As the COVID-19 pandemic has made very clear, viral spillover from animal hosts to humans is a serious threat. However, viruses are also capable of spilling back into wildlife from humans, establishing reservoir populations in wildlife in new geographic regions. This idea of "spillback" is also an important aspect for scientists to study in our efforts to prevent pandemics.

A paper recently published in PLOS Neglected Tropical Diseases looked at viral spillback in the Neotropics—Central and South America, as well as Mexico and the Caribbean. They used yellow fever as a case study, as it has spilled back and established a persistent enzootic reservoir in primates and mosquitoes. They wanted to understand what ecological factors may cause other viruses to do the same. They highlight the endemic circulation of the chikungunya, dengue, and Zika viruses as other points of concern for South American countries. All of these are spread by mosquitos and have primate hosts.

The researchers used mathematical modelling to explore how a spillback event could play out. Parameters such as the virus' extrinsic incubation period, mosquito lifespan, and primate population size, predicted the effects a single infected primate might have in creating an enzootic reservoir. Evidence showed that given the right conditions, spillback events can occur and the chikungunya, dengue, and Zika viruses could establish persistent reservoirs in the Neotropics. This possibility is of great concern to epidemiologists, given the danger of these four mosquito-borne viruses to humans.