Vector potential of stable flies (Stomoxys calcitrans) for transmission of porcine reproductive and respiratory syndrome virus (PRRSV)

PRRS virus is responsible for important financial losses in the swine industry in the U.S. and worldwide. The virus replicates in macrophage cells of infected pigs resulting in pneumonia and late-term abortions in sows. The link between outbreaks in separate farms within an area despite biosecurity measures remains unclear. House flies (Musca domestica) and mosquitoes (Aedes vexans) can transmit the virus from pig to pig under laboratory conditions. Stable flies are commonly found around swine facilities and their persistent biting gives them potential as mechanical vectors of PRRS virus. We have investigated the vectorial potential of stable flies in the transmission of PRRS virus under laboratory conditions. Groups of stable flies fed blood containing PRRS virus or flies intrathoracically inoculated with the virus were sacrificed at different time intervals to determine virus persistence in the gut and hemolymph.  In transmission experiments, groups of virus-fed flies were also placed on naïve pigs for four subsequent blood meals, and the pigs monitored for signs of the disease. Active virus was recovered from stable fly guts, but detectable virus decreased with time, suggesting no virus replication in fly tissues. Intrathoracically inoculated stable flies had ~1,500 times more PRRSv copies/ml 48 h post inoculation compared to virus-fed flies. Transmission of the virus to naïve healthy pigs was unsuccessful under the current experimental conditions although all fly groups tested positive with virus isolation.