With the Zika virus now circulating in two dozen countries and territories across the Americas, the mosquito-borne pathogen seems destined to reach the United States and likely sooner rather than later.
What is far less certain, say public health and infectious disease experts, is Zika’s potential reach and impact here. The South is seen as especially vulnerable because of its warm, humid climate and pockets of poverty where more people live without air-conditioning or proper window screens. Plus, the region is already home to mosquitoes that can transmit the virus.
Some models estimate as many as 200 million people live in areas that might be conducive to the spread of Zika during summer months — including along the East and West coasts and much of the Midwest. That makes for a huge target as researchers scramble to determine exactly how the virus manifests itself in the human body, who is particularly at risk, and why.
Their urgency comes amid increasing reports from Brazil, the current epicenter of Zika, of thousands of newborns with a rare condition involving brain damage. Doctors there have also seen a surge in another rare syndrome known as Guillain-Barré, which can lead to paralysis. With both, a link to Zika is suspected.
Already, Brazil, Colombia, El Salvador and Jamaica have urged women to postpone pregnancy. The Centers for Disease Control and Prevention has an ever-expanding advisory urging U.S. women who are pregnant from traveling to those 24 countries and territories in the Caribbean and Central and South America that have local transmission.
President Obama received a briefing Tuesday about the situation from his health and national security teams, including Health and Human Services Secretary Sylvia Mathews Burwell, CDC Director Thomas Frieden and Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
A day earlier, the World Health Organization’s director-general warned that the disease’s spread is “explosive.”Margaret Chan also described the circumstantial evidence connecting Zika to cases of microcephaly, in which a baby’s head and brain are abnormally small, as “extremely worrisome.”
So far, the experts are divided about what lies ahead for the United States. More than a dozen cases of the virus have been confirmed to date, including two announced Tuesday by Arkansas and Virginia state health officials. In each, the person was believed to have been infected while out of the country.
Fauci is downplaying the potential for a significant eruption of Zika here. He notes that dengue and chikungunya, diseases transmitted by the same kinds of mosquitoes, are widespread in Latin America. But their foothold in the United States has been well controlled, with only small clusters of cases.
“It is unlikely that we will have a major outbreak of Zika in this country,” Fauci said.
Yet Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, fears the opposite is true. He recalls spending much of 2014 insisting that the Ebola outbreak would not become a significant problem in the United States. He gives no such assurance now.
“I think we’re in for real trouble in the United States,” he said, considering how swiftly Zika can spread. He focuses on conditions throughout the Gulf Coast, where stagnant water sources — in uncollected garbage, discarded tires, untended bird baths — can be ideal breeding grounds for mosquitos much of the year.
“You’ve got to assume the worst-case scenario,” Hotez said.
Aedes albopictus, the Asian tiger mosquito, which can transmit the virus, is present from Florida to Connecticut and as far west as Illinois, said Laura Harrington, chair of Cornell University’s entomology department. It bites people as well as animals. A second species, Aedes aegypti, the yellow-fever mosquito, has a more limited geographic footprint and only bites people. But it is the primary “vector” for Zika’s transmission.
Taken together, “there are many parts of the United States that are vulnerable because of where the mosquito populations are,” Harrington said.
The experts acknowledge that any projections are hampered by their lack of knowledge about the disease. It takes between 10 to 11 days for a mosquito to become fully infected to pass on the virus, according to Nikos Vasilakis, an arbovirologist at University of Texas Medical Branch in Galveston, where researchers are studying Zika in collaboration with scientists at Yale’s School of Public Health and in Brazil. They don’t yet know what the risk is for pregnant women who are bitten by an infected mosquito and contract the virus — much less what the risk is for their fetus.
The CDC has issued guidelines for both pregnant women and newborns. For example, it recommends testing for the virus when babies have microcephaly and their mothers traveled to or lived in Zika-virus regions while pregnant. On Tuesday, agency officials held a telebriefing with clinicians nationwide to go over the guidelines as part of increased vigilance and public education.
In the Brazilian cases, most of the exposure to Zika appears to have been during the first trimester of pregnancy. But CDC officials have said there is also some evidence that a fetus can be in danger into the second trimester.
There is no drug to treat Zika or vaccine to prevent it, although Fauci said researchers are working on the latter. The most common symptoms of infection are fever, rash, joint pain and red, itchy eyes. Symptoms are usually mild and last several days to a week.
One of the biggest problems in researching the possible link between Zika and cases of birth defects has to do with the widespread presence of dengue disease in the countries with high Zika infection, said Lyle Petersen, director of CDC’s vector-borne disease division. Humans produce antibodies in response to both viruses, but current tests cannot really differentiate between them.
“In people with previous dengue exposure, there’s no test to be able to sort that out,” Petersen explained.
Major past outbreaks of Zika occurred in the Pacific islands among small populations, and uncommon kinds of birth defects were not noticed right away. After Brazil began reporting increased prevalence of microcephaly last October, authorities in French Polynesia went back and analyzed their 2013-2014 Zika outbreak involving more than 30,000 people. They found an increase in microcephaly cases and of Guillain-Barré syndrome.
Brazil has said it wants to expand an experiment using genetically modified mosquitoes produced by a British company called Oxitec. The company says they can be effective at controlling pest populations: Altered male mosquitoes are released into the wild to mate with females. The offspring never reach adulthood, blocking the next generation from carrying diseases like dengue, chikungunya and Zika virus.
Oxitec says that trials involving its mosquitoes in Brazil, Panama and the Cayman Islands “all resulted in a greater than 90 percent suppression” of the wild Aedes aegypti population. This month, the company announced that Piracicaba, a city in the Brazilian state of Sao Paulo, would expand its effort to eradicate disease-carrying mosquitos.
But it’s unlikely that genetically engineered mosquitoes will be buzzing around the United States anytime soon.
Beth Ranson, a spokeswoman for the Florida Keys Mosquito Control District, which has partnered with Oxitec for a proposed trial, said that before any altered bugs could be released there, the Food and Drug Administration must first complete an environment assessment of the project. Its findings must then be opened to public comment. Even once finalized, a local board would need to approve the trial — over the expected opposition of some Keys residents.
In the meantime, Ranson said, local inspectors are pushing forward with more conventional methods such as treating and eliminating breeding areas for mosquitoes.
“We’ve always been aggressive . . . [but] we’re stepping up the aggressiveness,” she said. “We don’t want dengue, chikungunya or Zika here – we don’t want any of it.”
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