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Even at the Zoo, Mosquitoes Favor Local Wild Birds for Meals

A bright red cardinal, viewed from the side and looking to the right, perches on a dead tree branch in front of a blurry brown and gray mottled background.

In a study of mosquitoes in and around the Nashville Zoo, northern cardinals (Cardinalis cardinalis) were found to be the most common source of the mosquitoes’ blood meals, despite more than 300 species of animals on exhibit. None the less, the study suggests zoos are a valuable resource for monitoring mosquito species diversity, biology, and pathogen presence. (Photo by Eric Isley via iNaturalist, CC BY-NC 4.0)

By John P. Roche, Ph.D.

As is the case in most habitats in North America, zoos are home to numerous mosquito species. An understanding of the diversity of mosquitoes in zoos, the hosts they select, and the pathogens they spread is valuable for both the humans who visit and work at zoos and the animals that live there. To advance this understanding, a group of researchers from Cornell University and the Tennessee Department of Health collaborated with the Nashville Zoo to investigate the mosquitoes, hosts, and pathogens present there.

Mosquitoes can spread a range of human pathogens, including West Nile virus and St. Louis encephalitis virus. Researcher Cierra Briggs and colleagues studied mosquitoes in the Nashville Zoo because they are “sentinels for emerging pathogens,” she says. They shared their findings in a study published in August in the Journal of Medical Entomology.

“Previously, zoos have served as a valuable resource during epidemics (especially when West Nile virus was first detected in the U.S.) due to extensive animal health records and testing,” says Briggs, a graduate research assistant at Cornell at the time of the study and now a program associate in the Department of Entomology and Plant Pathology at the University of Arkansas. “However, through this study we demonstrated that zoos are also a valuable resource for invasive species monitoring, mosquito biology investigation, and pathogen surveillance in non-epidemic times.”

In their study, Briggs and colleagues placed traps in 10 sites at the Nashville Zoo and at two sites in nearby residential areas. When the study was conducted in 2020, the zoo held more than 300 species of animals. The investigators collected a total of 9,308 mosquitoes from 27 species.

The most abundant species collected were Culex pipiens, Culex quinquefasciatus, Culex restuans, Aedes albopictus, Aedes vexans, Culex erraticus, and Anopheles punctipennis. One of the species collected, Culex nigripalpus, was a first for Davidson County, where Nashville is located.

In the zoo, the investigators looked for viral pathogens in 5,032 Culex mosquitoes in 565 mosquito pools from 10 sites. Twenty-one of the 565 pools were positive for Flanders virus and four pools were positive for West Nile virus, while no pools were positive for St. Louis encephalitis virus. Flanders virus does not cause illness in humans, but it can serve as a “sentinel” for the presence of West Nile virus, showing up in testing prior to West Nive virus emerging in the same area.

In the residential sites, they examined 1,495 mosquitoes in 137 mosquito pools at two sites. Eleven of 137 pools were positive for Flanders virus. No pools were positive for West Nile virus or St. Louis encephalitis virus.

For West Nile virus (WNV), Flanders virus (FLAV), and St. Louis encephalitis virus (SLV), the investigators calculated the number of pools positive for specific viruses per number of mosquitoes tested, a variable known as the minimum infection rate (MIR). Minimum infection rates were listed as virus positive individuals per 1,000 mosquitoes tested.

A line chart titled "WNV and FLAV MIRs of The Nashville Zoo by week of trapping. Left vertical axis is titled "MIR Value," which even-numbered labels of 0 at bottom through 16 at top. Horizontal axis is titled "Epidemiological Week," with labels for every week, beginning with Week 24 at left and ending with Week 40 at the far right. The yellow line on the chart, for WNV, is flat at zero except for Week 26 (7.61) and Week 39 (1.29). The blue line, for FLAV, is zero except for Week 24 (13.39), Week 33 (11.09), Week 34 (3.09), Week 37 (1.17), Week 38 (6.85), and Week 39 (3.86).

In weekly measurements of the minimum infection rates for West Nile virus (WNV) and Flanders virus (FLAV) in mosquitoes sampled at the Nashville Zoo, June through October 2020, the highest weekly FLAV rate occurred 12 weeks before the highest weekly WNV rate, a pattern that has been seen in other studies, supports the hypothesis that peaks in FLAV infection may serve as an indicator of later WNV infection peaks. (Chart originally published supplemental to Briggs et al 2023, Journal of Medical Entomology)

West Nile virus mean infection rates were numerically higher at the zoo than in surrounding residential areas, but this difference was not statistically significant.

Interestingly, the highest weekly FLAV MIR occurred 12 weeks before the highest weekly WNV MIR, a pattern that has been seen in other studies. This supports the hypothesis that peaks in FLAV infection may serve as an indicator of later WNV infection peaks.

Briggs and her colleagues examined 100 engorged mosquitoes to examine what hosts the mosquitoes were feeding on. The investigators were able to match the DNA from host species in the blood meals of the engorged mosquitoes. Twenty-three different host species were identified, only eight of which were zoo species. The most frequently used hosts were birds, with the highest percentage of mosquitoes carrying blood from northern cardinals (Cardinalis cardinalis). Meanwhile, 17 percent of the blood meals were from humans.

While northern cardinals were common hosts, it is unclear how they contribute to West Nile virus transmission. The researchers observed that it is possible that northern cardinals have a suppressing effect on transmission of West Nile virus if they are only moderately competent reservoirs of West Nile virus. More research is required to draw more conclusions about the role of northern cardinals in the West Nile virus transmission cycle. Such research could include measuring how common northern cardinals are compared to other host bird species; this could help investigators assess if the proportion of blood meals from northern cardinals correlates with the population frequency of cardinals.

What were the big conceptual discoveries of this study? One was that in the data collected, mosquitoes were more diverse at the zoo than in residential areas. Another was that the mosquitoes fed less on zoo animals than on native bird species. Other important findings were that mosquitoes were more abundant in the zoo sites than in the residential sites and that the most frequent host overall was the northern cardinal.

The investigators conclude, “Further research should be performed to investigate wild bird populations at the Nashville Zoo and provide more insight into the repeated detection of WNV within the zoo.” Additional studies could also sample more residential sites. The higher diversity seen at the zoo in this study could have been a product of the higher proportion of sites sampled at the zoo versus residential areas.

This was one of the first comprehensive studies of mosquitoes, hosts, and pathogens in a zoo setting. One ingredient that is critical to help keep this type of research going, though, is adequate funding. “Since feeding and caring for so many animals is not a cheap endeavor,” Briggs says, “many zoos do not have funds available to carry out this research. Therefore, it is through relationships with health departments or universities that funding and the required manpower can be secured.”

John P. Roche, Ph.D., is a book author, biologist, and science writer with a Ph.D. and postdoctoral training in the biological sciences and a dedication to making rigorous science clear and accessible. He authors books and articles and writes materials for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the UMass Chan Medical School, and serving as a visiting professor at four tier-one colleges and universities.

1 Comment »

  1. Although well written and addressing a somewhat unique opportunity, I think this authors over state the significance of their findings. Their conclusions regarding host selection were based on very low sample sizes [largest here n = 36 for Culex pipiens]. As host availability can vary dramatically over time and space, I think the authors should be a bit more cautious in their interpretations and less bold in their claims concerning the importance of this study. Other research [not cited by the authors] addressing the same objectives [perhaps not in a zoo setting] with the same species have tested hundreds of blood fed females before drawing conclusions concerning host selection.

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