Where did the 'sandwich theory' come from? What are the ways in which swine fever can infect?

BarcelonaA disease that had gone undetected in the region for three decades: now, at least nine positive cases and at least fifty dead wild boars in the Collserola Natural Park. This is the tally, as of Thursday, of the return of African swine fever (ASF) to Catalonia, less than a week after the outbreak was detected. Beyond the economic repercussions for the Catalan pig farming sector, which is anxiously following the race against time to control the infections, the priority is to break the chains of transmission and prevent the virus from spreading. Professionals at the Animal Health Research Center (IRTA-CReSA) are working tirelessly sequencing and isolating the virus to understand its genome, compare it with those circulating in other countries, and pinpoint its origin—that is, determine where it may have come from.

The most plausible explanation is that it is similar to viruses circulating in European or Asian countries, such as Italy, Poland, Germany, Romania, Russia, or China. However, Quim Segalés, a swine health specialist and researcher at IRTA-CReSA, warns that there is a risk of not being able to detect outbreaks in other countries where it has been impossible to pinpoint its origin. One reason for this is that some strains are very similar to one another. For now, we will have to wait for scientists to sequence the virus to see if there is a circulating strain that is the same as the one detected in our country. Regarding how it reached Catalonia, the main hypothesis so far is... sandwich theoryThis means the virus arrived via a food sample containing pork contaminated with the virus.

Segalés acknowledges that, without context, this explanation might seem implausible, so he considers it crucial to understand how the disease is transmitted and explains that there are three main routes of infection. The first is through contact with a sick animal, which experts rule out because it would mean an infected wild boar crossed several borders undetected until reaching Collserola and spreading the virus. African swine fever can also be transmitted through a tick that acts as a vector, but only a very specific species is capable of transmitting the disease, and according to Segalés, it has never been detected in Catalonia. This also rules out this second route.

Therefore, the last and final route of transmission is the most plausible for experts: that it occurred through the ingestion of products contaminated with the virus, such as a sandwich with cold cuts from a country where swine fever is circulating unchecked. Furthermore, the researcher emphasizes that the epicenter of the swine fever outbreak "is a crossroads of roads and people" and is near a university campus with an influx of people "from all over the world and from many different countries," which, in his opinion, reinforces the experts' hypothesis. According to the Ministry of Agriculture, the most common entry point for the virus in a disease-free country is also through pork products that have been used as feed for other pigs.

Security at the center

However, once the disease is reported, as has already happened in Catalonia, contact between healthy and sick pigs is the most important factor in its spread. Therefore, it is crucial to establish a perimeter to try to curb the virus's expansion. It must be taken into account that, once recovered from the disease, the animals are asymptomatic carriers and play a "fundamental role" in transmission, the Ministry warns in a document on combating African swine fever (ASF). The director of the ASF virus laboratory at the Severo Ochoa Molecular Biology Center (CBMSO), Yolanda Revilla, warns that, apart from contaminated food, there are other potential routes through which the virus could have arrived in Catalonia, such as transport vehicles like trucks or ships. "Barcelona has a very active port; I wouldn't rule it out," the expert cautions. Segalés explains that, along the same lines, there have also been outbreaks in the Dominican Republic and Haiti, most likely stemming from a container of contaminated meat that arrived from China. "These jumps are not unusual. Until now, we had avoided them, but it's happened to us now. It's a bit of a lottery," says the IRTA-CReSA researcher. In the last decade, the virus has also suddenly appeared in countries like Belgium and Sweden, as has now occurred in Catalonia, and in those two cases, they were able to eliminate it because they only had cases in wild boar and acted quickly, which is what they are trying to do now.

Regarding the messages linking the origin of the infections to the samples used at IRTA-CReSA for their research, the head of the IRTA-CReSA High Biocontainment Unit, Xavier Abad, insists that they have analyzed all the actions carried out with this virus in the last three months and "it has not been detected while working with biosafety protocols. All samples used in the laboratory are destroyed to guarantee that no viable trace remains and thus avoid any possible incident," he adds. However, he acknowledges that "zero risk does not exist." Revilla, who has no connection whatsoever with IRTA-CReSA, also questions this information and defends the Catalan center: "The virus has been spreading for years."

While scientists continue working to sequence the virus, Rural Agents, Forest Defense Groups (ADF), the Civil Guard, the Military Emergency Unit, Firefighters, and the Mossos d'Esquadra (Catalan Police) continue working within the delimited perimeter in Collserola where Both have appeared. The goal is to detect infected animals and prevent the spread of African swine fever (ASF). Once animals are found, samples are sent to IRTA-CReSA for PCR testing to determine whether or not the animal is infected, while awaiting virus sequencing to shed light on its origin.