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Zika Defense

We don’t have a cure for Zika virus, but we have potent tools to control its evolution and block its path

by Wendy Orent

If you were a child living in Central Africa, you wouldn’t pay any attention to Zika virus. In Uganda, in Nigeria, in the Central African Republic, where the virus has been lurking for decades or centuries, you’d get bitten by a mosquito when you were running through a field or playing among the trees. Maybe you would come down with a low fever and a little rash. Or you’d feel nothing at all — and that would be that. When you got pregnant, years later, you’d never fear that your child might be born with a small head and tragically damaged brain, because that wasn’t anything you’d ever seen. You, and all your friends, wouldn’t be susceptible to Zika virus infection while you were pregnant because, very likely, you’d already be immune.

That, as John Brooks of the Centers for Disease Control and Prevention in Atlanta, Georgia, argues, is the natural history of Zika virus — a relatively benign infection, apparently originating in monkeys, which didn’t cause much trouble as it moved around into people. If, on occasion, a pregnant woman had a microcephalic child because she’d escaped infection earlier, that was a tragic event, and no one associated it with a mosquito bite in a place where mosquito bites were endless.

But, Brooks explains, when the virus moved into Brazil, spreading among people who’d never been infected with it before, that hideous side effect erupted. It’s not that the virus became more deadly; it’s that it wreaked grotesque havoc on a virgin population. The world froze. What would happen when athletes from around the world converged in Rio for the Olympic Games? Where would the virus go, and how would it evolve? Were we prepared for a pandemic of Zika virus, which not infrequently produces, among pregnant woman, such a dreadful, irremediable condition as microcephaly?

Then things got even more frightening. It wasn’t just mosquitoes that spread the virus: people spread it too, via sexual transmission. It seems, says Brooks, that Zika virus, like Ebola virus, can apparently reproduce floridly in the male testes, a protected environment that the immune system does not penetrate. It’s expressed in semen and spreads through male-female and male-male contact. It’s also been found in vaginal secretions, and in one documented case, a woman passed the infection to her male partner. Blood transfusions, too, can spread the infection; viral RNA has been found in saliva, but -as of now – oral contact risk remains theoretical.

Making things still more worrisome, only 20% of people infected with Zika show any symptoms at all. That means that 80% of infected people could be silently spreading the disease through mosquitoes or through sex. Zika has appeared in several districts in Miami and elsewhere in the United States, and the numbers are increasing. With the threat of silent spread, are we looking at the evolution of a new, sexually transmitted disease like AIDS?

Not likely. Zika isn’t going to take over the United States or much of the Western world any time soon. As Stephen Morse of the Mailman School of Public Health at Columbia University points out, the Zika virus shares the inherent limitations of all mosquito-borne infections when they run up against our infrastructure, our screens and air-conditioning: mosquitoes can’t get through. They can’t infect enough people to maintain the disease. West Nile virus, which has caused – as of November 7, 2017 – 22,738 neuroinvasive (brain-affecting) cases and 2121 deaths in the US since its introduction in 1999, is, in some ways, more of a threat than Zika because it lives in the blood of certain common birds, blue jays and crows among them. Zika doesn’t have any natural hosts besides primates and humans. So it has to rely on infected humans as its hosts, and those hosts, in the air-conditioned West, are hard to get to.

But not impossible. Local transmission has appeared already, in the Wynwood district of Miami and elsewhere. That trendy neighborhood has its rougher patches, where some houses are unscreened. Areas of the country — parts of Texas, the Gulf Coast, the Mississippi Delta — have plenty of homes with no screens or air-conditioning. As evolutionary biologist Paul W. Ewald of the University of Louisville points out, small pockets of dengue fever, spread by the same mosquito that spreads Zika, pop up in those regions periodically. We can expect to see pockets of Zika as well.

Also, the mosquitoes that spread Zika and dengue are day-biters — they can attack you while you’re working in your garden or walking to the car. But that doesn’t change the risk profile too much, says Ewald: “If you’re sick, you stay indoors, where if you’ve got screens you won’t get bitten.” That makes it harder for mosquitoes to get infected in the first place, and greatly reduces the chances of spread, as we see with dengue. It’s not known whether asymptomatic people  – who would be expected to have a lower level of viremia (bugs in the blood)  – can play a significant role in transmitting Zika virus, at least through mosquitoes.

As for Zika evolving into an efficient sexually transmitted germ, that too is unlikely. Brooks points out that all the data now available indicate that the presence of live Zika virus in semen drops off sharply after 40 days or so. Viral RNA can be detected in semen for many months, but that doesn’t mean a man could infect his partner with live virus. The CDC, cautions Brooks, advises any man infected with Zika virus to avoid unprotected sex for at least 60 days after recovery. And women, says Ewald, are likely a dead-end in most cases, because there doesn’t seem to be any evidence that the virus reproduces in vaginal tissues. The message is obvious: infected men need to use condoms, and couples living in areas with heavy transmission should likely use condoms throughout the course of a pregnancy.

For mosquito-borne transmission, the message is clear too, though it’s probably not something that people want to hear. Many people would first think of vaccination. Vaccines, however, must first be developed and shown to be safe and effective — a long, onerous, expensive process, which works for one germ at a time. And in some cases, dengue and malaria for instance, no one has ever been able to develop a safe, reliable vaccine at all. But building up infrastructure throughout the nation would reduce the threat of all mosquito-borne diseases to almost nothing. “I would like to see a federal program that is designed to make all houses in the United States mosquito-proof,” says Ewald. “Throughout the Southern states, Texas to Florida — the government should set up screens and/or air-conditioning.”

And it’s been done before: the Tennessee Valley Authority screened houses (each for less than the cost of a case of malaria) and essentially eliminated malaria from the United States before World War II. The same thing happened after the War in Italy.

What about the rest of the world? Screens on every hut and hovel? Well, yes. Dr. Andrew Haddow of the United States Army Medical Research Institute of Infectious Diseases describes essentially screening the world as the passion of his life. Haddow, whose grandfather was the co-discoverer of the Zika virus back in 1947, realized on a visit to Africa that bed nets are badly employed by many people, who toss them off as too hot or turn them into fishing nets. He points out that screens, combined with tiny solar-power units to be used to air-condition huts or supply them with fans, might go a long way to minimizing the threat of all mosquito-borne agents: malaria, yellow fever, dengue, chikungunya, Rift Valley fever, mosquito-borne encephalitis, and West Nile virus as well as Zika. Malaria alone kills millions: we are dependent now on drenching huts with DDT, passing out bed nets with all their liabilities, and treating the sick with less and less effective drugs as the malaria parasites develop resistance. Malaria vaccines consistently defeat our best efforts, and no current vaccines are available for most of the other diseases.

But screens are effective not just against one or two pathogens but all. Haddow plans to apply for a pilot project to test his “screens and air-circulation” idea in one village in Africa, to show how his proposed method would work. Screens block some air circulation, leaving tropical huts hotter than ever, but adding some air-conditioning, or even just electric fans, would go a long way toward replicating the safety of Western infrastructure.

And there’s another, evolutionary reason to try out Haddow’s method, and to switch to screens and air-conditioning as primary prevention. According to Ewald, by largely preventing seriously ill people from being bitten, you’d be reducing the virulence of the disease, thus forcing the Zika microbe to evolve. Imagine that the sickest people are lying supine on their beds, too weak to swat away foraging mosquitoes. Now think what sort of strains those deathly ill patients would likely be harboring. If bugs couldn’t bite the sickest people, and were forced to dine only on the blood of those healthy enough to be walking around outside, the strains those mosquitoes would transmit would be likely much less virulent ones. Those people harboring the most virulent disease would be out of the reach of mosquitoes: they’d be hidden away behind their screens, unbitten. So malaria and other mosquito-borne diseases would be passing around parasites from the blood of those less seriously infected. This, according to Ewald, would force the evolution of milder strains of malaria and other mosquito-borne diseases, a double benefit for the screening protocol.

Screens are passive protection — but these and other barrier methods continue to work: There have been a total of 347 cases of Zikavirus infection reported in the United States in 2017,  and of those cases, only two (one in Texas, one in Miami-Dade County, Florida) seem to have come from local mosquito-borne transmission. Three came from sexual transmission. So 342 cases appeared in people who’d traveled to areas where Zika remains prevalent. Despite climate change, despite the presence of the day-biting, Zika-hospitable Ades Aegypti mosquito in the United States, Zika virus isn’t spreading here – and we can thank our screens and air-conditioners for that.

Even better, rates throughout the Americas have plummeted:  as Jon Cohen reports in Science magazine, the 205,578 probable Zika cases in Brazil, the most severely-affected country, had dropped to 13,253 as of August 16th., 2017. Some scientists attribute this drop to the spread of asymptomatic cases throughout the region:  enough people are immune so that the virus no longer spreads effectively.

In the U.S. territories of Puerto Rico, the Virgin Islands, and American Samoa, though, there have been 583 cases so far this year – all of them attributed to local transmission. Two catastrophic hurricanes have destroyed much if not all of what infrastructure existed in Puerto Rico and the US Virgin Islands. It’s terrible to think that a lasting legacy of Hurricanes Irma and Maria may be the birth of children with tragically-small heads.

CDC map of Zika

CDC map of Zika worldwide, 2016

 

Featured Image: 3-D representation of the Zika virus courtesy of Manuel Almagro Rivas
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