Though the West Nile virus outbreak in 2018 was the fourth worst since the virus was introduced in 2002, all current indicators suggest that the threat has abated. The Louisiana Department of Health reported a total of ten human cases of WNV year-to-date. Of the 65 WNV-infected mosquitoes pools collected this year, only two were collected in the month of September (down from 12 in August). Pooled samples of mosquitoes are tested weekly by the Louisiana Animal Disease Diagnostic Laboratory (LADDL), at the LSU School of Veterinary Medicine. Two of the 628 (0.03%) total pools of mosquitoes tested positive for WNV infection in September. The WNV-infected mosquitoes were collected in Madisonville and Covington. Mosquito traps set over 196 trap nights, at various locations across St. Tammany Parish, collected 214,136 mosquitoes (up from 47,126 in August) or 1,092.5 mosquitoes/trap night (up 426.3% from 207.6/trap night in August).
Despite waning WNV risk, a new crop of nuisance mosquitoes including an occasional WNV vector, Culex nigripalpus, have taken our attention. The significant increase in mosquitoes collected in our surveillance traps is largely due to the unprecedented populations of Cx. nigripalpus, Cx. salinarius, and a few other species. Though the WNV risk may be slight, we still recommend taking all precautions to avoid mosquito bites including: wearing mosquito repellents that contain DEET, Picaridin, or oil of lemon eucalyptus, avoiding peak biting times, and wearing long sleeves and pants can reduce your personal risk.
Continuing the weather pattern that we have seen since May, the month of September was drier and warmer than the ten-year historical average for both parameters. Total rainfall during September (4.29 in) was below the ten-year historical average (5.1 in) for the month. In addition to drier conditions, ambient temperatures structure the development time of this mosquito. Mean temperatures in September ranged from 73-90 ̊F, with an average of 81 ̊F, which is considerably warmer than the ten-year average for September of 78.1 ̊F. Staff Entomologist Nick DeLisi measured the time required for this mosquito to mature from egg to adult at an average of seven days in September.
Given the increase in nuisance mosquito species, STPMAD control efforts operated at moderate levels during the month of September. The Twin Otter flew two missions in September providing service to 40,439 acres. A contract aerial adulticide operator flew two additional aerial missions (62,903 acres) while the Otter was out of service. Night ultra-low volume (ULV) spray trucks sprayed an additional 132,654 acres (down 42.6% from 231,166 acres during August) in 170 separate missions during September.
Kevin A. Caillouet, Ph.D., M.S.P.H.