In Baltimore, Maryland, people in low-income neighborhoods are at higher risk of contracting West Nile virus, a mosquito-borne disease, than people living in more affluent neighborhoods. So reports a new study published in the Journal of Medical Entomology.
Lead author Sarah Rothman, a PhD student in the Department of Environmental Science and Technology at the University of Maryland College Park, says: “Our study is the first in Baltimore to document how mosquito infections are related to West Nile virus infection to the socio-economic neighborhood where mosquito incidence is high and what diseases they carry can help focus monitoring and management programs where they are most needed. “
Mosquito-borne diseases are a growing threat in cities in the United States. Vacant lots and abandoned buildings can create environmental conditions that aid mosquitoes and the diseases they carry. Overgrown vegetation, stagnant water for hatching, and access to blood meal from rodents, cats, and birds can put nearby residents at risk of developing mosquito-borne diseases such as Zika, Chikungunya, and West Nile viruses.
This study builds on previous research that found mosquito-borne diseases to be an environmental justice issue in Baltimore. Co-author Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies, says, “Previous work has shown that lower-income neighborhoods tend to have more mosquito habitats than wealthier neighborhoods, which puts people at higher risk of who are already at risk due to their limited access to health care. We have also found that larger mosquitoes, which may have greater infection potential, thrive in less affluent areas. “
The most recent study took place over three years and focused on five neighborhoods in Baltimore that represent a socio-economic area. The focus areas included two districts with incomes below the median, two in the median and one above the median annual household income. All five districts are made up of similar blocks of terraced houses and are within 2 km of each other, which minimizes environmental fluctuations.
In 2015 and 2016, the team examined adult mosquitoes in all five districts. All but the wealthiest neighborhoods were also sampled in 2017. From June to September, the team set traps every three weeks (4-6 traps per neighborhood; a total of 26 traps) and let them lure mosquitoes for 72 hours. The traps were lured with carbon dioxide and a mammalian attractant and placed in shady, protected locations.
Mosquito species targeted included the invasive tiger mosquito (Aedes albopictus) and indigenous species of culex (mainly the northern house mosquito, Culex pipiens), which are known to thrive in cities and transmit diseases to humans. Back in the lab, mosquitoes were tested for the pathogens that cause West Nile virus, Chikungunya virus, and Zika virus.
The team discovered the West Nile virus in Ae. Albopictus and Culex were collected in all the districts examined in 2015 and 2017. The West Nile virus was not detected in any part of the city in 2016. One possible explanation could be the extreme heat recorded this summer, which can kill mosquitoes before they can spread disease. Chikungunya and Zika were never spotted. In 2015, the mosquito infection rate was highest in the two least affluent neighborhoods. In 2017, mosquito infection rates rose significantly in middle-income neighborhoods and only slightly exceeded those in the least affluent neighborhoods.
LaDeau says, “This study highlights the discrepancy in the health risks Baltimore residents face. Less affluent communities are at greater risk of being bitten by a West Nile virus and negative health outcomes are compounded by a lack of medical resources in these neighborhoods. Monitoring neighborhoods for mosquitoes is a step we can take to find out where mosquitoes breed and where residents are at risk. This information can be used as measures to mitigate these conditions and protect residents serve. “
Protecting residents begins with thorough surveillance. Since the number of mosquitoes and the infection rates vary from block to block, it is easy to overlook the sources of infection by taking rough samples. Effective management requires consistent, small-scale monitoring, even in locations that are difficult to access. “
Paul Leisnham, Senior Writer, Associate Professor, University of Maryland College Park
LaDeau summarizes: “This study will help us understand how the risk of mosquito-borne disease differs across urban landscapes and in socio-economic differences. As the rest of this work, we aim to provide city government authorities with information on factors that affect the number of Mosquitoes and increasing infections are helping to solve this growing challenge of environmental justice. “
Cary Institute of Ecosystem Studies
Rothman, SE, et al. (2020) Higher West Nile Virus Infection in Mosquitoes from Aedes albopictus (Diptera: Culicidae) and Culex (Diptera: Culicidae) from low-income areas in Urban Baltimore, MD. Journal of Medical Entomology. doi.org/10.1093/jme/tjaa262.