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How does African swine fever spread?

27 June 2019, at 1:00am

Dr Liz Wagstrom, chief veterinarian with the National Pork Producers Council in Des Moines, Iowa, USA, explains how African swine fever (ASF) is being transmitted across different parts of the world.

Dr Liz Wagstrom, chief veterinarian with the National Pork Producers Council, speaks to The Pig Site's Sarah Mikesell in Des Moines, Iowa, USA

"African swine fever is transmitted through close contact. Unlike some of the diseases that we know that are aerosol and move great distances, this is close contact, nose-to-nose; bodily secretions can transmit it. But another concerning transmission factor is the fact that it can be transmitted through meat," said Dr Wagstrom.

Movement of African swine fever

ASF virus can survive for long times in uncooked or even cured meats. Reports indicate that it's survived in serrano hams for over a year, according to Dr Wagstrom. Transmission in meat and/or meat scraps being fed to pigs is one of the ways ASF has been moving throughout eastern Europe and in China.

Another unique route of transmission in some sub-tropical areas is through soft-body ticks. Once the ticks feed on a pig, it will stay infected for the rest of its life. Dr Wagstrom noted that the some of those same types of ticks can be found in southwestern US states. She said ticks have played a role in outbreaks in Mediterranean countries as well.

ASF virus is a very environmentally hardy virus that can remain alive even in dead animal carcasses.

“We have reports in eastern Europe where they have found carcasses that have over-wintered, and they're nothing but hides and bones, and they're still finding live virus in the bone marrow of those bones,” she explained.

If those carcasses are eaten by wild boars, the virus would have a new host and could continue to be spread, even through faeces.

Another concern with ASF is not only transmission through cannibalism but also with people tracking manure or other bodily secretions from location to location. They could easily serve as a mechanical vector of the virus.

“In Europe, there’s some concerns with truckers that may be coming across from African swine fever positive areas to negative areas,” she said. “Are they bringing sausages or sandwiches, and throwing some garbage out? And with the large wild boar population they have, those scraps could easily be picked up by wild boars.”

The large wild board population is a significant area of concerns for many like Russia, eastern Europe and the Baltic nations. Even in Belgium where they are fighting ASF, wild boar populations are a challenge in their national forest. If ASF were to enter the US, the southeastern states have a very large feral pig population that would be vulnerable as well.

Biosecurity and diagnostics opportunities

The basics of biosecurity should be followed:

  • Don’t track anything into barns.
  • If possible, bring animals inside to ensure they don't have any contact with wild boar.
  • Make sure the boots and clothing that are worn outside are not worn inside with an ASF-free pig population.

“The other thing we are hearing and that we are concerned about in the US is feed ingredients and feeding,” Dr Wagstrom said. “In many of these small holdings, they feed garbage or plate waste containing meat, and that could surely serve as a vector to bring the virus into those animals.”

During Dr Wagstrom’s trip to eastern Europe, she said grass cuttings from ditches and fields to feed smallholding animals was also considered a risk factor. Anything brought onto the farm should be evaluated as to whether it's a risk, then consider how to minimise the risk, she explained.

“The other thing that we worry about is once you get a sick animal, how do you diagnose it?” she said. “The symptoms of ASF can mimic other diseases. Whether it's a salmonella, septicaemia or some of those things we'd see fairly regularly, it's really important to have your veterinarian on board and make sure that they're sending in the correct diagnostic samples.”

In the US, if there's a suspicion that a sick pig might have African swine fever, notify state or federal veterinary authorities, and they will initiate and conduct a foreign animal disease investigation. This ensures the correct types of samples are being sent to diagnostic laboratories using validated tests. In the United States, the samples of choice are spleen, tonsils and whole blood.

"Europe has had a lot of experience with doing a great amount of surveillance. So they're testing wild boar, and they're using PCRs," Dr Wagstrom said.

They are also looking at antibodies as well as the antigen that you'd detect with a PCR. Having robust surveillance, including a lot of tests and testing capacity, and being comfortable running those tests is going to be really valuable as you look at controlling an outbreak situation.

"It's also really critical to have a test that you're comfortable that a positive is truly is positive, and that a negative truly is negative," she explained. "That's really important when you're talking about a disease that has the trade impacts, as well as the animal health impacts that African swine fever has."