2
With the spread of African swine fever (ASF) in European countries has come a new discussion on how the disease should be handled. Professor Zygmunt Pejsak headed the swine diseases department at the National Veterinary Institute in Poland and here gives his insights about the choice of strategy.
How persistent is the ASF virus and what other characteristics dictate our control options?
ASF virus may persist for up to five months in the bone marrow of a wild boar carcass during the winter. So a population of wild boar can serve as a reservoir of the virus for a very long time. What is more, due to slow spreading among infected pigs and the initial lack of clinical signs in many animals, it is not easy to detect the disease on an affected farm. Rapid laboratory diagnosis is needed to differentiate it, especially from erysipelas. Classical swine fever is absent in Europe, but should be included in the differential diagnosis in other countries and we must also differentiate from porcine dermatitis and nephropathy syndrome (PDNS).
Europe follows the OIE policy of eradicating ASF, but one alternative tried in China is partial depopulation, in which only the infected pigs are culled so the farm can continue production. Is this a way of living with ASF?
Maybe partial depopulation makes sense from an economical point of view because eradication kills not only affected pigs but many others as well. But from an epidemiological perspective, it's a very big mistake. If you decide to control ASF by partial depopulation, you also decide to live with the disease for a very long time. I'm sure no one can accept such a method in the European Union or countries like the USA.
Do you believe applying it would result in the virus staying longer in that country?
Yes, much longer, maybe forever. Some day they may decide to change strategy, but by that time probably a majority of their swine herds will already be infected and they will have a classic endemic form of ASF. Maybe in 5,10, or 15 years, the clinical picture will be different, making detecting an affected farm is even more difficult than it is today.
Could control also become harder? So far we have two ASF genotypes outside Africa, with genotype-2 in Europe. Do we need to monitor other genotypes in case they evolve and present a threat in the future?
While ASF virus genotypes 1 and 2 have been responsible for most outbreaks in domestic pigs until now, at least another 23 genotypes are present within Africa. Genotypes other than 1 and 2 have been isolated from disease incidents and clinical trials conducted, for example genotypes 9 and 10 in Uganda and the recently detected genotype-23 in Ethiopia. This suggests the ASF virus could be virulent to domestic pigs independently of which genotype is involved. Therefore, effective surveillance based on the conserved P72 gene is required, in any affected or potentially affected country to control the possible introduction of a novel virus genotype.
What is the conserved P72 gene?
This occurs in all 23 ASF genotypes. We have to do molecular diagnostics based on the detection of this gene because we never know when new genotypes will appear in Europe or elsewhere.
Why is there still no internationally accepted vaccine for ASF?
The potential for finding a vaccine is complicated by the fact that wild boar and pigs infected by the field virus do not produce neutralizing antibodies. It seems clear that any future vaccine should be a live attenuated type from a genetic engineering approach, with a supplementing strategy for a DIVA system allowing us to differentiate between vaccinated and infected animals.
Will a vaccine actually happen?
Some day we will produce some good vaccines, but there will still be practical issues to resolve. Not least, to control ASF among wild boar we will need to apply any vaccine through a form of bait in the ground. It has been done before, in Germany against classical swine fever. But this showed that the distribution of the baits was crucial. The aim will be to immunize wild boar of different ages. But sows and weaners, for example, root in different levels of the soil. We will have to distribute this vaccine manually, by putting bait in various places underground. It's time-consuming and very expensive. So even if we had a good vaccine and a DIVA system, it would be difficult to vaccinate at least the majority of wild boar, which is the proportion necessary to control the disease.
At least in some countries, could vaccination eventually take over eradication in the fight against ASF?
You have to consider legislation and rules of trade. In the European Union it will be very difficult to introduce and use a vaccine against ASF. Probably it will be like with classical swine fever and Aujeszky’s disease or pseudorabies, in that if a country decides to use an ASF vaccine it would be treated as if affected by the disease.
What is our most applicable option to beat ASF, then?
Significantly reduce the wild boar population and push all farmers to apply biosecurity; it is the only tool we have to control the disease among swine farms. Also the hunters out in the field should remember about biosecurity. At times they’ve even been a vector of the ASF fever virus spreading.
