Mosquitos are responsible for over 1 million deaths each year due to the diseases they transmit.
A major problem with the current Zika outbreak is that there is, at present, no vaccine for the virus. Researchers worldwide are stepping up efforts toward a vaccine, but such research invariably requires a lot of time and money. For now then, the focus on tackling the disease turns elsewhere.
Following an Emergency Committee meeting, the World Health Organization (WHO) Director-General, Dr. Margaret Chan stated that the most important protective measures to be taken were "the control of mosquito populations and the prevention of mosquito bites in at-risk individuals, especially pregnant women."
The mosquitos that are behind the transmission of the Zika virus are those belonging to the Aedes species, namely Aedes aegypti and Aedes albopictus. These mosquitos spread the virus by feeding on people already infected with Zika, becoming infected themselves and then passing the virus on when feeding on another human.
In this spotlight, we take a look at why mosquito-borne diseases such as those spread by the Aedes species have risen to prominence, as well as examine methods that are being suggested for halting the spread of these worrying viruses.
Mosquito-borne diseases have spread over past 20 years
Last week, Dr. Tony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that "we need to look at Zika virus in its context as the latest in a series of mosquito-borne diseases that expanded their reach in the past 20 years or so. These include, as you heard, dengue, West Nile virus just last year. There will be others."
In a world full of dangerous creatures such as venomous snakes and powerful crocodiles, it is the mosquito that is deadliest to humans, with over 1 million people dying as a result of mosquito-borne diseases such as malaria each year.
Another prominent mosquito-borne virus is dengue fever, a severe condition that has increased in incidence by 30 times over the past 50 years. WHO estimate up to 50-100 million infections occur each year, with a case fatality rate that can be as high as 10%.
Medical News Today spoke to Devika Sirohi, a graduate research assistant in molecular virology at Purdue University in West Lafayette, IN, about mosquitos and their role in the spread of disease. She offered a number of possible explanations that may be behind the growing prevalence of mosquito-borne disease.
"There's just so many factors," she explained, "like climate change, increased travel and increased urbanization."
- Climate change: "A lot of these mosquitos prefer warmer climate, so as the climate becomes warmer, they will become prevalent so to speak," said Sirohi. Richard Duhrkopf, an expert on mosquitos and associate professor of biology at Baylor University in Waco, TX, believes that "as the weather warms up and there is a greater flow of the virus into the [US], I am confident we will see transmission this summer."
- Increased travel: According to WHO, Aedes aegypti mosquitos will spend their lifetime in and around the area that they emerge as adults, with studies indicating that they usually fly an average of 400 meters in their lifetimes. As a result, it is the movement of humans rather than mosquitos that is behind the spread of the virus. "As people travel, these mosquito-borne viruses travel with them," explained Sirohi, and upon returning home mosquitos endemic to their region can pick up with viruses and begin transmitting them.
- Increased urbanization: Densely populated cities, such as those in Brazil where the Zika virus has spread, provide ideal conditions for virus transmission. "If people are very close together as they are in metropolitan areas, they are more likely to encounter an infected mosquito," Sirohi said. Buildings also protect mosquitos from the weather, increasing their longevity.
It is this latter point that has Dr. Anne Schuchat, the principal deputy director of the Centers for Disease Control and Prevention (CDC), confident that the US will not experience a Zika outbreak on quite the same scale as elsewhere in the Americas, despite being home to Aedes mosquitos:
"We do think the living conditions in general in the United States, the lack of urban density in those areas where the mosquitos are circulating and the air-conditions and screens will hopefully keep us in better shape compared with what's beginning on in some of the hot spots in South America or the Caribbean."
What is being done about Zika in Brazil?
As mentioned before, there is no vaccine available for Zika at present. Another complicating factor is that the majority of people, around 75-80%, who become infected with the virus are asymptomatic. This fact presents difficulties for health authorities wishing to keep track of the virus' spread.
Densely populated urban environments like these slums in Rio de Janeiro enable the rapid transmission of mosquito-borne viruses such as Zika.
Mosquitos are the most visible target, and the Brazilian government has made eradicating the threat that the Aedes mosquitos pose a priority.
Brazilian health authorities have estimated that up to 1.5 million people have been infected with Zika virus in the country. In response, a national mobilization day will be held on Saturday, with soldiers and state employees being sent out into homes and workplaces searching for potential breeding grounds for mosquitos.
"I will insist, since science has not yet developed a vaccine against the Zika virus, that the only efficient method we have to prevent this illness is the vigorous battle against the mosquito," stated President Dilma Rousseff in a recently televised address to the nation.
An emergency decree signed by President Rousseff has also made it compulsory for health workers to be granted access to homes and properties in order to inspect for still water deposits.
Could DDT be used to fight Zika?
The mosquito problem has led to renewed calls for the use of DDT (dichloro-diphenyl-trichloroethane). DDT is a pesticide that was first introduced in the 1940s to protect populations from insect-borne diseases such as malaria and typhus but has since been banned in many countries due its effects on the environment and its toxic properties.
DDT was originally used to prevent insect-borne disease but has since been banned in many countries.
The chemical is currently classified as a "probable human carcinogen" by many international authorities, but the gravity of the Zika situation is causing some experts to raise DDT as an option.
"That concern about DDT has to be reconsidered in the public health context," Dr. Lyle R. Petersen, director of the Division of Vector-Borne Diseases at the CDC, told The New York Times.
Dr. Petersen's argument is that previous concerns with DDT arose through using the pesticide widely outdoors within agricultural practice, whereas small-scale focused use of DDT inside homes to kill mosquitos would not prove to be anywhere near as problematic.
This tactic is used in Africa in areas where disease-carrying mosquitos feed on people while they are asleep at nighttime. Conversely, the Aedes mosquitos typically feed during the daytime. According to Lynn Goldman, dean of the George Washington University School of Public Health, more research is needed to indicate whether DDT would be an effective solution.
"You have to be using the right weapons for the right mosquito," Goldman told TIME. "Firing at any mosquito we see is not going to be useful."
Are mosquitos both the problem and the solution to the Zika outbreak?
An alternative strategy that Dr. Schuchat of the CDC described as "intriguing" and "a really exciting idea" involves utilizing mosquitos themselves to put an end to transmission. The mosquitos in question are being developed by a company called Oxitec, and their research involves genetically engineering male mosquitos so that their offspring will die before reaching sexual maturity.
Scientists have genetically engineered a strain of Aedes mosquito that dies before reaching sexual maturity.
The company states that this method has an advantage over the use of conventional pesticides in that it only targets a specific species, increasing its efficacy while reducing the potential for harming the environment.
Oxitec have conducted field trials of their genetically modified Aedes mosquitos in Brazil to reduce the spread of dengue fever. In 2013, the company reported that a trial funded by Brazilian government had achieved a 96% reduction of the wild mosquito population in Mandacaru, a village near the city of Juazeiro, after 6 months. The researchers estimate that around 3,000 people were protected from the dengue mosquito during this time.
There is also another technique involving mosquitos that is in development. Devika Sirohi, who along with her colleagues is researching how antibodies bind to Zika virus using structural approaches, explained to MNT how a bacteria called Wolbachia could be used "like a parasite for mosquitos."
Wolbachia is naturally present in many insect species but not the Aedes mosquitos. Research suggests that the presence of this bacteria in Aedes mosquitos can stop them from transmitting the dengue virus, although at present researchers are unsure precisely what causes this to happen.
A drawback with using Wolbachia, however, is the possibility exists that mosquitos may develop resistance to this bacteria and that in time it may not be able to block viral transmission.
It is likely that we will hear more of the GM mosquito strategy in the coming months; Oxitec announced last month that they would be setting up a new mosquito production facility in Piracicaba, Brazil, declaring that it would "have capacity to protect over 300,000 people."
How can I protect myself from mosquitos?
While researchers and health authorities debate and develop new ways to tackle mosquito-borne infection, the general public can take matters into their own hands by following a number of precautionary steps to avoid mosquitos.
Richard Duhrkopf suggests the following measures to reduce the risk of mosquito bites:
- Remove temporary standing water from your home
- Keep artificial containers clean
- Dry up natural containers of water
- Avoid going outside at dusk and dawn
- Maintain upkeep of your home
- Do not wear clothes that expose your skin
- Utilize insect repellents when outside.
There have been reported cases of non-mosquito transmission - through blood transfusion, sex and from mother to baby during pregnancy - though these suspected methods of transmission are either rare or require more research to be understood fully.
For now, transmission from mosquitos is by far and away the most common method of transmission and the one that most is known about. As a result, it is common sense that the focus should be on Aedes mosquitos, at least in the short term when so much remains unknown about the Zika crisis.