brand logo

Surface-to-insect-to-human Covid transmission possible?

28 Jun 2021

By Ruwan Laknath Jayakody   Although the SARS-CoV-2, the virus that causes the infectious and deadly Covid-19, is a pandemic spread by air when people are in physical proximity, since it may also be transmitted via contaminated surfaces, even though this has not been conclusively demonstrated, insects in contact with or feeding on such contaminated surfaces may play a role in Covid-19 transmission, a review of the possibility of insects transmitting such observed. The review, whilst noting that, at present, the link is theoretical and under possible investigation, elaborated however that, in the past, insects and pandemics seem to have been inextricably linked in a negative manner in the people’s minds.   These observations were made by Prof. Chandima Dangalle (attached to the Colombo University’s Science Faculty’s Zoology and Environment Sciences Department) in a review titled “Insect vectors of human viral diseases: Can they transmit Covid-19?” which was published on 26 January 2021 in the Sri Lankan Journal of Biology.  According to M.J. Miller’s “Viral taxonomy”, a coronavirus is a ribonucleic acid (RNA) virus of the family coronaviridae, genus coronavirus, which as per P. Dehghani and H. Kassiri’s “A brief review on the possible role of houseflies and cockroaches in the mechanical transmission of Covid-19”, was first isolated in 1937. Dangalle explains that coronaviruses, which are the largest known RNA viruses, are divided into four genera, namely, alpha-coronavirus, beta-coronavirus, gamma-coronavirus, and delta-coronavirus.  There have, to date, been six human coronaviruses (HCoVs) identified, which according to D. Wu, T. Wu, Q. Liu, and Z. Yang’s “The SARS-CoV-2 outbreak: What we know”, includes the SARS-CoV and the Middle East respiratory syndrome-CoV (MERS-CoV). Wu et al., further explain that new coronaviruses appear to emerge periodically among humans, primarily due to the high prevalence and wide distribution of coronaviruses, the large genetic diversity and the frequent recombination of their genomes, and the increase in human-animal interface activities.  Covid-19, reported in late December 2019 is, Dangalle adds, the seventh member of the family coronaviruses, and is a beta-coronavirus with over 70% similarity in genetic sequence to the SARS-CoV. Covid-19 is transmitted from person to person via respiratory droplets, close contact, aerosols, and according to Dehghani and Kassiri, H. Eslami, and M. Jalili’s “The role of environmental factors to the transmission of Covid-19” and T.M. Nath and B. Gupta’s “Covid-19: A Review”, the faecal-oral route in a limited number of confirmed patients (cases detected in Wuhan and Shenzhen in China and in the US) with the virus in a person’s stool remaining viable for days under favourable atmospheric conditions, and indicative that the virus can exist and replicate in the digestive tract. However, Wu et al., have observed that it is not certain as to whether it is the consumption and ingestion of virus-contaminated food that causes the infection and transmission.  Bats, as per Wu et al., are the most possible host of this pathogen, while snakes, minks, and pangolins are also potential hosts. However, Dehghani and Kassiri note that there have been no reports of Covid-19 transmission through blood sucking arthropods such as mosquitoes or any other such insect. That said, V.M. Carn explains in “The role of dipterous insects in the mechanical transmission of animal viruses” that viruses are transmitted by arthropods, biologically or mechanically (insects become contaminated with viruses during normal feeding behaviour, and the virus persists on their mouthparts or body until the next feed). Dehghani and Kassiri further explain that insects such as cockroaches and houseflies, which are major mechanical vectors of pathogens, can transmit diseases by contact with contaminated surfaces while beetles and domestic insects too are mechanical carriers of pathogens.  The SARS-CoV, the MERS-CoV, and the influenza virus too can, depending on the strain variation, titre (concentration), surface type, suspending medium, mode of deposition, temperature, and relative humidity of the environment, according to J.A. Otter, C. Donskey, S. Yezli, S. Douthwaite, S.D. Goldenberg, and D.J. Weber’s “Transmission of SARS and MERS CoVs and influenza virus in healthcare settings: The possible role of dry surface contamination”, survive on surfaces for extended periods, even on occasion, going up to months.  Therefore, Dangalle argues, any organism which is in contact with or feeding on human faeces may play a role in Covid-19 transmission. In this regard, the role of insects such as houseflies and cockroaches becomes important, as they are, according to Dehghani and Kassiri, capable of transmitting more than 100 pathogens which may occur on human food waste, sputum, nasal secretions, fresh and dried blood, and stool, through their legs, body hair, mouthparts, faeces, and vomit. Currently, however, Dangalle reiterates, no insect-based transmission of viruses of the family coronaviridae has been documented. She further noted that with regard to deadly pandemics, those caused by viruses were few and that when the viruses in question were the pathogens, transmission was usually airborne. C.R. MacIntyre, A. Das, X. Chen, C. De Silva, and C. Doolan’s “Evidence of long distance aerial convection of the variola virus and implications for disease control”, cited the case of the since eradicated smallpox, where the disease-causing variola virus was transmitted via the airborne respiratory route. Hence, Prof. Dangalle emphasised that even though the biological transmission of Covid-19 by insects may not be a possible route for the disease to spread, the mechanical transmission of the virus by insects other than mosquitoes, on the other hand, cannot be entirely ruled out, and since the mode of infection transmission has implications for the containment of diseases by way of public health interventions, the methods of Covid-19 transmission should be further examined and understood.


More News..