If it seems you’re hearing less about West Nile virus (WNV) this summer, you are not imagining it. Although mosquitoes have been abundant this year, for some reason the virus has remained relatively quiet in 2019.
Where has WNV gone?
A paper written by epidemiologist Dr. Wendy Chung and colleagues in 2013 may offer some insights on the absence of the virus this summer. Those of us who lived in Dallas in 2012 may remember that summer as the worst human outbreak of WNV ever. Nearly 400 cases were reported in Dallas County alone, and 19 people died of the disease. The epidemic was so bad that Dallas county resorted to spraying the entire county for mosquitoes by plane–something not seen in north Texas since an encephalitis outbreak in 1966.
Chung and colleagues charted the course of the disease during 2012 and saw high infection rates of mosquitoes early in the summer, followed by a rapid increase in human cases. Looking back over previous years and case numbers, the researchers concluded that an unusually mild winter followed by rainfall patterns ideal for mosquito breeding in the spring (and a very hot summer–West Nile virus multiplies quickly in mosquitoes at higher temperatures) created ideal conditions for an outbreak.
So what’s different about 2019? We had a relatively mild winter, with only three days at or below 28° F, and a wet spring–both conditions mosquitoes love. But the summer, at least by Dallas standards, has so far been cool. Until this week, the DFW Airport weather station saw only two days over 100° F. By the end of July the area usually has experienced more than seven days over 100° F.
One of the tools used by health departments to predict disease risk for WNV is the vector index (V.I.). The V.I. is calculated weekly from mosquito trap data, and combines information on both average abundance of Culex quinquefasciatus (the main carrier of WNV) and disease incidence in the trapped mosquitoes. A V.I. of 0.5 or higher for two or more weeks is considered a crisis indicator by health officials. Despite higher mosquito numbers this summer, the V.I. hasn’t ventured above 0.1 for either Dallas or Tarrant counties. Most of the summer the V.I. has been closer to zero. Hence fewer news reports about mosquito spraying or people getting sick. In Dallas county this year there have been no human cases of WNV. Tarrant County (Fort Worth) reports only one case this year with a very low V.I., near zero most weeks (top graph).
With this week’s string of 100° days will risk go up? Certainly West Nile virus is a threat through the end of the summer and into the fall; but this late in the season the chance of a major outbreak is probably low. On the other hand, hot weather favors the virus. It’s no time to forget about mosquitoes. Indeed, I expect Aedes mosquitoes (yellow fever mosquito and Asian tiger mosquito) to become more abundant after last weekend’s rains. This week is a good time to get out and dump standing water. Although Aedes mosquitoes are not major disease risks, they cause most of the itchy mosquito bites we get during the day–and we don’t want that.
To learn more about mosquitoes, and best ways to manage and repel them, check out the Mosquito Safari website, where you can take a virtual tour of a field and backyard and learn important facts about mosquitoes. For public health professionals and pest management professionals, Extension medical entomologist, Dr. Sonja Swiger, will be offering preparatory classes for pesticide applicators wanting their Public Health (Category 12) license, and a 3-day Master Vector Borne Disease Management Course. To learn more, and to register, go to https://livestockvetento.tamu.edu/workshop-registration/ .