BOTULISM – Can you see it and is it dangerous?

Medicine is the restoration of discordant elements; sickness is the discord of the elements infused into the living body - By Leonardo da Vinci

Quick Notes:
Botulism is a disease caused by a soil bacteria, which can get into hay, haylage, and silage. This can be caused by decaying carcasses, soil contamination, damaged wrapping, or poorly made products. Disease in the UK is rare, with most adult horses being immune, but where disease does occur it is usually fatal with horses becoming paralysed and dying rapidly. To survive, they need intensive care for many weeks. Unfortunately, it is not possible to ‘see’ the bacteria and so forage can look okay and still be toxic.

More Detail:
Botulism is caused by a bacteria called Clostridium botulinum, which lives in the soil. Under some circumstances C. botulinum produces a neurotoxin (a toxin that damages nerves) – this toxin is infamous for it use in the cosmetic industry as ‘botox’. Yes, if you have a botox injection you are actually injecting a small amount of bacterial neurotoxin into your body!

There are different strains of the toxin, and in some countries outbreaks of disease occur. For example, in the USA in 2008 one breeding farm lost over 100 brood mares because of botulism in a bad batch of haylage, with many more very ill. In the UK, the disease tends to be more sporadic (i.e. not affecting large number of horses at a time), but can still occur.

How can a horse get infected?
There are several ways that horses can get infected:

  • Adult horses are usually immune to the toxins, but under some circumstances they can become infected. Infection in adult horses is usually because of contaminated forage.
    • In hay, this is usually due to an animal carcass being included in a bale of hay while it was being made (e.g. a rabbit). Or when hay is fed on muddy ground in the paddock during winter. The risk of botulism from hay is usually lower than from haylage, as the bacteria prefer living conditions without oxygen.
    • In haylage and silage it is usually due to the packaging being damaged before feeding, but can also be due to problems when it was being made; an animal carcass; or feeding off muddy ground. Wrapped bags of haylage or silage that are made properly have a low pH and any fermentation stops. If the wrapping is damaged, then fermentation starts again and there is an increased risk of bacteria (such as the one causing botulism) producing toxins.
    • There is a greater risk from large bales compared to small bales, as they are less likely to reach a low pH quickly.
    • Note that lawn mowings can contain botulism toxin too, as can any wilted vegetation.
  • ‘Shaker disease’ occurs in a foal up to 8mo if the bacterial numbers increase in its intestines after eating soil containing the bacteria. This usually occurs in foals that are born to dams that have recently been moved to a problem area, so there has been no immunity passed from mare to foal. This has also been linked to grain feeding and fast-growing foals.
  • Rarely, infection can occur through deep wounds (in the same way as tetanus), and also via gastric ulcers in foals.

Once the neurotoxin has entered the body, it stops nerve-muscle junctions from working properly, and this causes paralysis. If the horse doesn’t die quickly, then the paralysis can last for weeks, so the horse will need intensive care until it recovers.

What are the symptoms?

  1. Classic Botulism (paralysis)
    Within hours to days of eating the contaminated feed, the toxin will start to paralyse muscles within the body. This often shows as signs such as problems eating and swallowing (with drooling and reflux from the nose); problems pulling the tongue back into the mouth; muscle tremors and weakness; problems walking and getting up; drooping eyelids, and a flaccid (limp) tail. Although the muscles are paralysed, the horse can still feel everything as usual, so will feel pain and fear.
    The paralysis progresses to all of the muscles and the horse will die once the respiratory muscles are affected (as the horse can no longer breathe). The horse may also die from complications from being ill (e.g. colic, or pneumonia caused by food entering the lungs because the horse can’t swallow properly).
    To survive, the horse needs intensive care for anything ranging from days to weeks, until the toxin no longer affects the body.
  1. Equine Grass Sickness (EGS)
    One of the strains of C.botulinum toxin has been linked to EGS, which causes paralysis in the intestinal system (i.e. the gut stops working). Unlike classic botulism however, EGS is not usually linked to the feeding of haylage or hay and may be related to soil contamination instead. Symptoms include signs of colic, difficulty swallowing and reflux, drooping eyelids, muscle tremors, rapid weight loss, and sudden death.

Is there a vaccine?
Toxoid vaccines can be expensive, and need to contain multiple strains, but they will protect horses in areas where outbreaks of botulism are common. They are commonly used in some areas of the US, but are not in common use in the UK.

Antitoxin can be given for sick horses; it does reverse the paralysis that already exists, but does stop any loose toxin from making things worse. (For the difference between an antitoxin and a toxoid, see the article on tetanus).

Out of interest, has ‘botox’ ever been used in horses?
Yes, but not for the reasons you might think! Difficult foaling’s can sometimes cause a tear between the vagina and the rectum, which need stitching. The pressure of going to the toilet causes the stitching to breakdown, and one research trial used low doses of botox to decrease the pressure. However, its use is not without side effects, so it is unlikely that horses will be routinely ‘botoxed’.

How can I avoid botulism?
Botulism cannot be completely avoided, but you can reduce the risks:

  • Do throw away any forage if you find that it has a decaying carcass (e.g. rabbit/ bird/ mouse) inside it. Don’t assume that the other end of the bale is safe.
  • Do avoid buying any haylage that has damaged wrapping.
  • If you damage the wrapping yourself, then do ensure that the haylage is completely fed out within a few days.
  • Do feed all haylage from a bale within a few days of opening (usually 3-5 days is the recommended maximum). Large bales are not suitable for only 1-2 horses, as they cannot be fed within a reasonable period of time, so small bales should be used.
  • Do avoid silage at all times. Note that cattle are more resistant to botulism than horses. Horses are believed to be the most susceptible species! If silage must be fed due to a feed shortage, then ensure that horses are vaccinated beforehand.
  • Do buy forage from a reputable supplier. Those experienced in making forage for horses will be aware of the risks (e.g. raking hay in a way that doesn’t drag soil into it; drying hay or haylage to the correct moisture percentage; etc.).
  • If feeding forage in the field, then do use hayracks or similar where possible, and move the rack regularly to non-muddy areas of the field. If feeding off the ground, then choose clean areas each day, and not areas poached into mud.
  • Do ask your vet about vaccination if you are in an area where botulism outbreaks have occurred.

And a final very important point:

DO NOT assume that just because the contents ‘look ok’ that it is safe to feed. You cannot see bacterial spores or the neurotoxin and the bale does not have to be mouldy to contain toxin!

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