Toxicoinfectious Botulism – Shaker Foal Syndrome

Botulism is a neurotoxin produced by the Clostridium botulinum bacterium. Horses are particularly sensitive to botulism. The bacterium itself is widely found in soils and the intestinal tract of animals. When exposed to the right environmental conditions, the bacterium sporulates and releases the botulin toxin. Once the toxin is released in the intestinal tract it quickly moves into the bloodstream and targets nerve cells, causing a neuromuscular blockade that leads to generalized weakness and eventual paralysis.

The most common cause of botulism toxicity in horses is the ingestion of contaminated hay. The second most common cause is known as Shaker foal syndrome.

Shaker foal symdrome affects young foals that are 10 days old or younger. During the first few days of the foal’s life, it ingests a bit of soil that contains the bacterium Clostridium botulinum type B. Once in the intestinal tract, the bacteria proceed to colonize and grow, releasing toxins that are readily transferred into the bloodstream. Affected foals develop central nervous system symptoms such as an inability to swallow. Milk may be seen dripping from their nostrils as they nurse, and they may cough repeatedly, trying to clear the milk from their trachea. Drooling is often present. Other symptoms include muscle tremors (hence the name Shaker foal), poor tongue and tail tone, abnormal gait, and progressive weakness. It is not uncommon to see a Shaker foal simply collapsing due to muscle weakness instead of lying down in a controlled manner. This disease progresses rapidly in foals and often results in death.

The best way to avoid Shaker foal syndrome is to vaccinate the mare prior to foaling so she can pass a level of immunity on to her unborn foal. Foals born to mares that have been recently moved into an area where Shaker foal syndrome is prevalent and have not been vaccinated during pregnancy are at the greatest risk of developing the disease. Type B bacterium is prevalent in Kentucky, the Midwestern and Atlantic seaboard states; however, cases have been reported throughout the USA. The best form of defense is to vaccinate your mare against the disease.

The American Association of Equine Practitioners recommends the following vaccination schedule:

Previously vaccinated pregnant mares:

Vaccinate annually with a single dose 4 to 6 weeks before foaling.

Previously unvaccinated pregnant mares:

Vaccinate during gestation with a primary series of 3 doses administered at 4-week intervals and scheduled so that the last dose will be administered 4 to 6 weeks before foaling to enhance concentrations of immunoglobulin in colostrums (i.e., months 8, 9, 10 of gestation).

Foals of vaccinated mares (in endemic areas):

Administer a primary series of 3 doses, at 4-week intervals, starting at 2 to 3 months of age. As maternal antibody does not interfere with vaccine response, foals at high risk may have the vaccination series initiated as early as 2 weeks of age.

Foals of unvaccinated mares (born in, or moving to, endemic areas):

Administer a primary series of 3 doses, at 4-week intervals, beginning at 1 to 3 months of age. Foals at high risk may have the vaccination series initiated as early as 2 weeks of age.