Foal immunodeficiency syndrome (FIS), formally known as fell pony syndrome, is an inherited condition of progressive anemia and immunodeficiency in foals. Initially FIS was thought to occur only in the Fell Pony breed of horse, but in 2009 a case was reported in a Dales breed pony. A mutation associated with FIS in both the Fell and Dales Pony breeds was identified in 2011. Other at risk breeds of FIS are the Clydesdale, Highland Pony, Exmoor Pony, Welsh Pony, and colored horses and ponies.
Affected foals appear normal at birth but at 2 to 8 weeks of age begin to develop severe anemia and immunodeficiency.