A virus moves north

The sudden emergence of an insect-borne disease of livestock in northern Europe has plunged the agricultural sectors of three countries into crisis. At least 45 farms in the Netherlands, Germany, and Belgium have so far been affected by bluetongue disease, which infects ruminants such as cows, sheep, goats, and deer. Scientists are trying to find out how the disease, which had made inroads into southern Europe over the past 8 years, made the giant leap north, and what its potential for further spread is. The disease was first discovered in sheep on a farm in the Netherlands on 14 August. On 21 August, the European Union announced a series of measures to contain it, including an export ban on ruminants–as well as their semen, ova, and embryos–in a 150-kilometer radius around stricken areas.

The Bluetongue Virus, carried by tiny insects called biting midges, causes severe and sometimes fatal disease–including a blue tongue, caused by bleeding–in sheep and goats; cows are reservoirs but usually don’t get sick. The virus occurs in many parts of the world, including sub-Saharan Africa, Turkey, and the Middle-East. But until recently, it was rare in Europe. From 1998 on, however, different subtypes of the virus started spreading into Greece, Italy, Spain, Portugal, and the Balkan countries. Still, its sudden jump north–by some 10 degrees of latitude–is “very surprising,” says Bethan Purse of the University of Oxford, who studies bluetongue epidemiology.

In southern Europe, bluetongue’s main vector is a species called Culicoides imicola, which doesn’t occur in the newly affected countries. A team led by medical and veterinary entomologist Willem Takken of Wageningen University in the Netherlands is trapping insects around Dutch farms to determine the most likely vector there. So far, the team has found predominantly C. obsoletus–which lab studies have shown to be a potential vector for bluetongue–as well as nine other Culicoides species, Takken says. Studies to determine whether they carry the virus are still underway.

Meanwhile, virologists are trying to determine which of the 24 subtypes of the virus is involved. The fact that several cows in the Netherlands have fallen ill suggests that it may be an unusual one, Takken says. Researchers aren’t sure why bluetongue is moving north. In a paper published last year, Purse and colleagues suggested climate change might have triggered its recent spread into southern Europe. Other explanations, such as a more virulent strain or changes in land use, agriculture, or animal health systems, seemed implausible, they said, and the disease had struck primarily in those areas that had heated up the most.

September 12, 2006

Original web page at ScienceNow