A new study published in the journal PLOS ONE has found a link between West Nile virus prevalence and land use, climate and wildlife populations in the northwest United States.
The authors collected samples of mosquitoes to determine West Nile virus (WNV) prevalence in Washington, Oregon and Idaho, which were then compared with land use type (vegetable/forage crops, orchards and natural); climate (temperature and precipitation); and the distribution of vector and host species, especially birds and mosquitoes.
Their results show that there are more mosquitoes, and more mosquitoes carrying WNV, in orchard and vineyard habitats. These habitats also appear to have higher populations of house sparrows and American robins, two common reservoirs for WNV.
While there appeared to be no correlation between temperature and WNV prevalence, the study did find that areas with less precipitation had higher virus prevalence in human, horse, and bird populations.
WNV killed 243 people in the United States last year. The virus, which is spread to humans through mosquitoes that bite infected birds, causes symptoms that can include high fever, coma, and paralysis in the most serious cases, and headaches, nausea and rash in less serious cases. People who are over age fifty are at greater risk of infection.
The disease first appeared in the United States in New York in 1999, and researchers are still working to understand the ecology of the disease.
David Crowder, an entomologist at Washington State University and author of this study, cautioned that the link between orchards and WNV prevalence has yet to be explained in full. Some have hypothesized that mosquitoes are drawn to the orchards’ plant nectar and that birds use these areas for nesting and feeding. It is also possible that less precipitation can reduce the populations of mosquito predators.
The authors of the study explained that further research is necessary but also emphasized that, “predicting the spread of arboviruses [viruses transmitted by arthropod insects] requires a system-based approach that explores ecological interactions between vectors, hosts, and pathogens across landscapes.”
The CDC recommends reducing contact with mosquitoes as the best way to prevent WNV infection. They advise using insect repellent, wearing protective clothing and getting rid of standing water where mosquitoes can breed.