Marty Misener from South West Ontario Veterinary Services,Moderate Virulence ASFV Spread: “An Even Slower Fuse”

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African Swine Fever Virus (ASFV) is most often described as being highly contagious and causing virulent disease with very high mortality. The fact of the matter is that ASFV is not highly contagious. It certainly is contagious but only a relatively low percentage (30 %) of animals exposed to the virus will actually become infected from that exposure. To some extent this low level of spread presents some very real disease early detection and control problems. People usually like to say that if there was a Transboundary Disease like ASFV that got into a naïve population you would know for sure that something special was happening because the disease would spread like “wild fire” and mortality would rise very quickly. Not so true for ASFV. Once in the barn the disease spreads somewhat slowly until it gradually builds to a higher number of sick pigs and then later on a higher level of mortality. In the beginning it would be easy to miss that the virus was already in the barn based on the initial daily mortality records. The problem is compounded further with some of the newer and very pesky moderate or low virulence ASFV strains that are becoming more common.  Early detection will require very good observation of an acute change in health and mortality in a relatively small population in the barn. It won’t likely be detected early on in a population by simply reviewing mortality statistics.

Understanding ASFV transmission dynamics within a herd is necessary in order to prepare for and respond to an outbreak. Although the transmission parameters for the highly virulent ASF strains have been estimated in several studies, there are relatively few studies focused on moderately virulent ASFV strains. These researchers from the University of Minnesota and APHIS estimated the adequate contact rate for moderately virulent ASFV strains and determined the statistical distributions for the durations of mild and severe clinical signs using individual, pig-level data. An individual pig based disease transmission model was then used to estimate the time to detect ASF infection based on increased mild clinical signs, severe clinical signs, or daily mortality. How long does it take to spot ASFV after it makes its way into the barn?

The researchers found the following:

  • It may take 2 weeks or longer to detect ASF in a finisher barn via mild clinical signs or increased mortality beyond levels expected in routine production.
  • The long latent infected period for an individual pig (mean 4.5, 95% P.I., 2.4 – 7.2 days) will slow down the appearance of detectable clinical signs. Latent period is the time between infection and onset of sickness.

Take Home Message:

  • ASFV is very much like a slow burning fuse that is difficult to spot in a pig barn and it requires a keen eye to spot some unusual clinical signs or mortality but you can expect a particularly bad outcome. Finding out sooner rather than later will be paramount in stopping spread to the next barn.

Ref: Sasidhar Malladi , Amos Ssematimba , Peter J Bonney , Kaitlyn M St Charles  Timothy Boyer, Timothy Goldsmith , Emily Walz , Carol J Cardona , Marie R Culhane   Predicting the time to detect moderately virulent African swine fever virus in finisher swine herds using a stochastic disease transmission model  BMC Vet Res . 2022 Mar 2;18(1):84. doi: 10.1186/s12917-022-03188-6.