Lesions of spinal cord parelaphostrongylosis in sheep. Sequential changes following intramedullary larval migration

Spinal cord nematodiasis epidemiologically, clinically, and histologically consistent with Parelaphostrongylus tenuis infection was noted in two flocks of sheep. Spinal cords from two sheep with active infection and one from a partially recovered animal were studied in an effort to determine the sequence of lesions following larval invasion of the central nervous system. In the former two sheep, migration of larvae within the spinal cord induced asymmetrically irregular tracks of disrupted and necrotic tissue, primarily in white matter. Subsequently, macrophages infiltrated these regions and phagocytized the necrotic tissue, which led to cavity formation. Swelling and loss of axons, diminished myelin staining, mononuclear cell infiltration and increase in astrocytic fibers were often seen in adjacent tissue. Only occasional coiled larvae were found in these actively infected animals. Late stage lesions in the white matter in the partially recovered sheep included multiple small astrogliotic regions with diminished myelin and axonal content, and a single large multicavitary, atrophic, gliotic zone.