The West Nile virus is the most widely distributed virus in the world and it is transmitted by mosquitoes. Exactly how it spreads is complex because it is influenced by factors including weather conditions and urban environmental settings such as storm water management ponds (SWMP).
Professor Huaiping Zhu, professor of mathematics and director of the Laboratory of Mathematical Parallel Systems at York University and Tier 1 York Research Chair in Applied Mathematics (commencing July 1, 2018), looked at the impact of these two factors on the transmission of West Nile virus.
Using mathematical modelling, Zhu led a study in conjunction with Peel Public Health and the Toronto and Region Conservation Authority (TRCA). The study, funded by the Canadian Institutes of Health Research (CIHR) and the Natural Sciences and Engineering Research Council of Canada (NSERC), discovered some ways to reduce the spread of the virus. The findings, published in Royal Society (August 2017), could inform local health units.
“We discovered that moderate temperature and precipitation will increase the mosquito population and the potential for an outbreak of West Nile virus. However, excess precipitation could reduce the mosquito population,” Zhu explains. “This information is valuable because it could identify measures to control larval abundance in these ponds and, subsequently, control the transmission of West Nile,” he adds.
Zhu is an expert in developing mathematical models, theories, methodologies and tools for the prevention and control of vector-borne diseases.
West Nile virus spread rapidly after first occurrence in 1999
The West Nile virus is transmitted when female mosquitoes are infected by feeding on the blood of birds carrying the virus, these mosquitos then transmit the virus to humans and other animals. There are no vaccines or treatments to date.
The first case was recorded in New York City in 1999. From there, the virus spread rapidly. It reached Ontario only two years later. Since 2001, human infections in this province have occurred yearly. The figure below illustrates the human infections in the Greater Toronto Area (GTA), June to October, 2002 to 2011. Midway, week 34 (July), marks the virus’ peak.
Mosquitos breed in stationary water, like ponds and swamps. SWMPs are artificial ponds designed to collect, retain and filter storm water run-off. Ontario municipalities started building them in the 1980s. Today, there are more than 1,000 in the GTA. They’re not supposed to retain stagnant water, but they do when they’re improperly designed or maintained.
The authorities are aware of this risk. In fact, TRCA has been running a mosquito larval monitoring and surveillance program in natural wetlands and SWMP on TRCA lands in the GTA since 2003. Their results showed that the mosquitoes collected from these SWMP were principally West Nile vector species. SWMP can be used to predict adult mosquito emergence and the potential for human infections.
This is where Zhu’s research began. His study sought to explore the impacts of SWMP, temperature and precipitation on West Nile vector abundance and the transmission of the virus between mosquito and bird populations. The mathematical model he developed was used to analyse how weather conditions and SWMP can influence an outbreak, to predict or control the virus with greater accuracy.
“Having a better understanding of the mechanism of an outbreak and, in turn, a more reliable evaluation of transmission risk will greatly help to control the spread of the virus and human infections,” Zhu explains.
Research team factored in weather from data collected at Pearson Airport
Zhu’s research team split the mosquito population in two stages, then considered the intraspecific competition of mosquitos in the aquatic stage. (Intraspecific competition occurs when members of the same species compete for limited resources.)
They found that (1) the abundance of pre-adults was closely related to intraspecific competition and (2) intraspecific competition was associated with standing water developed from the water in SWMP.
The researchers also factored in weather, where SWMP in conjunction with precipitation determines the water habitat for mosquito larvae. They used weather data from June to October gathered from Toronto Pearson International Airport Station, as well as weather data from Meteorological Service of Canada (EC), in particular, the Ontario Climate Data Portal (OCDP). “The OCDP, established by continuous funding support by the Ministry of the Environment and Climate Change since 2011, serves as a platform for the impact studies of climate change in Ontario,” Zhu explains.
Findings have application for local health units
The researchers discovered that moderate temperature and precipitation increases the potential for an outbreak of West Nile because these factors increase the mosquito population. Of particular interest, they found that excess precipitation could reduce mosquito population, which would lead to fewer infectious mosquitoes and birds, and fewer outbreaks.
This new knowledge can help to identify measures to control larval abundance in SWMP and the transmission of West Nile. “This work can be used to guide programs in local health units where monitoring standing water is used to control mosquito populations and the spread of West Nile virus,” says Zhu.
Future work may involve other factors such as land use, wildlife species distribution, wind patterns and elevation on the abundance of mosquitoes and the transmission of West Nile virus.
By Megan Mueller, manager, research communications, Office of the Vice-President Research & Innovation, York University, email@example.com