Virginia has seen 12 human cases of West Nile virus so far this year, the state’s Department of Health reported Tuesday.
Last year, Virginia had 13 West Nile cases, up from only eight in 2016, according to the Centers for Disease Control and Prevention.
The number of West Nile cases vary widely from year to year, CDC data shows, sometimes rising as high as 30 in 2012 and swinging back down to only six in 2013.
The research group Climate Central released a report earlier this month showing that the number of so-called mosquito danger days — when temperatures are high enough to make it easier for mosquitoes to spread diseases like West Nile — have been going up.
But, even as the summers warm, other environmental factors influence mosquito activity. Rain, for example, often wipes out the sewers where mosquitoes like to lay their eggs.
Mosquito activity changes from year to year, and earlier this month some localities were reporting that there were too few mosquitoes to even test West Nile virus, possibly due to the rain Virginia has had this summer.