Introduction

Bluetongue disease is an insect-borne multiserotype viral disease affecting sheeps, cattles, goats and other ruminants. The control of Bluetongue disease relies essentially on vaccination. Bluetongue virus is the type-species of the genus Orbivirus in the family Reoviridae. Bluetongue is an OIE listed viral disease of ruminants and camelids that has 24 known serotypes and occurs predominantly in warm temperate parts of the world. Transmission of the Bluetongue virus is usually vector mediated (Culicoides midges) but may also be transmitted by the transplacental route (new scientific data on serotype 8), contaminated germplasm, or by iatrogenic means. Affected sheep may have erosions and ulcerations on the mucous membranes, dyspnea, or lameness from muscle necrosis and inflammation of the coronary band.

The Bluetongue virus may cause illness leading to loss of production or even death in sheep, deer, and cattle.The consequences of direct losses of livestock (illness in infected sheep and cattle, infertility, abortion and neonatal deaths, losses due to decreased production, lameness), welfare implications, bans on exports of live ruminants and germplasm to some destinations, and movement restrictions are likely to have severe economic impacts. This is because affected countries have to import livestocks from neighbouring countries for food consumption, which eventually results in economic loss.

                            The bluish discoloration observed on the hooves of an infected sheep.


In sheep, the severity of disease varies with the breed of sheep, virus strain and the stresses posed by the environmental conditions. The morbidity rate can be as high as 100% in this species. The mortality rate is usually 0-30%, but can be up to 70% in highly susceptible sheep. Similar morbidity and mortality rates are seen in bighorn sheep.

 The world map showing the Bluetongue virus serotypes which affects specific countries.

Bluetongue virus occurs predominantly in warm temperate parts of the world.  It has been observed in Australia, the USA, Africa, the Middle East, Asia and Europe. Its occurrence is seasonal in the affected Mediterranean countries, subsiding when temperatures drop and hard frosts kill the adult midge vectors, that may promote viral survival and vector longevity during milder winters. Since Singapore does not rear any sheeps, it is one of the countries which appeared to be unaffected by the Bluetongue virus.

Prevention

Prophylactic immunization of sheep remains the most effective and practical control measure against Bluetongue in affected regions. Three polyvalent vaccines, each comprising 5 different bluetongue virus serotypes attenuated by serial passage in embryonated hens’ eggs followed by growth and plaque selection in cell culture. The control of bluetongue differs in areas where the disease is not epidemic. During an outbreak, when one or a limited number of serotypes may be involved, vaccination strategy depends on the serotypes that are causing infection. Use of vaccine strains other than the ones causing infection affords little or no protection. Although a number of non-infectious vaccines are in development, they are not yet commercially available. Control of vectors by using insecticides or protection from vectors by moving animals into barns during the evening hours lowers the number of Culicoides bites and subsequently the risk of exposure to bluetongue virus infection.                                         Control Strategy There are various methods that act to control the spread of the Bluetongue virus. Introduce a combination of quarantine and movement controls to prevent spread of virus throughout the sheeps. Possible treatments and husbandry procedures can also be implement to control vectors, reduce transmission and protect susceptible animals. Surveillance was implemented in determining the extend of virus and vector distribution. Lastly, zoning of the sheep farm would prevent the widespread of the disease by separating the infected and the disease-free areas.