By Vital Strategies Environmental Health Division staff: Russell Dowling MPH, Thomas Matte MD MPH and Daniel Kass, MSPH
Many vector-borne diseases (VBDs), like Zika, West Nile fever and dengue fever are spreading to new places, as parallel trends of increasing travel, urbanization and climate change alter the geographic and seasonal distribution of the mosquitos, ticks, fleas, and other insect vectors that transmit them to people. This increased mobility of populations also makes it more likely for infected hosts to travel from a disease endemic region to an area where the disease is not found, thereby creating the potential for local transmission.
Urbanization creates both opportunities and challenges for the control of vector-borne diseases. Lessons from the explosive emergence of Zika virus in the Americas shows how urban public health agencies need to work across sectors and use a range of strategies aimed at people, insect vectors, the urban physical environment and the relationships among them.
The 2015–2016 epidemic of Zika virus in the Americas began in April 2015 in Brazil and spread throughout most of South and Central America and the Caribbean. Nearly 200,000 confirmed cases, and a half million suspected cases of Zika virus were confirmed over the course of the following year, and several thousand cases of a rare birth defect known as microcephaly resulted.
Fortunately, the incidence of disease has decreased significantly over the past 18 months, likely as a result of populations developing immunity and decreasing circulating virus. In 2016, the CDC reported more than five thousand symptomatic cases of Zika virus infection in US states alone. Of these, 95% were associated with travel to Zika endemic areas. These numbers do not include the presumably thousands of asymptomatic individuals who were also infected. Through October 18 of this year, there have only been 311 symptomatic cases of Zika virus infection in the US. Nearly all were travel-related, 3 cases were acquired through sexual transmission and none were locally transmitted by mosquitos. While this dramatic decrease is not solely the result of public health response, it is clear that a range of public health measures and factors in the physical environment limited local transmission in the US. To understand public health strategies used to control Zika and other VBDs, it is helpful to know how climate, social factors and the built environment can affect the vector, pathogen and human host.
Zika virus is primarily transmitted by the Aedes aegypti mosquito. Local climate, especially temperature and precipitation, affects not only mosquito survival, reproduction rate and activity but also reproduction of the Zika virus within the mosquitos. Small temperature increases can substantially increase the abundance of mosquitos and climate change has expanded the geographic range of the mosquito vectors. In the United States, the range of Aedes aegyptimosquitos is extending northward. In a given location, climate change can also lengthen the mosquito season from weather conducive to reproduction and disease transmission.
In a rapidly urbanizing world, the built environment of cities and social factors can greatly affect vector habitat and the risk of vector-borne disease transmission.
The urban heat island phenomenon, for example, can create a local climate more hospitable for mosquito vectors than in outlying, cooler rural or suburban areas. Inadequate sanitation and drainage in cities can create standing water conditions suitable for mosquito breeding. Buildings without air conditioning or screens, high population density and crowding increase the chances of mosquitos biting and transmitting infections. A recent studycomparing the incidence of dengue fever in Laredo, Texas and an adjacent city just across the Mexican border found that mosquitos that carry dengue were more abundant in Texas, but the incidence of dengue fever was eight times greater on the Mexican side, likely due to fewer air conditioners and unscreened windows and more crowding in Mexico when compared to Texas. As is often the case, poverty and social disadvantage underlay the differences in living conditions and disease risk.
This is a modern example of a historical pattern: transformation of the urban physical and social environment can either amplify or help to reduce the risk of vector borne diseases. A classic example is yellow fever, which caused severe outbreaks in US cities in the 19th century. Though a vaccine was not created until the 1930s, the last major outbreak in the United States was in 1905, as the risk abated with improvements in urban sanitation, housing and social conditions.
Controlling Zika is especially difficult, given the long lag between transmission and testing among those who develop symptoms.
The ubiquity of the mosquitoes makes it difficult to pinpoint where transmission is occurring. And, because Zika is the only arbovirus known to be transmitted sexually, even effective control measures cannot prevent all transmission. Cities have therefore utilized a range public health interventions to attempt to control the spread of Zika. They all fall into four broad categories: 1. mosquito control through a combination of eliminating breeding grounds, targeted application of larvicides and adulticides (commonly referred to as source reduction); 2. surveillance of human cases as well as of mosquito abundance and the presence of virus; 3. public education in measures for personal protection and reduction of mosquito habitat; and 4. public health response guided by data on locally and travel acquired cases. A city-wide effort needs to involve the collaboration of several arms of local government, including sanitation, environmental protection, housing maintenance, and maternal-child health service deliverers.
Preventing the breeding of mosquitoes is critical to prevent mosquito borne illness in cities. This can be done largely by eliminating standing water sources. The Baltimore and New York City Health Departments, for example, train city employees to identify and eliminate standing water sources as well as report large pools of water that are unreachable. They also encouraged local residents to call a toll-free number to report standing water so citations can be rapidly issued. A similar “tip ‘n toss” program was enacted in Savanna, Georgia, where sanitation workers were given extra shifts in high-risk neighborhoods to collect waste that were potential breeding grounds.
Many cities coupled such programs with mosquito control contractors to apply insecticides on streets and in public spaces. In New Orleans, Louisiana, the city’s Mosquito, Termite and Rodent Control Board received an additional $500,000 to target Zika carrying mosquitos with ground and air spraying. Other cities worked to install aeration systems in fountains, lakes and waterways to keep water flowing, creating an unhospitable environment for mosquito reproduction. In Laredo, Texas, thousands of minnows were placed into city waterways to eat larvae before they mature into biting adults. Laredo also instituted a city ordinance in 2015 to reduce illegal tire dumping, in which water pools. The city reported a 45% increase in used tires delivered to landfills since the ordinance was enacted.
Of equal important to source reduction is surveillance. Tracking both the prevalence of the vector and the incidence of disease are paramount to effectively controlling vector-borne diseases. Surveillance should be used to predict the risk of local transmission and inform the targeting of prevention and outreach strategies. New York City, for example, was fortunate enough to have an infrastructure in place from its ongoing West Nile Virus control efforts. The city simply expanded surveillance efforts with the creation of a three-year plan to combat the spread of Zika. The program retooled to also trap and analyze albopictus mosquitoes, adding staff positions and enhancing laboratory capacity.
Surveillance efforts in cities around the country have also incorporated a range of predictive modeling techniques to further estimate the effects of climate on vector-borne diseases. One form, statistical modeling, maps the geographic distribution of vector species in relation to climate variables. Another form, known as landscape-based modeling, utilizes satellite and weather data to more accurately predict mosquito distribution and transmission patterns. Coupled with surveillance, predictive models can inform both current response efforts and future policy options.
Because personal actions are critical to mitigate risk of vector-borne transmission, public health departments provide outreach and health education to affected communities. In Houston, Texas, an approach referred to as “3D” was used to remind the public to protect itself: drain, dress, DEET. The city used materials translated into several languages to accommodate a diverse population, and placed signs in buses, trains and airport terminals. The city also established a health alert system for physicians to keep them apprised of new and important information. In New Orleans, the city health department went door to door in high-risk neighborhoods. And in New York City, health education materials from the “Fight Back NYC” awareness campaign were translated into 16 different languages and included general information posters, a pregnancy travel warning, and information on how and where to get tested. Alternative methods such as a ‘train a trainer’ model and a program that enlisted ‘community Zika ambassadors’ have also been successful. And in the case of Zika, health departments can draw on their programs that monitor and educate on sexually transmitted diseases to assist with communication.
Response to Identified Cases
Finally, timely response to locally acquired cases is paramount to all vector-borne disease control. Potential cases should always be referred to the local department of health for testing. The city of Baltimore took case detection a step further, requiring teams to complete an area survey to look for potential breeding grounds and educate neighbors. Responses to potential cases may include rapid diagnostic testing (when available), laboratory confirmation, treatment and targeted education.
Strengthening Future Preparedness
The threat to cities from endemic vector-borne diseases such as dengue fever, chikungunya and Zika requires resources and retooling of urban public health systems. As climate changes, the challenges will only become greater. But their impact on human health can be controlled by strengthening public health systems and using a combination of well-established and novel control principles to improve urban environments and living conditions. While the recent Zika virus epidemic was met with a strong urban public health response in some US communities, others remain unprepared should a true vector-borne epidemic occur. The epidemic identified challenges and capabilities that need to be bolstered to strengthen readiness to prevent and respond to future threats, especially in low-and middle-income countries. Health systems should be strengthened, physical housing conditions improved, overcrowding addressed and water and sanitation systems upgraded and integrated. More research is needed to refine knowledge of how urban built environments can alter the reproduction and survival of vectors within the built environment.
Growing cities, particularly in low- and middle-income countries, have a real opportunity to utilize urban planning to control vector-borne diseases and minimize the risk of future epidemics.
Early and sustained efforts in public health infrastructure including cross-sectoral engagement, laboratory capacity, and a strong connection to preventive and clinical services are paramount to adequately addressing the threat of Zika and other vector-borne diseases in cities. Efforts should be supplemented with surveillance capable of early detection and a strong communications system to help people understand appropriate personal and institutional responses. The tools required to mount a suitable response have been tested and are readily available with proper leadership and investment. While the results will not be felt overnight, the benefits can be great.