When Lithuania began fortifying its border with Belarus in July 2013, the fear wasn't soldiers or tanks, but an invasion of a different kind: African swine fever (ASF), a notorious viral disease that kills both farm pigs and wild boar. Lithuanian border guards sprayed trucks with disinfectant, wildlife biologists experimented with animal repellents and fencing, and officials designated a 20-kilometer-wide surveillance zone.
It was to no avail. In January 2014, two dead boar found on the Lithuanian side of the border tested positive for ASF, marking the first arrival of the disease in the European Union in decades. It has continued moving westward since then, putting the European Union on high alert. "The concern is increasing and increasing," says Dolores Gavier-Widen, a wildlife pathologist at Sweden's National Veterinary Institute in Uppsala and chair of an EU research network studying the disease.
ASF has engulfed three Baltic countries and far-eastern Poland, and this summer, it suddenly appeared in wild boar in the Czech Republic. In November, it also surfaced around Warsaw, increasing the alarm in nearby pig-producing countries such as Germany and Denmark. ASF "is the disease with the highest concern right now, no doubt about that," says Jens Munk Ebbesen, chief veterinarian of the Danish Agriculture and Food Council in Copenhagen. Pork exports were worth $4.8 billion last year, 19% of Denmark's food and agricultural exports.
The threat has researchers scrambling to pin down how ASF spreads—a puzzle complicated by the role of wild boar—and redoubling efforts to develop a vaccine. The economic stakes are high; even a single ASF case in a wild boar can lead other countries to ban imports of pork. "Whenever we have the first case, the damage is already done," says Willie Loeffen of Wageningen Bioveterinary Research in the Netherlands.
First reported in domestic pigs in eastern Africa in 1921, ASF is harmless to humans but kills up to 90% of pigs, causing internal bleeding and fluid-filled lungs. Herds with infected animals must be culled to fight it. The virus spreads through sick animals' secretions and can survive for long periods on workers' clothes and shoes or hay, which helps it move from farm to farm. It can also leap even farther when people transport contaminated pork, which will infect pigs or boar if they eat the scraps.
ASF became increasingly common in sub-Saharan Africa over the 20th century and first struck Europe in 1957, arriving in Portugal from Angola. A subsequent outbreak in Spain took 35 years to end, and the disease remains endemic on the Italian island of Sardinia, where many pigs roam free. The current outbreak originated in Poti, Georgia. A virus matching strains from southeast Africa showed up there in 2007, apparently brought in with food waste discarded from a ship. It quickly spread throughout Georgia and neighboring countries. After entering Russia, it hopscotched vast distances in shipments of military food, and reached Ukraine and Belarus.
The consequences have been dramatic. In Estonia, 22,000 pigs were slaughtered in 2015; pork prices collapsed, and more than a third of pig farms went out of business. Nearly all backyard farms gave up their pigs as well. "It has changed life completely," says Arvo Viltrop, an epidemiologist with the Estonian University of Life Sciences in Tartu.
The virus's presence in wild boar complicates the fight. Feces, urine, or nasal secretions from sick boar contaminate soil or plant material, which dog walkers or mushroom pickers, for example, might carry out of the forest. Hunters who kill an infected animal pose a bigger risk, as blood is highly infectious. But exactly how the disease spreads among wild boar remains mysterious. Family members seem to infect one another, and young boar may run an increased risk because they will nose around infected carcasses and sometimes gnaw on bare bones; that's why it's recommended policy to remove boar carcasses.
Scientists don't have a good handle on wild boar populations either. They appear to be booming in Europe, in large part because hunting clubs feed them in winter, and their range is expanding northward. Existing population models are sophisticated, but lack essential biological data, such as reproductive rates of boar in northern Europe, says wildlife epidemiologist Vittorio Guberti of the Italian National Institute for Environmental Protection and Research in Rome."It's like having a Ferrari, but no gas." Guberti hopes that further studies and improved models could reveal how many carcasses must be removed and boar killed to stop the disease.
The surest way to get rid of ASF in wild boar would be with a vaccine packaged in bait. There is a precedent: In parts of Germany during the 1990s and 2000s, biologists delivered a vaccine against classical swine fever, a disease that also afflicts pigs and boar, in almond-flavored balls of fat and corn flour that were covered with soil. (Some were also dropped from planes.) After several campaigns, up to 80% of wild boar had become immune in areas with dense populations, which helped eliminate the virus. But safe and effective vaccines for ASF don't yet exist; attempts to create one from inactivated virus particles have failed. Researchers have also been trying live attenuated viruses, but even the most viable candidates are probably still a decade from market, says virologist Linda Dixon of The Pirbright Institute in the United Kingdom.
In the meantime, surveillance is crucial, because quick detection boosts the odds of stamping out viral incursions. In June, ASF was detected in two wild boar in the Czech Republic, about 400 kilometers from the nearest infected population at the time. Hunting groups and biologists then set up electric fences and boar repellents to contain the animals within a 50-square-kilometer area. In September, officials decided to let trained hunters kill boar in that zone, in a bid to eliminate the disease. But it's only a matter of time before infected animals escape, says Radim Plhal, a wildlife biologist at Mendel University in Brno, Czech Republic.
The jump to the Warsaw area, where two dead wild boar were reported on 17 November, was another "huge surprise," says Grzegorz Woźniakowski of the National Veterinary Research Institute in Puławy, Poland. By this week, at least 40 cases had been confirmed around the city, raising concern for the large pig producers in western Poland.
Veterinary authorities and trade groups in Denmark and Germany are now reminding farmers and hunters to keep biosecurity as tight as possible. Animal transport trucks from affected countries are disinfected before they enter Denmark. Germany and several other countries at risk are testing every wild boar that is found dead, and they offer bounties to encourage boar hunting, says veterinarian Klaus Depner of the Friedrich Loeffler Institute, Germany's national institute for animal health, in Greifswald. "It is hard to stop the introduction of the virus," he says. "We hope that we will have enough time to prepare properly."