The West Nile Virus (WNV), a mosquito borne virus, first occurred on the US East Coast in 1999. Since then, the disease has transmitted across the entire country in just a few years, infecting not just humans, but animals such as birds and horses as shown in the figure below.

The WNV spreads to people via the bite of an infected mosquito and can cause fevers, body aches, rashes, diarrhea, long-term fatigue, and, in some cases, inflammation of the brain and its membranes (Pennisi, 2018). Since 2003, human infections have exceeded 7,000 cases and caused more than 300 deaths in California. It is not surprising that the WNV is the most common and serious vector-borne disease in California. However, studies have shown that this vector-borne disease has a new helper: light pollution. 

Artificial light pollution at night (ALAN) prolongs the infectious-to-vector period of the house sparrow, a host of WNV in urban areas. Many host and vector relationships are reliant on using light cues to coordinate their daily and seasonal rhythms (Kernbach et al, 2019). Additionally, light regimes also alter temperature fluctuations in immune defences and other factors that affect infection risks. So how exactly does light pollution affect the spread of the WNV?

 

Prolonged period of infection 

Figure (a) above shows that at day 6 of post exposure to WNV, house sparrows that were exposed to ALAN show significantly higher viral titres (as indicated by the blue points and lines) compared to control individuals without exposure to ALAN (Kernbach et al, 2019). More specifically, when we look a figure (c), after day 6 post exposure to WNV, we see that only house sparrows that are exposed to ALAN were infectious (the vertical dotted line shows the WNV transmission threshold: individuals to the right of this line are infectious to mosquitoes whereas individuals to the left are not). Other studies also stated that birds that were exposed to ALAN remained infectious for 2 more days compared to birds that were not. This has serious implications because this doubles the infectious period, which then doubles the number of infected mosquitoes that may transmit the WNV to both humans and birds, significantly increasing the number of cases.

Increased pathogen reproductive number (host infectiousness)

Additionally, to compare the changes in potential for WNV outbreak in the presence and absence of ALAN, this study uses the pathogen basic reproductive number, R0, to assess WNV transmission based on a single host and vector model. Results show that with ALAN, host infectiousness increased R0 from 8.95 to 12.66, indicating that ALAN would increase the reproduction of pathogens for WNV by 41% (Kernbach et al, 2019). ALAN alters the circadian rhythms of hosts to their environments, which can result in disruptions of gene expression and thus upsets the production of antiviral defences at appropriate times (Kernbach et al, 2019). Hence, ALAN not only increases the period of infectiousness but intensifies the infectiousness of pathogens for WNV. 

While the relationship between light pollution and the spread of diseases still remains relatively unknown, this study shows that more attention needs to be placed on investigating to what extent light pollution affects infectious disease risk as this affects the types of solutions to be put in place to curb the spread of infectious disease like the WNV. For example, in South Eastern US, attempts have been made to reduce lighting in vulnerable areas during seasons of high virus transmission in order to mitigate the vectored disease transmission.

 

Stay tuned for more! 

Jean 

 

References: 

• Kernbach, M. E., Newhouse, D. J., Miller, J. M., Hall, R. J., Gibbons, J., Oberstaller, J., & Martin, L. B. (2019). Light pollution increases west nile virus competence of a ubiquitous passerine reservoir species. Proceedings of the Royal Society. B, Biological Sciences, 286(1907).