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Medical Index
First Posted: February 2007
Updated: Feb 2014

Strangles (bacterium) aka Shipping Fever

by Debora Johnson
Update: Preventing Shipping Fever
Update: Strangles: Dispelling the Myths
Update: Strangles Guidelines Released to Public

Strangles is caused by a bacterium called, Streptococcus equi. Symptoms may include fever, lack of appetite, swelling and/or abscesses in the lymph glands under the jaw, and pusey discharge from the nostrils. Sometimes the abscesses break open and a thick yellow pus oozes out. Strangles is highly contagious. Therefore, affected horses should be isolated. In really severe cases the disease can cause prolonged illness and even death.

How Is Strangles Spread?

Sometimes the term "shipping fever" is used when referring to strangles. The bacterium spreads quickly from animal to animal and can cause epidemic outbreaks. Foals are very susceptible to strangles. Anything contaminated with the pus can cause Strangles to spread: Water troughs, feed buckets, brushes, tack of any kind, etc. Recovering horses can spread the disease for up to 8 months. Penicillin is the antibiotic of choice against S. equi. Penicillin shots, if given improperly, can cause death in horses as well. (Accidental injection directly into the blood stream) Often horses who have been infected with s. equi do not develop immunity. (approximately 25% do not)


Discuss with your vet the necessity of vaccinating your horse. When horses and foals are vaccinated against strangles for the first time, they require a primary vaccination course consisting of three doses of Equivac-S or Equivac 2 in 1. The three doses are given with an interval of two weeks between injections. Booster doses should be given at least annually and six-monthly re vaccination should be considered in circumstances where the risk of infection is known to be high. The vaccine is injected intramuscularly. The most convenient site for injection is the triangle injection site of the neck. The needle can be attached to the syringe and then administered to the horse or the needle may be inserted into the muscle and then the syringe attached and the vaccine injected. It is particularly important that booster doses be given prior to periods of greater risk of infection, such as the breeding or performance season. Pregnant mares may be vaccinated up to two weeks before foaling. Higher risk horses can be vaccinated every six months. There is a high incidence of reaction to the Strangles vaccine. Heat and swelling can occur at the injection site. Sometimes the horse may run a fever. The vaccine is not a total protection; however, it reduces the incidence of Strangles, and reduces the severity of the disease in a vaccinated horse allowing for a quicker recovery. This vaccine gives the best know protection possible.

Robert Holland, D.V.M. and an epidemiologist with the Maxwell Gluck Equine Research Center in Lexington advises, "Quarantine the horses for up to four weeks. They should then have two negative nasal swabs about one week apart before removing them from quarantine... If they are shedding—that is, if you have the disease actively coming out of their noses—I don't care how long it takes, they have to be in quarantine."

Holland warned horsemen that horses not exhibiting symptoms can still be capable of spreading the disease. He said, "We have just learned from European studies that horses can be asymptomatic shedders for an average of 10-to-11 months after they are done with the active infection. It can remain in the sinuses, but the gutteral pouch seems to be the main culprit." Precautions Urged for Horses Exposed to Strangles

Can the vaccine be used during an outbreak?

In the event of an outbreak of strangles, horses should be segregated into three groups:

  • Horses affected by the disease should be treated, but not vaccinated.
  • Horses with no known contact with the disease should be vaccinated immediately.
  • Horses known to have been in contact should be observed for seven to ten days and vaccinated only if they have a normal temperature and show no clinical signs of the disease.

Medical Index