Bovine viral diarrhea virus (BVDV) is one of the most important infectious agents of cattle. The annual economic loss caused
by BVDV is difficult to quantify but certainly is significant. The insidious nature of BVDV combined with the biology of the
virus and complex disease pathogenesis has made control and prevention of this virus challenging. BVDV has been associated
with many clinical diseases. There is little doubt that BVDV plays a role in bovine respiratory disease (BRD). This review
will present past and current information on the role that BVDV plays in BRD.
Role in Bovine Respiratory Disease
Bovine respiratory disease is the most frequent cause of morbidity and mortality in North American feedlots and is the major
cause of economic loss. It is generally agreed that Mannhemia haemolytica is the major contributor to pneumonic lesions. Considerable research has focused on the mechanisms by which M. haemolytica colonizes the lungs. Many predisposing factors have been implicated in reducing the local clearance mechanisms of the lungs
including stress from weaning, transportation, mixing of cattle, handling, and processing. Viruses, including parainfluenza-3,
bovine respiratory syncytial virus, bovine herpes virus-1, coronavirus, and BVDV, have been implicated as predisposing causes
BVDV has been implicated in bovine respiratory disease since it was first described by Olafson, MacCallum and Fox in 1946.
Although not conclusive, both circumstantial and experimental evidence suggest that BVDV is involved in BRD.
Clinical: Circumstantial evidence that BVDV is involved in BRD comes from clinical pathological observation. As mentioned earlier,
clinical descriptions of cattle undergoing acute BVDV infection often involve respiratory signs. In Sweden, severe respiratory
disease outbreaks were described involving both BVDV and PI-3. In the United States, BVDV has been reported as the most commonly
isolated virus from pneumonic lungs and in outbreaks of BRD.
Experimental studies attempting to reproduce respiratory disease with BVDV alone have resulted in mild lesions. In studies
by Potgeiter et al., calves infected with BVDV alone had less severe clinical signs and pulmonary pathology when compared to calves infected with
both BVDV and P. haemolytica. Synergistic effects between BVDV and P. haemolytica, BHV-1 and BRSV have been documented. Differences in pneumopathogenicity have been demonstrated for isolates of BVDV. These
findings suggest an immunocompromising role for BVDV in bovine respiratory disease.
Epidemiological: Epidemiological studies have both implicated and shown no evidence that BVDV is associated with outbreaks of respiratory
disease. Interpretation of results is often difficult because of the multiple etiologies of the BRD complex and the variability
of exposure to various pathogens prior to being studied. It is obvious from a review of seroepidemiological studies that the
risk of developing BRD in association with BVDV seroconversion varies. In two studies by Martin et al. involving feedlot calves, seroconversion to BVDV was significantly associated with the development of BRD. Similarly, young
calves seronegative to BVDV were at higher risk for developing respiratory disease while seroconversion to BVDV was associated
with respiratory disease. Protection from respiratory disease has been shown in calves entering a feedlot seropositive to
BVDV. Highlighting the importance of colostral antibodies, a protective effect against respiratory disease has been shown
in calves being born to BVDV seropositive dams.
In contrast, other studies have provided no evidence between seroconversion to BVDV and risk of developing BRD. Allen et al. reported a 51% rate of seroconversion to BVDV in both BRD cases and controls. In young dairy calves entering a commercial
calf rearing unit, BVDV was not identified as a risk factor for developing clinical respiratory disease. It should be noted
that in all of the above studies, other agents in addition to BVDV were identified as being associated with respiratory disease.
This highlights the complex nature of BRD and the fact that it is a multifactorial disease.Immunosuppressive Role: Several field studies have reported that clinical disease caused by a particular organism appears to be more severe when concurrent
BVDV infection is present. The compromising effect of BVDV is thought to be due to immunosuppressive effects of the virus.
The most important role that BVDV may play in BRD is in suppressing local immune system function in the lungs, thus allowing
pathogenic bacteria to become established.
Several reports have suggested synergistic effects between BVDV and other pathogenic organisms. In calves infected simultaneously
with BVDV and BHV-1, the latter virus became systemically disseminated as compared to remaining localized in the respiratory
tract in calves not infected with BVDV. BVDV infection has also been associated with concurrent Salmonella, E. coli, bovine papular stomatitis and rotavirus and coronavirus infections.