In This Issue
Fun Fact!
Botox is is actually Botulinum toxin Type A.  It is used to reduce muscle spasm (by blocking the nerves) and thereby reduces wrinkles!

 
April Newsletter
Botulism in the Horse 
  
  
It's April in New England and once again we find ourselves facing snow at the beginning of the month!  It's ok, though, the light is at the end of the tunnel.  Soon we'll have plenty of other things to worry about.  Speaking of things to worry about, let's bring you up to speed on another disease.  This month, we're discussing botulism.  

Many people have heard of botulism, but not everyone knows exactly what it is.  While we're focusing mainly on horses for this newsletter, keep in mind that botulism can affect any mammal.

While not common, it's a disease that lurks dangerously around the corner.  So read up and be knowledgeable!    
Enjoy the spring everyone!
Sincerely,
Drs. Matt and Ashley Kornatowski
Twin Pines Equine Veterinary Services, LLP
  
(Have an idea for a newsletter topic or an event?  We'd love to hear from you!  Just send us an email at [email protected])
Botulism: An Overview

Botulism is a very dangerous neuromuscular disease and is often fatal.  It progresses quite quickly and often times horses die because they lose the ability to breathe. 

The spores exist in the environment and are impossible to eradicate and there are various ways that the disease can be acquired.  Treatment options are limited, but effective, and prevention is possible.  We'll dive into all of these points in detail below!
A bit about Clostridium botulinum

 

The bacteria responsible for Botulism is a Gram positive, rod shaped anaerobe, Clostridium botulinum.  (Gram positive relates to how the organism shows up when it'

microbewiki.kenyon.edu

s stained for microscopic exam, and anaerobic means "without oxygen").  The organism exists in the environment across the world and is a close relative of another neuromuscular disease causing microbe - Clostridium tetani (tetanus). 

C. botulinum survives by forming spores that persist in the soil.  8 different neurotoxins are recognized and they are labeled Types A-G.  Botulinum toxin type B is most prevalent on the east coast and is the causative toxin in more than 85% of cases. (Type A is prevalent on the west coast).  This becomes important when we talk about treatment.   

 

While we often think about botulism being caused by dead animals (such as a rat rolled into a bale of hay), this situation results in a production of Type C toxin.  While this can still affect horses, this toxin is relatively uncommon and is a bit of a misconception with botulism.     

Really, C. botulinum thrives in fermented feeds.  Basically, any situation where moisture occurs and creates a warm environment without oxygen.  For example, large bales that get wet and the middles begin to ferment.  Haylage and silage are also culprits.  The toxin is formed by the bacteria and, even though the organism may not last when exposed to the air, the feed is already contaminated with neurotoxin.  

 

How does it work?


That's a really good question!  First, the horse has to become infected.  There are a few ways that this can occur. 

Most often, horses ingest the preformed toxin, as described above.  The toxin, being a protein, gets absorbed via the GI tract and then enters the bloodstream. 

In foals, the spore can be ingested whereby the organism then infects the GI tract.  Toxin is formed locally and gets absorbed as in the adult, however it should be noted that this is not the same as in adults, who ingest the toxin only (not the bacteria).  This is referred to as "Shaker Foal Syndrome." 

The least common avenue of infection is through an open wound.  C. botulinum can establish an infection with the wound and then the toxin gets absorbed into the bloodstream. 

In normal functioning muscle, a neurotransmitter called 'acetylcholine' is released from the axon of a nerve.  This signal is what tells a muscle to contract.  The botulism toxin blocks the release of acetylcholine, so the muscle cells never get the information telling them to get to work.  The muscle cells go on indefinite break and we end up with the clinical signs outlined in the next section!
Toxin's effect on Neuromuscular junction.
(Image from toxipedia.org)


Clinical Signs                 

 

When a horse becomes infected with botulism, the clinical signs can be acute and dramatic.  It is not uncommon to find the animals recumbent or dead.  Signs are related to loss of muscle tone over the entire body.  This can include:

 

Loss of tongue tone (www.vetstream.com)
  • Dysphagia (difficulty swallowing)
  • Weak tongue tone
  • Poor muscle tone
  • Generalized weakness
  • Muscle fasciculations/tremors
  • Recumbency
  • Colic
  • Death

While we don't like to include "death" as a clinical sign, it does occur.  Since the diaphragm is a muscle, when it loses the inability to contract and the horse becomes unable to breathe.  The problem here is that these signs can mimic other diseases.  Oftentimes, the presenting complaint is either "colic" or "choke".  While diagnosis is possible, treatment must be performed before the results are available.   

Diagnosis

   

As stated above, diagnosis is usually based on response to treatment.  A definitive diagnosis can be obtained, however it is usually well after the fact.   

 

Until recently, the only method available to test for Botulism was through a Mouse BioAssay.  Samples taken from the animal in question are injected into a mouse.  The mouse is then observed for several days to determine if they develop clinical signs.  This method, while effective, has a couple of issues.  First we're dealing with a live animal model and second, it can take 2-3 weeks for results.  Not ideal.    

 

A newer test developed at the University of Pennsylvania, uses polymerase chain reaction (PCR) to look for the neurotoxin genes.  This has been shown to be effective for Types A, B and C.  It still takes 5 days for results, however this is much reduced from the previous timeline.   

 

In the essence of time, there are a couple of quick tests that the clinician can due to determine if treatment for botulism is warranted.  The first is the tongue pull.  As can be seen above, horses with botulism have difficulty retracting their tongue.   

 

Additionally, a small amount of sweet feed can be offered.  A normal horse should be able to consume 8 oz of grain in less than 2 minutes.  Affected horses take much longer.  While these aren't perfect, sometimes they are all we have to go on and institute the appropriate treatment.        

Treatment                   

 

 There are two mainstays to treatment of botulism - antitoxin and supportive care.  Beyond that, there's not much else that can be done.    

 

 The antitoxin must be administered as soon as possible when a diagnosis is made.  This will neutralize circulating toxin and 1 dose provides protection for more than 60 days.  However, it does not neutralize toxin that has already been taken up by the nerve or toxin that is already been bound to the receptors.  Due to this, clinical signs may progress for another 12-24 hours.   

 

Supportive care depends on the severity of disease.  Fluids and nutrition may be necessary, and if the horse is having difficulty standing, it may need to be managed for that.  Additionally, complications may arise, however these do not typically affect the outcome of the case. 

www.vetstream.com

Treatment can often be successful as long as it is initiated quickly.  In a hospital setting, a 95% survival rate was noted if the horse was still able to stand (even if it is only for a few minutes in 24 hours).  If they completely recumbent, survivability reduces to 18%.  Overall, horses generally have a 48% survival rate.  (Please keep in mind that these numbers are from a referral hospital where 24 hour care and monitoring is available.  While no numbers are currently known, it is suspected that survival rate in the field is much lower.)     

  
Prevention
 
Botulism is a preventable disease.  Besides some husbandry techniques, there is also a vaccine available. 

With regards to feed, be sure to check your hay and grain for any signs of rot or for dead animals.  Round bales should properly processed and stored.  Try not to wrap them in plastic, and store them off the ground (on pallets) if outside.  Keep in mind that round bales are not the only way that horses can acquire botulism.

The botulism vaccine has been around since the 1980s.  It has been shown to be very effective in reducing the incidence of botulism.  In a majority of cases it has prevented the disease, and in a few the symptoms were minimized.  No deaths due to botulism have been reported in vaccinated horses.  It should be noted that the vaccine covers horses for Botulinum Toxin Type B and does not cover for any other toxin types.  However, this is the toxin type that is most prevalent. 

Vaccination is a little heavy on the front end, requiring once a month dosing for 3 months.  From then on it is once yearly.  This is a fairly inexpensive way to prevent a disease that  can be quite costly and fatal.  

Summary

Botulism
can be a devastating neuromuscular disease.  It doesn't take a lot of toxin to debilitate a horse. (1 tsp of toxin can kill 5,000 horses.)  While some populations are more at risk than others, the disease is prevalent in the environment making all horses susceptible.  Treatment is effective if instituted quickly, however the disease can mimic some other disorders thus making it hard to recognize. 

The biggest point is that this is a preventable disease.  If you have concerns about the risk to your horse, we recommend vaccinating.   It really can save your horse. 

If you'd like to read more, please refer to Neogen's write up: http://www.neogen.com/blog/equinebotulism/botulism-faq/