Bluetongue

Etiology

Pathogenesis

• There are quite similar pattern of pathogenesis in the host body but the magnitude of illness may vary from species to species.
• The virus gain entry to regional lymph nodes, following cutaneous instillation (through inoculation or biting of the virus affected vectors) to the host body. 
• The virus disperses to the mononuclear phagocytic cells, endothelial cells, lymphocytes and different parts of the body. 
• The virus is closely associated with endothelial cell and with erythrocytes during the course of infection. 
• Severe to moderate vascular endothelial damage resulting in changes to capillary permeability and fragility, with subsequent disseminated intravascular coagulation and necrosis of tissues supplied by damaged capillaries.
• Infection in ruminants is prolonged and circulates for several weeks with a concurrent high titre of neutralizing antibody.
• In cattle, clinical disease is thought to be a hypersensitivity reaction.
• Bluetongue virus Bluetongue virus (BTV) has been shown to be an abortifacient and is teratogenic.
• After transmission of BTV under natural and experimental conditions, the consequences of vertical transmission in pregnant cattle are variable.
  • Factors that influence reproductive consequences are the stage of gestation, characteristics of the virus, source and concentration of virus inoculum, placentation, season and the method and route of infection.
  • Reproductive consequences vary greatly in degree but in general include infertility, abortion, mummification of the fetus, stillbirths and congenital anomalies. 
• Immunological unresponsiveness, sporadic viremia and development of "latent infection" were consequences of vertical transmission in offspring of infected cattle. 
• Perpetuation of BTV through 3 generations in cattle has been documented.
• The presence of BTV in semen of infected bulls has also been demonstrated.

Etiology

• Bluetongue results from infection by Bluetongue virus Bluetongue virus, a member of the genus Orbivirus and family Reoviridae. 
• To date (2022), 29 serotypes have been identified worldwide.
• Isolates differ in virulence, and some strains seem to cause few clinical signs.
• Like some other viruses such as Influenza virus Influenza virus, bluetongue viruses can re-sort and recombine to produce new variants. 
• Bluetongue viruses are closely related to the viruses in the epizootic hemorrhagic disease (EHD) serogroup, a factor that can influence the development and/or selection of some diagnostic tests.

Timecourse

• The incubation period is estimated to be approximately a week, with a range of 2-10 days.

Epidemiology

• There are at least 29 serotypes worldwide, although not all serotypes exist in any one geographic area; e.g. 13 serotypes (1, 2, 3, 5, 6, 10, 11, 13, 14, 17, 19, 22, and 24) have been reported in the USA and 8 serotypes (1, 2, 4, 6, 8, 9, 11, and 16) in Europe. 
• Distribution of BTV throughout the world parallels the spatial and temporal distribution of vector species of Culicoides biting midges Flies, which are the only significant natural transmitters of the virus, as well as the temperatures at which BTV will replicate in and be transmitted by these vectors.
  • Of more than 1,400 Culicoides species worldwide, fewer than 30 have been identified as actual or potential vectors of BTV to date.
  • Continued cycling of the virus among competent Culicoides vectors and susceptible ruminants is critical to viral ecology.
  • In the USA, the principal vectors are C. sonorensis and C. insignis, which limit the distribution of BTV to southern and western regions.
  • In northern and eastern Australia the principal vector is C brevitarsis.
  • In Africa, Southern Europe, and the Middle East it is C imicola.
  • In northern Europe, the major vectors are species within the C obsoletus-dewulfi complex.
  • In each geographic region, secondary vector species may attain local importance.