The Zika (ZIKV) pandemic and its widespread implications have spurred increased activity on several fronts. In addition to countless hours of laboratory research being devoted to better understanding the virus and how to mitigate its effects, mosquito control professionals are likewise evaluating ways to prepare and bolster their current container mosquito programs.
Biostatisticians, too, have been hard at work. Two recent studies provide interesting insights into the breadth of the Zika challenge. “Mapping Global Environmental Suitability for Zika Virus,” (Messina et. al, 2016), addresses Zika risk on a global scale. “On the Seasonal Occurrence and Abundance of the Zika Virus Vector Mosquito Aedes Aegypti in the Contiguous United States,” (Monaghan et. al, 2016), layers in multiple variables to gauge Zika risk across the U.S.
“THE MESSINA STUDY CONCLUDED THAT A STAGGERING 2.17 BILLION PEOPLE WORLDWIDE ARE AT RISK FOR ZIKA TRANSMISSION – A NUMBER THAT CONSTITUTES MORE THAN 29% OF THE WORLD’S POPULATION.”
Zika (ZIKV) virus is vectored by container mosquitoes, most notably Aedes aegypti and, to a lesser extent, Aedes albopictus. In calculating global risk, the Messina study identified regions exhibiting a combination of environmental conditions known to be conducive to Aedes proliferation, using distribution models of dengue virus as an indicator. Vectored by the same species of mosquito, the spread of dengue provides researchers with historical data by which future Aedes-borne diseases can be predicted.
Of the conditions indicated, the study models showed cumulative precipitation to be dominant followed by temperature suitability for both Ae. aegypti and Ae. albopictus species. Other factors included relative population density (urban extent), Enhanced Vegetation Index (EVI) values, and minimum relative humidity. EVI goes beyond measuring the presence of vegetation to express differences in canopy type and structure which are key indicators of suitable habitats for container mosquito breeding.
The Messina study concluded that a staggering 2.17 billion people worldwide are at risk for Zika transmission – a number that constitutes more than 29% of the world’s population.
Not surprisingly, the study identified high levels of Zika risk in tropical and subtropical zones. While the recent emergence of the Zika in the Americas has garnered the most attention, the study found that Asia has the most people (1.42 billion) living in transmission-suitable areas, a large portion residing in India where more than 2 million square kilometers are suitable for transmission (see Table 1). The virus also poses risk to about 453 million Africans – the largest portion living in Nigeria. In the Americas, the study approximates that 298 million people are at risk, with about 40% of those residing in Brazil. The study goes on to estimate that 5.42 million births occurred in the Americas in 2015 within the areas and times suitable for Zika transmission.
The Messina study points out that not everyone will be exposed to Zika virus and that even in the most favorable environments, not everyone will be infected. Science dictates that increasing herd immunity to the virus will reduce the percentage of the population susceptible to Zika over time. The study also makes a strong statement about what this pandemic portends. In the early stages of their epidemiology, flaviviruses such as Zika are often viewed as being only minor contributors to a much larger pool of vector-driven mortality and disability, but the rapid spread and increasing virulence of Zika sheds light on our need to reassess how we consider these “minor” diseases.
MAPPING RISK IN THE U.S.
The Monoghan publication maps transmission risk by going deeper on a regional level, focusing specifically on the contiguous United States, particularly 50 major metropolitan areas (see Table 2). This study points out that multiple factors determine where outbreaks occur, thus layering in variables beyond environmental suitability that contribute to risk, namely travel and socioeconomic infrastructure.
As in the Messina study, the Monoghan study uses environmental suitability for Aedes species as a primary basis for Zika risk mapping in the U.S. Life cycle models were used to simulate life stages of the mosquito as a function of precipitation, humidity, temperature, and user-supplied container habitat information. For each of the areas in question, the variables combined to provide an Aedes potential abundance value of using warm season (May – October) conditions in Miami, FL as a baseline.
From a meteorological standpoint, only Southern Florida and South Texas displayed conditions suitable for Aedes aegypti during the winter months (December through March). Conversely, conditions were suitable for Ae. aegypti across all 50 cities during the summer months (July through September).
Next, the role of travel in disease transmission was figured in. As of today, no locally acquired cases of Zika have been reported in the U.S. – all known cases have been the result of either travel or sexual transmission. To understand the risk of Zika introduction from infected travelers, researchers focused on air travelers coming to the U.S. from CDC Zika Travel Advisory-listed countries – particularly to those large urban centers which serve as travel hubs. Also included in those calculations were the numbers of people crossing the border by land from Mexico – a number nearly five times greater than all air travelers arriving in the 50 focus cities combined.
In this case, July and August emerged as the periods of highest incoming travel – months that coincide with the most conducive meteorological conditions for Aedes (see Figure 1).
Lastly, the Monoghan study incorporated the likelihood of socioeconomic factors in Aedes proliferation. Aedes species thrive in urban environments, hence there are effects of poverty in urban centers that are believed to elevate mosquito exposure. Such indicators include reduced usage of air conditioning or other cooling options, disrepair in housing such as missing or damaged window screens, and decreased sanitation levels in general. The need for on-property storage of fresh water in these areas is also likely to be a contributing factor. From a high poverty rate perspective, the counties in South Texas that include Laredo and Brownsville, as well as Yuma, AZ and Miami, FL aligned with areas of prolonged, favorable meteorological conditions.
Although the study provides robust predictions, it cannot be assumed that the factors described in the publication could be the only factors that determine Aedes aegypti colonization. Aedes aegypti has been in USA for over a century but their distribution is patchy and restricted to warmer southern regions of USA. Alternatively, Aedes aegypti was mostly absent in California but in the past two years established populations have been discovered. It follows that changing environmental conditions could pave the way for more incidence of establishment in other regions.
While approaching Zika risk from two different perspectives, the implications of these studies are clear: The environmental factors contributing to Zika transmission are favorable for continued spread of the disease. As future mosquito control efforts are planned by public and private interests in the Americas and beyond, strict attention must be paid to the unique demands for controlling this new public health threat.
Mapping global environmental suitability for Zika virus; Messina, J.P.
On the Seasonal Occurrence and Abundance of the Zika Virus Vector Mosquito Aedes Aegypti in the Contiguous United States. PLOS Currents Outbreaks. 2016 Mar 16 . Edition 1; Monaghan, A.J