, the vaccine can lessen the severity of the disease. As I’ve said numerous times in this post, vaccination is a simple step you can take to prevent unnecessary suffering for your horse.

Thank you so much for reading about equine core vaccines. Let me know if any of the information in this post surprised you! I’d love to hear from you.

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