Ryan C. Smith
The Culex tarsalis mosquito is the primary vector of West Nile virus infection in Iowa, and the highest rates of transmission were found in the western region of the state, researchers reported in Nature.
“This study examines West Nile virus [WNV] transmission for the state of Iowa, defining the peak periods of human disease transmission, as well as where the majority of these cases occur,” Ryan C. Smith, PhD, assistant professor in the department of entomology at Iowa State University, told Infectious Disease News. “We also implicate the mosquito Culex tarsalis as being most likely responsible for human cases of WNV.”
WNV has caused approximately 50,000 infections and over 2,100 deaths in the United States since 1999, when the virus was introduced into the country. The transmission of WNV is complex, requiring a relationship between bird reservoirs and mosquito vectors, according the study.
From 2002 to 2016, a mosquito surveillance program led by the Iowa State University Medical Entomology Laboratory collected infection data from sentinel chickens, mosquito vectors and human WNV cases. Using a network of mosquito traps, the vector population is monitored annually. The traps are emptied regularly, the mosquitos are catalogued and some are tested for the presence of disease.
Smith and colleagues used these data to better understand the factors that drive transmission. Over the 15-year the study period, the researchers identified 498 human cases in the state. However, they noted that this number is probably a “dramatic underestimate” because many WNV cases are mild or asymptomatic and go unreported.
Larger populations of Culex tarsalis mosquitoes were found in Iowa’s western counties, putting the people who live there, especially along the Missouri River, at risk for WNV infections. Furthermore, the infections peak in early September, and C. tarsalis mosquitos have been found to feed more often on humans compared with other Culex species. Although Smith said there is still a risk for transmission from other Culex species in other regions of Iowa, the feeding pattern of Culex tarsalis implicates it as the primary vector of WNV.
Therefore, he recommends using DEET repellent when going outdoors for extended periods of time during summer months and to stay indoors during dusk and dawn — the prime feeding time for Culex mosquitos.
“These findings have significant public health relevance by defining where and when human WNV cases occur in the state of Iowa,” Smith said. “Further research is needed to better understand the year-to-year variation in cases of WNV and how environmental factors — such as rainfall and temperature — may influence this.”
Disclosures: The authors report no relevant financial disclosures.