Why is Zika ‘exploding’ across the Americas? The Aedes aegypti mosquito

Zika is exploding because this particular mosquito is a formidable opponent - and it is currently winning the battle.

by Randall Packard, Johns Hopkins University

Published Feb. 4, 2016, 6:30 a.m. ET

The rapid spread of the Zika virus through the Americas, infecting hundreds of thousands of people in Brazil alone since May 2015, is the latest episode in a long running battle between human populations and the ubiquitous Aedes aegypti mosquito. Zika is exploding because this particular mosquito is a formidable opponent - and it is currently winning the battle.

The Aedes aegypti mosquito is a very efficient transmitter of viruses. In addition to Zika, it is responsible for transmitting yellow fever, dengue and chikungunya viruses throughout the tropics. Epidemics of these other viral diseases have also been described as "explosive," spreading with amazing speed through exposed populations. The explosive character of these viruses is in large measure a reflection of the feeding and breeding habits of the Aedes eqyptei mosquitoes that transmit them.

The Aedes genus of mosquitoes are very skittish feeders. The female Aedes requires human blood to complete its reproductive cycle and lay its eggs. Females of the Anopheles genus, which transmit malaria, bite humans for the same reason. There is a difference, however, between the feeding behavior of Anopheles and Aedes mosquitoes. Anopheles often consume their blood meal by biting only one or two humans. The Aedes mosquito, by contrast, is easily disturbed and will frequently interrupt its feeding before consuming the blood it needs. This means that it often bites several humans in the course of a single feeding. This feeding pattern significantly increases the mosquito's potential to spread viruses among human populations.It also means that it takes relatively few Aedes mosquitoes to maintain virus transmission.

The Aedes aegypti species is also a ubiquitous breeder, and it loves living in close contact with human populations. It breeds best in densely populated cities where it can find open water sources. These may be created by rainfall collected in old abandoned tires in the slums of Calcutta, by water vessels employed by people who lack running water in the favelas of Rio de Janeiro, water left in coolers in Delhi, or in flower vases kept in middle class apartments in super clean Singapore. Its utilization of a wider range of breeding sites, including something as small as a bottle cap, makes it very difficult to prevent Aedes mosquitoes from breeding.

The recent history of dengue fever suggests that, in fact, once Aedes begins transmitting a virus within a city, it is almost impossible to root out the mosquito and the disease. Even a city like Singapore, which has one of the most highly developed health systems and public health infrastructures in the world has been unable to dislodge dengue. This is scary, because we currently have no vaccine to protect human populations from Zika, and thus the only way to control the spread of the disease is by eradicating the mosquitoes that transmit it.

History contains examples of campaigns that were in fact successful in eliminating Aedes populations and in eradicating yellow fever in the Americas. At the end of the 19th century, the United States invaded Cuba during the Spanish American war. Yellow fever was endemic in Havana, and frequently broke out in port cities in the southern United States. The disease decimated the occupying American troops. Gen. William Gorgas was put in charge of controlling the disease. Applying newly developed medical knowledge about the role of the Aedes mosquito in the transmission of yellow fever, Gorgas organized sanitary teams that went about fumigating houses and eliminating breeding sites. With in a year he had succeeded in eradicating yellow fever in Havana. He subsequently applied similar methods to eliminate yellow fever in Panama where the U.S. tates was constructing a transoceanic canal. Yellow fever was eliminated in other cities in Latin America during the teens and twenties by eradication teams organized by the Rockefeller Foundation's International Health Division. (Most famously, Frederick Soper eliminated yellow fever from Rio. Later, beginning in the 1940s, Aedes aegypti mosquitoes were eliminated from most of the Americas through the application of DDT.

So why is it so difficult to eliminate Aedes mosquitoes today in cities across the globe, when it was achieved with relative ease earlier the 20^th century? There are at least three reasons:

First: The urban environments of today's major cities, where Aedes mosquitoes transmit Zika, dengue and chikungunya, are much more challenging than the those of the cities where yellow fever was eliminated in the early 20^th century. The massive buildup since World War II of slums filled with overcrowded shacks and houses lacking access to sanitation services, such as those around Rio and Delhi, have created environments in which Aedes mosquitoes find abundant breeding sites. These include the thousands of open water vessels that compensate for the absence of running water. Zika, dengue and chikungunya are not diseases of poverty, but their persistence in urban environments has been fostered by conditions of poverty.
Second: Since the 1950s, there has been a dramatic increase in the production of disposable consumer goods, such as soda bottles and cans, soap dispensers, and canned goods. When improperly disposed of, these containers can collect water and create Aedes breeding sites.
Third: Public health authorities are unable, or refuse, to mobilize the legal and financial resources needed to address the conditions that facilitate mosquito breeding. General Gorgas and the Rockefeller Foundation's Soper were successful in large measure because they were able to exercise a high degree of authority over the populations of the cities in which they operated. In some cases this authority amounted to martial law. Households that refused to maintain mosquito free-houses faced heavy fines. Thanks in part to the support of the Rockefeller Foundation, health authorities also possessed the financial resources needed to mount military-style sanitation campaigns aimed at identifying and eliminating Aedes breeding sites.

Heath authorities in cities like Rio and San Salvador, by contrast,not only lack the power to fine violators, they lack access to large parts of their cities, which are run by street gangs and thus beyond the control of municipal authorities. In cities like Delhi, urban authorities have failed to invest in the necessary public health infrastructure needed to control dengue. Control efforts are episodic, put into place when an epidemic spike in cases occurs, but not sustained between outbreaks. At the same time, they have been slow to solve the city's housing problems and eliminate its slum areas. In Singapore, health authorities do possess the legal authority to fine residents who fail to maintain mosquito-free households as well as the manpower to engage in pervasive inspection campaigns. Yet outbreaks continue to occur, in part because the same authorities have not been vigilant in monitoring and eliminating breeding conditions in informal migrant worker camps associated with construction sites throughout the city.

These same economic and political conditions have also made it impossible for urban health authorities to spray their way out of the problem, as was done in the 1950s and '60s. Fumigation efforts have failed to dislodge dengue around the globe.

So what can be done?

There are efforts afoot to deploy transgenic mosquitoes to interrupt the reproduction of Aedes mosquitoes. This may have some impact. Given the widespread prevalence of Aedes across the Americas, however, it is doubtful that this technology will eliminate the problem, even if one ignores the possible adverse environmental impacts that this strategy might have. Better surveillance and sanitation efforts, combined with increased spraying will also help.

But as long as urban environments provide ideal breeding grounds for Aedes mosquitoes, diseases like Zika, dengue, and chikungunya will continue to thrive.

Randall Packard, a professor of the history of medicine at Johns Hopkins' Bloomberg School of Public Health, the author of "The Making of a Tropical Disease: A Short History of Malaria." He is working on global history of dengue fever.