West Nile Fever: Light Pollution Increases the Risk of an Outbreak by 41%

Disturbances caused by artificial nighttime lighting appear to increase the viral load of the West Nile virus circulating among wild birds, according to a study conducted by American researchers at the University of Florida.

This is not the first study on the harmful physiological and behavioral effects of light pollution on wildlife, and even on human health. But this one examines the impact on the risk of infectious disease transmission. It thus reveals serious epidemiological consequences regarding a zoonosis that is also present in France. While West Nile fever is a viral disease that primarily infects birds (reservoir hosts), it can also affect humans and equines (epidemiological dead-ends).

Scientists have observed that exposure to light at night significantly prolongs the infection period in the house sparrow (Passer domesticus), a reservoir host for the West Nile virus in urban environments.

Spotted sparrows in captivity were exposed to artificial light for 7 to 25 nights before being exposed to the virus. Blood samples were collected every two days during

10 days. All the birds were infected after 2 to 4 days of exposure.

The study’s authors also noted that these sparrows maintained transmissible viral loads for two days longer than the control birds, without showing a higher mortality rate. Furthermore, they emphasize that light pollution does not need to be excessive to produce such a result.

At the molecular level, artificial light at night affects gene cascades, particularly a group capable of influencing the rate of West Nile virus transmission. This comes as no surprise. Many vector-pathogen interactions rely on light signals to coordinate daily—and even seasonal—rhythms. Thus, since artificial light disrupts these rhythms, it would affect the immune response of passerines.

Furthermore, a mathematical model was used to estimate the increased risk of a West Nile virus epidemic under these conditions of viral circulation, which reaches 41 percent. This is a concerning finding given that this is a zoonotic disease that could potentially be present in areas with high human density, such as cities.

This study also echoes a previous one conducted on finches: birds with higher levels of cortisol (a stress hormone) were more likely to be bitten by mosquitoes, which are vectors of West Nile fever. It is well known that light pollution increases stress. And so the circle is complete…