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Streptococcus equi


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Strangles and vaccination

Why vaccinate against Strangles?

Vaccination provides the best possible protection against Strangles. As an outbreak of Strangles has both welfare and economic consequences to a herd, control of the disease is an important aspect of stable management. We recommend that all horses in a stable yard be vaccinated to provide the best protection against the disease.

Does the vaccine provide 100% protection?

A regular program of vaccination for all horses will usually control or very markedly reduce the incidence and severity of strangles. However the vaccine is not an absolute preventative. In registration trials, the vaccine had between 70% and 100% efficacy with an average of 75% protection compared to controls. Vaccination has good benefits and is the best protection possible. However vaccination does not replace the need for good stable management and procedures for introduction of new horses to a yard.

About the vaccine

How does this vaccine differ from Strangles vaccines in other parts of the world?

Equilis StrepE is a unique vaccine developed from a Dutch field strain. It is a live vaccine with the bacteria modified to prevent growth in mammalian cells. This, along with the unique method of administration, has lead to dramatically increased efficacy with reduced side effects. This is an innovative and newly developed vaccine.

This is a live vaccine. What is the chance of it introducing Strangles into the group of horses being vaccinated?

The bacteria used in the vaccine was taken from a virulent wild strain but has undergone subsequent modification. The vaccine strain cannot continue to grow in mammalian cells and trials have shown that it does not spread to other horses or introduce Strangles to a group.

Can the vaccine strain revert to virulence?

The stability of the strain has been shown to be very high and trials have failed to induce reversion to virulence. It has also been shown that the vaccine strain cannot compete with other bacteria in the environment so survival outside of the horse is considered unlikely. The strain is not considered to present a risk of reversion.

Will a vaccinated horse be able to pass vaccine strain bacteria to another horse?

It has been demonstrated that the vaccinal strain will not pass from one horse to another in normal circumstances.


What is the advantage of the three monthly boosters?

Boosters every three months will maintain the active immunity of the vaccinated horses. In a group this will mean that it is unlikely that an outbreak will be seen as this will reduce the overall challenge of bacteria in the environment. Boosters every three months are suggested for horses in high risk situations that require continuous immunity against strangles.

What is the advantage of 6 monthly boosters?

The duration of immunity following vaccination is three months. Therefore for the second three months the horse will be at increasing risk of disease should they be challenged with Strep. equi However a booster at 6 months will restimulate the immunity. Biannual boosters maintain a horse’s primed status, and if there is a strangles outbreak in the area during the second three months, a simple booster can be given immediately. This has been shown to be of benefit in the face of an outbreak and will significantly reduce the number of horses showing disease. With less horses becoming infected this should allow a faster resolution of the outbreak. Boosters every six months may be selected by veterinary surgeons for horses in medium risk situations where continuous immunity against strangles is not required.

My horse has been vaccinated today. Can I ride him now?

Generally vaccination has no effect on the rideability of a horse. In unusual cases following vaccination a horse may resent the bit for a day or two. Otherwise we do not see any requirement for resting the horse although we would advise against strenuous exercise for 24 hours following vaccination.

Can the vaccine be used in the face of an outbreak?

In cases where the horse has already had a primary course of vaccine it has been shown in trials that vaccination at the time of challenge will greatly reduced both the number of horses becoming infected and the degree of the signs in those that do develop Strangles. Only healthy horses with no signs of pyrexia should be vaccinated.

My horse is the only one on the yard to be vaccinated, is there any benefit in continuing?

The best way to control Strangles is to vaccinate all the horses on the yard as this will reduce the level of challenge. However, individual horses will still benefit from vaccination as their immunity will be higher than any unvaccinated horses. If the level of challenge is very high then there would still be a chance of them showing signs of Strangles as the protection provided by the vaccine is not 100% but even then we would expect the signs to be less severe and for there to be quicker resolution of disease.

Can a vaccinated horse become a carrier of the vaccinal strain?

Following normal doses of vaccine it has not been possible to isolate vaccinal strain bacteria from nasopharangeal swabs. It is therefore unlikely that a vaccinated horse can become a carrier of the vaccinal strain.

Risk categories

What is a “high risk” horse?

Examples of a high risk horse would include one in an area known to have a lot of Strangles circulating around the horse population or one that is being introduced to a large number of horses from an unknown source.

What is a “medium risk” horse?

A medium risk horse would be one in an area where there have been no recent reports of strangles and who occasionally comes into contact with new horses.

What is a “low risk” horse?

A low risk horse is one that infrequently comes in to contact with other horses.


Can my horse have his vaccination while he is on antibiotics?

No. This is a live vaccine and antibiotics would kill the bacteria before it was able to stimulate immunity.

Can the horse be injected in the lower rather than upper lip?

The vaccine has been tested in the lower lip. It was found that the lower lip would hang so that the mouth would be open for a number of days. Therefore vaccination into the lower lip is not recommended.

Post-vaccination observations

There seems to be a lump at the site where the vaccine was administered. Should this be of concern?

After vaccination we would expect a swelling of up to approximately 3-8 cms to develop at the site of injection, usually lasting 2-3 days. Rarely this could last up to three weeks. It should have no effect on your horse’s appetite.

There seems to be a cloudy discharge at the site where the vaccine was administered.
Should this be of concern?

It is common for there to be a slight cloudy discharge from the mucosal injection site at 3-4 days post vaccination.

About carriers

Could a vaccinated horse become a carrier of Strangles without showing signs of disease?

During trials no abscesses or chondroids were found in the guttural pouches so it appears unlikely that horses will become carriers.

Will vaccination reduce shedding of Strep. equi from carrier animals?

There have not been any trials to give any indication of this. As carriers only shed intermittently it would be very difficult to measure any possible benefit.

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