The Contra Costa Mosquito and Vector Control District announced that mosquitoes collected from a trap on Holland Tract this week tested positive for West Nile virus, marking the third group from that area to do so this year. Officials emphasized that these mosquitoes are not the same species that East Contra Costa County residents have been reporting. According to Scientific Programs Manager Steve Schutz, Ph.D., Contra Costa County is home to 23 native mosquito species, but only two, Culex tarsalis and Culex pipiens are efficient at transmitting West Nile virus. The mosquitoes residents have recently encountered, Aedes vexans, are aggressive biters but are not efficient at transmitting West Nile or other viruses. Reporting dead birds is also critical, since they are often the first sign of West Nile virus in a community.