Less virulent and never described: what do we know about the new variant of swine fever?

BarcelonaThirty-six days after the Government confirmed the first positive cases of African swine fever (ASF) The origin of the outbreak of this virus in Catalonia, which has been detected in two wild boars in Collserola, remains unknown. After the media focus shifted to whether the virus had escaped from the IRTA-CReSA laboratory in Bellaterra – this week the first analysis concludes that the Laboratory samples do not match those of the positive cases found outside.Experts insist that the most important thing now is not knowing where the outbreak originated, but finding the right strategies to prevent it from spreading. And in this containment phase, which so far has succeeded in preventing any infections from being detected outside the 6 km radius of ground zero, it is crucial to understand how the new variant of African swine fever (ASF) detected in Bellaterra behaves. "It's a strain that has never been encountered before," explained Toni Gabaldón this week, ICREA research professor at the Institute for Research in Biomedicine (IRB), the Catalan center that sequenced the virus samples to see if they matched those from IRTA. In fact, the researchers compared the Collserola samples with 800 ASF genomic sequences that exist in public databases worldwide and found that they did not match any of them. Therefore—and as already mentioned— the first report from the European Union laboratory pointed out Four weeks ago, it was discovered that this was a new variant, one that had not been described until now. But what does it mean for it to be a new variant? Is it common to find undescribed variants?

"The virus changes every time it replicates, that is, every time it infects a new individual," explains Julià Blanco, a researcher at IrsiCaixa and the Germans Trias i Pujol Institute. The virologist explains that a virus can only mutate if it infects a new cell—therefore, it cannot change when it is frozen in a laboratory—and, to do so, it must copy all of its genetic material, a process that requires thousands of copies and during which the virus "makes mistakes." During the replication cycle, the virus introduces changes (mutations): it can leave a piece of the genome uncopied—the genetic instructions of the DNA—it can copy the same piece of the genome twice (duplication), or it can even insert new elements into the genome. "This is where the variants occur, and they are completely random," Blanco explains. This randomness also makes it very difficult to calculate how long it might take for a new variant to develop and how many hosts it needs to stabilize. However, José Ángel Barasona, a Ramón y Cajal researcher at the Veterinary Health Surveillance Center of the Complutense University of Madrid, explains that for a new variant to mutate, the ASF virus needs "relatively long periods." "These are transmission cycles that must pass through different animals. You have to take into account the virus's incubation period, the shedding time, the contagion time... Therefore, it's not just a matter of days or weeks," Barasona points out.

In this regard, the researcher points out that since the African swine fever (ASF) virus entered the European Union in 2014 via two wild boars in Latvia, 29 new genetic groups of the virus have been detected, the most recent being the one in Bellaterra. "It may seem like a lot, but really, if you consider that eleven years have passed, it's not that many. What it does give you is an idea of the potential rate of emergence of new variants or genetic groups of the ASF virus," he explains.

Subtype 29

Although both Barasona and Blanco clarify that a genetic subtype is not the same as a new variant, and that experts will have to determine whether subtype 29 found in Bellaterra is a new variant on a scientific scale, some characteristics of the type of virus detected in Catalonia last month have already been detailed. The EU reference laboratory has already specified that the 2-29 genetic group was very similar to Georgia 2007, the strain used in laboratories to study African swine fever (ASF). However, Barasona warns that this similarity was to be expected: "Of course they look alike... all strains come from Georgia 2007, which has been the parent of all ASF viruses in the Northern Hemisphere for twenty years. All those found will be similar," he asserts. Gabaldón also explained at the presentation of the sequencing of the samples found in wild boar that the Bellaterra variant is "less virulent" than those previously detected. "The pathogenicity or virulence of the infection is also a factor that defines transmissibility. It's not the same if a wild boar becomes infected and dies after a week as if it dies after two weeks, because it has an extra week to transmit the virus to other members of the species," Blanco explains. Similarly, Barasona explains that, generally, "viruses don't benefit from a 90% lethality rate, as some highly virulent strains of African swine fever (ASF) do; instead, they prefer to be like human influenza, which adapts very well to its host but causes low lethality."

Serologies of live animals

Given this characteristic of subtype 29, the Government has already explained that serological tests are being carried out on wild boars captured alive to verify that no animals have developed resistance to the virus and are therefore infected and could transmit it to other animals. So far, more than 500 animals have been analyzed, and none have been found to be in this situation. In fact, Barasona asks that there be no alarm about the possibility that there are wild boars carrying the virus that do not die when "we are still not sure of anything." Regarding subtype 29, the IRB report revealed that the possible new strain had suffered a significant loss of a fragment of its genome compared to the variants described so far and that it contains "a set of unique mutations" that do not match the strains currently circulating in Western Europe. However, subtype 29 does have some similarities with strains described in Eastern European and Asian countries, such as Russia, China, and Thailand. In this regard, Barasona warns that in some Eastern European countries, "it's unknown what's happening with African swine fever (ASF) because in places like Belarus there's very little transparency in the reporting of animal health issues."