Most of the calves were recumbent or only able to stand with support, but none had clear macroscopic skeletal or visceral abnormalities. Upon necropsy it was found that all the calves had diffuse neuraxial hypomyelination and that BVDV antigen was present. A similar finding was observed by Brian Porter, Department of Veterinary Pathobiology, Texas A&M, in a longhorn calf submitted to the Texas A&M Veterinary Teaching Hospital. The newborn calf was unable to rise, exhibited generalized tremors, ataxia and horizontal and vertical nystagmus. The calf was euthanized at 8 days of age and necropsied. The brain and spinal cord appeared normal by gross examination. Macroscopically the most notable observation was that myelin sheaths were absent or markedly thinner in comparison with normal calves. IHC for BVDV antigen was positive for a number of cell types in the CNS. The infecting virus was identified as a BVDV2 strain by researchers at the National Animal Disease Center. These findings suggest that BVD should be considered as one of the main differential diagnoses of congenital tremor in calves.
Due to the sheer number of different clinical presentations existing under the BVD umbrella, diagnosing BVD based on clinical signs is not advisable. Thus diagnosis relies upon testing of samples in diagnostic laboratories. In the United States, most of the diagnostic effort has been focused on identifying and culling animals persistently infected with BVD pathogens.
The Academy of Veterinary Consultants recommends that testing for animals persistently infected with a BVD pathogen be incorporated into both beef and dairy biosecurity plans. However, as case #1 illustrates, outbreaks of acute BVD can result in substantial losses. Based on surveillance data, it is estimated that 85 percent of cattle in the United States have been exposed to a BVD pathogen, and 10 percent of herds currently harbor a persistently infected animal. The question is not whether practitioners will witness an outbreak of BVD but when they will witness it. Their ability to recognize BVD when they see it depends on regularly including BVD as a differential in their case workups.
Blanchard PC, Ridpath JF, Walker JB, Hietala SK: 2010, An outbreak of late-term abortions, premature births, and congenital deformities associated with a bovine viral diarrhea virus 1 subtype b that induces thrombocytopenia. J Vet Diagn Invest 22:128-131.
Evermann JF, Barrington GM: 2005, Clinical features. In: Bovine viral diarrhea virus: Diagnosis, Management and Control, eds. Goyal SM, Ridpath JF, pp. 105-120. Blackwell Publishing, Ames, IA.