Bluetongue

• The bluetongue virus belongs to the family Reoviridae and is classified under the genus Orbivirus.
• The virion is a nonenveloped, double-layered particle with an outer capsid surrounding a core containing a genome comprising segments of double-stranded RNA.
• Ten dsRNA segments include the virus, each coding for a distinct viral protein.
• The genetic content of the virus is encased in several protein layers that make up the capsid, or outer shell.
• The virus spreads locally and replicates in regional lymph nodes, generating viremia, and spreads systemically via viral replication in hematopoietic and endothelial cells.
• This results in tissue edema, vascular blockage, bleeding, endothelial damage, and epithelial sloughing from loss of microvasculature.
• The primary method of transmission is via its primary vector, Culicoides spp.
• Most often found in ruminants, particularly sheep and cattle, the bluetongue virus is spread by the saliva of many species of biting midges.
• Direct, indirect, or airborne transmission of the virus is improbable due to the low quantities of the virus in secretions and excretions.
• Through artificial insemination or mounting, cows may contract an infection from the semen of viremic bulls.
• Embryo transfer is safe as long as the donors are not viremic and the embryos are adequately sterilized.
• There have been reports of field strains of Bluetongue virus being transmitted transplacentally from the dam to the fetus in cattle, resulting in the birth of viremic calves; however, it is unknown how important this pathway is epidemiologically.
• There are at least 29 virus serotypes worldwide.
• The main vectors of bluetongue virus in the US are Culicoides sonorensis and Culicoides insignis, which restrict the virus's range to the south and west.
• Culicoides imicola is the primary vector in Africa, south of Europe, and the Middle East, while Culicoides brevitarsis is the primary vector in northern and eastern Australia.
• The important vectors in northern Europe are the Culicoides obsoletus-dewulfi group species.
• Secondary vector species have the potential to become locally significant in every geographic area.
• Bluetongue virus replicates within mononuclear phagocytic and endothelial cells, lymphocytes, and maybe other cell types in lymphoid tissues, the lungs, skin, and other organs.
• Ruminants that are infected may show signs of protracted viremia.
• When infected with bluetongue virus, viremia is largely cell-associated and lasts longer but does not persist.
• According to MacLachlan (2004), the bluetongue virus is most quantitatively associated with platelets and erythrocytes specifically.
• Because platelets have a short lifespan, the association with erythrocytes occurs later in ruminant bluetongue virus infection.
• The bluetongue virus causes damage to the tiny blood vessels in the target tissues.
• Disinfectants cannot prevent the virus from being transmitted between animals; however, sodium hypochlorite or 3% sodium hydroxide are effective.
• Rarely, cattle can have mild hyperemia, vesicles, or ulcers in the mouth, hyperemia around the coronary band, hyper-esthesia, or a vesicular and ulcerative dermatitis.
• Bluetongue is not a significant threat to human health.
• Attenuated and inactivated vaccines are available for bluetongue.
• Pregnant ewes infected during the first trimester may reabsorb the fetus, abort, or give birth to “dummy” lambs.
• Postmortem examination is necessary for the diagnosis of bluetongue.
• Clinical signs of bluetongue should be suspected when clinical signs are seen during seasons when insects are active.
• The head, ears, lips, and tongue may be very swollen.
• Insect control is important in limiting the spread of the disease; synthetic pyrethroids or organophosphates are effective against Culicoides.
• Laboratory tests for bluetongue include cultures, ELISA, immunofluorescence, immunoperoxidase, virus neutralization tests, and PCR.
• Initially, animals have a clear nasal discharge; later, the discharge becomes mucopurulent and dries to a crust around the nostrils.
• Infections in cattle are usually subclinical; often, the only signs of disease are changes in the leukocyte count and a fluctuation in rectal temperature.
• The muzzle, lips, ears, and coronary bands on the hooves are often hyperemic and the hooves painful; lameness is common and animals may slough their hooves if they are driven.
• A recent history of wasting and foot rot in the herd support the diagnosis of bluetongue.
• Animals are usually infectious to the insect vector for several weeks.
• There is no specific treatment for bluetongue.
• The mortality rate is usually 0-30%, but can be up to 70% in highly susceptible sheep.