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Hunting for a needle in a haystack: investigating inherited diseases of sheep

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The investigation of genetic diseases in animals can be a challenging and time consuming process. Published reports of genetic diseases in sheep are relatively uncommon, with only approximately 170 genetic disorders reported on the Online Mendelian Inheritance of Animals database compared with 470 genetic disorders in cattle (Nicholas 2017). This is likely because congenital anomalies in sheep are only investigated if perinatal losses are greater than expected, perhaps in part due to the potential cost of any investigation (Thompson and others 2008).

Congenital anomalies could be due to a genetic disease but may also be caused either by teratogens, such as viruses and plant toxins, or be secondary to nutritional deficiencies during gestation (Dittmer and Thompson 2015). Therefore, veterinarians called to investigate either isolated cases or clusters of congenital anomalies should take care not to suggest a genetic basis without sufficient evidence.

Obtaining a thorough history

The first step in investigating any outbreak of congenital abnormalities is to obtain a thorough history. This can be extremely helpful in determining between a genetic basis for the defect and a teratogenic or nutritional cause. Pedigree data for affected and unaffected animals should be obtained if possible, with the breeding of closely related animals suggesting an increased likelihood of a genetic cause for the abnormalities (Jolly and others 2016). History on trace element status, feeding and treatments during pregnancy, disease outbreaks on the farm or neighbouring farms, and known toxic plants on the farm should also be collected (Fig 1).

FIG 1:

Newborn Romney × Texel × Poll Dorset lambs born with congenital abnormalities. (a) Spine showing kyphosis, scoliosis and fusion of lumbar vertebrae. …

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