The possible role of Stomoxys calcitrans in equine sarcoid transmission

The association between bovine papillomavirus (BPV) and equine sarcoids is well established, but it is unclear how the virus spreads. Although evidence in support of viral spread through direct animal contact exists, this does not explain sarcoid development in isolated equids. BPV DNA has been detected in flies, which could indicate that these insects serve as a vector. This study aimed to investigate whether BPV-negative stable flies (Stomoxys calcitrans) become positive for BPV DNA after exposure to equine sarcoid or bovine papilloma tissue under experimental conditions and, if so, for how long. A total of 420 stable flies were caught alive and exposed to BPV positive equine sarcoid or bovine papilloma tissue. During the following week, dead flies were collected daily and BPV loads were determined by quantitative PCR. There was a significant rise in BPV load after tissue exposure both in sarcoid and papilloma exposed flies, but the viral load was higher and remained high for a longer time after exposure to papilloma tissue compared to sarcoid tissue. Within days, viral loads decreased again and became indifferent from loads before exposure. The results of these experiments indicate that BPV transmission by S. calcitrans seems possible and is more likely to occur after contact with bovine papillomas than with equine sarcoids. Transmission seems only possible shortly after tissue exposure. Further research could include experimental induction of sarcoids with BPV positive stable flies, or a repeat of the experiment with micro-dissection prior to PCR.