RESEARCH ▶▶▶


Flies can play a role in transmitting ASF into farms


Pigs can in theory get infected with African Swine Fever virus by eating infected flies. Danish researchers reached that conclusion on the basis of trials with stable flies. The outcome could explain ASF outbreaks in the summer on farms applying high biosecurity in e.g. the Baltic states.


D


Researcher René Bødker emptying a vector trap at an ASF-burial site in the Baltics.


BY ANJA PERNILLE JACOBSEN, CORRESPONDENT


r Rene Bødker has worked with herds in the Baltic states as well as other countries in Eastern Europe infected with African Swine Fever virus (ASFv). Be- ing associated with the National Veterinary Insti-


tute, Technical University of Denmark, for a long time he has been trying to find a pattern in the factors that can influence the infection between herds, to be able to help the non-in- fected herds to stay free of ASFv. Commonly known is that ASFv spreads by physical contact between pigs or if pigs are being fed with infected food. In that process, the role of flies intrigued him. He says, “Farms in the Baltic states have a very high biosecurity – many of them higher than for example farms in Denmark. The farms are closed with a fence around them. No trucks with animals, feed and so on are allowed. Recently some of them installed fly filters in the ventilation system to keep out the ASFv- infected flies, but they still got infected with the virus. But flies are difficult to keep out of the stables. One thing point- ing towards ASFv-infected flies as vector, is the fact that


Flies as mechanical vectors Blood-feeding horse flies (family Tabanidae) are known to be so-called ‘mechanical vectors’ for viral pathogens. These flies live in semi-aquatic habitats outside farms, where they may have contact with ASFv-infected wild boars before entering the houses containing domestic pigs. Those flies do not usually live and breed in domestic pig sta- bles. However, the flies are big enough to be chased and eat- en by the pigs or accidentally ingested during feeding, should they manage to get inside the pig houses. This might be a way the disease can spread from wild boars to herds and it could also explain large outbreaks in Europe during the sum- mer months. In order to test that hypothesis, Dr Bødker and his colleagues at the university studied experimental ASFv transmission to pigs via oral uptake of flies, which were previously fed on blood from ASFv-infected pigs.


Infected by eating stable flies with ASFv The researchers used the smaller stable fly (Stomoxys calci- trans), as a model for blood-feeding flies, as they are readily caught and fed. The transmission experiment showed that a pig easily gets infected with ASFv by eating stable flies fed on ASFv-spiked blood. In total, there were 3 groups with 4 pigs each in the experiment. • Group 1 was orally inoculated with 1 ml spiked blood, dose 5 log10


TCID50 . This was a positive control;


• Group 2 was orally inoculated with 20 homogenised flies, dose 5.1-5.3 log10


TCID50 . • Group 3 was feed with soft cake containing 20 intact flies;


In group 1, a total of three pigs showed different clinical signs of ASFv (fever, anorexia, depression, convulsions, vomiting and so on) from day 6 to 17. The animals were euthanised at


6 ▶PIG PROGRESS | Volume 35, No. 6, 2019


there seems to be a much higher rate of infection in the summer months.” That outbreaks apparently occur unpredictably on well-pro- tected farms, this provided more reason for Dr Bødker to sus- pect the influence of flies. He says, “In all researched coun- tries, populations of ASFv-infected wild boar could be found roaming outside closed farms. The different groups of wild boar are infected by direct contact. But also biting flies are considered to play a role. However, the biting flies do not have a huge living area.”


PHOTO: RENÉ BØDKER


PHOTO: SHUTTERSTOCK


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