Definition : Blue tongue (BT) is an economically important, infectious non-contagious viral disease of domestic and wild ruminants.BTV is transmitted by Culicoides midges and causes clinical disease in sheep, white-tailed deer, and subclinical manifestation in cattle, goats and camelids. The name BLUE TONGUE owes its origin from Dutch word- “blaau tong” means mouth sickness.
Characterized by 1) Fever, 2) swelling (because of inflammation of lips, tongue and head), 3) lameness ( because of inflammation of sensitive laminae of the foot i.e coronary band of hoof).
EPIDEMIOLOGY :
- BT has been recognized in 18th century in tropical and subtropical areas (Asia, Africa, Middle East, America, Europe, Australia etc).
- In 1934 the disease was considered as pseudo foot and mouth disease.
- BTV16 was first isolated in Pakistan in 1959.
- Mortality may reach up to 70% while 30% almost die.
ETIOLOGY :
- Blue tongue is caused by BT virus belonging to the genus Orbivirus, family Reoviridae and subfamily Sedoreoviridae.
- The virus is non-enveloped, icosahedral symmetry, about 80—90 nm in diameter, and having 10 segmented genome of linear dsRNA.
- The 10 viral genome segments encode 7 structural proteins (VP1—VP7) and 5 non-structural (NSI, NS2,
NS3/NS3A,NS4 and NS5) proteins.
- The mature BTV virion contains 3 concentric capsid layers.
- Globally, 28 distinct BTV serotypes have been reported.
TRANSMISSION : Mainly BTV is transmitted by several species of haematophagus bitting midges belonging to the genus Culicoides, order Diptera and family Ceratopogonidae. Culicoides)
PATHOGENESIS OF BLUE TONGUE VIRUS :
After cutaneous instillation by the bite of infected midges, BTV is transported from skin to the local lymph nodes (LNs) by dendritic cells (DCs) and initial virus replication occurs in LNs.
-Primary viremia which disseminates BTV to secondary organs such as LNs, spleen and lungs. BTV replicates mainly in vascular endothelial cells (ECs), mononuclear phagocytic cells like macrophages, conventional dendritic cells (cDCs), and lymphocytes of skin, LNs, lungs, spleen, and other tissues.
-severe hemorrhages (hemorrhagic fever), extensive edema and effusions in various parts of body including lungs, vascular occlusion and tissue infarction, resulting
Consumptives coagulopathy/disseminated intravascular coagulation (DIC) leading to damage of cells and tissues of infected animals.
BTV infection disrupts the follicular dendritic cells in sheep during early stages of infection thereby inhibiting B-cell division in germinal centers of lymph nodes resulting in delayed production of virus neutralizing antibodies and acute immunosuppression leading to predisposition to secondary microbial infections especially Pasteurella pneumonia.
