African swine fever outbreaks have occurred at high-biosecurity pig farms overseas. These farms use closed management, have fences around their perimeter, and prohibit trucks transporting animals and feed from entering. Some farms have even installed fly screens in their ventilation systems, yet pigs still became infected with African swine fever virus. Danish researchers conducted experiments using stable flies and found that pigs can become infected by eating flies carrying African swine fever virus (ASFV). This finding helps explain why African swine fever outbreaks occur at high-biosecurity pig farms in summer and suggests that flies may be an underestimated route by which African swine fever virus enters pig farms. This article will provide a summary and analysis based on the foreign-language literature “The Role of Flies in the Introduction of African Swine Fever Virus into Pig Farms_陈权.pdf”.
I. Background: Why do high-standard pig farms still experience outbreaks?
Dr. Rene Bodker previously worked at pig farms infected with African swine fever virus in the Baltic states and Eastern Europe. He has worked for a long time with the National Veterinary Institute at the Technical University of Denmark, seeking to identify factors that affect the spread of African swine fever virus between farms. It is widely believed that African swine fever virus spreads through physical contact between pigs or through feeding them food contaminated with the virus. However, outbreaks of African swine fever at farms that strictly implement protective measures seem to occur without warning. Investigations found that wild boars infected with African swine fever virus were active outside enclosed pig farms, and wild boar herds became infected through direct contact. Stable flies are also thought to play some role in transmission, but their range of activity is limited. The likelihood of a pig farm becoming infected with African swine fever virus is significantly higher in summer than at other times, providing grounds for suspecting that flies play a role.
Two. The mechanism of flies as mechanical vectors
Horseflies (Tabanidae) are considered “mechanical vectors” of viral pathogens. These horseflies live in semi-aquatic habitats outside farms and may have come into contact with wild boars infected with African swine fever virus before flying into domestic pig housing. They do not usually live and breed in domestic pig housing, but they are large, and if they fly into a pig house, they are likely to be chased and eaten by pigs or occasionally swallowed while pigs are feeding. This may be one route by which African swine fever virus is transmitted from wild boars to domestic pigs, and may also explain the large-scale outbreaks on European pig farms in summer. To test this hypothesis, Dr. Bodker and colleagues experimentally transmitted African swine fever virus by having domestic pigs eat flies. The stable flies used in the experiment were fed blood from pigs infected with African swine fever virus before being eaten by the pigs.
Three. Experimental Design and Results
Researchers used smaller individual stable flies (Stomoxys calcitrans) as a model for the transmission of African swine fever virus by blood-feeding flies, because they are easy for humans to catch and feed. The trial had 3 groups, with 4 domestic pigs in each group:
Only 1 experimental pigs in Groups 2 and 3 showed no clinical signs, and no ASFV DNA was detected in their blood after the trial. No antibodies against African swine fever virus were detected in the sera of all 12 pigs. Some pigs in each group showed delayed onset of clinical signs, indicating that 25%, 50%, and 50% of the pigs in Groups 1, 2, and 3, respectively, were infected through oral exposure to African swine fever virus, while the remaining pigs were likely infected through contact with infected pigs in the same group.
Four. Virus dose, fly species, and transmission distance
Earlier studies indicate that after a stable fly bites an infected pig with a blood virus titer of 5.8×10^g TCID50, it may carry 3.8×10^g–4.0×10^g TCID50 of the virus, accounting for the blood consumed by the fly itself. This is equivalent to the oral infectious dose for pigs; the infectious dose may be even lower for weak pigs. Horse flies can carry 5 times as much blood as stable flies, posing a greater infection risk to pigs.
Dr. Bodker’s research group noted that ingestion of blood-sucking flies is unlikely to be a common route for African swine fever virus transmission among wild boar or among domestic pigs within pig housing. However, Stomoxys flies are a plausible route for short-distance transmission of African swine fever virus, while larger individual tabanid flies may explain some longer-distance transmission cases, such as transmission from wild boar to domestic pigs on farms.
Five. The gap between test conditions and actual production
Dr. Bodker emphasized that the fact that domestic pigs became infected with African swine fever virus in the laboratory only after ingesting flies infected with the virus does not mean that this is likely to happen on pig farms. The routes by which pigs become infected with African swine fever virus on farms are influenced by many factors, which may explain the inconsistent patterns of transmission between pig herds in countries where African swine fever is endemic. More research is needed to fully identify how African swine fever virus spreads between pig herds. There may be other routes of transmission, and the main route needs to be identified. Until a vaccine is successfully developed, the problem should be addressed through other, lower-cost management measures. For larger farms with wild boars in the surrounding area, such as farms in the Baltic States and Eastern European countries, it is necessary to consider whether to invest more in keeping flies out of pig housing. As new or previously established animal species, such as wild boars, encroach, the risk to the livestock industry of costly, fatal animal diseases also increases. The Danish pig industry and Danish authorities have decided to build a fence along the German/Danish border to prevent wild boars from entering. If wild boars can be kept out, flies infected with African swine fever virus can also be kept out of pig farms.
Six. Prevention and control insights
In summary, flies—particularly stable flies and horseflies—can act as mechanical vectors of African swine fever virus, and pigs may become infected after ingesting infected flies. Although this may not be the most common transmission route on pig farms, it can explain outbreaks during summer at farms with high biosecurity standards. Prevention and control should combine fly control, wild boar exclusion, and biosecurity measures; relying only on fencing and vehicle controls is not sufficient.