Bovine viral diarrhoea
Bovine viral diarrhoea (BVD) and mucosal disease (MD) are clinically dissimilar disease syndrome yet have a common viral etiology. The acute disease is called as BVD. The term mucosal disease is reserved for chronic disease associated with persistent infection. The pathologic manifestations of infection in individual cattle vary with age and pregnancy status. BVD virus belongs to the genus Pestivirus in the family Flaviviridae. The disease affects dairy and beef-cattle cattle causing fever, explosive diarrhoea, buccal erosions and in adult cattle causing abortion, ocular and cerebral defects.
Clinical signs: Three situations are considered, which include postnatal infection in non-pregnant cattle, infection in pregnant cows and postnatal infection in calves, and mucosal disease in adult cattle.
Postnatal infection in nonpregnant animals, the disease occurs when maternal antibody levels decline by 3-8 months of age. There is fever, leucopenia, diarrhea, nasal and ocular discharge and immunosuppression.
Infection in pregnant animals results in transplacental spread of virus to the fetus. Infection in early pregnancy causes embryonic death and resorption. Infection before development of fetal immunocompetence (80-125 days) results in fetal death or growth retardation, congenital defects. Surviviing calves remain infected for life and never develop effective immune response to the virus. They shed virus in all body secretions and excretions. These animals may develop mucosal disease. Fetuses infected after
125 days of gestation usually survive and develop neutralizing antibody and eliminate the virus.
Persistent infection and mucosal disease develop in some calves. Mucosal disease occurs when two biotypes of BVDV (cytopathic and noncytopathic) are present. There is sudden onset, fever, profuse watery diarrhea, nasal discharge, erosive and ulcerative stomatitis and death.
Diagnosis: Virus isolation in cell culture from feces, nasal exudates, blood and tissues and aborted fetuses. Viral antigen detection can be done in tissues or cell cultures by immunofluorescence and viral RNA can be detected in tissues by reverse transcription (RT-PCR). Serology with paired serum samples using virus neutralization, agar-gel immunodiffusion, complement fixation and fluorescent antibody and enzyme- linked immunosorbent assay. However, immunological tolerant animals are not detected serologically.
Prevention and control: In most herds, immunization is the only control strategy used. Although vaccines were designed for its control, they have several drawbacks and are not very safe and effective. Vaccines are administrated at 6 months of age. Attenuated virus vaccines produced in cell culture are widely used but there is evidence that vaccination of presently infected immunologically tolerant animals can result in severe mucosal disease in bovine calves.