The number and range of mosquitoes has boomed across North America in recent years, and with it, the number and range of mosquito-borne diseases. Ticks and fleas are following their lead.
“Between the period of 2004 to 2016, the number of diseases caused by these insects— mosquitoes, ticks and fleas—has nearly tripled during this time period, and it is continuing to grow since then,” said Karen Holcomb, a biologist at the Center for Disease Control’s Division of Vector-Borne Diseases.
Those diseases include:
• flu-borne typhus for fleas;
• West Nile virus, dengue, malaria and chikungunya for mosquitos;
• lyme disease, Rocky Mountain spotted fever, babesiosi, anaplasmosis, erlichiosis for ticks.
“There’s a large number of diseases that these insects can transmit to humans, and climate has a big impact on vector-borne diseases, because it largely impacts where these vectors can live and how fast they can replicate.”
Scientists use the word “vector” for organisms that transmit diseases or parasites from one animal or plant to another. Climate change supports vectors in several ways, Holcomb said:
• As temperatures rise, mosquitos, ticks and fleas can develop faster, producing larger populations.
• At higher temperatures, viruses also spread faster, increasing the risk of infection for humans who get bit by an infected insect or animal.
• As temperatures rise, the habitat for these species expands.
“So for example, for mosquitoes as it rains more we get more water standing around that the mosquitoes can lay their eggs in,” she said in a recent seminar hosted by the National Oceanic and Atmospheric Administration, “and therefore we get larger populations of mosquitoes in different locations, and with the potential to transmit their diseases to humans overall.”
Holcomb worked with National Oceanic and Atmospheric Administration research librarian Trevor Riley to determine whether NOAA’s climate data could be better utilized by the CDC and others to predict and prevent vector-borne diseases. (Lots of detail on their research methods here).
In 2021 the Arizona Department of Public Health documented a record outbreak of West Nile Virus in Maricopa County, Arizona—desert home of the metropolis of Phoenix. More than 1,400 cases were reported, representing Arizona’s largest outbreak ever.
Holcomb studied the event and found that temperatures were normal during the outbreak, but that rainfall had dramatically increased from a prior dry year. Three times as much rain as normal fell in Arizona during the outbreak, and it fell over a greater number of days.
“The continual presence and increased amount of moisture maintained mosquito breeding sites and potentially facilitated a boom in the mosquito population when the virus was circulating in the local bird populations,” Holcomb said in a report published on climate.gov.
Birds serve as an intermediate host for West Nile, a pool from which the virus spreads among mosquitos who feed on the birds.
But weather variability, driven in part by climate change, adds complexity to the picture. Because 2020 had been dry, with fewer mosquitos, fewer birds had contracted West Nile in 2020. That means fewer birds had antibodies to West Nile in 2021. So when the rains came in 2021, the birds had less resistance to the virus.
“Therefore, in 2021, a large proportion of the bird population likely did not have antibodies to the virus,” Holcomb said, “thus allowing rapid amplification and spread of the virus among birds and mosquitoes, eventually spilling over to humans.”