Immunodeficiency/anaemia syndrome in a Dales pony

THE syndrome of anaemia and immunodeficiency was first recognised in Fell pony foals in the UK in 1997 (Scholes and others 1998) and has since been reported in the same breed in the Netherlands (Butler and others 2006) and the USA (Gardner and others 2006); there have been no reports of the syndrome in any other horse breed. Affected foals are apparently normal at birth, but the disease first manifests at two to six weeks of age; the reported characteristic clinical signs include weakness, dyspnoea, nasal discharge, poor growth, reduced appetite, diarrhoea and pale gums. A profound and progressive fall in red blood cell count (packed cell value [PCV] <20 per cent) is a notable early feature. The number of circulating lymphocytes is reduced and there is an increase in the number of peripheral blood polymorphonuclear cells (Dixon and others 2000). Analyses of lymphocyte subpopulations show normal numbers of circulating T lymphocytes (Bell and others 2001) but severely reduced numbers of circulating B lymphocytes (Thomas and others 2003); there are also low concentrations of circulating immunoglobulins in affected foals (Thomas and others 2005). These changes coincide with episodes of opportunistic bacterial, viral and parasitic infections. Typically, these changes persist for three to six weeks, with the foal becoming progressively weaker due to systemic infections and profound anaemia; the PCV can decrease to as low as 3 per cent. Attempts at symptomatic treatment have been made; including rehydration, antibiotics, enteral/parenteral nutrition, analgesia, supplementation with vitamins and selenium, blood transfusions and injections of erythropoietin. None of these is reported to have had any effect apart from a short-term delay in the death or euthanasia of the affected foal. No foals have been recorded as surviving the syndrome. At postmortem examination, there are many distinctive changes associated with the disorder, including glossal hyperkeratosis and diphtheritic fungal glossitis (usually associated with Candida species), colitis, pneumonia, inactive or aplastic bone marrow, and a hypoplastic thymus and lymph nodes (Scholes and others 1998, Richards and others