ASFV, a complex, cytoplasmic double-stranded DNA virus 175−215 nm in diameter, is the only DNA virus that uses arthropods as biological vectors [10]. At present, our understanding of ASFV is not comprehensive, and many aspects including virulence, pathogenesis and transmission mechanism remain poorly understood. The development of safe and effective ASVF vaccines may take a long time, hence understanding potential vectors is essential for preventing epidemic outbreaks and detecting potential epidemics. Thus, it is necessary to investigate the possibility of mosquitoes as ASFV carriers and vectors.
In this study, five pig farms in Jiangsu Province with ASF, or suspected of having ASF, were selected, 463 mosquitoes were captured, and they were confirmed to be Culex pipiens pallens by stereomicroscopic observation. A previous study counted the constituent ratio of mosquitoes in Jiangsu Province from April to November and found that Culex pipiens pallens was the dominant species, with indoor captures accounting for 75.07−92.86% of all mosquitoes [9], consistent with the results of the present study.
A number of criteria are usually met when judging whether or not mosquitoes are virus vectors; (1) viruses can be isolated from mosquitoes; (2) under experimental conditions, mosquitoes can be infected after feeding on blood of the viral host; (3) mosquitoes can transmit the virus by biting other mosquitoes during the experiment; (4) mosquitoes feed on blood from natural hosts and this can be proven [11]. If mosquitoes meet the above four criteria, mosquitoes can be considered a vector of the virus. The mosquitoes detected in the present study yielded negative results for ASFV, and did not satisfy criteria 1.
There have been very few studies on the potential role of mosquitoes in ASFV transmission, but one failed to validate the vector competence of mosquitoes and horse flies for the virus [12]. A recent study detected low viral doses in mosquitoes collected from an infected farm in Estonia, but virus isolation was unsuccessful, and the researchers believe that the viral DNA found in mosquitoes is likely to be due to general contamination of the environment by ASFV, but whether mosquitoes have transmission capacity requires further study [13]. Along with the present work, none of the above reports could confirm the role of mosquitoes as vectors in transmitting ASFV. Epidemiological surveys in China have shown that among the 155 ASFV outbreaks in which the epidemic source has been identified, 100 outbreaks have been caused by the allocation and transportation of pigs and their products, and the transmission of viruses by related personnel and vehicles, accounting for ~ 64.5% [14].
The transmission of ASF mainly depends on its strong resistance, since the virus can survive in the external environment and various biological media for a long time. Vehicles, personnel, tools and swill are the main sources of virus transmission. Therefore, we speculate that mosquitoes are not heavily involved in ASFV transmission, but the possibility of mechanical transmission cannot be ruled out. Large-scale pig farms still need to follow normal mosquito control procedures when formulating strategies for the prevention and control of ASF, but new virus-specific measures are not needed. Rather, attention should be payed to disinfection and epidemic prevention management related to personnel and vehicles entering and leaving farms.