Foal Immunodeficiency Syndrome, or FIS, is a devastating disease with a 100% mortality rate of affected foals within the first few months of life. Affected foals are unable to produce antibodies and will succumb to infection. It is a recessive genetic disease, meaning the foal will only be affected if BOTH parents are carriers of the mutation that causes the disease, AND have passed the abnormal gene on to the foal.
This mutation has been identified in Dales Ponies, Fell Ponies, and Gypsy horses. The good news about this mutation is that carriers are completely unaffected, and as long as at least one of the parents is not a carrier, it is impossible for the foal to have the disease. There is now a policy that all Dales Pony stallions MUST test negative for the FIS gene mutation in order to be licensed to breed. There are a few older stallions that were licensed before this rule went into effect and some of these have not been tested for FIS. If you are breeding a mare to a stallion that has not been tested or is a known carrier, it would be prudent to have the mare tested to make sure she is not a carrier of the FIS mutation.
On the off chance that an FIS-carrier mare would be bred to an FIS-carrier stallion, the resulting foal would have a 25% chance of bring FIS clear, a 50% chance of being a carrier like its parents, and a 25% chance of being born with the fatal disease.
Some breeders choose to test all breeding stock, and some only test stallions. As long as the current policy requiring stallions to be clear of the FIS mutation remains in effect, both strategies will prevent the birth of affected foals. Breeding unlicensed or unregistered stock could lead to foals being affected.
There is no advantage to breeding out FIS carriers, as long as breeders are thoughtful about which mares they breed to any remaining carrier stallions. In fact, eliminating FIS carriers would narrow the gene pool in this critically endangered breed, which could carry other unintended adverse genetic consequences.
Testing for the FIS gene mutation is available through multiple laboratories in the United States and one in the United Kingdom.
Links to testing laboratories with more information about the disease, and cost of testing: