As tropical diseases spread under the influence of climate change and increased global movement, the residents of one vulnerable city have found unusual protectors in the form of northern cardinals (Cardinalis cardinalis). 

West Nile Virus (WNV) is a mosquito-borne disease that has infected an estimated 780,000 North Americans since 1999. In severe cases, consequences include brain infections that have taken 1,700 lives in the United States alone.

With 3.3 infections per 100,000 people, Georgia has proven surprisingly immune compared to other US cities, considering that one-third of Atlanta-area birds tested for the virus have been infected.

Emory University’s Dr Rebecca Levine was struck by the fact that Chicago has five times as many human WNV infections per capita as Atlanta, even though birds in the area are less likely to be infected than those around Atlanta.

In the American Journal of Tropical Medicine and Hygiene, Levine attributes the difference to the type of birds in the area, and particularly the way that cardinals form a protective shield for Atlantans. The entire US southeast appears to be benefiting to some degree.

WNV is called a “spillover” disease because the virus circulates largely among animals, sometimes spilling into nearby human populations. American robins are particularly active facilitators of the disease, earning the tag “superspreaders”. They harbor the virus at high levels in their blood, giving it back to Culex mosquitoes that bite them, which then carry it on to humans.

"What we found is that, for some unknown reason, around the middle of July, mosquitoes in Atlanta seem to decide that they have had their fill of robins and they switch to feeding on cardinals," Levine said in a statement.  "But cardinals, even though they can be infected with West Nile virus, are much less likely to have enough virus circulating in their blood to transmit the disease back to feeding mosquitoes. That is why we called them 'supersuppressors'."

The timing of the shift couldn’t be better, occurring as mosquito populations approach their annual peak.

^{These birds really are guardian angels. Phil Lowe/Shutterstock}

The cardinals are probably not bearing the burden alone. Mockingbirds, gray catbirds and brown thrashers also appear to have a suppressing effect, showing why it really is a sin to kill a mockingbird.

The big question is what can be done to boost WNV suppression. Promoting suppressive species is one obvious option, but more information about why the mosquitoes change diet would probably help. Levine noted that WNV transmission appears to be lower from birds inhabiting old growth forests than those in other habitats. “So, we might find that keeping old growth forests intact, even in urban areas, can provide more than just an interesting piece of history," she said. Atlanta has an unusually large amount of urban tree cover for a major American city, almost four times Chicago’s.

An even more important question is whether there are species that act as suppressors for more dangerous mosquito-borne diseases. If so, these could become the modern equivalents of the geese that saved Rome.