day 9, 15 and 17 after inoculation, respectively. These three pigs also displayed ASFv DNA in the blood. The last pig in group 1 was clinically healthy, but viral DNA was detected in the blood. In group 2 and 3 in total seven pigs showed clinical signs of ASF from day 5-6 or 11-13. They were euthanised at day 7 and 12-14, respectively. These seven pigs also had viral DNA and infectious ASFv in blood and serum samples. Only one pig in group 2 and 3 did not show clinical signs of ASF during the trial and did not display ASFv DNA in the blood after the trial. No anti-ASFv antibodies were detected in the serum from any of the 12 pigs. Within each group, the delayed time of infection in some pigs indicate that only 25% (group 1) and 50% (group 2 and 3) of the pigs were infected by the oral ingestion of the virus. The rest of the pigs were
most likely infected via contact with the infected animals in their group.
Dose, fly and distance According to older research in this field, a single fly could car- ry some 3.8-4.0 log10
TCID50 , which corresponds to the infec-
tious dose by oral inoculation, when taking into account how much blood S. calcitrans consume, biting an ASFv infectious pig with a blood titre of 5.8 log10
TCID50 . Weak animals are in-
fected by even lower dose. The Tabanidae can carry up until five times more blood than stable flies, demonstrating a higher risk for the pigs. Dr Bødker and his team call it unlikely that ingestion of blood-fed flies is a common route for transmission of ASFv between wild boar or between pigs within a stable. They ar- gue, however, that the results indicate that Stomoxys flies could be a possible route of transmission over short distanc- es, while larger flies, such as the Tabanidae, might explain some longer distance examples of ASFv transmission (e.g. from wild boar into farms).
Caged flies feeding on ASF-infected blood.
A lab result – not in practice Commenting on the results, Dr Bødker says, “That pigs can be infected just by eating ASFv-infected flies in the lab, does not necessary mean that this happens easily in herds. The path of infection is influenced by many factors on farms and proba- bly explains the inconsistent spreading of the infection seen between herds in countries with ASFv. New studies are
▶PIG PROGRESS | Volume 35, No. 6, 2019 7
A close up of the stable fly Sto- moxys calcitrans, the type of fly that was used in the research.
PHOTO: ANN SOFIE OLESEN
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