The legendary Quarter Horse sire Impressive is known for his versatility and was a world champion in both barrel racing and halter. He was also a very successful breeder, producing a large number of outstanding horses. But recently his descendants were linked to a genetic mutation that causes a deadly muscle condition called hyperkalemic periodic paralysis, or HYPP. This disease is very dangerous for horses and their handlers, as it can cause severe muscle twitching and weakness that can lead to respiratory failure, and the disease can be fatal for some of them.
The disease is caused by a mutation in the gene responsible for making acetylcholinesterase, an enzyme that breaks down nerve impulses in the body. This mutation leads to the accumulation of a harmful compound called ammonium ions in the cells, which cause the muscles to spasm. The symptoms of the disorder can range from mild to severe and can affect both males and females. The disease can be difficult to diagnose, as many of the symptoms are similar to those of tying up syndrome and colic. It is thought that the disease is passed on through a recessive genetic mutation, which means that only one of a horse’s parents needs to carry the mutation in order for their foals to be affected by it.
While horses with HYPP are still being used in both the barrel race and halter disciplines, the disease has become an issue of concern for both owners and breeders. Fortunately, a test has now been developed that can identify the horses that carry the mutation. This is a major step forward, as the disease has been extremely hard to trace until now.
One of the reasons why HYPP has been so difficult to trace is that the majority of horses affected by it have been descendants of a single sire called Impressive. This famous stallion was the sire of 2,250 foals, and many of his offspring went on to become world champions in both barrel racing and halter. In 1992, researchers discovered that Impressive was the carrier of a genetic mutation that caused the disease, and they were able to link all affected horses back to him.
Thankfully, there are ways to lower the risk of passing the mutation on to your horse, including linebreeding and crossing with horses that do not have the mutation. However, it is important to note that no dilution of bloodlines will remove the risk completely, and even if you are careful, you can end up with a HYPP-positive foal. The best way to reduce your risk is to not breed a horse with the mutation and use only purebreds that are tested free of it. AQHA now requires all horses with Impressive bloodlines to be tested for the disease before they can be registered. If they test positive, AQHA will not register them. This is a big step forward, and it may eventually be possible to eradicate this terrible disease altogether.