By Kelvin Obambon
Seasonal weather patterns, rapid agricultural expansion, and the escalating effects of climate change have emerged as the primary catalysts driving the spread of Lassa fever across West Africa, according to findings from a landmark systematic review by Cochrane.
The insights were disclosed by Dr. Ekpereonne Esu, a Senior Research Associate at Cochrane Nigeria, during a two-day media roundtable held between Tuesday 14 and Wednesday 15 July, 2026, in Calabar, the Cross River State capital.
The comprehensive 2025 systematic review synthesized over five decades of global research spanning 1969 to 2023, offering an unprecedented look at the complex environmental and ecological dynamics fueling the circulation of the Lassa virus.
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Lassa fever, according to Esu, is a severe viral hemorrhagic illness endemic to parts of West Africa. “It is primarily transmitted to humans through contact with the excreta – such as urine, feces, and saliva – or blood of the multimammate rat (Mastomys natalensis). Because this rodent is both a common agricultural pest and highly adaptable to human environments, understanding its behavior is critical to predicting and preventing future outbreaks,” he explained.
Esu, an Associate Professor of Public Health at the University of Calabar, stated that the systematic review identified three primary environmental drivers of Lassa virus transmission.
“While rainfall does not directly alter the virus’s ability to infect humans, seasonal rain patterns dictate the movement and population cycles of the rodent reservoir. Human Lassa fever cases typically peak during the dry season. During this agricultural off-season, Mastomys natalensis rodents frequently migrate out of fields and into residential homes in search of food, significantly increasing the likelihood of human-rodent contact.
“As agricultural activities expand and natural habitats are converted into human-dominated landscapes, favorable conditions are created for the rodent host. Fragmented habitats and poor food storage practices in residential areas encourage M. natalensis to cluster inside homes. The review strongly links increased habitat fragmentation and human development to a higher risk of the virus spilling over from rodents to humans.
“While the multimammate rat remains the primary reservoir, the review notes that other rodent species, such as Mastomys erythroleucus, are also capable of carrying the virus, raising concerns about host-switching. Conversely, researchers point to a potential “dilution effect” driven by biodiversity. High biodiversity may lower infection rates by introducing competing species – such as the invasive black rat (Rattus rattus) – which compete with the primary reservoir for food and territory,” he explained, citing well-referenced studies on the subject matter.
Despite these critical insights, Esu noted that significant knowledge gaps remain, particularly regarding the precise biological mechanisms behind seasonal fluctuations and the long-term impact of biodiversity loss.
Looking forward, the threat is expected to intensify. As climate change continues to alter global temperatures and rainfall patterns, scientists project that the ecological niche suitable for the Lassa virus will expand. This expansion threatens to put millions of previously unexposed people across the region at risk.
To combat this escalating threat, researchers are calling for the urgent adoption of an interdisciplinary “One Health” approach – a framework that connects human, animal, and environmental health – to design robust, science-based policies capable of protecting vulnerable populations.
Addressing participants on the purpose of the workshop, Professor Angela Oyo-Ita, Director of Cochrane Nigeria, emphasized that scientific data must move out of academic silos to be useful.
“Any research that ends up only in the paper or in the researcher’s drawer is not worth its salt,” Professor Oyo-Ita stated. “We bring in the journalists because you are there to tell the story, and you know how best to communicate with the people. Evidence-based medicine pulls every available piece of information on a specific question, critiques it for bias, and summarizes it so that clinicians, policymakers, and patients can make informed decisions.”
By empowering the media to translate technical systematic reviews into clear public knowledge, Cochrane Nigeria aims to foster a more data-conscious society capable of tackling emerging health threats like Lassa fever from the ground up.
