A 2018 study, “Infection of pigs with African swine fever virus via ingestion of stable flies” is making the rounds again as several industry experts try to make sense of the unexpected spread of African swine fever to farms in Eastern Europe with relatively decent biosecurity.

To investigate this possibility further, Danish researchers allowed pigs to ingest flies that had fed on ASFV‐spiked blood, which had a realistic titer for an infected pig. Some of the pigs became infected with the virus.

While it’s unlikely that ingestion of ASF-infected flies is a common route for transmission, the scientists say the results indicate that Stomoxys flies could be one possible route of transmission over short distances (e.g., within farms), while larger flies, such as the Tabanidae, might explain some longer distance examples of ASFV transmission (e.g., into and between farms). The researchers conclude that blood‐feeding flies could be a route for the observed, but unexplained, introduction of ASFV into farms with high biosecurity.

We know that ASF can be transmitted to pigs through feeding of food waste containing contaminated pork products as well as direct contact with infected pigs, their waste, blood, contaminated clothing, feed, equipment and vehicles, and in some cases, ticks. But we really haven’t talked about flies yet.

According to Peter Fernandez, a former Animal and Plant Health Inspection Service veterinarian, at present the only blood-feeding insect demonstrated to be a vector of ASF is the stable fly (Stomoxys calcitrans), which can maintain high levels of virus for two to three days and inject or transmit the virus to pigs up to 24 hours after virus ingestion.

“In experiments with purposefully infected stable flies, pigs that ate flies could also become infected,” Fernandez says. “Both of these are examples of mechanical transmission.”

The infectious disease epidemiologist says a number of experiments have attempted to investigate the possibility of other insects as competent vectors for ASF such as: hard ticks, blow fly larvae, lice, mosquitoes and mites. However, all provided little or no evidence of effective transmission.

“ASF virus is known to be transmitted by soft ticks of the genus Ornithodoros both in a natural sylvatic cycle among African warthogs and Ornithodoros moubata and also in domestic swine and Ornithodoros erraticus in the Iberian Peninsula,” Fernandez says. “This is the only known biological vector of ASF, which means the virus actually replicates in pigs and also in soft ticks. All other transmission mentioned is mechanical transmission which means there is no replication in the insect vector and the insect acts as a small ‘flying syringe.’”

While the mechanical transmission by flies seems pretty low on the totem pole in terms of transmission, future studies on the role of blood-feeding flies may be warranted.

“This study is another important demonstration of the ability for ASFV to be transmitted through the oral route and provides additional evidence that the environment within a stable fly is capable of supporting viral stability for transmission when naturally consumed,” says Megan Niederwerder, Kansas State University assistant professor of diagnostic medicine and pathobiology in the College of Veterinary Medicine.

Niederwerder says it would be interesting to investigate if consumption of a single fly or a few flies (<20) are capable of causing transmission, which may be more representative of field conditions.

“The potential for flies to play a role in mechanical spread of the virus would most likely be short distances due to the biology and normal behavior of the stable fly,” Niederwerder says. “However, longer distances may be possible in association with wind.”