In early 2020, a new virus called SARS-CoV-2, the cause of Covid-19 disease, was spreading rapidly around the world. Everyone wanted and needed information on what it was, what it did and how it was transmitted.
Since then, the global scientific community has accumulated a wealth of information about the virus, and the Agricultural Research Service (ARS) contributes to this knowledge base.
Scientists continue to research whether the virus originally transmitted from an animal to humans. Researchers at the ARS Arthropod-Borne Animal Diseases Research Unit (ABADRU) in Manhattan, KS, in collaboration with Kansas State University, are investigating whether insects are somehow involved in the transmission of this virus. The team recently published two papers outlining their studies and findings.
In March 2021, the first study was published online in the Journal of Medical Entomology. It focused on biting insects, specifically mosquitoes and two species of mosquitoes, as they are known to ingest disease-causing viruses and transmit them to humans and animals. The insects were allowed to feed on blood spiked with SARS-CoV-2, which is a natural route of infection, and were later tested at different times for the presence of viral RNA and infectious virus. Several insect cell lines were also tested to see if the virus could multiply in them.
The team found that virus replication was not supported in any of the insects or cell lines tested.
“We conclude that these biting insects pose no risk of transmitting SARS-CoV-2 to humans or other animals after a SARS-CoV-2 infected blood meal,” said ARS research molecular biologist and study author Dana Mitzel.
The second study, published in April 2021 in the journal Parasites and Vectors, focused on houseflies for their association with mechanically transmitting bacterial, parasitic and viral pathogens to humans and animals. The team wanted to see if houseflies could pick up SARS-CoV-2 and transmit it to their environment.
The houseflies were exposed to SARS-CoV-2-spiked culture media or virus-spiked milk. (The team used reconstituted milk powder as the substrate because it’s also used as a food source for rearing the flies, and they knew the insects would feed on it easily.) The flies were tested for viruses 4 and 24 hours after exposure. All flies exposed to the virus tested positive for viral RNA, but infectious virus was only present in the flies exposed to the virus-spiked milk.
In the next experiment to check transmission, the flies were again exposed to virus-spiked milk and tested positive for the virus 24 hours later. However, the flies were only able to mechanically transfer viral RNA, not infectious virus, to unvaccinated (clean) milk and test surfaces. Therefore, although some flies appeared to be able to carry infectious virus, they were unable to transmit the virus, only the RNA of the virus, which is non-infectious.
“We would like to emphasize that this research was carried out under carefully controlled laboratory conditions with high levels of virus – much more than what flies could encounter naturally. More research is needed to determine whether houseflies can transmit infectious SARS-CoV-2 in a natural environment, and if so, what the public health implications are,” said ARS entomologist Dana Nayduch, ABADRU principal investigator and correspondent author of the study. “On the positive side, since we were able to detect both virus and viral RNA in flies, we may be able to use flies to monitor and detect viruses in the environment.” With this type of surveillance strategy, the housefly does all the work by visiting animals, including humans, and their droppings. Then we can examine flies in the lab for evidence of the virus and know if it’s out there in places the flies have visited.”
Source: USDA-ARS
