As the world braces for the impact of the El Niño in the coming months scientists have warned changes in rainfall can trigger increases in rodent populations and subsequent spikes in infection, posing new risks for human health.

The research, led by the Natural History Museum, London, and University of Oxford with collaborators at University College London and other institutions, developed a modelling framework using almost 30 years of data collected by the University of Antwerp on the Natal multimammate mouse (Mastomys natalensis), a widespread African rodent and the main host of Lassa virus.

When tested against more than 6,000 recorded Lassa fever cases in Nigeria, the model accurately predicted the seasonal timing of outbreaks, suggesting it could help scientists identify periods of heightened disease risk.

Lassa fever is a rodent borne zoonotic disease endemic to West Africa. Humans can become infected through contact with infected rodents or food and household items contaminated by their urine or faeces. Severe cases can be fatal. Understanding the environmental conditions affecting these animals is therefore important for identifying when the risk of disease spillover may be greatest.

Using more than 20,000 rodent captures from Tanzania alongside climate and pathogen exposure data, the researchers found that rainfall was a key driver of rodent populations, with wetter conditions likely increasing food availability and stimulating breeding.

Dr Gregory Milne, postdoctoral researcher at the Natural History Museum, who co-led the study said: “The planetary emergency is changing the conditions in which people, wildlife and pathogens interact. To understand what that means for human health, we need to understand the ecological processes connecting environmental change to disease.

“Our research shows how changes in rainfall can drive rodent population booms and influence infection, helping explain why disease risk varies over time. When we applied the model to Nigeria, the timing of peaks in infected rodents closely matched the seasonal timing of human Lassa fever outbreaks.

“Understanding how environmental change affects wildlife populations is essential for anticipating future zoonotic disease risks, and could help scientists and public health authorities identify periods of heightened risk.”

Prof Kate Jones, Director of the UCL People and Nature Labopens in a new window said: “This shows the value of putting ecology at the centre of zoonotic disease research. Climate alone cannot predict the size of an outbreak, but understanding the ecological processes connecting environmental conditions, wildlife populations and pathogens could help identify periods when the risk of spillover is higher.”

The researchers also found that the virus could persist in rodent populations by passing from mothers to their offspring. Their model estimated that almost 80% of infected pregnancies resulted in this form of transmission, helping the virus survive between breeding seasons.

The impact of rainfall was particularly clear during the 2015–2016 El Niño event, when unusually heavy rainfall in East Africa coincided with a predicted rise in young rodents followed by an increase in infected animals.

Peaks in infected young rodents tended to occur around a month before peaks in human cases. In 83% of comparisons, the predicted peak in infected rodents and observed peak in human cases were within 28 days of each other.

The new framework goes beyond identifying correlations between climate and human disease outbreaks by examining the ecological processes connecting them. Future climate projections could therefore be used to investigate how environmental conditions may alter wildlife populations, pathogen transmission and zoonotic risk.

The approach could also be applied beyond Lassa fever. Rodents host more zoonotic pathogens than any other group of mammals and respond rapidly to environmental change. Similar models could investigate other climate sensitive host pathogen systems, particularly where human disease data are scarce.

As the planet changes, establishing how climate affects animal populations and pathogen transmission could help scientists anticipate and prepare for future changes in zoonotic disease risk.