David L. Morris: Vet Column 4-4-11
Bovine viral diarrhea virus (BVDV) articles are commonly observed when reading many of the lay publications involving cattle management, as well as within the advertisements. There is little doubt that BVDV is an important disease of cattle, but how does one differentiate the terms genotype from noncytopathic from subgenotype? How does one make sense of the labels on various vaccine packages when attempting to make purchasing decisions? Vaccines do cost money.
BVDV causes inapparent to severe disease involving one or more organ systems, and can be fatal. This virus primarily affects the respiratory and digestive tracts, with the reproductive tract and fetus being quite susceptible at critical gestational stages. Because BVDV can suppress the immune system, it often renders an animal more susceptible to other viral and bacterial diseases.
Fetuses infected with BVDV in susceptible and pregnant heifers and cows can result in abortions, stillbirths, congenital (present at birth) abnormalities, or birth of persistently infected (PI) calves. PI calves are born tolerant to the infecting strain of the virus and shed the virus throughout the life of the animal. Persistently infected animals are considered the principal source of virus for exposure of susceptible cattle and are considered reservoirs of BVDV. In the south central United States, a prevalence study indicated that 16.7 percent of herds tested had at least one PI animal, with a within-herd prevalence of 0.3 percent to 5.2 percent.
PI animals are created when susceptible females are exposed to noncytopathic (isolated virus does not kill the cells upon which the virus is cultured outside of the animal) strains of BVDV at approximately 42 to 125 days of gestation. Fetuses exposed to BVDV after approximately 125 days of gestation typically react by generating an immune response that eliminates the virus.
BVDVs are classified as cytopathic (isolated virus does kill the cells upon which the virus is cultured outside of the animal) or noncytopathic biotypes. BVDVs are then further classified as genotypes, such as BVDV 1 and BVDV 2, on the basis of genomic or genetic differences in the virus. Various genotypes, however, have varying responses immunologically and are then classified into subgenotypes. There are 12 different BVDV 1 subtypes and two different BVDV2 subtypes.
In the United States, BVDV1a, BVDV1b, and BVDV2a are commonly isolated. Although BVDV2b has been isolated, BVDV1a, BVDV1b, and BVDV2a were obtained from samples submitted from PI cattle with 77.9 percent of the three isolates being BVDV1b. Most of the commercial vaccines licensed by USDA in the United States contain BVDV1a and BVDV2a strains. Importantly, studies have indicated that vaccines containing BVDV1a and BVDV2a do protect against BVDV1b.
BVDV infection remains a complex issue. Understanding and controlling potential exposure to BVDV-PI cattle is important in managing biosecurity risks for acquiring the disease. Optimal vaccination against BVDV is also an essential tool in mitigating risk. To best evaluate and assess BVDV risk for your herd, consultation with your veterinarian regarding BVDV should be an integral part of your annual bovine health program assessment.