The Council on Foreign Relations has posted a useful summary for environmental health professionals on recent Zika virus developments. This article, authored by Laurie Garrett, Senior Fellow for Global Health, is reposted with permission.
In 2014, Brazilian health officials reported 147 cases of babies born with microcephaly, a congenital deformation of the skull that compresses the infants’ brains. In 2015, some 3,000 microcephaly cases were reported in the country, along with 500 other infections; about a quarter of the 4,000 were detected in the month of December. That represents a twenty-fold year-to-year increase in an extreme birth defect; the cause should be identified, preventing it from claiming more babies.
Though Koch’s Postulate has not been put to the task, the correlation between the 2015 explosion in microcephaly cases in Brazil and nineteen other countries in the Americas and the spread of the Zika virus is so strong that most public health leaders accept the hypothesis that maternal infection with the mosquito-carried virus during pregnancy causes the skull deformations. Zika virus RNA has been found in the amniotic fluids of mothers delivering microcephaly babies, and autopsies of two babies that died of the ailment in the United States discovered Zika viruses in their brains.
This X-ray picture from Colombia compares the skulls of a normal baby to "Patient 1" born to a mother who was infected with Zika Virus during pregnancy.
Though several microcephaly cases associated with Zika have appeared this year in the United States, none of them were acquired in the country and no infected mosquitoes have been found in the fifty states—all of the mothers had traveled during their pregnancies to Zika-hit countries. But the infected insects lurk nearby, as locally acquired Zika cases have emerged in Puerto Rico and Mexico. As of January 24, the United States and Europe have identified a mounting toll of Zika-infected babies—in all cases, the mothers traveled in Zika-infected countries during their pregnancies. The United States has identified several new cases; the United Kingdom, three; Spain, two; and Portugal, four.
The U.S. CDC will shortly announce that suspected Zika infections are "mandatory reportable" diseases, thereby compelling all of the nation's health agencies to monitor and compile information. Until that goes into effect, the best reckoning, according to CDC officials, is that "close to two dozen" cases of travel-related Zika have appeared in the United States, including babies with microcephaly. There is little immediate concern that the virus will spread across the United States because mosquitoes are dormant during winter, and this year's winter weather has been harsh even in the southernmost parts of the continental United States. However, as temperatures rise and spring rains create potential breeding sites, there is genuine cause for concern. This CDC map demonstrates why the agency is fretting about Zika (and Chikungunya and dengue)—both Aedes mosquito species are in the country, especially across southern states.
Other than the spread from mother to fetus in utero, there is no evidence of person-to-person transmission of the Zika virus; to be infected, a person must have been bitten by a feeding mosquito, or hypothetically, receive a contaminated blood transfusion. Once infected, the individual’s immune system determines whether the illness is so mild as to go unnoticed, or more severe.
There is no vaccine or treatment for Zika and the skull malformations in infected babies cannot be reversed. Nevertheless, most Zika fever cases are mild enough to be confused with the flu or a bad cold and rarely merit hospitalization. In countries such as Brazil, where the other rapidly spreading mosquito-carried viruses—dengue and chikungunya—are found, the flu-like symptoms are so similar that blood tests are needed to distinguish the causes. All of these pathogens cause headaches, fevers, joint aches, malaise, weakness, and nausea in milder cases. In very rare cases, Zika can cause the neurological disorder known as Guillain-Barré syndrome, which can be permanently debilitating. (This is, however, so rare that of the 270,000 Zika cases in the 2013 French Polynesia outbreak, just seventy-three developed the neurological syndrome, or 0.0003 percent.) In addition to rising microcephaly rates in Brazil, Colombia, and at least a dozen other countries in the hemisphere, there are new reports of rising numbers of Guillain-Barré syndrome cases. Both Zika and chikungunya can cause the syndrome. French Polynesia is still struggling with Zika after nearly three years, and recently reported an increase in brain defects in newborn babies.
In 1947, disease hunters from the Rockefeller Foundation were in Uganda, studying yellow fever using rhesus macaque monkeys. The animals were placed in cages that were perched in trees in the Zika Forest (meaning "overgrown" in the Luganda language). On April 18, 1947, one of the Asian Rhesus monkeys took ill with a previously unknown virus, which the researchers dubbed Zika. In a lab in Entebbe, the Rockefeller scientists injected the new virus into mice, which then developed the feverish illness, thereby proving the virus they had discovered was the cause of the disease. The first human case of Zika fever was identified in Nigeria in 1954.
In Africa, the virus is carried by the Aedes africanus mosquito, which is classically found along the continent’s equatorial belt stretching from Cape Verde to Kenya. Few human beings ever acquired Zika because the africanus mosquitoes mostly fed on wild monkeys and chimpanzees. David Morens and Anthony Fauci (who heads the U.S. National Institutes of Allergy and Infectious Diseases) argue that changes in lifestyle in Africa in the later twentieth century afforded the virus the opportunity to infect another, more globally common mosquito, Aedes aegypti. As villagers built rainwater-catching devices and uncovered containers full of water became common across Africa, the aegypti mosquitoes placed their larvae in the fresh, clean still water. The aegyptipopulations swelled all over the continent, expanding into Aedes africanus territory and likely feeding on primates that had been infected with Zika by africanus.
In 1978, a handful of Zika cases were identified in Indonesia, but the disease seemed to disappear without any public health intervention. The first serious outbreak of Zika outside of Africa occurred in April 2007 on Yap Island, part of Micronesia, eventually spreading to neighboring islands of the nation. Frustrated scientists from the U.S. Centers for Disease Control (CDC) discovered dengue blood tests falsely tested positive for Zika. It remains difficult to discriminate between them with rapid commercial tests, and clear diagnosis requires use of genetic RNA tests (polymerase chain reaction).
Since the Yap outbreak, Zika has emerged eastward across the Pacific in the Cook Islands, Tahiti, and, last year, the Easter Islands off Chile. Until 2015, Zika had never been seen in the American hemisphere.
For unknown biological reasons, Zika appears to follow the emergence of chikungunya, which in turn follows new spread of dengue.
In the unfortunate case of the Americas in 2015–2016, a perfect storm of biological events has unfolded—any one of which by itself would have been relatively harmless. Introduction of Asian and African mosquitoes, spread of yellow fever, then dengue, followed by chikungunya and then Zika; the El Niño climate event; and a Brazilian crisis in both economics and politics. Combined, these have fueled explosive spread of the disease to twenty nations, twenty of which have severe enough outbreaks to warrant public health emergencies and travel advisories from the U.S. CDC.
Morens and Fauci warn that another, highly aggressive mosquito, Aedes albopictus, a.k.a. "Tiger Mosquito," is found all over the Americas today. Were it to take up the Zika virus, as aegypt did, the spread of the disease could reach far greater proportions.
Because there is no human-to-human transmission of the virus outside of the womb, the smart place to focus public health attention is on those Aedesmosquitoes, none of which originated in the Americas. As these insect species have appeared in the hemisphere, public health entomologists have warned of their tremendous potential to spread life-threatening disease. The warning, which typically included calls for resources to kill off the mosquitoes, have largely gone unheeded by politicians that control purse strings across the region. In the United States, politicians have faced another non-fiscal threat: the NIMBYs who fought all efforts to use chemicals to kill insect larvae and spray adult mosquitoes.
The aegypti mosquito was first brought to the Americas in 1648, a hitchhiker aboard slave ships from Africa. Across the hemisphere, the mosquito spread during the eighteenth century, spawning huge yellow fever epidemics. In 1793, one out of every ten inhabitants of Philadelphia—then the capital of the United States—died of yellow fever. In the late 1890s, Cuba’s William Gorgas spotted the association between yellow fever and the aegypti mosquitoes, executing the first campaign to drain swamps and kill off the insects. His effort proved so successful that it was mimicked by the U.S. Army in 1904 in Panama during the construction of the canal.
In 1789, American physician Benjamin Rush described another seemingly new disease, dubbed break bone fever. Like yellow fever, break bone was chiefly seen during summer months, could kill nearly a third of those that took ill, and could not be cured. Today, break bone is called dengue, which is caused by four different subtypes of viruses. In its most severe form, typically resulting from coinfection with more than one subtype or sequential infections with different forms of dengue, the virus causes an Ebola-like hemorrhagic disease that is most often fatal. In 1997, the CDC’s Duane Gubler showed that dengue became a global killer during World War II, when soldiers hunkered across the Pacific theater were exposed to multiple subtypes, carrying the viruses in their bodies from one battle site to the next where they were absorbed by biting mosquitoes.
Widespread aegypti eradication campaigns in Asia and Latin America after World War II dramatically reduced dengue and yellow fever, as did rising use of window screens and elimination of open water storage. But in 1997, the world experienced the largest El Niño event in history (until the current one), bringing record rainfalls across Southeast Asia, Singapore, and Malaysia. With the rain came more breeding grounds for aegypti in the form of puddles and pools, and a major regional dengue epidemic ensued. Complicating the control efforts was albopictus, the tiger mosquito, which had become another vector for dengue.
Tiger mosquitoes are larger, tougher insects that travel greater distances in their feeding compared to aegypti and can bite more people in a given day, thereby taking up more viruses. As a result, viruses evolve faster in albopictus, mutate, and become more virulent, as happened with dengue.
That year, another African virus appeared in the Americas for the first time, West Nile virus, also spread by the Aedes aegypti mosquitoes. First seen in New York City during the summer of 1997, West Nile virus is now endemic across the continental United States and parts of Canada. Fortunately, the disease is rarely a severe ailment in human beings, but its nearly two decades of endemicity in the United States illustrates the gaps in the country’s mosquito abatement programs, and vulnerability to dengue, chikungunya, and now Zika.
Chikungunya exploded in 2006 across southern India and nations along the Indian Ocean from Madagascar to Sri Lanka. It was carried by both albopictus and aegypti, causing tens of thousands of cases of the self-limiting, flu-like ailment. Fortunately, chikungunya is rarely, if ever, fatal. But India suffered nearly 1.5 million cases of the disease in 2006, with an attack rate of 45 percent, affecting the economy due to medical costs and lost worker productivity. Like Zika, chikungunya is frequently misdiagnosed in areas that also have dengue. From 2007 to 2009, the virus spread across Asia and the Indian Ocean, reaching Singapore in 2008 and then Malaysia and Thailand. By 2010, the virus was endemic across southern Asia, carried primarily by the tiger mosquito.
Thanks to globalized trade, the tiger mosquito spread across the Pacific and reached Brazil via recycled tires in 1986. Worn-down tires from Malaysia, Thailand, and Singapore were loaded into ships, often during heavy rains. Water pooled inside the tires, where tiger mosquitoes laid their eggs. The larvae emerged as flying insects on arrival in Rio de Janeiro, Brazil, where the tires were off-loaded for rubber reprocessing. The albopictus thrived and spread, soon dominating mosquito niches from southern Argentina all the way to Central America and the Caribbean. During the 1997 El Niño climate event, the tiger mosquito took hold in Cuba, spawning an epidemic that sickened three thousand people, causing hemorrhagic fever in two hundred of them and killing twelve.
In late 2015, the record-breaking El Niño was wreaking havoc across South America, forcing evacuation of more than 150,000 people in Paraguay due to torrential downpours and flooding and causing a drought in northern Brazil. Much of Brazil has been hit by heavy rains, however, causing significant agricultural losses and prompting President Dilma Rousseff to declare emergencies after Christmas in the country’s southernmost states. The rains left more puddles, pools, and ponds for mosquito breeding, allowing both Aedes species to surge across the region. And all three diseases—dengue, chikungunya and Zika—are now prevalent. In the first quarter of 2015, Brazil suffered 460,000 cases of dengue,120,000 more than in the same period of 2014.
A new climate model predicts the aggressive tiger mosquito will greatly benefit from changes in rainfall and heat patterns: "From the climate projections for 2050, and adopting a habitat suitability index larger than 70 percent, we estimate that approximately 2.4 billion individuals in a land area of nearly 20 million square kilometers will potentially be exposed to Aedes albopictus." Moreover, albopictus will be more likely to spread disease and have a distinct competitive advantage over other mosquito species as it exploits new habitats.
Albopictus has taken hold on Hawaii’s Big Island, where it spawned a 2015 dengue outbreak that continues at this time, with more than 220 cases reported as of January 21. The tiger mosquito also emerged in Southern California during the 2015 summer, brought from Asia in U.S. Navy ships to San Diego, and has subsequently spread overmuch of the state.
In the absence of vaccines or treatments for the three diseases, public health officials are taking drastic steps to slow the epidemics. The CDC this week issued a warning to American women of child-bearing age, suggesting they not travel to countries that have Zika. The Brazilian government has warned women to avoid getting pregnant until the outbreak is controlled.
Perhaps the most extreme government action comes from El Salvador, which strongly advises women to delay pregnancy until 2018 due to Zika risks. Since 1988, it has been illegal to obtain an abortion in El Salvador, even in cases of rape, incest and danger to the mother’s life, prompting women’s health advocates to insist the law be revoked in cases of Zika infection. Under El Salvador’s laws, women are imprisoned for two to eight years if they undergo an abortion, all pregnancies that terminate before the fortieth week are investigated for possible murder prosecution, and health professionals that perform abortions face eight to twelve years of imprisonment.
For Brazil, the Zika crisis comes as the country is spiraling into economic and political disorder, the president faces threatened impeachment, and the nation hopes to impress the world as host of the 2016 Olympics in Rio. Desperate times call for desperate measures: the government is now considering release of genetically modifiedaegypti mosquitoes that die before reaching adult stage.
It is summertime in Brazil, with Carnivale season not far off. Perhaps the country’s greatest hope is that the chill of winter in June will drive the mosquitoes into dormancy during the July Olympics. The Brazilian government is concerned that the billions of dollars’ worth of tourism expected for the Rio2016 Olympics will be erased by global Zika panic. Some three million travel agencies worldwide have received letters from Rio, promising that there would be no Zika threat during the Games, thanks to aggressive mosquito control efforts. And the Ministry of Sports issued a memo that noted the Olympics commence on August 5th, in the middle of Brazil’s winter when the mosquitoes are rarely seen, and "only a minor incidence of cases of dengue" are found nationwide.
The final word goes to Morens and Fauci:
"In our human-dominated world, urban crowding, constant international travel, and other human behaviors combined with human-caused microperturbations in ecologic balance can cause innumerable slumbering infectious agents to emerge unexpectedly. In response, we clearly need to up our game with broad and integrated research that expands understanding of the complex ecosystems in which agents of future pandemics are aggressively evolving."
As always, I will strive to keep you informed of major issues in global health.
Senior Fellow for Global Health
Council on Foreign Relations