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.

What happens if we eat mutton from an infected sheep?

The Bluetongue virus does not affect humans. Eating mutton or drinking the milk of a sheep infected with the virus does not pose any risk of humans contracting the disease. The meat and milk of a sheep that is vaccinated against Bluetongue, is not affected. . Therefore, no labelling is done for products from Bluetongue-vaccinated animals.

Symptoms and Diagnosis

Major symptoms of Bluetongue are excessive salivation, high fever, swelling of the face and tongue as well as cyanosis (blue or purple discolouration) of the tongue. In minority of the animals, the tongues and lips will swell, giving it the tongue its typical blue appearance. Prominent nasal symptoms are nasal discharge and heavy breathing.

Foot lesions beginning with coronitis (inflammation of the coronary band) and ending up in lameness will develop in some animals. In sheep, This can lead to knee-walking. While all symptoms usually develop within a month, the incubation period is 5-20 days. Despite the normally low mortality rate of Bluetongue disease, susceptible breeds of sheep in Africa face much higher mortality rates. In Africa, local breeds of sheep may not die from the disease but the death rate in imported breeds may go as high as 90%.

Life Cycle of Blue Tongue Virus

Life cycle of the Bluetongue Virus

Bluetongue viruses are generally found within the latitudes 40°N to 35°S  (from 

Southern Spain to Southern Africa), although during the current outbreak the virus spread as far as 53°N. 

Bluetongue is caused 
by a virus within the Orbivirus genus of the family Reoviridae.  24 distinct serotypes of the virus have been identified. (shown by the Figure on the left)

The bluetongue virus is made up of three layers. In the centre is the nucleic acid (the genetic blueprint) which carries all the information needed to  make more copies of the virus. This is surrounded by two layers, each made up of two proteins. There is a further outer coat that is lost when the core moves into an animal cell.

The BTV RNA is packaged inside two protein layers (pictured on the left). A third layer is lost when the virus enters the host cell.










Pathogenesis and Duration of *Viremia

fig. A The transmission cycle of bluetongue virus (BTV)
fig. B  The life cycle of the Culicoides vector


The extrinsic incubation period of BTV involves the entry of virus into the Culicoides vector, dissemination through the *haemocoel and subsequent infection of the salivary glands. Temperature affects most stages of the Culicoides life cycle, including the survival of adults and larvae through the winter months (enhanced by high winter temperatures), recruitment to the adult population and activity rates of adult Culicoides. Most stages of the Culicoides life cycle are also affected by the availability of moisture. Breeding habitats are semi-aquatic — larvae and pupae require moist habitats and adults are prone to *desiccation.


The cattle are implicated as the asymptomatic reservoir hosts of the virus, they do not get the disease but is able to produce the virus. The sheep are "target" species, they suffer from the disease. The virus co-circulates with the sheep's antibodies.

Bluetongue Blight-er

Biting Midge (Culicoides obsoletus)

Biting midges act as vectors of important livestock diseases. The biting midge Culicoides obsoletus is believed to transmit the bluetongue virus. This virus, now widespread throughout Europe, causes a potentially fatal disease of ruminants (sheep, goats, cattle and deer) but cannot be transmitted directly between animals, only via the midges. The midges can, in suitable weather conditions, be carried more than 200 km in air currents and this is probably how the disease was recently introduced into the UK. Bluetongue poses a serious threat to livestock production in Europe and it’s all because of these midges!




* - definitions taken from http://dictionary.reference.com/


viremia - presence of virus in the blood


ruminant - any even-toed, hoofed mammal of the suborder Ruminantia,being comprised of cloven-hoofed, cud-chewing quadrupeds,and including, besides domestic cattle, bison, buffalo, deer,antelopes, giraffes, camels, and chevrotains


haemocoel - the body cavity of many invertebrates, including arthropods and molluscs, developed from part of the blood system


desiccation - to preserve by removing moisture ; dehydrate