Mechanical transmission of SARS-CoV-2 by house flies

Authors: BALARAMAN, VELMURUGAN - Kansas State University; Drolet, Barbara; Mitzel, Dana; Wilson, William - Bill; Owens, Jeana; GAULTIERO, NATASHA - Kansas State University; MEEKINS, DAVID - Kansas State University; BOLD, DASHZEVEG - Kansas State University; TRUJILLO, JESSIE - Kansas State University; Noronha, Leela; RICHT, JUERGEN - Kansas State University; Nayduch, Dana

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2021
Publication Date: 4/20/2021
Publication URL: https://handle.nal.usda.gov/10113/7346752
Citation: Balaraman, V., Drolet, B.S., Mitzel, D.N., Wilson, W.C., Owens, J.L., Gaultiero, N.N., Meekins, D.A., Bold, D., Trujillo, J.D., Noronha, L.E., Richt, J.A., Nayduch, D. 2021. Mechanical transmission of SARS-CoV-2 by house flies. Parasites & Vectors. 14:214. https://doi.org/10.1186/s13071-021-04703-8.
DOI: https://doi.org/10.1186/s13071-021-04703-8

Interpretive Summary: SARS-CoV-2 is a recently emerged coronavirus that is the causative agent of the global COVID-19 pandemic. COVID-19 in humans is characterized by a wide range of symptoms ranging from mild to severe illness that can result in death. SARS-CoV-2 is highly contagious and is transmitted to new hosts via the respiratory route through aerosols, or after contact with items contaminated by infected persons. House flies transmit various bacterial, parasitic, and viral agents to humans and animals as mechanical vectors. Previous experimental studies have shown that house flies can mechanically transmit turkey coronavirus; however, the house fly’s role in SARS-CoV-2 transmission has not yet been explored. Therefore, the goal of this work was to determine whether house flies can acquire and transmit the SARS-CoV-2. virus. The objectives of this study were to systematically assess whether house flies can acquire SARS-CoV-2, from a substrate, harbor infectious virus, and mechanically transmit the virus to naïve substrates and surfaces. Two independent studies were performed to address these objectives. The first study aimed to determine fly acquisition of virus. Flies were exposed to SARS-CoV-2-spiked culture media or 10% milk substrates and tested for virus at either 4 or 24 hours after exposure. In the second study, flies were exposed to a substrate containing virus for 24 hours (as well as positive and negative control substrates). Flies were transferred to a clean container with naive substrate, and after 4 and 24 hours flies, container swabs and the naive substrate were removed and tested for SARS-CoV-2-. All flies exposed to SARS-CoV-2- inoculated media or milk were positive for viral RNA at 4 hours and 24 hours post-exposure. However, infectious virus was detected only from the flies exposed to virus-spiked milk. Moreover, the virus' nucleic acid (e.g., viral RNA) was detected in environmental samples (swabs, naive substrates) after contact with SARS-CoV-2 exposed flies, but no infectious virus was recovered. Under laboratory conditions using high amounts of virus, house flies were able to acquire and harbor infectious SARS-CoV-2 for up to 24 hours post-exposure. In addition, the flies were able to mechanically transmit SARS-CoV-2 to the surrounding environment up to 24 hours post-exposure. Further studies are warranted to determine if house fly transmission occurs naturally and to assess the potential public health implications of these results.

Technical Abstract: Background SARS-CoV-2 is a recently emerged coronavirus that is the causative agent of the global COVID-19 pandemic. COVID-19 in humans is characterized by a wide range of symptoms ranging from mild to severe illness that can result in death. SARS-CoV-2 is highly contagious and is transmitted to new hosts via the oral-nasal route through aerosols, or after contact with contaminated fomites from infected persons. House flies transmit various bacterial, parasitic, and viral diseases to humans and animals as mechanical vectors. Previous experimental studies have shown that house flies can mechanically transmit turkey coronavirus; however, the house fly’s role in SARS-CoV-2 transmission has not yet been explored. Therefore, the goal of this work was to investigate the potential of house flies to transmit SARS-CoV-2. The objectives of this study were to systematically assess whether house flies can acquire SARS-CoV-2, harbor live virus, and mechanically transmit the virus to naïve substrates and surfaces. Methods Two independent studies were performed to address the study objectives. In the first study, flies were tested for virus after exposure to SARS-CoV-2-spiked media or milk substrate. In the second study, environmental samples were tested for virus after contact with SARS-CoV-2-exposed flies. During both studies, samples were collected at various time points and evaluated by RT-qPCR and virus isolation by immunofluorescent assay. Results All flies exposed to SARS-CoV-2-spiked media or milk substrate were positive for viral RNA at 4 hours and 24 hours post-exposure. However, infectious virus was detected only from the flies exposed to virus-spiked milk. Moreover, viral RNA was detected in naïve environmental samples after contact with SARS-CoV-2 exposed flies, although no infectious virus was recovered. Conclusions Under laboratory conditions using high titer (105 TCID50) virus, house flies were able to acquire and harbor infectious SARS-CoV-2 for up to 24 hours post-exposure. In addition, the flies were able to mechanically transmit SARS-CoV-2 to the surrounding environment up to 24 post-exposure. Further studies are warranted to determine if house fly transmission occurs naturally and the potential public health implications of these results.