Efi Tsouri, BSc Molecular Life Sciences
In October of 2020, the first Dutch patient was infected with the West Nile virus (WNV) in Utrecht [1]. Subsequently, five additional cases of WNV infection were reported in the Netherlands [1]. What did these patients have in common? They were all previously stung by WNV-carrying tropical mosquitoes. Such mosquitoes are not commonly found in the Netherlands, raising concerns about the spread of tropical diseases in Europe.
WNV belongs to the arboviruses (arbo: arthropod-borne), a group of RNA-viruses that can infect vertebrate hosts [2]. But what is so special about them? As their name indicates, arboviruses are transmitted through blood-sucking arthropods; the most important of them all: mosquitoes (Figure 1) [2]. Not all mosquito species can be infected with and transmit arboviruses; three species, Anopheles, Aedes, and Culex, are responsible for the transmission of most arboviruses [2]. These so-called tropical mosquitoes originate from tropical regions in Asia, Africa, and America [3]. However, due to different factors, such mosquito species have spread to Europe as well. In general, viral genetic variability, natural adaptation to various mosquito species, and climate change are the most important driving forces behind the emergence of arboviruses in Europe [3].
Arthropod-human and human-human transmission are vital in the spread of arboviral diseases (Figure 1). It is estimated that 17% of the infectious diseases caused worldwide are a consequence of arboviruses, causing almost 700,000 deaths globally per year [5]. Arboviral infections are associated with different symptoms, ranging from mild to life-threatening. For example, WNV is characterized by flu-like symptoms such as fever, headache, and back pain [6]. However, more severe symptoms can develop too after infection with the virus, including rashes, myocarditis, hepatitis, and even encephalitis. Therefore, the severity and global spread of such viruses underline the need for more effective strategies to limit arboviral infections.
Several methods to control arboviruses already exist. Similar to most infectious diseases, researchers have focused on developing vaccines and anti-viral drugs [6]. However, a controversial but theoretically more effective method to limit the spread of such diseases is vector control [7]. Vector control, instead of focusing on the virus itself, targets the vectors, i.e. the mosquitoes. In this way, the transmission of arboviruses to humans can be prevented. An example of this strategy is gene drive, which can be used to introduce new genetic traits in mosquitoes [7]. Mosquito fertility genes can be silenced using the CRISPR-Cas9 system, whereby female mosquito populations can be suppressed. Alternatively, gene constructs providing resistance to arboviral infections or transmission can be introduced to mosquitoes’ genome. Over time these modified mosquitoes can replace the pathogenic mosquito populations and ultimately control arboviral spread. Yet, the feasibility and ecological impact of such methods remain to be evaluated. New strategies, similar to the ones described, are continuously being developed.
In conclusion, arboviral infections represent an emerging public health problem worldwide that should be addressed urgently in the following years. So next time you take a stroll through your backyard, and you spot a mosquito, you will know to duck for cover.
Figure 1| Schematic representation of arboviral transmission cycle.
References:
[1] National Institute for Public Health and the Environment. Westnijlkoorts [Internet]. Amsterdam [updated 13-11-2020; cited 28-01-2021]. Available from: https://www.rivm.nl/westnijlkoorts.
[2] Kuno, G. & Chang, G.-J.J.J.C.M.R. Biological transmission of arboviruses: reexamination of and new insights into components, mechanisms, and unique traits as well as their evolutionary trends. 18, 608-637 (2005).
[3] Medlock, J.M., et al. A review of the invasive mosquitoes in Europe: ecology, public health risks, and control options. 12, 435-447 (2012).
[4] Gould, E., et al. Emerging arboviruses: why today? 4, 1-13 (2017).
[5] Organization, W.H. & Unicef. Global vector control response 2017-2030. (2017).
[6] Gould, E. & Solomon, T.J.T.L. Pathogenic flaviviruses. 371, 500-509 (2008).7. Achee, N.L., et al. Alternative strategies for mosquito-borne arbovirus control. 13, e0006822 (2019).