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November 01, 2020

Modified mosquitoes may reduce disease risks for humans, animals

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Updated October 15, 2020

Female Aedes aegypti mosquito
A female Aedes aegypti mosquito acquires a blood meal from a human host. (Photo by James Gathany/CDC)

Plans to release genetically modified mosquitoes in the Florida Keys should benefit humans and may also benefit animals.

In August, Florida Keys Mosquito Control District officials approved plans to work with biotechnology company Oxitec to distribute the company’s genetically engineered Aedes aegypti mosquitoes starting in 2021, with the goals of reducing the population of that mosquito species and the risk of human illnesses from dengue fever and Zika virus infection, according to a company announcement and regulatory filings. The target mosquito species primarily feeds on humans, although studies found variation in how often they feed on other animals.

The mosquitoes produced by Oxitec are modified with a genetic kill switch for female larvae. Those larvae survive in the presence of tetracycline but die when the drug is removed from their diet, letting the company maintain a breeding population and distribute an all-male population to mate with wild female mosquitoes, according to filings from the company with the Environmental Protection Agency.

The males are homozygous for a dominant gene that kills female offspring, so their heterozygous offspring carry the kill switch and continue reducing the wild population for generations. Oxitec filings indicate the company saw reductions in A aegypti populations in testing previous versions of the mosquitoes in Brazil, the Cayman Islands, and Panama.

Company spokeswoman Meredith Fensom said two years of trials in Brazil reduced local mosquito populations 96%.

Rhoel R. Dinglasan, PhD, is a professor of infectious diseases at the University of Florida, associate chair of research at the university’s College of Veterinary Medicine Department of Infectious Diseases and Immunology, and director of the Centers for Disease Control and Prevention’s Southeastern Regional Center of Excellence in Vector Borne Diseases. Citing an article published this April in the journal Viruses, he said not all A aegypti populations have the same feeding habits.

In that study, researchers in Texas who captured A aegypti mosquitoes near the southern border found that 50% of blood meals came from dogs and 30% came from humans. But the article notes that the study came from a region with low human density relative to wildlife and that, in 18 previously published studies on blood meal analysis from A aegypti, the mosquitoes fed on humans 84% of the time.

Dr. Dinglasan said the A aegypti that feed on dogs can transmit Dirofilaria immitis larvae—a risk increased by stray dogs in the Florida Keys—and those feeding on birds can transmit avian malaria parasites that affect perching birds. One of those avian malaria parasites, Plasmodium relictum, has been found among sparrows in Tampa, he said.

Reducing the A aegypti population could reduce those disease risks to dogs and birds, including migratory birds that travel through Florida’s wetlands, Dr. Dinglasan said. The intervention could have additional unidentified benefits for animals, he said.

John Beckmann, PhD, assistant professor of biotechnology in the Auburn University Department of Entomology and Plant Pathology, expects no impact on animal populations. He said A aegypti mosquitoes are adapted to feed on humans, and the viruses carried by those mosquitoes, too, are adapted to humans and other primates.

In response to questions from JAVMA, Dr. Beckmann talked with his medical veterinary entomology class about the potential effects of the mosquito release and provided a video of the discussion. In the video, he said the modified mosquitoes likely would be safer and more effective than insecticides at reducing the A aegypti population, without the off-target effects of spraying insecticides on ponds and lakes.

“I would vote to release these mosquitoes in my home town if I was plagued by Aedes aegypti,” he said.

Correction: A previous version of this article incorrectly said the mosquitoes were gene edited, rather than genetically engineered.