With two cases of mosquito-transmitted malaria about a mile across the Potomac River, public health officials have begun testing mosquitoes near Potomac for malaria last week.
Montgomery County began testing trapped mosquitoes for the malaria virus on the weekend of Sept. 28, according to Dr. Carol Garvey, chief medical officer of Montgomery County.
Two teenagers in Loudoun County, Va. were diagnosed with malaria in August; neither had traveled overseas. Then on Sept. 25, mosquitoes captured in Loudoun County were tested positive for malaria, according to the Loudoun County Health Department.
Montgomery County will begin testing in Poolesville, although Loudoun County borders both Potomac and Poolesville.
"We're starting our testing in the northwest part of the county because of its proximity to Loudoun County," said Garvey.
The threat of malaria comes at the end of a summer that included the death of a Bethesda woman from West Nile Virus. Crows and other birds in Potomac, Bethesda and Rockville tested positive for the virus, and the county has stopped collecting and testing dead birds in most areas because the presence of the West Nile Virus has already been confirmed.
While it’s not unusual to see cases of malaria in Montgomery County, those cases virtually always involve people who had traveled to parts of the world where malaria is common.
"We see 35-50 cases of malaria in Montgomery County every year," said Garvey, who said that nearly all cases involve people who have traveled abroad. "Because malaria is a very common disease in some parts of the world, we expect to keep seeing it. … We've always known that local mosquitoes could pick it up and transmit it."
While regional transmission of malaria has long been a theoretical possibility, there has been no confirmation of it happening in recent years.
"This is the first time in over 20 years that positive mosquitoes have been found in conjunction with a human case in the United States," said Dr. David Goodfriend, director of the Loudoun County Health Department.
Most West Nile-related cases of encephalitis occur in late summer and early fall, and a cold spell will minimize the threat of mosquitoes, Garvey said.