Study Shows Flies, Cockroaches Do Not Transmit Coronavirus
By Andrew Porterfield
Ever since the arrival of the COVID-19 pandemic in 2020, scientists and public health officials have looked for the various ways the disease could be transmitted. While airborne transmission is the most significant form of transmission, the virus (or at least viral RNA) has been observed on surfaces and can infect domestic dogs and cats as well as humans.
But many have also wondered: Could insects also transmit the virus? Insects reside in nearly every home, and some do transmit pathogens, so it’s a fair question. But, so far, studies show that SARS-CoV-2 does not replicate in insects (including mosquitoes and biting midges, which carry other diseases).
That leaves the question of mechanical transmission, in which non-biting flies can carry viruses or other pathogens on their mouthparts or bodies. So, to see if common household insects could mechanically transmit the coronavirus that causes COVID-19, a team from Texas A&M University looked at house flies and cockroaches at homes in Texas where resident people or pets had been infected. Their results, published May 30 in the Journal of Medical Entomology, found that none of the insects tested had the virus or its RNA.
The study, led by Gabriel Hamer, Ph.D., associate professor of entomology at Texas A&M, also examined the potential of using cockroaches and flies for xenosurveillance, the practice of sampling DNA from insects or arthropods to monitor for presence of disease pathogens. Xenosurveillance can be a valuable tool for gathering DNA or RNA, especially from species that are elusive, small in size, or live in physically complex habitats.
The researchers placed 133 sticky and liquid-baited insect traps in and around 40 homes in Brazos, Bell, and Montgomery counties in Texas. The traps captured 1,345 insects belong to 11 families of Diptera (flies) and Blattodea (roaches). In addition, they set 14 traps in seven homes on the day a cat or dog tested positive for SARS-CoV-2 RNA. They then used reverse transcriptase quantitative PCR (RT-qPCR) to find evidence of the viral RNA. In all of this testing, no sample tested positive for SARS-CoV-2.
The results were at least, on the surface, a reversal of at least one earlier laboratory study showing SARS-CoV2 RNA on (but not in) insects. “Given that we were sampling insects in homes with recent COVID-19 cases, some of which also had active animal shedding of SARS-CoV-2, we expected to find the nucleic acid from the virus on these insects,” Hamer says. “Instead, we did not detect evidence of the virus in the sample insects from these homes.” In addition to showing that insects didn’t transmit SARS-CoV-2 in a field setting, it also ruled out the insects as a potential surveillance tool for the virus.
The earlier laboratory study, conducted by researchers at Kansas State University and the USDA Agricultural Research Service in Manhattan, Kansas, “was an experimental infection study to see if house flies could mechanically transmit SARS-CoV-2 in a captive and controlled environment,” Hamer says. That study showed evidence of infectious virus and viral DNA 24 hours after exposure. Hamer’s field study was “complementary to this lab study and from a high-risk environment. Both lab and field studies are needed to incriminate mechanical or biological transmission by insects.”
Both biological and mechanical transmission of pathogens are important to understanding how disease can spread through insect vectors (among other animals). Biological transmission involves the entry of the pathogen into the insect, in which it grows and then can be transmitted to a new host. Mechanical transmission involves carrying the pathogen on an animal’s surface anatomy—in which the pathogen does not grow but can be transferred to a new host.
“Most vector-borne pathogens that we think of are biological modes, such as mosquitoes transferring West Nile virus, dengue virus, or the agent of malaria. Examples of mechanical transmission would be flies transmitting various enteric bacteria such as Salmonella or Shigella,” says Hamer.
The new study also underscores the need to find insect candidates for xenosurveillance. Many insects can be used to detect human pathogens. “This concept of xenosurveillance will likely work in different contexts but is dependent on the insect being used, the pathogen being tested for, and the environment being sampled,” he says.
Journal of Medical Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.