Two years after Miami-Dade County became ground zero for the first outbreak of the deadly Zika virus in the continental U.S., county efforts to combat the disease-carrying mosquito that spreads it and a steep decline in infections across the U.S. have sharply reduced the disease.
Only 22 travel-related infections and two undetermined cases have been confirmed this year, with South Florida’s rainy season coming to a close last week.
Statewide, just 82 travel cases were reported this year, including 63 pregnant women. One baby was born with microcephaly. Two years ago, the county had 300 local cases and two active transmission zones in Miami and Miami Beach amid a spiraling statewide outbreak that left at least 1,600 people infected.
Across the U.S., the Centers for Disease Control and Prevention reported no locally acquired infections in 2018 and 46 travel cases, with 98 infections in U.S. territories. In 2016, 225 people in the continental U.S. were infected by mosquitoes, nearly 4,900 travelers returned carrying the disease and more than 36,000 people infected in U.S. territories.
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Despite the positive numbers, the county’s new mosquito control chief is hesitant to announce the war won.
“We see Zika and dengue are both circulating in the region, in the Caribbean and probably in South and Central America,” said zoologist Bill Petrie, who was hired last year after the county increased its mosquito control budget from about $1.6 million to more than $16 million in an effort to amp up measures after sharp criticism for doing too little in advance of the outbreak.
Some health experts attribute the drop in numbers to a process called herd immunity, when infections decline because cases are so high. But Petrie said that as long as Miami-Dade remains a gateway for frequent travel to and from countries with infections, the risks persists.
“We have to be prudent,” he said. “We know the dengue and Zika are still coming in from overseas, so we still have the threat.”
When a suspected case is reported, Petrie said the county continues to respond in full, sending out inspectors to search for mosquitoes and breeding places and spraying larvicides and pesticides to kill both adults and eggs. Between the various diseases, staff has responded to far more than the 22 confirmed cases, he said.
The county has also expanded its surveillance and detection network, routinely monitoring about 180 traps and keeping lab techs busy sorting through mosquito carcasses and larvae to try to identify the known virus carriers, like Aedes aegypti, from less dangerous mosquitoes like those in the Culex family. In the lab where tech Maday Moreno works, a small temporary shipping container, the walls are lined with pictures for easy reference.
“There are some mosquitoes that look almost exactly the same and the difference is minute,” she said.
One wall is referred to as the “serial killer wall” where culprits are displayed.
While the staff has increased from 16 to 55, Petrie said the more robust department is still not quite fully up and running. Efforts to improve communication have helped with the hire of PR specialist Michael Mut, who maintains an active @305Mosquito Twitter feed posting when scheduled treatments are planned and frequently updates web information. But Petrie said lab staff still needs more training.
The county is also awaiting the results of a field trial in South Miami that involved releasing male mosquitoes infected with Wolbachia bacteria, which prevents mosquitoes from reproducing. Early results are so far promising: The number of mosquitoes declined where the infected mosquitoes were released. But whether that’s statistically significant remains to be seen, Petrie said. Results from an independent survey should be completed in November.
Petrie said the agency is also keeping a close look on Aedes aegypti for signs of change after the mosquitoes began turning up in unexpected places. The mosquitoes normally only breed in clean water. But inspectors are starting to see them in dirty water on construction sites and even paint.
“It’s not the same as saying it’s invaded and it’s adapting to its new conditions, because it’s not new,” he said. “So is it changing its behavior? We don’t know. And if it is, that’s the question.”
The county has not sprayed by air for mosquitoes since July 2017 — a controversial option because it involves spraying the pesticide Naled which kills butterflies, bees and other pollinators and is banned in Europe. But it continues regularly applying organic larvicides by truck. Spraying occurs regularly on Miami Beach and in Wynwood, where active transmission zones appeared during the Zika outbreak, and intermittently, depending on the number of mosquitoes found, elsewhere. After an increase on the Venetian Isle earlier this year, truck spraying was used for about a month before numbers again dropped, Mut said.
But the lull doesn’t appear to have eased Petrie’s worries.
“Certainly the numbers are way down, so we don’t have the same level of concern as before, but we still need to pay attention,” he said. “The diseases have not gone away. It’s people who bring them in. Not the mosquitoes.”