By Josh Lancette
In the past few decades, outbreaks of multiple mosquito-borne viruses have been popping up throughout North and South America. Some of these viruses are well known to the public, such as Zika, chikungunya, and West Nile, while others such as Eastern equine encephalitis and Mayaro haven’t quite broached mainstream news cycles. Furthermore, other viruses that have been in the Americas a long time continue to cause problems, such as dengue and yellow fever.
But why have there been so many outbreaks, specifically of diseases new to the Americas?
While the reasons likely are myriad, a team of researchers from Brazil and Argentina propose several ideas in a paper recently published in the Journal of Medical Entomology.
In the article, the authors point to four main factors influencing the spread of mosquito-borne viruses.
- A diverse mosquito and host fauna. According to mosquitocatalog.org, 862 mosquito species can be found in South America, 462 in continental Central America, and 192 in North America. Furthermore, the continents host thousands of species of birds and mammals, the primary hosts for mosquito-borne viruses. While not all of the mosquito species are vectors of diseases, such a large population offers the viruses ample opportunity to adapt to new vectors and infect new hosts.
- Global warming. Mosquitoes require external heat sources to regulate their temperature, so they are very dependent on climate to function. While the ecology of viruses and their vectors is complex, a warming climate could be one factor aiding vectors’ range expansion.
- Increased international travel. Mosquitoes and viruses have taken advantage of global travel for centuries. For example, the slave trade may have introduced yellow fever and dengue to the Americas. Later, the Asian tiger mosquito, Aedes albopictus, which is a vector of multiple viruses, arrived in the U.S. in ships returning from the Pacific theater of World War II. More recently, Zika is believed to have arrived in Brazil through an international canoeing competition or the soccer World Cup.
- Urbanization. In the past 50 years, the world’s population has grown by more than four billion people, and now more than half of those people live in urban areas. However, inadequate infrastructure in urban areas in developing or underdeveloped countries can create prime breeding sites for mosquitoes (such as in rainwater collection containers in areas without a reliable water supply) and increase human contact with mosquitoes (such as in houses without air conditioning and with screenless windows).
These four reasons for the increase in mosquito-borne disease outbreaks in the Americas are broad, and the factors that influence each one are complex. Due to the complexities, preventing further dispersal of vectors and the diseases they spread isn’t an easy task. Attempts at control require broad integration of multiple control techniques and can differ greatly by species.
Some of the techniques the authors list that can be employed include:
- using insecticides
- ensuring adequate vaccinations
- infecting male mosquitoes (particularly Aedes aegypti) with Wolbachia, a naturally occurring bacteria that prevents the mosquito from being able to reproduce
- releasing male mosquitoes that have been genetically modified to be sterile or to produce nonviable descendants.
- improving water supplies
- preventing and removing inadequate water storage vessels
- elimination of potential larval habitats
- releasing larvicide dust particles into oviposition sites, which kill the larvae and get carried by female mosquitoes to other sites
- using repellents
- wearing protective clothing
- putting screens on windows
- using bed nets
- improvement of sewage and wastewater systems
- educating the public on prevention and control options
Unfortunately, there are no easy answers for why outbreaks of mosquito-borne diseases new to the Americas continue to occur, and there are no easy answers on how to prevent or control them.
Because there aren’t easy answers, the authors argue that vigilance for diseases and their vectors is necessary for everyone, from mosquito control professionals and doctors to people living in or traveling to areas where diseases are present. Furthermore, control efforts should not be abandoned or neglected after a disease is thought to be controlled but rather should be continued to prevent future outbreaks.
Journal of Medical Entomology
Josh Lancette is manager of publications at the Entomological Society of America.