Vaccine Guidelines – Optional

Optional or 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 which vaccinations are appropriate for your horse. The following websites provide disease information in particular areas of the world. The diseases mentioned are only those reported and generally underestimate the level of disease:

Diseases for which the vaccines are optional are those that;

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

Equine influenza

Equine influenza is an acute, contagious, respiratory disease caused by two distinct subtypes (subtype 1: H7N7 and subtype 2: H3N8) of influenza A virus. Only influenza subtype A/Equine 2 has been isolated over the last 20 years worldwide.

Vaccination with an A/equine 2 vaccine (with the most up-to-date North American strains) is recommended for use;

  • especially in horses 1 to 5 years of age, since they seem to be more susceptible to the disease
  • 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, the disease in vaccinated horses is less severe and the signs last a shorter period than when horses are unvaccinated
  • A modified live equine A/Equine 2 influenza vaccine for intranasal administration is commercially available in Canada

(What are the benefits of a modified-live vs. a killed equine influenza virus vaccine?)

Given the highly contagious nature of the disease and the impact on horse health and industry economics, some racing regulators and racetracks as well as organizations including the United States Equestrian Federation, Federation Equestre Internationale (FEI) and Equestrian Canada have rules requiring vaccination against equine influenza.–woodbine-entertainment-and-hbpa.html

Equine herpesvirus

Equine herpesvirus was previously known as equine viral rhinopneumonitis. There are numerous strains of Equine Herpesvirus (EHV) that can be transmitted between horses by body fluids, including nasal secretions. It is estimated that approximately 70% 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 can in people. It is then shed to horses in close proximity. In broodmares is can cause abortion, and in all horses it can cause respiratory or neurological disease. Vaccines contain EHV-1 and EHV-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.

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 5th, 7th, and 9th months of pregnancy.

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


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

Strangles is most common in animals less than five years of age and especially in groups of weanling foals or yearlings.

There is currently only an intranasal attenuated live Strangles vaccine available in Canada. Vacccination for strangles prevents infection in many horses and in others reduces the severity and duration of infection.

Potomac horse fever (PHF)

Potomac horse fever (PHF) is caused by the bacteria Neorickettsia risticii (N. risticii) It is maintained in nature in a complex aquatic ecosystem.

A primary two-shot immunization, followed by an annual booster, should be considered for horses in areas where PHF has previously been diagnosed.


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 paralysis.

Horses are the most sensitive of the domesticated animals to botulism. Hay and especially hay silage can be contaminated with the bacteria during the raking and baling process. Hay silage can be a great feed when preserved properly but carries the risk of botulism.

A toxoid vaccine should be used three times initially, one month apart, followed by an annual booster if hay silage is going to be fed. The vaccine protects against type B botulism only. Foals can be especially vulnerable. Discuss the need for this vaccine on your farm 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 of the head, neck and trunk.

After an incubation period of 3-14 days, clinical signs can include any combination or all of the following:

anorexia; fever up to 41°C for 1-9 days; depression; limb edema, especially of the hind limbs, scrotum and prepuce; stiffness of gait; nasal and ocular discharges; skin rash; abortion in the mare; and, infrequently, respiratory distress, coughing and diarrhea in the young foal.

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, from either natural infection or vaccination, to this virus may prevent a horse or its semen being exported to another country.

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

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


Guidelines for Vaccination of Horses. Guelph; Dr. Bob Wright (OMAFRA) and Dr. Dan Kenney (Ontario Veterinary College), 2004

Guidelines for The Vaccination of Horses. Lexington; AAEP, 2008

The Ontario Health Network

For more information, contact

Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: [email protected]