African Swine Fever spreads slowly until human activity helps things along

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By George Charbonneau, DVM

Like most idioms in the English language there are several theories about the origin of the term ‘Shooting yourself in the foot’. The theory that always made the most sense to me was related to the widespread use of handguns in the wild wild west of the US prior to the establishment of an effective police force.

Because the speed of your draw was directly correlated to your probability of survival in a gun fight it was important to be able to synchronize the draw, cock, and fire sequence to maximize the speed of your draw. An inept gun fighter would sometimes manage to inadvertently pull the trigger before the pistol had actually cleared the holster. Most often the gunslinger’s foot was directly in the line of fire. The saying eventually became a more colourful way of indicating that ‘someone was the author of their own misfortune’.

It can be argued that the rapid spread of African Swine Fever is an example of how society can be the author of its own ‘disease control’ misfortunes.

Dr. Klaus Depner is a German veterinarian that works at the German National Reference Laboratory for Classical Swine Fever (CSF) and African Swine Fever (ASF) at the Friedrich-Loeffler-Institut (FLI). He has worked with ASF in Africa and Europe and has continued to work at the forefront of understanding the spread of ASF. Depner argues that there are three main disease traits that affect how the ASF virus can spread in the wild boar environmental habitat cycle. These traits allow ASF to persist in the environment such that prolonged natural transmission can occur.

  • Contagiosity – Perhaps the least understood trait of ASF by most people is that ASF isnot very contagious. There is very little viral shedding of any kind from a sick animal. Only 30% of animals will actually become infected after a single exposure to ASF virus in contrast to a virus like PED where it is almost 100 % infection rate.
  • Tenacity – ASF virus, unlike most other viruses, survives for very long periods of time in blood, meat and the decomposing carcass of an affected animal. This high environmental tenacity ensures the long-term persistence of the virus in the environment. The virus is therefore available to the next pig host that unwittingly stumbles upon the rotting pig carcass in the wild.
  • Case fatality rate – a 90 to 100% case fatality rate makes the ASF virus largely available for spread in the form of many infected carcasses.  Given that the virus can be found in almost every tissue in the carcass, a naïve pig only needs to nibble on a small portion of a dead pig to be exposed to the virus. The interaction of these three parameters maximizes local persistence in the wild boar population and normally limits the rate of geographical spread of the virus to a slow and persistent march.

When asked how fast ASF can actually spread Depner says that it depends whether you are talking about the natural spread from pig to pig, via soft-sided ticks, or the spread associated with human intervention and stupidity. Natural spread is actually quite slow. When humans get involved, however, Depner’s answer is that ASF disease can spread as fast as you can drive a truck down the road. The spread of ASF in the country of Georgia in 2007 followed the major roads of Georgia.

In most developing nations when the ASF virus strikes the first producers don’t necessarily recognize it as ASF right away. Most people think of the disease as being an explosive outbreak of high mortality from the outset but it can take several weeks before high mortality becomes an issue. When they do recognize that something is happening they sometimes start selling live pigs to other producers or slaughtering animals that are incubating the virus.

Producers start to liquidate their affected and unaffected herds in an effort to prevent financial losses before the price tanks or they are shut down by the government. All of this pork flooding the market depresses the local meat price. Because of this low meat price any visitors to the ASF-infected regions are compelled to purchase and move infected cheap pork back to an as-yet uninfected region. Poor farm biosecurity then allows the virus to enter a barn or be exposed to the as-yet unaffected wild boar population. This starts the entire cycle all over again in a new region.

Depner states that “By transporting contaminated meat and discarding meat products so that they end up as waste or kitchen leftovers in pig stables or in nature inhabited by wild boar, humans have managed to spread the disease from the point of introduction in Georgia to Belgium in the west, the Arctic circle in the north, and China in the east.”

This is one disease where the epidemiology of the disease is actually well understood but the barrier to control is going to be the development of a better understanding of the management of the social, economic, religious and political variables that almost inevitably lead to human-assisted spread. Our future will be much brighter if we can just avoid “Shooting ourselves in the foot!”

The writer is a swine veterinarian with South West Ontario Veterinary Services

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