Human spread
Humans were the "most likely route of infection" for the boars in Belgium last week, believes Linda Dixon, researcher in genomics of African swine fever at the Pirbright Institute in the UK.
Since the infected pigs were found in a forest area, more than 1,000 kilometers (621 miles) from any infected territories, she suspects that people consumed infected meat products and then left them in the forest where wild boars ate them.
There are no risks associated with eating infected meat, Dixon said.
In the rest of Europe the main factor for spread has been wild boar movements, illegal pig and pork meat trading and the movement of people and vehicles between countries, according to Andriukaitis.
Climate change and "absolutely different weather conditions" have helped African swine fever spread, he said, explaining that the virus is a very heat- and cold-resistant one.
Thousands infected in China
China has also been hit hard.
As of September 25, eight provinces were also reporting cases of African swine fever thousands of kilometers apart, with 2, 283 pigs infected. The country has seen 29 outbreaks, according to the World Organisation for Animal Health's database.
China is one of the world's largest producers and consumers of pork products, according to the UN Food and Agriculture Organization.
Stone believes that the movement of live pigs or pig meat has been "instrumental in both initiation but also propagation" of the virus in China.
Dirk Pfeiffer, chair professor at City University's College of Veterinary Medicine and Life Sciences in Hong Kong, believes that the wide geographical spread is due to the "extensive live pig trade network in China."
"Food waste is being widely fed to domestic pigs, which if contaminated with the virus, will greatly facilitate spread."
Where there is "significant illegal trade in live pigs, pork or food waste for feeding pigs," it becomes "virtually impossible" to find the source of the virus, Pfeiffer added.
Medical workers participate in a swine fever prevention exercise in China in 2014.
But investigations into how the virus was introduced are vital to prevent further spread.
China is undertaking standard procedures such as a movement ban of pigs and pork products from affected to unaffected provinces and culling on at-risk farms. "Forward and backward" tracing is also ongoing to identify the source of the virus and which other areas could have undetected infections spreading. The feeding of pig swill (food waste) has also been banned, Pfeiffer said.
However, given the size of the country and the number of outbreaks, Pfeffer believes there is a need for more trained veterinary staff who are familiar with the virus.
Given the number of countries now affected, preparedness programs, such as awareness campaigns warning people not to bring meat products from infected areas, are ongoing in most countries, Dixon said.
Schengen struggles
"Europe is very worried about further spread," Dixon said, with the large number of wild boars that roam freely being a primary concern, aided by the Schengen zone that allows Europeans to travel between 26 countries without any border checks.
Within the free movement zone, seven countries have reported outbreaks: Belgium, Lithuania, Latvia, Estonia, Poland, Hungary and the Czech Republic.
Ways to keep wild boars from spreading the virus must include an understanding of the animal's movement, Stone said.
In summer 2017, the Czech Republic managed an outbreak of the deadly virus in its wild boar population by early prevention methods like targeted hunting, increased biosecurity, awareness campaigns and training of local people.
Other cases of African swine fever, like a 1980s outbreak in Spain, were also eliminated thanks to a reduction in free-range pig farming in the south of the country. But today's conditions make containing the virus harder.
"Now, we have different circumstances because of the single market, free movement of goods, the Schengen area and the different legal environments," Andriukaitis said.
Rest of Europe builds defense
The European Union has laid out contingency plans for dealing with African swine fever, including hunting bans in affected areas, movement controls, surveillance, ensuring high levels of biosecurity in all pig holdings, fighting illegal trade in meat or pig products and raising awareness in farmers and the local population.
In order to prevent spread of the virus to unaffected countries, Germany issued a decree allowing for hunting of wild boar populations during the entire year and has rehearsed response tactics in the case of an African swine fever outbreak. Denmark has approved plans for a fence along its border with Germany to stop wild boar movements between the countries as a precaution.
Andriukaitis has "doubts" about using a fence between countries as a prevention strategy because it wouldn't stop movement by humans or vehicles, which can still transmit the virus.w
The next hope is a vaccine.
African swine fever is a highly contagious DNA virus, the UK's Department for Agriculture, Environment and Rural Affairs said.
The European Union issued a research grant in 2018 for the development of an African swine fever vaccine in its Horizon 2020 program, the biggest EU research and innovation program.