Sickly swine

 作者:贝鄢     |      日期:2019-03-07 03:18:16
By Debora MacKenzie in Brussels SOARING numbers of wild boar roaming Europe’s forests are prompting the European Commission to consider developing and releasing a genetically engineered virus to control a plague that is deadly to pigs. Classical Swine Fever (CSF) has been largely eradicated from Europe’s pig farms following years of stringent controls, but outbreaks do still occur. An epidemic in the Netherlands in 1998 led to the slaughter of more than a million animals, at a cost of £1.5 billion. But pigs are no longer vaccinated against the disease because even vaccinated animals can still carry the CSF virus and pass it on to other pigs. This makes it hard to completely stamp out the disease. Wild boar are just as susceptible as domestic pigs to CSF, but until the early 1990s they were too thinly spread to sustain infection. Sick animals would die before they could infect others. However, in a recent report from the European Commission’s Scientific Committee on Animal Health and Animal Welfare, Volker Mönnig from Hannover University says that CSF has become endemic in wild boar. This is partly because numbers of boar have skyrocketed. There are now more than a million animals in France and Germany, and hundreds of thousands in Italy, Spain, Portugal and Eastern Europe. Mild winters may have contributed to the population explosion. Ineffective hunting is another factor that is being blamed. In Germany, hunters are allowed to take 70 per cent of all young boar, but they seldom manage to hit that many, according to Volker Kaden of the German Federal Research Institute for Animal Viruses near Greifswald. The animals tend to feed in fields of corn or potatoes, which provide good cover. But enough young pigs die to provoke sows to have a second litter each year. Hunting can also disrupt herds of boar, dispersing the animals and any infection that they carry. CSF has become milder in wild boar than in pigs, which has further helped it to spread. Since 1993, 80 per cent of CSF outbreaks among pigs have been in areas where CSF is endemic in boar, and 60 per cent—including one in August in the Brandenburg region of Germany, near Berlin—were definitely due to contact with sick boar or their droppings. “There are no strategies known for the efficient control of endemic CSF in wild boar,” Mönnig warns. Efforts by Kaden’s team to vaccinate boar using edible baits containing weakened live CSF virus have only been partially successful. The tasty baits are hogged by older animals that have already acquired immunity to the disease. But improving baits and their delivery is difficult when scientists have no way of assessing how well a vaccination programme is progressing. Current oral vaccines trigger pigs to produce antibodies that are indistinguishable from those produced when the animals are infected with the disease itself. Mönnig sees most potential in vaccines based on genetically engineered live viruses, perhaps consisting of non-pig viruses carrying CSF proteins. An alternative approach would be to use altered versions of CSF that produce distinctive antibodies in vaccinated animals, marking them out from animals that become infected. Such “marker” vaccines are available for farm pigs. Whether a biotech-wary Europe will permit the release of such viruses in the forests is another matter,