Mechanical transmission of SARS-CoV-2 by house flies

Abstract

Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a recently emerged coronavirus that is the causative agent of the coronavirus disease 2019 (COVID-19) pandemic. COVID-19 in humans is characterized by a wide range of symptoms that range from asymptomatic to mild or severe illness including death. SARS-CoV-2 is highly contagious and is transmitted via the oral-nasal route through droplets and aerosols, or through contact with contaminated fomites. House flies are known to transmit bacterial, parasitic and viral diseases to humans and animals as mechanical vectors. Previous studies have shown that house flies can mechanically transmit coronaviruses, such as turkey coronavirus; however, the house fly's role in SARS-CoV-2 transmission has not yet been explored. The goal of this work was to investigate the potential of house flies to mechanically transmit SARS-CoV-2. For this purpose, it was determined whether house flies can acquire SARS-CoV-2, harbor live virus and mechanically transmit the virus to naive substrates and surfaces.

Methods: Two independent studies were performed to address the study objectives. In the first study, house flies were tested for infectivity after exposure to SARS-CoV-2-spiked medium or milk. In the second study, environmental samples were tested for infectivity after contact with SARS-CoV-2-exposed flies. During both studies, samples were collected at various time points post-exposure and evaluated by SARS-CoV-2-specific RT-qPCR and virus isolation.

Results: All flies exposed to SARS-CoV-2-spiked media or milk substrates were positive for viral RNA at 4 h and 24 h post-exposure. Infectious virus was isolated only from the flies exposed to virus-spiked milk but not from those exposed to virus-spiked medium. Moreover, viral RNA was detected in environmental samples after contact with SARS-CoV-2 exposed flies, although no infectious virus was recovered from these samples.

Conclusions: Under laboratory conditions, house flies acquired and harbored infectious SARS-CoV-2 for up to 24 h post-exposure. In addition, house flies were able to mechanically transmit SARS-CoV-2 genomic RNA to the surrounding environment up to 24 h post-exposure. Further studies are warranted to determine if house fly transmission occurs naturally and the potential public health implications of such events.

Keywords: Coronavirus; House flies; Mechanical vector; SARS-CoV-2; Transmission.

Fig. 1

House fly mechanical transmission of SARS-CoV-2 study design. a Acquisition study: house flies were exposed to virus-spiked substrate or positive and negative controls on wicks for 4 and 24 h, following which the flies were collected. b Transmission study: house flies were exposed to the same set of substrates on wicks for 24 h and transferred to a second container containing virus-free substrate on wicks. At 4 and 24 h post-transfer, the flies, swabs and exposed substrate on wicks were collected. Sample analysis: the  flies, container inner surface swabs and substrate samples collected from the studies were processed and tested for SARS-CoV-2 nucleocapsid gene by quantitative reverse transcription-PCR (RT-qPCR) and for infectious virus by virus isolation