Flies ‘possible route’ of African swine fever spread

Flies that have been in contact with swine infected with African swine fever (ASF) could be responsible for spreading the disease between herds.

Researchers at Denmark’s National Veterinary Institute found that pigs can become infected with the virus if they ingest flies carrying the virus.

The results pose extra biosecurity challenges for producers, who should employ good disinfection practices to limit fly numbers, in addition to ensuring wild animals are kept out of farms, the researchers said.

ASF has been spreading across Eastern Europe in recent months, with wild boar populations and contaminated meat identified as the main factors behind infections. Outbreaks have also been reported in China.

Biting flies

Scientists wanted to identify the risk biting flies pose to spreading the virus, as larger insects such as horseflies could feed on dead wild boar before finding their way onto farms.

In the study, researchers divided 12 piglets into three groups, housing each group in a separate room.

• Group 1 was inoculated orally with blood taken from pigs infected with ASF.
• Group 2 was inoculated orally with homogenized flies that had fed on ASF-spiked blood.
• Group 3 was fed intact infected flies — 20 per pig — which were added to 100 grams of soft cake.

Following daily monitoring, the scientists found that 25% of piglets in Group 1 and 50% of piglets in Group 2 and Group 3 were infected by orally ingesting the virus. The remaining pigs were believed to have been infected via contact with infected pigs in their group.

‘Readily preventable’

The results show that not only can biting flies carry ASF by feeding on infected pigs, they can also infect healthy animals if they are eaten.

“It is unlikely that ingestion of blood‐fed flies is a common route for transmission of ASFV between wild boars or between pigs within a stable,” the researchers said.

However, when bearing in mind the biology of the flies, the results indicate that flies could be one possible route of transmission over short distances.

“Such transmission seems readily preventable [through] using ventilation filters,” they added.

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