Equine Guelph

The immune system is the body’s defense against infection. Vaccines help the body to develop immunity by imitating an infection and are intended to create and maintain immunity against specific diseases for a period of time. Together with good management and biosecurity practices aimed at preventing and controlling infection, a vaccination program can minimize the horse’s risk of getting sick, as well as lessen the severity of sickness and reduce the risk of spread to other animals if sickness does occur.

There is no standard once-size-fits-all vaccination program for horses. Each situation should be evaluated based on several factors which include the risk of infection, consequences of the disease, potential for serious adverse reactions, and anticipated effectiveness of the product. The costs involved in vaccination in terms of time, labour, and vaccine expense should be weighed against the potential costs of contending with the disease including lost competition time, treatment and control of an outbreak, or even loss of life.  

Every horse owner should understand vaccine basics, and all the horses in a herd should be vaccinated at intervals based on manufacturer recommendations and the professional judgment of the attending veterinarian. Let’s take a closer look.

CORE VACCINES

Core or basic vaccines provide protection against diseases that have a significant health impact (including death) for the horse and/or for humans. The agents causing disease are often present in either wildlife, insects, or the environment and are therefore not easily controlled. These are the “no-brainer” vaccines and should be administered to all horses. They include rabies, tetanus, eastern equine encephalitis and West Nile virus.

Rabies

Rabies is a well-known cause of fatal disease affecting the nervous system in mammals. The virus is found in saliva and transmitted to other animals and people by a bite, saliva exposure to open cuts or wounds, and/or saliva entering the eye, nose, or mouth. In Canada, the most common wild animals to transmit rabies to domestic animals or humans are foxes, skunks, racoons, and bats.

The red fox is one of Canada’s most widespread mammals. Foxes are among the most common wild animals to transmit rabies to humans and domestic animals. Photo: Canstock/DragoNika

Rabies is a rapidly progressive, fatal neurologic disease that can be prevented by vaccination. Following an initial booster series, rabies vaccines are administered to horses annually and they must be administered by a licensed veterinarian.

Fortunately, rabies is not often diagnosed in Canadian horses; yet its importance cannot be understated. To find out the number of confirmed rabies cases in Canada, the number involving horses, and the province they were from, go to the Canadian Food Inspection Agency website and search Rabies cases in Canada. 

Tetanus

Tetanus or lockjaw is a life-threatening disease caused by the anaerobic bacteria (grows in low oxygen conditions), Clostridium tetani. The spores of C. tetani are commonly present in the soil where they can remain for years, and can contaminate wounds such as puncture wounds, crushing wounds, open lacerations, surgical incisions, and the umbilici of foals.

Related: What Are You Vaccinating Your Horse Against?

Tetanus is caused by the anaerobic bacteria Clostridium tetani, which can contaminate wounds such as this puncture wound. The stick was successfully removed from behind this horse’s eye. Photo: Clix Photography 

Upon gaining entrance to the body, C. tetani produces powerful neurotoxins, which result in muscle contraction and spasm (tetany). Horses of all ages can be affected. Horses are the most susceptible of all the animal species.

Initially, all adult horses should be vaccinated for tetanus twice at intervals of four to six weeks using tetanus toxoid, and boostered annually or as recommended by a veterinarian. Horses that have a wound or are undergoing surgery six months from the time of their tetanus vaccination should receive a booster of tetanus toxoid.

Tetanus antitoxin (technically not a vaccine) is administered to unvaccinated horses, or those of unknown vaccination status, if they have sustained a wound which has become contaminated. It is prepared from the blood of healthy, hyperimmunized horses and provides immediate passive immunity lasting seven to fourteen days. At the same time, these horses are administered the initial booster of a tetanus toxoid to stimulate an immune response. This should be followed by a second vaccination four to six weeks later. Owners of horses administered tetanus antitoxin should be informed about the rare but serious risk of serum hepatitis (Theiler’s disease).

West Nile virus (WNv)

West Nile virus (WNv) is spread by mosquitoes and causes inflammation of the brain and spinal cord in horses. Since WNv can be a fatal disease and survivors can have residual neurological deficits lasting from a period of months to a lifelong disability, it is important to protect horses against this virus. 

Horses should be vaccinated annually and within three months of mosquito season for the greatest protection. Previously unvaccinated horses or those with unknown vaccination status should be administered two primary doses, with the time between doses depending on the vaccine used. Following the booster series, annual vaccination is recommended.

Eastern Equine Encephalitis (EEE)/ Western Equine Encephalitis (WEE)

Eastern equine encephalitis (EEE) virus is spread by a mosquito that breeds in hardwood swamps and normally feeds on birds infected with the virus but not affected by it. People and other animals can also develop EEE when bitten by infected mosquitoes.

The EEE virus has a range from southeastern Canada to southeastern United States, as well as the Caribbean and South and Central America. Some areas of a region have higher risk for exposure to the virus and this risk is based on populations of the offending mosquito species (Culiseta melanura) and the migratory patterns of certain reservoir birds. For example, in Ontario there have been more EEE cases in Eastern Ontario, around the Niagara region, and on the eastern edge of Georgian Bay. To find out where EEE has been diagnosed in Canada please go to the Canadian Animal Health Surveillance System website.  

The mosquito Culiseta melanura breeds in hardwood swamps and feeds on birds infected with eastern equine encephalitis (EEE). People and animals can develop EEE when bitten by infected mosquitoes. Photo: Entomology Today

Eighty to ninety percent of infected horses develop acute and fatal neurologic disease, namely ataxia, paralysis, and seizures with death usually occurring within 48 hours. Survivors may have persistent neurologic signs. Fortunately, there are effective vaccines available to prevent EEE. After an initial booster series, horses should receive an annual vaccination three months prior to the onset of mosquito season.

Western equine encephalitis (WEE) has been diagnosed in the western United States, Western Canada and South America, although cases have not been reported in Canada in decades. Similar to EEE, the WEE virus is transmitted by mosquitoes; however, small wild animals may also be involved in transmission of WEE virus.

Similar to EEE, WEE causes sudden and often severe neurologic signs although the mortality rate is lower than with EEE. WEE vaccines are very effective and almost always combined with EEE in a vaccine. After the initial booster series, annual vaccination is recommended.

RISK-BASED VACCINES

Knowing the risk of particular diseases in your area, or the area to which you are planning to travel with your horse, can help you and your veterinarian decide on the most appropriate vaccinations. The following websites provide disease information in specific areas of the world. The diseases mentioned are only those reported, and the level of disease is generally underestimated:

Equine Disease Communications Center: Disease Alerts (US and Canada)

Canadian Animal Health Surveillance System

Diseases for which vaccines are risk-based/optional are those with:

• variable impacts on the health of the animal;
• a low risk of causing life-threatening disease;
• a less-than-perfect response to the vaccine’s ability to prevent disease (due to the biology of the agent);
• primary occurrence in a specific regional or geographic area; 
• a management factor that significantly increases the risk of developing the disease, e.g., botulism and feeding silage.

Equine Influenza

Equine influenza is a contagious, respiratory disease caused by two distinct subtypes of the influenza A virus: H7N7 and H3N8. Only influenza subtype H3N8 has been isolated over the last 20 years worldwide; therefore, vaccination with an H3N8 vaccine (with the most up-to-date North American strains) is recommended. To understand what strains of influenza are circulating worldwide and the current recommendations for vaccines go to OIE Expert Surveillance Panel on Equine Influenza Vaccine Composition – OIE Bulletin, search Equine Influenza.

An equine influenza vaccine should be administered primarily:

• to horses one to five years of age, since they seem to be more susceptible to the disease, and;
• in situations where there are frequent contacts with large numbers of horses, e.g., new arrivals to the barn/track, attendance at shows. 

While vaccination does not necessarily prevent influenza due to the change in strains (antigenic drift), the disease in vaccinated horses is less severe, the clinical course is shorter, and there is less virus shed from infected horses thereby reducing spread of the virus.

At risk horses should be vaccinated annually at a minimum, and for some horses every six months when the risk of exposure continues. Usually influenza is combined with EHV-1/4 in the vaccine so more frequent vaccination is not recommended due to the combination.

Equine Herpesvirus (EHV)

Equine herpesvirus (EHV) was previously known as equine viral rhinopneumonitis. There are numerous strains of EHV that can be transmitted between horses by body fluids including nasal secretions. It is estimated that approximately 70 percent of the horse population is infected with equine herpesvirus-1 (EHV-1), which most likely occurred at birth or during the first few months of life. This virus lays dormant or asleep in a nerve cell body in the head, and also in some lymph nodes. It can awaken during times of stress or illness, much like the shingles virus in humans, and is then shed to horses in close proximity. In broodmares it can cause abortion and in all horses it can cause respiratory or neurological disease. Vaccines contain EHV-1 and EHV-4 (EHV-1/4), and when administered to horses provide them with some protection against the respiratory and abortion forms of the disease. No vaccine is presently labelled for protection against the neurological form of the disease.

Related: Horse Care Through the Seasons

Equine herpesvirus clinical signs can include fever, nasal discharge and cough, stumbling, weakness in the hind limbs which may progress to all limbs, and the inability to rise. Photo: Dreamstime/Mariya Shustova

Pregnant mares should be vaccinated with a killed EHV-1 vaccine prior to breeding if they have never received an EHV vaccine, and in the fifth, seventh, and ninth months of pregnancy.

Horses older than a year should be vaccinated once or twice annually based on risk. As with influenza, given the highly contagious nature of the disease due to EHV-1 and EHV-4, and the impact on horse health and industry economics, some racing regulators and racetracks, as well as sport organizations, have rules requiring the administration of an EHV-1/4 vaccine.

Strangles

Strangles is a highly contagious and serious infection of horses and other equids caused by the bacterium Streptococcus equi. The disease is characterized by severe inflammation of the mucosa of the head and throat, often with extensive swelling and rupture of the lymph nodes below and behind the jaw which produce large amounts of thick, creamy pus. 

Strangles is more common in animals less than five years of age and especially in groups of weanling foals or yearlings, although horses of any age can become infected.

There is currently only an intranasal attenuated live strangles vaccine available in Canada. This vaccine is administered to competition and other horses at risk of exposure to disease. The risk of exposure increases at comingling sites such as shows and auctions. Some vaccinated horses will show mild signs of strangles when exposed to the bacterium. As with other vaccines, the shedding is much reduced, which limits the potential for an outbreak. Strangles vaccination should not be done at the same time as other injections as there is a potential for contamination of those injection sites. As well, in rare cases, vaccinated horses have developed mild, noncontagious, signs of strangles shortly after vaccination.

After an initial booster series of two doses given two to three weeks apart, horses should be vaccinated annually based on risk.

Potomac Horse Fever (PHF)

Potomac horse fever (PHF) is caused by the bacteria Neorickettsia ristici, and in Ontario by a newly identified organism Neorickettsia findlayensis. It is maintained in nature in a complex aquatic ecosystem. Transmission to horses can occur through accidental ingestion of insects such as caddisflies, damselflies, dragonflies, mayflies, and stoneflies containing Neorickettsia sp. The vaccine is made from a single strain of N. risticii; therefore, there has been a history of vaccine failure. Anecdotally, veterinarians in endemic areas for PHF feel that vaccination reduces the severity of disease in many situations.

Potomac horse fever is transmitted to horses through accidental ingestion of insects such as dragonflies, mayflies, and caddisflies. Photo: iStock/William Krumpelman

After a primary two-dose series, horses should be vaccinated annually in areas where PHF has been previously diagnosed and where veterinarians have seen a positive response to vaccination. Your veterinarian will have an appreciation for the efficacy of the vaccine in your area.

Botulism

Botulism is a disease that occurs when toxins produced by the bacterium Clostridium botulinum enter the horse’s body causing weakness, which may progress to an inability to swallow, paralysis, and death. The botulism bacterium is a spore-forming anaerobic bacteria (grows in the absence of oxygen), which can occur in decaying plant material. The bacteria may also grow in wounds or in the intestinal tract of foals, releasing toxins.  Neurotoxin serotypes A, B, and C are associated with most botulism outbreaks.

Related: Understanding Processed Feeds

Improperly preserved hay, haylage, and silage carries the risk of botulism, to which horses are highly sensitive. Photo: Canstock/Dotweb

Of all domesticated animals, horses are the most sensitive to botulism. Hay and especially haylage or silage can be contaminated with the bacteria during the raking and baling process when animal carcass remnants may be incorporated in the forage. Hay silage can be a great feed when preserved properly, but it carries the danger of botulism. 

A toxoid vaccine against C. botulinum serotype B is available and should be used three times initially, four weeks apart, followed by an annual booster if haylage or silage will be fed. The vaccine protects against type B botulism only, and foals and horses continue to be susceptible to serotypes A and C. Foals can be especially vulnerable. Discuss the need for this vaccine with your veterinarian.

Equine Viral Arteritis (EVA)

Equine viral arteritis (EVA) was first identified in 1953 following an extreme respiratory-abortion syndrome on a Standardbred farm in Ohio.

EVA causes panvasculitis (inflammation of the veins and arteries) that results in edema of the limbs and an urticaria-like reaction (allergic/hive-like) of the head, neck, and trunk.

After an incubation period of three to fourteen days, clinical signs can include any combination or all of the following:

• anorexia;
• fever up to 41 degrees C for one to nine days;
• depression;
• limb edema, especially of the hind limbs, scrotum, and sheath;
• stiffness of gait;
• nasal and ocular discharges;
• skin rash;
• abortion in the mare;
• respiratory distress, coughing and diarrhea in the young foal (infrequent).

EVA can result in the establishment of the carrier state, with shedding of virus into the semen in a significant percentage of infected stallions. A positive titre (test to determine the level of antibodies to specific pathogens) for EVA, either from natural infection or vaccination, may prevent a horse or its semen from being exported to another country.

Prior to vaccination, refer to importation guidelines of the destination country should exportation be contemplated.

RISK FACTORS BY HORSE

Adult Horses 

Risk Factor: Geographic Location

Horses maintained on rural properties with little or no contact with other horses have minimal risk of coming in contact with disease agents. However, if they do, they often exhibit severe signs of disease since their immune systems are relatively naïve (i.e., little or no immunity). It is recommended that horses exposed to minimal risk be vaccinated with the basic or core vaccines. 

It is also important to recognize that different disease agents have different geographic distributions, which can be related to the ecology of the vector. For example, Culiseta melanura, the mosquito vector of EEE, seems to be confined to some swamps and is probably related to the migration of birds from infected areas in the southern USA to the same general summer ranges.

Disease agents can also have different geographic distributions. Often this can be related to the ecology of the disease vector such as mosquitoes or wildlife. For example:

• PHF is generally reported from Eastern Ontario. This may be related to the complex aquatic ecosystem associated with the insect vectors (caddisflies, damselflies, dragonflies, mayflies, and stoneflies). Horse owners report very large hatches of these insects at certain times of the year in these areas.
• In Canada, EEE has only been diagnosed in Ontario and Quebec (as of 2022).

Broodmares

Risk Factor: Pregnancy

Pregnant mares are at risk of being infected by bacteria and viruses, which can either kill the fetus in the uterus directly or cause physiologic changes in the mare, resulting in abortion. The emotional and financial losses are significant, and welfare of the mare is compromised.

Some infectious agents, such as EHV-1, can lay dormant (latent) in horses until they become stressed by circumstances such as shipping. The virus becomes active again and is shed from the nose. Other mares can become exposed to the virus through nose-to-nose contact and the subsequent infection can lead to a single abortion or “abortion storms” when a number of mares abort. The aborted fetus, placenta, and placental fluids contain a large amount of virus, putting other mares that are in contact with them at risk for infection. Biosecurity is extremely important when managing these disease situations — vaccination is just one aspect.

Related: Biosecurity on the Horse Farm

Foals

Risk Factor: Age

Protective antibodies and other factors ingested from the mare’s colostrum (passive immunity) protect the foal during the first nine to twelve weeks of age. Foals begin to produce their own antibodies (active immunity) immediately after birth when they are first exposed to antigens; however, these antibodies don’t reach a protective concentration until the foal is two months of age or older.

Foals receive maternal antibodies in colostrum, and vaccination of pregnant mares should be timed for the most favourable protection of both mare and foal. Photo: iStock/Kent Weakley

To maximize the amount and type of antibodies in the mare’s colostrum in order to protect the foal, mares should be vaccinated four to six weeks prior to foaling. The foal is then vaccinated at six months of age when the antibodies passed on from the mare are waning and less likely to interfere with the foal’s ability to produce antibodies to vaccines.

Foals from non-vaccinated mares should receive their first vaccination between three and four months of age; the second between four and five months of age; and the third between five and six months of age.

• Foals from vaccinated mares should receive certain first vaccinations at four to six months followed by a second dose four to six weeks later and a third dose at 10 to 12 months of age;
• Foals vaccinated with the WNv Flavivirus vaccine only require one vaccination at five to six months of age followed by a booster at 10 to 12 months of age prior to the next mosquito season.

Traveling Horses

Risk Factor: In-and-Out Traffic

When there are horses coming and going from a facility, there is an increased risk of exposure to viral and bacterial pathogens, not only for those horses leaving the property and mingling with other horses at events, but also for the resident horses. Of main concern are the contagious respiratory pathogens such as influenza, EHV-1/4, strangles, and equine rhinitis virus A. Both competition horses and resident horses, if exposed to the competition horses, should be vaccinated for influenza and EHV-1/4, as well as strangles if it’s a risk in the appropriate activity. There is no vaccine in Canada at present for equine rhinitis virus A but, for the other diseases, please see the information under risk-based vaccines.

To learn more about how to decrease the risk of spreading diseases when traveling, check out Equine Guelph’s Biosecurity Risk Calculator.

Related: Equine Vaccine Q&A

Please note: This information provides guidelines only and should never replace information from your veterinarian.

Published with the kind permission of Equine Guelph.

Main Photo: Clix Photography