Susceptibility of Midge and Mosquito Vectors to SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2, also known as SARS-CoV-2, is the notorious coronavirus responsible for the COVID-19 pandemic. As is the case with any disease, understanding how SARS-CoV-2 spreads through a population is one of the most important steps in slowing transmission. Early on, scientists determined that the primary mode of infection for SARS-CoV-2 is through exposure to respiratory droplets carrying infectious virus (CDC 2021). This means the most likely route of infection would be through direct, indirect, or close contact (<6 feet) of an infected person through infected secretions.

In 2020, scientists published findings that SARS-CoV-2 viral RNA (genetic material of the virus) was identified in the blood and serum of infected patients, suggesting that the virus could also be present in the bloodstream (Chen et al. 2020, Hogan et al. 2020, Young et al. 2020). This led researchers to question if blood-sucking arthropods like biting flies could also serve as a mode of biological transmission for SARS-CoV-2. Initial experiments testing this hypothesis confirmed that the virus did not replicate in three common mosquito species (Aedes aegypti, Ae. albopictus, and Culex quinquefasciatus) after the virus was injected directly into the mosquitoes (Huang et al. 2020). Additional research by Xai et al. (2020) showed that the virus did not replicate in cells collected from Aedes mosquitoes, and SARS-CoV-2 was never recovered from field-caught Culex or Aedes mosquitoes from Wuhan. These results were promising, but only a few species were tested, and researchers injected the mosquitoes with the virus rather than allowing them to ingest an infected bloodmeal. 

Balaraman et al. (2021) set out to address some of the holes left in previous research by testing if more species of biting flies were susceptible to the SARS-CoV-2 virus after ingesting an infected blood meal. The research team expanded on the list of biting insects investigated to include two previously untested flies, a biting midge (Culicoides sonorensis) and another species of Culex mosquito (Cx. Tarsalis), along with the previously tested Culex species Cx. quinquefasciatus.

To get a better idea of susceptibility of these insects to the virus, the research team measured the infection potential of these biting insects in two different ways. First, cultured cells from all three species were exposed to SARS-CoV-2 to determine if the cells from any species were vulnerable to infection or damage. Results from these experiments were negative for all insects, meaning they showed no signs of cellular infection or damage.

The next step was to test if any of the three species would be susceptible to infection after ingesting an infected bloodmeal. Researchers fed the insects on blood spiked with SARS-CoV-2 and held them for 10 days. After the waiting period, all blood-fed insects were tested for the presence of SARS-CoV-2 viral RNA and positive samples were set aside for further evaluation. It is important to note here that viral RNA is not the same as infectious virus. This means that simply confirming the presence of viral RNA would not also guarantee that infectious virus particles were present. To determine if any of the samples with viral RNA also contained infectious virus, another set of experiments were conducted that measured cellular damage to confirm viral infection. Those results were all negative meaning that none of the samples tested positive for infectious virus. 

It may seem like a no-brainer that the mosquito, an insect crowned the world’s deadliest organism because of the diseases it is known to spread, should also be able to biologically transmit the SARS-CoV-2 virus. But the disease transmission process can be extremely complicated. The overarching conclusion we can draw from these studies is that the biting insects studied so far most likely do not play a role in biological transmission of SARS-CoV-2. While we cannot rule out the possibility of arthropod-borne transmission of SARS-CoV-2 all together, there is no evidence so far to suggest this route of transmission is possible.

Research is currently ongoing to measure the potential for some insects (mostly cockroaches and flies) to mechanically spread the SARS-CoV-2 virus after contacting a contaminated surface or material. While this route of viral spread may be more plausible, the likelihood that a person contracts COVID-19 after contacting a contaminated surface is low CDC (2021).

Mike Bentley, PhD, BCE


References:

Centers for Disease Control and Prevention. (2020). Science Brief: SARS-CoV-2 and Potential Airborne Transmission. https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/scientific-brief-sars-cov-2.html.

Centers for Disease Control and Prevention. (2021). Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments. https://www.cdc.gov/coronavirus/2019-ncov/more/science-and-research/surface-transmission.html.

Chen, W., Y.  Lan, X.  Yuan, X.  Deng, Y.  Li, X.  Cai, L.  Li, R.  He, Y.  Tan, X.  Deng, . et al.  2020. Detectable 2019-nCoV viral RNA in blood is a strong indicator for the further clinical severity. Emerg. Microbes Infect. 9: 469–473.

Huang, Y. J. S., D.L.  Vanlandingham, A.N.  Bilyeu, H.M.  Sharp, S.M.  Hettenbach, and S.  Higgs. . 2020. SARS-CoV-2 failure to infect or replicate in mosquitoes: an extreme challenge. Sci. Rep. 10: 1–4. 

Hogan, C. A., B. A.  Stevens, M. K.  Sahoo, C.  Huang, N.  Garamani, S.  Gombar, F.  Yamamoto, K.  Murugesan, J.  Kurzer, J.  Zehnder, . et al.  2020. High frequency of SARS-CoV-2 RNAemia and association with severe disease. Clin. Infect. Dis. 1–5. 

World Health Organization. (2021). Malaria. https://www.who.int/news-room/fact-sheets/detail/malaria.

Xia, H., E.  Atoni, L.  Zhao, N.  Ren, D.  Huang, R.  Pei, Z.  Chen, J.  Xiong, R.  Nyaruaba, S.  Xiao, . et al.  2020. SARS-CoV-2 does not replicate in Aedes mosquito cells nor present in field-caught mosquitoes from Wuhan. Virol. Sin. 35: 355–358. 

Young, B. E., S. W. X.  Ong, S.  Kalimuddin, J. G.  Low, S. Y.  Tan, J.  Loh, O. T.  Ng, K.  Marimuthu, L. W.  Ang, T. M.  Mak, . et al.; Singapore 2019 Novel Coronavirus Outbreak Research Team. 2020. Epidemiologic features and clinical course of patients infected with SARS-CoV-2 in Singapore. JAMA  323: 1488–1494.

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