Congenital tremor type AI: light and electron microscopical observations on the spinal cords of affected piglets.

The histology and ultrastructure of the spinal white matter from the dorsolateral funiculus of the third cervical segment was studied in normal control pigs and pigs whose dams were inoculated with the Weybridge congenital tremor strain of swine fever virus in early pregnancy. Only inoculated sows produced abnormal piglets. These showed congenital tremors and ataxia. The severity of clinical signs was related to the degree of spinal myelin deficiency. Morphologically this was quantified by determination of the thickness of myelin investing axons classed according to their diameter. In clinically affected pigs fewer axons were myelinated than normal. Though the myelin sheath thickness increased with increasing axon diameter in all pigs whether clinically normal or not, the increase was less in moderately affected and much less in severely affected pigs. The deficiency of spinal myelin was probably due to delayed or sub-normal myelination accompanied by paranodal myelin abnormalities, myelin degeneration and remyelination.