African Swine Fever made its entrance into Western Europe as well as China this fall, causing a lot of nervousness amongst pig professionals worldwide, even in North America. Time to make clear where the real risks are. Pig Progress editor Vincent ter Beek enquired with ASF experts in various countries and summarised his take in this overview.
It was first described in Kenya in 1921. Until 1957 it stayed on the African continent. As it was a relatively hidden problem for a long time, the virus escaped the attention of many animal health companies. Between 1957 and the 90s, it existed in southern Europe, like Portugal and Spain, until it was eradicated there.
In terms of clinical signs, ASF looks a lot like its namesake. The disease comes with fever, dullness, breathing difficulty, vomiting, coughing, nasal and ocular discharge (conjunctivitis), skin haemorrhages and death within one to two weeks. One difference between the viruses is that CSF can give neurological signs, whereas ASF can lead to bloody diarrhoea. Normally it takes a laboratory to figure out which of the 2 viruses caused the disease.
The causative viruses are not related, however. CSF is an RNA virus, whereas ASF is a large and complex DNA virus. For CSF, vaccines exist. For ASF, there is no vaccine available.
Domestic pigs and wild boar, as well as several ‘cousins’, like bushpigs and warthogs. In sub-Saharan bushpigs and warthogs, the virus has become endemic – the animals serve as a reservoir, showing no or few clinical signs of disease. In addition, soft ticks (Ornithodoros species) can act like a vector. These ticks typically occur only in (sub-)tropical climates.
Definition matters here. ‘Contagiousness’ means the degree to which a disease can be transferred from one body to the next. While most experts would agree that the virus is contagious, few would qualify the virus as ‘highly contagious’. Although scientific knowledge is still building up on this topic, it is generally accepted that close contact – if only limited – is needed for infection, as the virus transmits with a certain difficulty. This close contact could be with:
For much more on ASF, including up-to-date interactive outbreak maps, visit the special African Swine Fever page within Pig Progress
The ASF virus is not considered to be as extremely contagious as viruses like Porcine Reproductive and Respiratory Syndrome virus (PRRSv), Porcine Epidemic Diarrhoea virus (PEDv) or Foot-and-Mouth Disease virus (FMDv), the latter being very contagious. In addition, ASFv is not likely to travel far in the atmosphere, like e.g. PRRSv can.
Yes, in nearly 100% of the cases, the virus is lethal for pigs (for exceptions, see question 8).
In this context, it is good to remember the definition of the word ‘virulence’: the degree of disease a pathogen can cause. The ASF strain that is currently circulating in Europe and Asia, genotype II, is known as ‘virulent’ or ‘highly virulent’. In other words, it does create the disease described above at question 2 to a high degree.
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The virus is known to be resistant and persist long time in the environment and different meat products; the virus can even lose its outer membrane and still be infectious. ASFv is able to persist in ticks for years and it can stay on fomites for a long time, some experts mention a year or even longer. Much depends on circumstances like e.g. the temperature. In addition, the presence of protein (blood or meat) helps it stay viable for a longer time.
Recent US research, carried out by Scott Dee and others, seems to suggest that ASFv is also capable of remaining infective during feed transport.
The situation in Russia, as well as some countries in Central and Eastern Europe, like Poland and the Baltic states, demonstrates how difficult it is to eradicate the virus from wild boar populations. There, an endemic status of the virus in the wild boar population already appears to be the case. From these areas, it takes only one contaminated sausage to infect a herd in a completely different area.
Things are not all doom and gloom though. In the Czech Republic, attempts to contain the virus in the area around the city Zlín appear to be successful, so the virus doesn’t necessarily have to become endemic in wild boar populations once it is found and immediate adequate control measures are applied.
Whether or not ASF will become endemic in certain populations, once more, may also depend on the environment, with factors like wild boar density and virus pressure playing a role. Also important in this respect is how quickly the virus was found and which eradication measures were taken. It is a notifiable disease in the EU.
Viruses do change character over time, but the speed with which they do so is different. Influenza viruses are known to change relatively quickly, hence human flu can come in different forms. ASFv on the other hand, because it is a large DNA virus, does not change very quickly. Evidence from Spain showed that only after 30 years of circulation, some variants had lost key virulence factors and had become attenuated.
If ASFv will change over time, it is obvious that this process can only occur in the wild boar population, as domestic pigs will be culled once ASF is found in a herd. In an attenuated form, the variants will still be capable of infecting, but they may do so without killing the pigs. That will eventually lead to persistence in the presence of antibodies.
Also a wild boar population can adjust. This process will probably commence with some animals with a strong innate immunity being sub-optimally infected. That would give some a chance to survive and transmit some of the tolerance to their offspring.
Above all, in capital letters: BIOSECURITY. A good biosecurity will protect a farm against ASF. What to think of?
As a next step, all sectors need to work together to one common goal. That, of course, is a responsibility that exceeds that of swine farmers, but also includes authorities and wildlife rangers.
Read more on what to do to keep ASF virus away from farms