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September 22, 2016: Predict Zika’s Spread? It’s Hard Enough to Count the Cases

How far will the Zika outbreak spread, and for how long?

“Predicting Zika’s course in the continental United States is difficult. Health experts have never confronted a virus quite like this one: a mild infection that can nonetheless devastate unborn infants, and that is transmitted by both mosquitoes and sex.

“Even tracking cases is hard because so few cause symptoms. ‘This is such a new thing entomologically that we’re all speaking above our pay grade,’ said Joseph M. Conlon, a technical adviser to the American Mosquito Control Association.

“Still, there is growing agreement among some experts that the virus may be, at best, slowed by aggressive mosquito control. Nothing short of winter will stop it, they said, and how many cases are mounting up is still hotly debated.

“Virtually no entomologists believe that the transmission of Zika is limited to a few square miles of downtown Miami and Miami Beach, no matter how vigorously state officials insist it is.

“’That’s just dreaming — it’s totally unrealistic,’ said Duane J. Gubler, a former director of the vector-borne diseases division of the Centers for Disease Control and Prevention. ‘Mosquitoes move around, people move around. Mosquitoes even move by car sometimes.’

“Nonetheless, the C.D.C. on Monday lifted its travel advisory for the Wynwood neighborhood of Miami, saying that no new locally transmitted cases had been detected there since early August and traps there had few mosquitoes since the spraying of two pesticides, naled and Bti, began.

“Elsewhere, Florida has been reporting new locally transmitted cases almost every day. There were 79 as of Sept. 16. As of Friday, it was investigating 17 infections to ascertain whether each was a lone case or part of a wider cluster.

“The state has refused to reveal the five places where it has trapped mosquitoes that tested positive for the virus, saying the information was “not necessary to public health.” Last week, arguing that it was, The Miami Herald sued Miami-Dade County to force it to name the sites. And it’s likely that some local transmission is not even known about yet. According to local doctors, hundreds of women are waiting weeks to get test results from overwhelmed state laboratories.”

“Experts have long predicted that other Gulf Coast cities might have outbreaks like Miami’s. Indeed, they may be having them now without realizing it because of the testing lags and asymptomatic cases.

“’Every week there’s another ‘Little Shop of Horrors,’ and everyone seems surprised,” said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine. ‘But finding virus in both people and mosquitoes suggests intense transmission.’

“But a wildfirelike spread on the Gulf Coast, such as Puerto Rico is experiencing, is not expected, largely because air-conditioning and screens are more common.

“Also, Zika usually smolders for months before exploding. For example, Puerto Rico reported its first locally transmitted case in December, but did not have thousands each week until the summer. Florida’s first local case was in late July, so cold weather may break the cycle.

“Since January, the CDC has predicted that Zika would not spread faster on the American mainland than dengue and chikungunya, both of which are also carried by the Aedes aegypti mosquito.

“But some scientists argue that dengue and chikungunya are poor models for Zika. Lab animals with Zika develop more virus in their blood than they do when infected with the other viruses, said Rebecca C. Christofferson, a disease transmission specialist at Louisiana State University’s veterinary school. And, she said, Zika appears to be unusually quick to move from a mosquito’s gut to its salivary glands, where it is injected into the next victim.

“Dengue takes seven to 10 days to make that trip; chikungunya takes as few as five days, depending on temperature, mosquito size and other factors. ‘Zika is looking like chikungunya,’ Dr. Christofferson said.

“Also, Zika flies below the public health surveillance radar. Dengue’s nickname is “breakbone fever,” and chikungunya’s is “bending-up disease.” Both can be excruciatingly painful, so victims often see doctors quickly and their test results are reported to the state.”

“By contrast, 80 percent of Zika victims lack symptoms and may never see a doctor. ‘The symptoms are so diffuse that many, including many physicians, fail to recognize that a person has it,’ said Manuel F. Lluberas, a former Navy entomologist now in the private sector.

“As a result, a Zika cluster can grow without being noticed until someone infected, usually a pregnant woman, is tested.

“Also, the state’s biggest dengue outbreak began in 2009 in Key West, a tiny vacation island with a relatively rich and educated population of 25,000. It still took two years to contain, with 90 confirmed cases.

“The state’s first Zika outbreak is in Miami, an area of 5.5 million. Cities are more likely to have residents, some with guns or dogs, who refuse to open their doors to mosquito inspectors, said Mr. Conlon of the mosquito control association. One property’s pools and gutters can produce enough mosquitoes to blanket a neighborhood.

“’What’s getting to me is how complacent people in Miami are about this,”’he said. ‘That does not bode well for containment.’

“For example, he said, mosquitoes breed in flowering bromeliads. Yet despite threats of $1,000 fines, some Miami Beach residents resist ripping up their gardens.

“Half measures will not slow the virus. Aedes aegypti lay eggs that can cling to a dry surface until the rain they need arrives. It is not enough merely to empty the rows of conch shells, for instance, that decorate outdoor restaurants in Miami. ’”You have to scrub each one out":https://youtu.be/EacJsCirabE, Mr. Conlon said.

“Predictions of Zika’s course are also made difficult by a monumental wild card. Unlike other tropical viruses, it can be sexually transmitted.

“’That makes Zika a different breed of dog,’ Dr. Gubler said.

“If an infected person passes it to a household sexual partner, the virus will be in human blood for up to 20 days at that location for local mosquitoes to pick up and pass on.

“The long-term picture is even fuzzier. Experts are divided on whether the virus could become endemic, recurring summer after summer.

“The virus itself has two ways to survive winter. A recent laboratory experiment showed that about one mosquito in 300 passes it on to the next generation in her eggs. And Zika can last for six months in men’s semen.

“Either route could seed a new domestic outbreak next year — or the virus could be reimported from Latin America or the Caribbean.”

Source: The New York Times

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