The potential for SARS-CoV-2 to be transmitted biologically or mechanically by arthropods

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which resulted in the coronavirus disease 2019 (COVID-19) pandemic, demonstrates how inadequate knowledge and readiness for new viruses can have damaging worldwide consequences.

Background

SARS-CoV-2 is related to two additional coronaviruses that appeared on a much lesser scale from animal reservoirs: severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV). SARS-CoV generated an outbreak of severe respiratory syndrome that began in China in 2002 and spread to 17 countries, resulting in over 8000 cases and over 800 deaths.

Surprisingly, the outbreak ended almost as swiftly as it began, with no new cases reported since 2004. The viruses are thought to have originated in horseshoe bats using civets as a bridge host to humans. In 2012, a new virus caused Middle East respiratory syndrome. However, unlike SARS-CoV, MERS-CoV has continued to create human cases, with a case fatality rate of around 35%. Cases have been reported from 27 countries, most of which are in the Arabian Peninsula, with the largest outbreak occurring in the Republic of Korea in 2015.

The earliest human illnesses were most likely spread by camels near domesticated animals. Because SARS-CoV-2, SARS-CoV, and MERS-CoV are related, it was fair to predict that SARS-CoV-2 would share some aspects of these other beta coronaviruses, such as those of these other beta coronaviruses the routes of transmission. SARS-CoV-2  is disseminated through aerosol and exposure to infected individuals, and it can be passed on to other animals.

Findings

In a recent review published in Viruses, researchers examined results from field studies, laboratory-based research, and environmental studies to analyze the potential of SARS-CoV-2 transmission either biologically or mechanically among arthropods. SARS-CoV-2 was found to be unable to replicate in three mosquito species: Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus, in the first published vector competent investigation.

Because it is known that bypassing the typical infection mechanism of hematophagous arthropods might sometimes allow a virus to infect an incompetent mosquito species or even non-hematophagous insects, this radical strategy to challenge mosquitoes was adopted. If this method failed to infect mosquitos, the presumption was that oral contact with the virus in blood would not lead to infection. Because SARS-CoV-2 failed to remain or reproduce in these mosquitoes after intrathoracic inoculation, the possibility of biological transmission of SARS-CoV-2 by these mosquitoes was successfully eliminated.

After eating on blood infected with SARS-CoV-2, Cx. quinquefasciatus, Cx. tarsalis and the midge Culicoides sonorensis did not become infected, according to a recent report. This research also showed that SARS-CoV-2 replication did not occur in insect-derived cell lines.  The authors concluded that when these mosquito species were given SARS-CoV-2 infected blood, the insects did not circum to infection. Mosquitoes that consumed the infected blood were allowed to lay eggs, which then developed into larva, some of which were kept to generate adults. SARS-CoV-2 was not vertically transmitted under these experimental settings, as evidenced by the negative results of RT-qPCR tests on larvae and adults.

SARS-CoV-2 shares numerous aspects with viruses capable of mechanical transmission. SARS-CoV-2 has been shown in multiple experiments to be infectious for several hours, depending on the environment. Many arthropod species were tested for their capacity to promote SARS-CoV-2 mechanical transmission.

The tests also looked into the possibility of mechanical transmission by enabling Ae. albopictus to probe many times on an infected blood meal before feeding on uninfected blood tested for the virus. The second bloodmeal contained no traces of the virus, which led the authors to conclude that mechanical transmission of SARS-CoV-2 from an infected individual to a healthy host via an insect vector is not possible. Although the virus was found in Aedes albopictus, mechanical transmission is unlikely due to the low viral load.

Previous research showed that house flies may remain positive for up to 24 hours after being exposed to SARS-CoV-2 contaminated milk and that viral RNA could be found on areas with which they had come into contact. The authors determined that the low quantity of infectious virus carried by flies limits their potential to transmit SARS-CoV-2 since the infectious virus could not be detected on these surfaces. In another recent investigation, over 1,345 arthropods were obtained from the homes of SARS-CoV-2 infected persons and tested for the virus via PCR; the results showed none were positive.

Implications

SARS-CoV-2 cannot be biologically or mechanically transmitted by hematophagous arthropods, according to experimental evidence and observations from nature. Further research into ectoparasites may be necessary because it is uncertain how wild animals become infected. Although the virus may continue to be infectious on some materials and be found in human secretions and feces, there is no evidence that arthropod-mediated virus transmission plays a large role in virus transmission and human infection.

Written by

Colin Lightfoot

Colin graduated from the University of Chester with a B.Sc. in Biomedical Science in 2020. Since completing his undergraduate degree, he worked for NHS England as an Associate Practitioner, responsible for testing inpatients for COVID-19 on admission.