Mycotoxicosis and Mycotoxins:
How do they affect YOUR horse?

Mycotoxicosis is the disease caused by ingestion of mycotoxins, which are toxins produced by fungi living on or in a plant at harvest time.

This disease is very serious because of the possible severe reactions (including death in some cases) in horses. Thankfully, it is also a fairly rare disease.

Some of these toxins are fatal within hours of ingestion, such as ergotamine, produced from ergots from the fungi Claviceps spp. This fungi has been found contaminating fresh pasture in Europe.

Red clover, which can harbor mycotoxins that produce mycotoxicosis.

Other mycotoxins, however, do not have such serious consequences of ingestion. These include slaframine, produced by Rhizoctonia leguminicola which is found in red clover and alfalfa. This mycotoxin causes excessive salivation and increased water intake. However, it breaks down in stored forages over time, so its less likely to cause problems.

The biggest problem, from a management standpoint, is the fact that there are no reliable predictors of mycotoxin infestation. A number of plant variables, such as plant health, growth status, and plant maturity all affect whether or not mycotoxicosis is a potential problem.

Conditions at harvest, including weather and how quickly the feed is stabilized also play a major role.

Temperature and moisture content at harvest time, as well as during storage, are two of the biggest indicators of whether or not mycotoxicosis might be a problem.

However, the various temperature and moisture combinations at which the different fungi produce harmful levels of mycotoxins is largely unknown. Complicating the matter even further is the fact that there are so many various fungi that produce mycotoxins, creating a wider range of conditions under which problems may occur.

Mycotoxins can be found in fresh pasture, stored forage, or grains. The fungi need heat and moisture to thrive and produce the toxins, so mycotoxins will usually not be found in feeds that have been properly harvested and stored. This is one reason it is so important to only feed your horse feeds that you know have been properly harvested and stored. When in doubt, don't feed!

Below is a table that summarizes some of the various fungi and their toxins. Don't be alarmed by the size of the I mentioned above, mycotoxicosis is a pretty rare disease, despite the various organisms and plants involved.

Fungus Family Specific Fungus Source Specific Plant Toxin Toxin Symptoms
Claviceps spp.   Pasture Ryegrass;
Native European species
Toxic alkaloids  
Rhizoctonia spp.   Pasture
  R. leguminicola Pasture
Red Clover;
slaframine Excessive salivation;
Increased water consumption
Fusarium spp.   Pasture
  tricothicenes Kidney damage;
Nervous system problems
        fumonisins Blind staggers;
  F. roseum;
F. graminearum
Reproduction problems
Asperigillus spp.   Hay
  aflatoxins Carcinogenic;
Liver problems
Penicillium spp.   Hay
  dicoumerol Nasal bleeding;
Joint Swelling;
Respiratory problems
        patulin Carcinogenic

Like most diseases that face our horses today, mycotoxicosis is much more easily prevented than treated. Use common sense when buying feed products for your horse, especially hays. Ask the producer how it was stored and what the weather and drying conditions were when it came out of the field.

Store feeds properly yourself...keep them off the ground and in a place where they won't get rained on or other moisture drawn into them. I personally also salt my hay bales for extra protection, giving me one less thing to worry about.

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