Foal immunodeficiency syndrome (FIS), formally known as fell pony syndrome, is a fatal autosomal recessive disease that affects young foals. Foals with FIS appear normal at birth, but soon develop a profound anemia which causes them to start to weaken 2 to 8 weeks later. The anemia is caused by the complete absence of B lymphocytes in the circulation or tissues which has an affect on their ability to produce their own antibodies.

The first reported case of FIS was in a Fell Pony in 1996. In 2009, a confirmed case of FIS was seen in a Dales pony foal. 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.