Bluetongue – the facts

With news that a fourth case of Bluetongue has been confirmed in Britain, many farmers are wondering what exactly is this infection which is posing such a threat to agriculture and how it can be controlled. Bluetongue – the facts Bluetongue is a viral infection of most domestic and wild ruminants such as cattle, sheep, goats, deer etc. There are 24 serotypes of BTV and strain variations within these serotypes. Depending on the strain of virus it may affect some species more than others. Six serotypes of BTV have been found in sheep in the Mediterranean countries of the EU for a number of years. Beginning in August 2006, an unprecedented disease epidemic spread through sheep and cattle in the Netherlands, Belgium, Luxembourg, Germany and France. Now, the disease has spread to Britain, with a fourth case of the virus identified yesterday. This epidemic was caused by Serotype 8, previously known to have been in sub-Saharan Africa, but a new serotype in Europe. BTV has never been recorded in Ireland but recent events in England mean that there is the potential for this to happen. How do animals become infected? The virus is only transmitted by biting midges (Culicoides species). It is not shed into the environment, so cleaning and disinfection procedures will not control its spread. The midges that spread infection are active between April and October in Ireland and are commonly found around farms. Of the 16 most common midge species in Ireland, approximately eight are potential vectors for BTV.  They feed on livestock mainly in the hours around dusk and dawn but, if conditions suit, they can be active almost 24 hours a day under Irish weather conditions. Infection is transmitted when an uninfected midge takes a blood meal from an infected sheep or cow. If the temperature is high enough (high 20’s centigrade) for several days the virus will develop inside the midge and be transmitted when the midge feeds on a new host. With the arrival of cooler autumn or winter weather, midge activity and virus transmission will cease. Culicoides midges will emerge again in Spring from larvae that overwintered in the soil. The larvae do not carry the BTV virus. How the virus survives the winter is not fully understood but it is probably as a sub-clinical infection in cattle or sheep. The newly emerged midges then become infected after feeding on these carrier hosts to start a new cycle of transmission. How is BTV controlled?  The EU and national legislation to deal with the control of Bluetongue broadly follows the arrangements in place for other diseases such as Foot and Mouth Disease and Avian Influenza etc., but are more extensive given the way in which the disease can be spread. How does it effect livestock? Most animals show no sign of illness (sub-clinical infection). For example, in Belgium most flocks had one or two sick sheep but only one third of flocks had mortalities. Cattle rarely show signs of illness with BTV infection. However, during the serotyope 8 epidemic in northern Europe 80 percent of cattle herds had one or two sick animals but most of these made a full recovery.  In those animals that become sick, the virus causes a fever and will damage blood vessels causing them to leak fluid into the tissues of the face and extremities. Therefore, affected animals may have a swollen muzzle, lips and tongue (the swollen blue tongue that gives the disease its name is rarely seen). Some animals will have eroded gums and drool profusely. Animals will be off their feed and may lose condition rapidly, including muscle degeneration, and those that survive face a prolonged recovery. Eyes may be reddened with tear staining of the face. Others will have a swollen udder and teats, or swollen fetlocks with inflammation around the coronary band of the hoof and laminitis causing lameness.  The following list summarises those clinical signs that one might expect to see in an animal with acute infection:  Fever with depression Increased breathing rate,  Off feed and loss of condition as the disease progresses,  Reddening of the lining of the mouth leading to swelling of the tongue, lips, and face,  Sometimes sheep have a blue swollen tongue sticking out of the mouth,  Reddening of the eyes and surrounding face with tear staining, runny nose, and drooling Sore muzzle with open sores on lips, tongue, or around the nostrils Bottle jaw and sometimes swollen ears Reddening and swelling of the udder and teats Swelling of the hocks and coronary band of the hoof causing lameness Hunched-up gait and reluctance to move  Abortion or birth of malformed lambs or calves Secondary pneumonia, How long does it take animals to become sick if bitten by a BTV-infected midge? Between four and 20 days, depending on the type of animal, its age, the strain of the virus and the dose of the virus received.  Are people at risk from BTV? BTV does not affect or infect humans and, consequently, the disease has no public health significance. Furthermore, the virus cannot be acquired from foods such as meat or milk.  How could BTV get to Ireland?  By three routes:  Firstly by importing an animal that is carrying the virus in its blood. If this animal was bitten by an insect vector (midges of the Culicoides species) and the environmental conditions were favourable, the midge could transmit infection to other animals.  Secondly – potentially, prevailing winds could spread infected midges from affected areas in Europe, across the English Channel or Irish Sea. Again the environmental conditions would have to favour survival of the midges and allow transmission.  Thirdly though least likely – through the importation of infected semen or other biological products How would it affect trade? An outbreak in Ireland could have significant impact, in particular, on our livestock export and semen/embryo trade due to restrictions that would be put in place. Such restrictions may need to be in place for a long period of time. It is important to note that BTV cannot be transmitted in meat or milk and thus such trade should not be affected.