On Thursday, August 17th, a mosquito pool located north of Bishop tested positive for Saint Louis encephalitis, or SLE. I spoke to Rob Miller, Operations Supervisor of the Owens Valley Mosquito Abatement Program (OVMAP) on August 29th—the same afternoon that 3 more samples left the hands of his personnel to be tested in UC Davis laboratories for SLE, West Nile virus and West equine encephalitis. All three are mosquito-borne diseases.
This morning, August 31st, one of those samples tested positive for West Nile virus—the first occurrence of the disease in Inyo County since 2019. The positive sample was collected from the same trap location in which SLE was detected.
“I’ve been in this program for 22 years,” said Miller, the organization’s longest working employee, “and I’ve never seen a case of it. I reached out to old managers. None of them have any record of any SLE-positive pools, ever, in Inyo County.”
For Miller, the abatement of disease-carrying insects is personal.
“When I was younger, before West Nile came to the States, SLE was the big mosquito virus that was prevalent in the valley. I had a schoolmate die from it in the ‘80s.”
Miller remembers that, years later, when West Nile was first identified in the New York metropolitan area in the summer of 1999, it sort of “outran” Saint Louis encephalitis.
“Now, we’re starting to see a lot of SLE cases. It’s because of the mosquito abundance, and the water on the river being uncontrolled,” said Miller.
Miller identified that the unprecedented levels of runoff from this year’s long winter have created large pools of standing water where vectors thrive—vectors, as defined by the World Health Organization, as “living organisms that can transmit infectious pathogens between humans, or from animals to humans.”
Often, these vectors are the same bloodsucking creatures we encounter on wooded hikes, or around late-summer campfires.
When I mentioned to Nate Reade, Agricultural Commissioner of Inyo County, that I had a constellation of mosquito bites from one such hike by Emerald Lake, he put my fears at ease given the current danger. Reade explained that those mosquitoes are likely a different species from the vectors identified in the Bishop pool.
“The high altitude mosquitoes are some of the most interesting ones. They breed only in water trapped in holes and trees and snow caps melting in the sun,” explained Reade. “Often, they’re not vectoring disease, but they are aggressive.”
Reade agreed with his colleague, Rob Miller, that the water levels resulting from this winter’s thaw have made for unprecedented mosquito hatches.
Reade and Miller work closely with the Los Angeles Department of Water and Power to tailor flows out of reservoirs in the area and manage river levels.
“Long story short,” explained Reade, “certain species of mosquito hatch out of the mud. The water needs to be going up and down on a regular interval to allow those to hatch. Depending on the temperature, we can guess that interval.”
With cooperation from both departments, intentional flooding of local waterways is implemented to prevent hatching.
But what if prevention fails?
“[We] stock up on larvicide; stock up on personnel,” said Miller.
As with the containment of any disease, monitoring is everything. According to Miller, OVMAP closely surveils over 200 mosquito sites in Inyo County. Altogether, his program inspects over 1700 square miles of land on a weekly basis.
“The program follows integrated test management protocol,” says Reade. If mosquitos do hatch, “the attempt is to kill them when they’re larvae with an incredibly selective larvicide. It doesn’t harm people.”
The larvicide both Miller and Reade are referring to targets and kills young mosquitoes before they grow into adults. When samples test positive for disease, however, extraordinary precautions are taken.
“When we start dealing with human health issues and concerns, we’ll do the adulticide treatments,” said Reade.
The day after the pool north of Bishop tested positive for SLE, the Owens Valley Mosquito Abatement Program performed fogging treatments around the Dixon Lane and Bear Creek areas.
With the recent positive for West Nile virus, fogging applications are planned to take place around the Tri County Fairgrounds and surrounding residential areas on Friday, September 1st, and Saturday, September 2.
“It’s kind of like the last tool left in the box,” said Reade.
Visit the California Department of Public Health Department’s website for more information related to Saint Louis encephalitis and West Nile virus (https://www.cdph.ca.gov/).
You can report any mosquito-related concerns to the Owens Valley Mosquito Abatement Program by calling (760)-873-7853, or visiting inyomonoagriculture.