Cerebellar Hypoplasia Caused by Congenital Bovine Viral Diarrhea Virus Infection

Lisle W. George

A complete discussion of bovine viral diarrhea-mucosal disease (BVD-MD) can be found in Chapter 32. BVD virus infec­tion of susceptible cows from 90 to 170 days of pregnancy results in abortion or stillbirth, hydranencephaly, or cerebellar hypoplasia in the fetus.^1,2 The BVD virus infects the developing germinal cells of the cerebellum and kills Purkinje cells in the granular layer, which results in necrosis and inflammation.³ The microscopic lesions include necrosis of the external germinal cells, focal parenchymal hemorrhages, and folial edema.

The signs of cerebellar dysfunction are usually present at birth and include truncal ataxia, falling backward, opisthotonos, basewide stance, coarse intentional head tremors, hypermetria, hyperreflexia, and nystagmus or strabismus (Fig. 35.17).^4,5 If severely affected, the animal may be unable to stand or lie in sternal recumbency. Excitatory stimuli in these animals precipitate wild oscillations and side-to-side movements of the head, which can be mistaken for convulsions. The affected calves may have a deficient menace response and appear to be blind, especially with concomitant hydranencephaly or microphthalmia. The neurologic condition rarely improves after birth. Other fetal changes that may be induced by the BVD virus include thymic atrophy, retinal degeneration, corneal opacity, and failure to grow.^1,4 The diagnosis of cerebellar hypoplasia is based on identification of the specific clinical signs and the recognition of BVD antibodies in precolostral blood specimens. The virus may be cultured from the blood of some affected calves, but this appears to be dependent on whether an immune response to the virus has developed in the affected calf.

FIG. 35.17 Characteristic head position and basewide stance of a calf with cerebellar hypoplasia.