The Strangles vaccine is given based upon the risk associated with each situation.
This vaccine does not necessarily prevent disease but does lessen the severity of disease should a horse contract Strangles. This is because of the way that the bacteria attacks the body.
The Strangles vaccine also comes with increased risk of adverse reaction as compared to the vaccines for things like EEE, WEE or Tetanus. Horses who have had Strangles in the last year (even such a mild case that it wasn't noticed) can have a severe adverse reaction to the vaccine. If giving the intranasal vaccine, there is also the risk of having the bacteria get on the skin and contaminate injection sites if other vaccines are given at the same time.
You should always discuss the pros and cons of this vaccine with your veterinarian prior to giving it. Adult horses are at much lower risk of contracting Strangles than young horses. Horses that kept in facilities where lots of horses pass through such as breeding facilities, show barns, sale barns, etc are at increased risk of infection. Horses who have had Strangles in the past 5 years are much less likely to contract the disease again. So the factors that contribute to the risk of infection all have to be weighed against the chance of infection.
Licensed Veterinary Technician