New low virulent strains of African swine fever (ASF) could spread in Ireland undetected, a Canadian pig veterinary expert has warned.

“These low virulent strains, if they were to come to Canada or Ireland, could really hamper our early detection efforts,” pig veterinary expert and veterinary counsellor for the Canadian Pork Council Egan Brockhoff has said.

The comments came during the first session of the Irish Pig Health Society virtual symposium series 2021, which highlighted global perspectives on ASF.

“If we've got low virulent strains, it could take weeks longer, perhaps even more than a month longer, before we notice or find the virus and by then we could have significant spread,” Brockhoff warned.

“We use this term all the time in Canada called ‘masking diseases’, and if ASF is changing, are we going to have more opportunities for people to miss the virus?”

Cause for concern

Researchers from the Harbin Veterinary Research Institute in China identified 22 strains of ASF during an epidemiological survey carried out from June to December 2020.

“There's a lot of discussions now in both China and eastern Europe that this virus is changing. I think this change has given me as much concern as anything recently,” Brockhoff continued.

“We’re talking about a longer course of infection, more asymptomatic pigs and some pigs completely recovering and potentially being carriers of the virus.”

The study found that there are at least four natural variants of the low fatality strain in the field in China.

Unsafe vaccines

Brockhoff went on to highlight his concern around the use of unapproved vaccines in the field.

“We’re getting reports of gene-deleted vaccine viruses being used illegally in the field, resulting in chronic infection and increased virus transmissibility.

“Mid-2019 there was already reports then of six to eight vaccines being tested in the field, with no safety and efficacy trials.

“That’s a very similar story to what happened when the PRRS virus [Porcine Respiratory and Reproductive Syndrome] circulated in China in the previous decade.

“There’s no question that [illegal vaccines] resulted in faster spread of the virus, and more variants being moved throughout the country.”

Early diagnosis and culling of infected pigs are the main strategies for prevention and control.

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