- 10 minutes
- Control | Expertise video
A Polish study of African swine fever published in 2020 has found important new evidence on the infectivity of the ASF virus. With a virulent viral strain tested in an experimental infection by the researchers, even an extremely low titre or dosage was enough to infect pigs at six weeks old.
Results from the study have been described by research team leader Professor Grzegorz Wozniakowski in an episode of “Meet the Expert”. He is Head of the Department of Swine Diseases at the National Veterinary Research Institute in Puławy, Poland, and also Director of Poland’s National Reference Laboratory on African swine fever.
In the following extract from the podcast, Professor Wozniakowski talks to journalist Peter Best.
What was the main aim of your study?
We tried to examine the virulence of the African swine fever virus strains recently isolated in Poland, from wild boar and from pigs. As well as determining the pathogenicity of these strains, we wanted to figure out what was the minimal dose which was capable of infecting the pigs and to reproduce the clinical signs in experimental groups.
What did you find?
The first result to mention is that, up to five days after experimental infection, there were no specific clinical signs for African swine fever apart from an elevated body temperature. But we also identified that the infection of pigs is possible with only five units of the virus. So the titre needed to produce an infection can be really low. It tells veterinarians and farmers that the potential introduction of ASF to a farm may involve a very small dose of the virus.
That dosage of five units, would such a small dose depend on intranasal infection or direct contact or aerosol? How would it have to reach the pig?
We looked whether it was possible to infect the pig by the intranasal route and indeed this was true, intranasal infection is important in the spread of ASF. Aerosol and airborne routes were also tested and the results are still being analysed, but we have evidence that they could be other possibilities. Of course, five units of the virus is very little. We believe this low dosage suggests that the different routes of infection all need to be taken into account, something which is another important aspect for biosecurity.
Would the dosage explain a difference in clinical forms? In other words, would five units produce a lesser clinical effect than if there was a higher dosage?
No, we observed the same clinical course, independent of the primary dose or titre of the virus in the infection. But your question tackles the important issue of virulence in strains of African swine fever virus. Currently in Poland we have only very virulent ASF strains, which cause close to 100 per cent mortality in infected pigs or wild boar. Different variants found, for instance, in the Baltic states and probably also in Russia, have moderate virulence and they may lead to a subclinical or chronic form of the disease.
In our experiment, we observed an interesting phenomenon because in one infected pig we saw chronic disease. That pig survived until the 32nd day of the experiment and showed the classical clinical course of African swine fever. This shows that in some conditions pigs or wild boar may survive the infection.
You saw acute and chronic disease. Did you also see subacute cases, where the signs were slightly less than with acute ASF?
Yes, indeed. For instance, in an experimental group infected with about 500 doses of the virus, we observed both the acute form and the subacute form of the disease. But it’s extremely difficult to say when clinical signs differentiate between these two forms. Subacute disease may relate to a fever occurring later for that affected pig than with an acute form, but really the border between acute and subacute is hard to establish.
The virulence of the isolate was the same for each of the pigs experimentally infected. Were the pigs all of a similar age? The dosage given them differed, but what else could have affected the outcome?
They were six weeks old. Before the animal trial, we tested for potential co-infections and we were able to exclude them in our experimental conditions. But at farm level anything is possible because there are many other infectious agents that could be involved in a co-infection. All we can say is that they were not a factor in our case.
From your study then, would the main points be that a very low dose rate could be sufficient to create an infection, but also that there can be different clinical outcomes which are regardless of the virulence or the dosage?
Yes. For us, the real surprise was that as low as five doses of the virus was enough to reproduce the clinical course of the disease. I think for Poland and the neighbouring countries this was the most important message. It influences how to prevent the potential spread of ASF to domestic pigs from a population of infected free-ranging wild boar.
How can they take that information about the low dosage and use it to protect their domestic pigs?
Sometimes it’s difficult to convince people that certain procedures regarding biosecurity and activities are crucial to prevent the spread of the virus to pig holdings. At the end, I think, the key point is awareness. We are doing our best to organize campaigns to present our results from our animal trials, to raise consciousness about the potential infectivity of the virus, the danger of ASF, which may occur in almost any pig holding.
Should the information from your study influence routine monitoring and surveillance on farms?
Yes, our results showed the clinical signs were non-specific and variable, but in general the first clues could be observed five days after infection. Some signs seen in practice could suggest a disease which may be cured with antibiotics, for instance erysipelas. But the ASF virus spreads only slowly from pig to pig. Sometimes an elevated temperature, over 41.5 degrees, in a single pig may be the first clinical sign of the beginning of an outbreak of African swine fever.