BY VACCINE - Optional Vaccines Guidelines

Optional or Risk-based Vaccines

Diseases for which the vaccines are optional are those that;

Veterinarian and horse

• 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 viruses. Only influenza subtype AE-2 has been isolated over the last 20 years worldwide.

horse's nostral

Vaccination with an AE-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 AE-2 influenza vaccine for intranasal administration is now commercially available

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

Equine herpesvirus

Equine herpesvirus is also known as equine viral rhinopneumonitis. There are numerous strains of Equine Herpesvirus (EHV) that are passed between horses by body fluids, including nasal secretions. Horses commonly have antibody titres to EHV subtypes 1, 2 and 4. 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.

horse's nostral

Herdmates in contact with pregnant mares should follow the same vaccination protocol. Mares may still abort despite being vaccinated as per the manufacturer’s recommendations. Some horses will be chronic shedders of the virus and spread the virus to other herd mates when stressed.

This indicates that they have been exposed to the viruses at some time in their life. Subtypes 1 and 4 can cause central-nervous-system disease, respiratory disease and abortion. Neurologic disease may occur after a herpesvirus respiratory disease infection. Only killed-virus vaccines are available in Canada. These vaccines are labeled for the prevention of respiratory disease and abortion. The manufacturers do not make claims about the prevention of neurologic disease.


foal and mare

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, with extensive swelling and, often, rupture of the lymph nodes, which produces large amounts of thick, 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. This vaccine has the potential to cause adverse reactions.

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Potomac horse fever (PHF)

Potomac horse fever (PHF) is caused by the bacteria Neorickettsia risticii. 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, and stoneflies, containing infected E. risticii.

There are thought to be at least six strains of the agent. The vaccine is made from a single strain. Fully vaccinated horses have developed Potomac horse fever.

PHF seems to be most prevalent in two areas of the province (Brighton, Cannington). A primary two-shot immunization, followed by an annual booster, should be considered for horses in areas where PHF has previously been diagnosed. There is varied data in the field regarding the effectiveness of this vaccine, so please discuss this with your veterinarian.


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.

wrapped bale

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, releasing toxins.

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 danger 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.

resting horse

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

For more information, contact:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
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