Botulism in Haylage Kills Horses

In October, 2008, 100 horses died in Ocala, Florida as a result of botulism, a form of food poisoning that occurs when toxins produced by the clostridium botulinum bacterium enter the horse’s body and cause weakness which may progress to paralysis.  

Clostridium botulinum is familiar to most of us as the bacterium used to prepare the “Botox”  that we inject to cause temporary paralysis in the muscles that cause wrinkles.

The bacteria that killed the Florida horses were reportedly located in the hay silage (haylage) fed to recipient mares–100 of whom died as a result–at the EquiTransfer embryo transfer facility owned and operated by two veterinarians.  The source of the haylage was not disclosed by the veterinarians/owners who cannot be compelled by the state of Florida to disclose the source because the state of Florida does not regulate grass.  The poisoned horses were insured, according to the owners.

In an earlier post, I referred to a Dow Chemical company webpage that included a statement by researcher Manfred Coenen of the Institute of Animal Nutrition, University of Veterinary Medicine, Hanover, that

some potential issues linked to feeding haylage to horses are object of discussions and further research is needed to elucidate best practices.

I congratulated Dow for publishing the warning, but a closer look shows that the Coenen article containing the warning is not prominent on the company’s website.  It is contained in an article published in the Autumn 2005  Silage Insights Newsletter, but is not included on the “Practical Tools and Tips/Experts Advice/Haylage for Horses” page accessible from the main site.  

The Dow “Haylage for Horses” page does feature several opinions from other researchers that endorse the practice of feeding hay silage to horses, despite qualifications, such as this one:

Risks in the production and feeding of silage result from mould growth, rotting areas, dust, soil inclusions, elevated acid concentrations, poor fermentation, insufficient fermentation, mouse faeces, toxic plants and dead animals. Many of these factors lead to contamination with the toxin Clostridium botulinum – botulism. Carefully prepared bale silage exhibits no contaminants, rotting or whitish areas. In addition, high-quality silage is characterised by an aromatic scent that is pleasantly acidic to bread-like.

Silage chopped into small particles smaller than four to five centimetres should not be fed to horses because the chewing response and saliva formation among horses are too low at this consistency. This fodder can cause gullet blockages and congestion among highly voracious horses. 

The Dow site also includes an “economic benefit tool” for calculating the cost/benefit of feeding bale silage.  The only study I could find on the subject of feeding haylage to horses, published in October 2001, clearly condemned the practice, concluding

haylage is not recommended when feeding horses due to the danger of botulism.

According to Dr. B. Wright – Veterinary Scientist, Equine and Alternative Livestock/OMAFRA and Dr. Dan Kenney, Staff Veterinarian, OVC/ University of Guelph–the veterinarians who conducted the study–horses “are the most sensitive of the domesticated animals to botulism. 

 Affected horses

  • usually have muscle tremors.
  • may be so weak that they cannot stand up.
  • lose control of their tongue so it may hang from their mouth.
  • can’t eat and they drool because they can’t swallow.
  • may walk stiffly with a short stride or they may be weak and stumble. Their tail may lose its tone.”

What is the current veterinary “best practice” on the advisability of feeding haylage to horses?

If proper processing and storage of haylage are essential to avoid the possibility of poisoning horses who are fed haylage, why do so few states regulate this food product?

If a horse poisoned by contaminated haylage is fully insured, how does that affect the cost/benefit analysis?


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