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Foal immunodeficiency syndrome (FIS), a fatal autosomal recessive disease found in three breeds of horses, was first reported (in the Fell pony) in 1996, and it soon became apparent that significant numbers of syndrome foals were being born each year. In each FIS case, the foals are clinically normal at birth, but start to weaken at 2–8 weeks (Scholes and others 1998) as they develop profound anaemia (Dixon and others 2000) and do not have the ability to produce their own antibodies (Thomas and others 2005), due to the almost total lack of B lymphocytes in the circulation or tissues (Thomas and others 2003), but with apparently normal levels of functional T lymphocytes (Bell and others 2001). The outcome is persistent opportunistic infections with no effective treatment; euthanasia is the preferred option. FIS has also been reported in Fell ponies in The Netherlands (Butler and others 2006), Germany (May and others 2011) and USA (Gardner and others 2006). In 2009, we confirmed a case of FIS in a Dales pony foal (Fox-Clipsham and others 2009).
The search for the genetic lesion thus became paramount, as the carrier parents were clinically normal, and it was feared that there could be high carrier …