Feeding the MIM (PSSM2) Horse

PSSM2/MIM symptoms are caused by constant muscle breakdown. This causes the horse to have a permanently elevated need for protein to build and repair muscle. Simply put, protein consists of a chain of amino acids. When one or more amino acids are missing, this chain will not be complete, which results in there not being enough protein to build muscle cells. Essential amino acids are amino acids that cannot be created by the horse’s body, meaning the horse has to get them through its food. The essential amino acids that are most often lacking in roughage and have a bottleneck function in the building of protein are lysine, methionine and threonine. This makes these amino acids extra important for horses with PSSM2/MIM. Numerous owners of horses with PSSM2/MIM report that supplementing just these three amino acids has resulted in a major improvement in their horse’s muscle health. The standard dosage of these amino acids is 10 grams of L-lysine, 5 grams of DL-methionine and 2 grams of L-threonine. Mares can have hormonal reactions to methionine, in which case the dosage of methionine can be lowered to 2 grams.

Many horses with PSSM2/MIM benefit from extra natural vitamin E (2000-6000 IU). Vitamin E is a good anti-oxidant, which helps to prevent muscle episodes. Many horses with PSSM2/MIM also benefit from additional magnesium (8-15 grams) and salt (15-40 grams). Extra fat is not necessary, but a source of omega 3 combined with low omega 6 can help prevent and fight inflammation. Inflammation in the body can cause a negative nitrogen balance, which can trigger an episode. Micronized linseed or linseed oil are suitable sources high in omega 3.

Examples of feeds high in protein, suitable for horses with PSSM2/MIM are alfalfa, esparcette, and soy, but it needs to be mentioned that not all horses do well on these legumes. Hemp (whole ground up plant with neglectable THC levels) has also been used successfully to provide PSSM/MIM with additional protein. Protein can also be added in the form of a supplement. Most protein supplements are based on whey, which has a very complete protein profile. Plant based alternatives that are suitable for horses are brown rice protein and pea protein. Note that peas are legumes as well. The protein need of a horse with PSSM2/MIM is usually between 50-200 grams a day.

Horses with the Px variant need a bit of a different approach. Their optimal diet is more of a combination of the PSSM1 and PSSM2 diet: low in sugars and high in protein. Horses with Px can display explosive behaviour when their diet contains too much calcium and/or potassium. For this reason, alfalfa and soy are usually not suitable for horses with the Px variant. Salt can help lower potassium uptake from a horse’s feed which makes supplementation extra important for horses with the Px variant. A salt lick is usually not sufficient. Adding a tablespoon of salt to the horse’s feed is recommended, or as much as the horse will accept.

Usually, horses with PSSM2/MIM tolerate grass well (as opposed to horses with PSSM type 1), with the exception of horses with the Px variant, though some do tolerate a few hours on pasture