Botulism is caused by toxins that are produced from the spore-forming bacteria, Clostridium botulinum. The bacteria are widely distributed in the environment and can survive for long periods.  Different sub-types of the bacteria produce different types of toxins, with the type B toxin causing most disease in horses.  Types C and A have also been documented to cause disease in horses.  There are different ways that horses can acquire botulism.  Wound botulism occurs when spores vegetate in a contaminated wound and produce toxins.  Forage poisoning usually results from ingestion of decaying plant material contaminated by pre-formed toxin due to improper storage of hay, haylage, silage or grain.  Animal carcasses decaying in feed material can also be a source of toxin.  Shaker Foal Syndrome results when foals ingest spores from the environment that then vegetate and produce toxins in the intestinal tract.  Adult horses seem to be resistant to the proliferation of ingested spores and absorption of toxins in this manner.

Botulism toxin acts by blocking nerve impulses, leading to weakness, difficulty swallowing and poor muscle tone.  Eventually horses become so weak that they cannot rise and they will often die of respiratory failure.  Treatment is often unrewarding and involves the use of antiserum and supportive care.

There is a vaccine available for horses against the type B toxin.  It was designed to be given to pregnant mares to prevent disease in their foals.  We do carry this vaccine and do sometimes use it to prevent botulism in adult horses and foals.  It will only prevent botulism caused by type B toxin.  In general we only recommend this vaccine for horses on farms that feed round bales, haylage or silage, and on farms that have a recent history of a botulism problem.  The bacteria that cause botulism are anaerobic, meaning that they need an environment without exposure to air, so the bacteria often can live in the center of large round bales that are consumed slowly by horses.  Haylage and silage can present a problem due to the anaerobic environment in which these feeds are stored.  We do not routinely vaccinate all horses or pregnant mares because the incidence of the disease is low in our practice area.  

Subepiglottic cysts are fluid-filled structures in the throat area, underneath the epiglottis, which is the flap that moves to cover the trachea (wind pipe) upon swallowing.  These cysts can be quite large and obstruct airflow and swallowing function.  They are believed to be a problem that is usually present at birth.  They may result from cystic distortion of embryonic remnants of the thyroglossal duct.  Cysts acquired later in life are hypothesized to result from obstruction or inflammation of the mucus glands.  Very large cysts in foals can cause difficulty breathing and difficulty swallowing shortly after birth.  More commonly, horses show no signs of a problem until they are placed into work.  Upon exertion, one may notice abnormal respiratory noise as well as exercise intolerance.  Occasional coughing may be noted.  The diagnosis is usually made via endoscopy (scoping), although x-rays and other diagnostics may be needed.  Treatment involves surgical removal of the cyst.  There are different procedures available and the best one to use depends on one’s budget as well as the shape, size and location of the cyst.  Other than traditional surgery which involves incising the throat area to gain access to the cysts, one may also try laser removal or removal via the oral route using a snare.  There has also been a recent report of a subepiglottic cyst being treated with formalin injection with a good outcome.  The prognosis after surgery is usually favorable.  Dorsal displacement of the soft palate (flipping the palate) and/or epiglottic dysfunction may become a problem after surgery, secondary to excessive scar tissue formation.