Veterinary Preventive Medicine-II

INTRODUCTION

• Bovine viral diarrhoea (BVD) and mucosal disease (MD) are clinically dissimilar disease syndromes, and were originally described as separate diseases, but they are now known to have a common viral etiology. The virus can cause both acute disease, bovine viral diarrhoea and a protracted form of illness, mucosal disease, arising from persistent infection.
• BVD may occur at any age. MD is a persistent infection acquired in utero and characterized by high mortality and low morbidity.
• The BVD virus can be either non cytopathogenic or cytopathogenic. Isolation and demonstration from both BVD and MD were serologically similar and gave the same experimental disease.
• The clinical evidence that abortions were constant finding of field outbreaks of BVD and also the demonstration of transplacental transfer of the virus to the fetus.
• Early fetal infection with BVD virus may lead to persistent viraemia and a failure to develop antibodies.
• Persistently viraemic animals could subsequently develop MD. Persistently viraemic animals are infected with only non-cytopathogenic virus, whereas those developing MD are infected with both non-cytopathogenic and cytopathogenic forms.
• Mucosal disease is almost invariably fatal disease with low morbidity. It occurs in cattle, which have a persistent BVD infection acquired as fetuses characterized by a specific immune tolerance to the infecting virus strains and consequent lack of antibody to it.
• In MD, if the fetus infected at 110-120 days of gestation, i.e., before the age of immunocompetence, the virus become accepted as self and persist through out the life of the animal. This explains the lack of antibody to persisting virus and the continued state of immuno tolerance.
• Sometimes after birth, when the animal is about 6-18 months of age, super infection of these persistently viraemic animals with the cytopathogenic bio-type may occur causing MD.
• Bovine viral diarrhoea virus (BVDV) is a SS-RNA virus, genus Pestivirus in the family Flaviviridae and is closely related to classical swine fever and Ovine Border disease viruses. This virus affects domestic and wild ruminants and pigs. There are two genotypes of the BVDV recognized.
• Both viral genotypes have been co-circulating in cattle throughout the world. This disease reported from most regions of the world. Natural infections and disease occur not only in cattle but also in sheep, pigs, goats and a wide range of captive, free-living ruminants.
• BVDV also classified into two biotypes-Non cytopathogenic and cytopathogenic biotypes
• The most important sources of BVDV in nature are immuno-tolerant, persistently infected animals.
• High virus levels are usually present in their nasal secretions, saliva, tears, semen, milk, urine and faeces.
• Animals in contact with a persistently infected animal rapidly become infected. Vertical and horizontal transmission may occur. Disease transmits by direct or indirect contact.
• The virus may be isolated in a number of bovine monolayer cell cultures (e.g. kidney, lung, testis or turbinate). Growth of both biotypes is usually satisfactory.