Affected foals appear normal at birth, but become weak, and either die or must be euthanized by three months of age due to persistent infections caused by immunodeficiency.
By one month of age, foals become depressed, lose weight, and have diarrhea and a cough with nasal discharge.
[1] The cause of disease was tracked down using a genome-wide association study, which implicated a region on horse chromosome 25.
[4] Shortly after this, a genetic test was launched for horse owners to identify whether their ponies were carriers.
Over time, the frequency of the disease gene will decrease, without having an adverse impact on the genetic variation of the pony population.