Mosquitos Thrived at Oxford Basin During West Nile Scare
Clogged lagoon bred “millions and millions” of them, though none tested positive for virus that proved fatal in Marina del Rey and Venice
By Gary Walker
Marina del Rey’s denuded Oxford Basin Lagoon was a breeding ground for millions of mosquitos last year during an uptick in local West Nile Virus infections and fatalities, but none of those mosquitos tested positive for the virus, according to public health officials.
The Los Angeles County West Vector & Vector-Borne Disease Control District began investigating Oxford Basin as a possible source of West Nile-infected mosquitos after county health officials reported that a Marina del Rey resident had contracted the virus.
In the fall and winter of 2015, four Marina del Rey residents were diagnosed with West Nile and one of them died from the virus, said Dr. Rachel Civen, a medical epidemiologist with the Los Angeles County Department of Public Health. There was also a fatal case of West Nile in Venice and two that were not fatal in Santa Monica, part of a 2015 countywide total of 299 West Nile cases resulting in 22 deaths.
“Last year we had the highest number of cases of West Nile virus and of deaths from West Nile in Los Angeles County and the Westside since 2004,” Civen said.
Oxford Basin, which was under flood control renovations following the removal of vegetation in early 2015, “was found to be breeding millions and millions of mosquitoes on Nov. 18, 2015,” according to a report by agency Executive Director Robert Saviskas.
The Los Angeles County Department of Public Works moved quickly to eliminate conditions that were breeding mosquitos at Oxford Basin after they were notified of them, said Public Works spokesman Kerjon Lee.
Although the Los Angeles County West Vector report identifies Oxford Basin as a possible source of area West Nile infections at the time, further testing did not find any evidence that these mosquitos carried the disease.
“None of the mosquitoes that were captured at Oxford Basin tested positive for West Nile virus,” Saviskas told The Argonaut earlier this year.
Authorities remain uncertain about how or where local West Nile victims contracted the virus.
“They could have been bitten by a mosquito at other breeding sites,” Saviskas said. “There’s really no way to know.”
At Oxford Basin, mosquitos had been breeding in standing water that pooled after the basin’s tidal ocean flow became cut off.
According to the West Vector report, “this breeding most likely started in late June 2015, when Los Angeles County Beaches & Harbors and Public Works closed off the ocean water from the marina to the basin and blocked-off the storm drain inlets from miles and miles of surrounding neighborhood storm drains.
“This is important because the previous mixture of saltwater and [the] neighborhood’s freshwater runoff in the basin prevented mosquito breeding,” the document continued. “By changing these dynamics by blocking off the freshwater from the saltwater, it created an extensive mosquito breeding problem not only at the mouth of the basin inlet but miles and miles back upstream in the neighborhood storm drains that were no longer able to drain freely.”
Saviskas said vector control officials also found pools of water near residences in the nearby Oxford Triangle that they thought could be potential sources of mosquito breeding but found no infected mosquitos.
Lee said Oxford Basin project managers not only moved to eliminate mosquitos but also restored seawater circulation to prevent standing water.
“Currently, as part of the department’s regular operations, seawater is being circulated through the basin on a nearly daily basis to prevent standing water conditions. Measures like the circulation berm were included to ensure that water within the basin would not be allowed to pond or stagnate,” Lee said.
“There were also a number of measures taken to prevent mosquitos from breeding during construction, including pumps that continually removed groundwater from the basin,” he added.
Saviskas said vector control workers are inspecting the Oxford Basin for mosquitos on a weekly basis.
“We haven’t seen any adult mosquitos with the virus,” he said.
The West Nile virus survives in birds and can be transmitted to humans who are bitten by mosquitos that feed on infected birds.
A dead bird found in Marina del Rey tested positive for West Nile on Sept. 22, according to a County West Vector year-end report.
In Venice, four birds and five chickens tested positive for West Nile in September and October, and two mosquito pools tested positive for the virus on October, according to the report.
Two birds and a chicken in Playa del Rey, six birds in Mar Vista and Del Rey, three birds in Westchester and eight birds in Santa Monica also tested positive for the virus last year.
Civen said it was unclear whether El Niño storms could facilitate early mosquito breeding this year.
“There will be more moisture with the rain, but there’s also more of a chance for washout of any mosquito eggs with the rain. It’s really hard to say,” Civen said.
Most people who are infected with West Nile will not become ill, but some may suffer severe neurological symptoms.
“The elderly and those who have immune systems that are compromised — people with diabetes, those who have had organ transplants, cancer patients and people who are on dialysis — are the most vulnerable,” Civen said.