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Item Identifying risk factors for Lassa fever infection in Sierra Leone, 2019-2021(2024-04-12) Sama, Daniel JumaBackground Lassa fever (LF) virus (LASV) is endemic in Sierra Leone and poses a significant public health threat to the region; however, no risk factors for LASV infection have been reported in Sierra Leone. The objective of this study was to identify the risk factors for LASV infection in an endemic community in Sierra Leone. Methods We conducted a case-control study by enrolling 37 laboratory-confirmed LF cases identified through the national LF surveillance system in Sierra Leone, and 140 controls resided within a one-kilometre radius of the case household. We performed conditional multiple logistic regression analysis to identify the risk factors for LASV infection. Results Of the 37 cases enrolled, 23 died (62% case fatality rate). Cases were younger than controls (19.5 years vs 28.9 years, p<0.05) and more frequently female (64.8% vs. 52.8%). Compared to the controls, LF cases had contact with rodents (rats or mice) in their households more frequently in the preceding three weeks (83.8% vs.47.8%). Households with a cat reported a lower presence of rodents (73% vs 38%, p<0.01) and contributed to a lower rate of LF (48.6% vs 55.7%) although not statistically significant (p=0.56). The presence of rodents in the households (Matched Adjusted Odds Ratio \[mAOR\]: 11.1), and younger age (mAOR: 0.99) were independently associated with LASV infection. Conclusion Rodent access to households is likely a key risk factor for LASV infection in rural Sierra Leone and potentially in other countries within the West African region. Controlling rodent access to households might help reduce household-level LASV infection in Sierra Leone.Item The enduring story of Lassa fever: Advancing the case for sustainable management strategies(Elsevier BV, 2025-07-03) Awoniyi, Adedayo MichaelLassa fever (LF) is an acute viral hemorrhagic illness caused by the Lassa virus (LASV), an enveloped, spherical virus belonging to the Arenaviridae family. LASV possess a single-stranded RNA genome of negative polarity and exhibits high genetic diversity, corresponding to the geographical distribution of its seven principal distinct clades across West Africa\[1\]. LASV was first isolated in 1969 from an American missionary nurse stationed in the rural town of Lassa, Borno State, Nigeria, following her return from a brief vacation in the United States\[2\]. Although she had travelled through Sierra Leone and made a stop in Jos, Nigeria, before arriving at her station, the virus was contentiously named after Lassa town. LF is endemic in several West African countries, particularly within the Mano River Union countries (Guinea, Liberia and Sierra Leone) and Nigeria, where annual outbreaks are consistently reported. Additionally, cases of LF or the detection of the virus have been reported in other West African countries, including Benin, Côte d’Ivoire, Mali and Togo. Sporadic exported cases have also been reported in Asia, Europe and North America, highlighting LF’s potential as a global health threat if not adequately addressed\[3\]. LASV is primarily hosted by the multimammate rat Mastomys natalensis, which typically remains asymptomatic and can harbor the virus throughout its lifespan. Infected rats shed LASV through their urine, faeces\[4\], saliva, and respiratory secretions, contaminating household surfaces, foodstuffs and other environmental materials. Human infection occurs predominantly through direct contact with infected rats or indirectly via exposure to contaminated items, especially the consumption of contaminated foodstuffs, and rarely through human-to-human transmission via contact with bodily fluids of an infected person or contaminated medical equipment\[3\]. LF presents with a wide clinical symptoms, ranging from asymptomatic to mild or severe illness and in severe cases, LF can lead to life-threatening complications including hemorrhage, respiratory distress, neurological disorder and multiple organ failure, particularly when diagnosis and treatment are delayed\[5\]. LF affects an estimated 897700 people annually, with 18000 deaths in the West Africa region\[6\]. The true burden of LF is likely underestimated due to several factors: early symptoms often mimic those of other common febrile illnesses, diagnostic capacity in many endemic areas is limited, and underreporting is exacerbated by social isolation “stigma” stemming from LF’s association with rats–disease of poverty, and certain cultural/religious beliefs seldom discourage victims from seeking medical care. Fatal outcomes are particularly high among pregnant women, where LF can lead to spontaneous abortion and elevated maternal mortality\[7\]. Survivors may suffer from long-term complications, such as sensorineural hearing loss, neurological impairments and other chronic health conditions, all of which can significantly diminish quality of life and increase economic burden on individuals, families and the government\[5,7\]. At a broader level, LF imposes a substantial socio-economic burden on affected countries through both direct costs such as medical treatment and indirect costs, including lost productivity due to the illness, long-term disability, and premature death. These impacts are especially devastating for families where the affected individual is the primary breadwinner, often leaving dependents in poverty or financial instability. Given the substantial socioeconomic burden posed by LF and other zoonoses, exacerbated by recent global experiences such as the COVID-19 pandemics, there is growing interest in strengthening the management of zoonotic threats. While advancements in vaccine development and public health interventions have led to notable reductions in the morbidity and mortality of many infectious diseases, similar progress has been limited for rodent-borne diseases like LF. To date, there is no World Health Organization (WHO)-licensed vaccine against LASV\[8\]. This general bottleneck in LF management could be in part due to LASV’s extensive genetic diversity, complex transmission dynamics shaped by environmental and socioeconomic conditions, and the persistent difficulty of controlling the rat reservoir responsible for maintaining viral circulation in endemic regions. As efforts towards vaccine development continue, it is important to implement integrated preventive strategies, such as improved housing and sanitation, public education to minimize human exposure to rats, to reduce human risk of LF and other rodent-borne diseases. In contrast to many high-income countries, many nations in the Global South, particularly in LF-endemic regions lack national or regional specific policies dedicated to rodent surveillance and population management, despite prominent rodent public health- and economic-related problems in the regions. Funding and institutional support for rodent- and rodent-borne disease research remain minimal if not non-existent, especially when compared to vector-borne diseases such as malaria. Additionally, there is marked shortage of trained rodent control specialists within endemic regions. Consequently, vulnerable communities are often left without effective or sustainable mitigation strategies. In many cases, local populations resort to the indiscriminate use of rodenticides, which are not only ineffective in the long-term but also pose significant risks to non-target species, including humans, due to accidental poisoning. The continued escalation of factors driving rat proliferation, such as climate change, rapid urbanization, overcrowding, increasing slum inhabitants, inadequate personal hygiene, precarious sanitation infrastructure, poor waste management and deepening poverty, pose a growing threat, particularly in LF endemic regions. These deteriorating conditions, common in both rural and urban communities across West Africa, could increase the frequency and intensity of human-rat interactions. Given the well-documented association between rat abundance and rodent-borne disease transmission risk, public health experts are sounding alarm over the heightened potential for future zoonotic outbreaks. It is no gainsaying to warn that LF-endemic regions are metaphorically “sitting on a powder keg” poised for emergence of potentially new zoonotic threats unless substantial interventions are prioritized. Such event could have devastating socioeconomic consequences, especially in communities of West Africa that are already grappling with persistent LF outbreaks and the recently declared viral mpox outbreak\[9\]. In conclusion, the complex transmission dynamics of LF, compounded by diagnostic challenges, limited treatment options and the lack of WHO-licensed vaccine, underscore the need for a comprehensive and multisectoral response. With case number rising steadily over the past decade, effective mitigation must extend beyond clinical management to address the socio-environmental determinants of transmission. Specifically, governments/policymakers should implement (a) sustained health education campaigns to raise community awareness about LF risk factors, transmission pathways and preventive practices; (b) coordinated investment in integrated research and action involving key stakeholders, including local community, academia, healthcare providers, ministries of health, agriculture, information and education, waste management authorities and the currently non-existent national rodent and vector control agencies. Such collaboration should aim to develop locally tailored protocols for rodent surveillance and population management to reduce human-rodent interactions and subsequent spillover of pathogens into human population; (c) enhanced case surveillance, early detection and rapid response mechanisms to limit potential outbreaks; (d) national policies to promote improved personal and community hygiene and sanitation practices to minimize LASV contamination; (e) stigma reduction strategies, including anti-discrimination policies and post-recovery reintegration programs to encourage timely health-seeking behavior and community support for survivors; (f) policies to discourage the consumption of bushmeat, particularly rodents, and encourage safer agricultural practices–such as storing harvested crops in rodent-proof containers; and (g) investment in the laboratory infrastructure and workforce (re)training to strengthen prompt confirmation and treatment. Collectively, these interventions will not only curb LF incidence but also reduce the risk of other zoonotic disease spillovers, thereby enhancing the overall resilience and wellbeing of the vulnerable populations, especially in endemic regions. Conflict of interest statement No conflict of interest was declared by the authors. Funding The study received no specific funding for the study. Authors’ contributions AMA conceived and designed the study and wrote the first draft of the paper. Both AMA and UB contributed to literature search, critically revised and improved the presentation of the paper and approved the final manuscript. Publisher’s note The Publisher of the Journal remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Edited by Zhang Q, Lei Y, Pan YItem Drivers of Lassa fever in an endemic area of southwestern Nigeria (2017–2021): An epidemiological study(Public Library of Science, 2025-06-25) Cadmus, SimeonBACKGROUND: Reporting two million human Lassa fever (LF) cases with around 10,000 associated annual mortality, the West African sub-region is endemic for Lassa fever virus (LASV). The true incidence of LF is difficult to determine because most LASV-infected individuals show no differentiating clinical signs and symptoms. We investigated the distribution of cases, post-hospitalization survival patterns, and evaluated factors contributing to infection and clinical course of the disease during an outbreak of LF in Ondo State, Nigeria, from 2017 to 2021. METHODS: We extracted LF data from the Integrated Disease Surveillance and Response weekly report of the Nigerian Centre for Disease Control for 2017-2021. Kaplan-Meier estimate was used to describe the probability of survival among the LF cases. Also, a univariable binary logistic regression was used to explore factors associated with mortality among the study participants. Key informant was interviewed and environmental assessments were also done. RESULTS: LASV infection was confirmed in 1,115 (24.5%) of 4,551 cases with clinical signs suggestive of LF (age 35.24 ± 20.77) and case fatality rate of 25.5%. Hospitalized patients who did not recover within 17 days had less than 50% chance of survival. Age is a strong predictor of survival; hospitalized patients >40 years were significantly more likely than younger ones to experience mortality (Odds ratio:2.46; 95% CI = 1.67-3.62; p < 0.01). Similarly, male patients were significantly less likely than the females to survive beyond 10 days of hospitalization. Open sun drying of food items and congested urban residential settings with history of frequent rat sightings are possible factors for the increase of LF cases in the study area. CONCLUSION: Current case definition in Ondo State identified close to 25% of laboratory confirmed LASV infection. Human activities during the dry season (October-March) are associated with increased LF cases. We propose a One Health disease surveillance approach that synchronizes farming activities with educational campaigns as a mitigation strategy against LASV infection and mortality in Nigeria.Item Contact networks of small mammals highlight potential transmission foci of Lassa mammarenavirus(African Field Epidemiology Network, 2025-08-10) Simons, DavidIntroduction Lassa fever, caused by Mammarenavirus lassaense (LASV), is an endemic zoonosis in several West African countries. Human infections primarily arise from rodent-to-human transmission, with Mastomys natalensis, a synanthropic species, serving as the principal reservoir. In Sierra Leone, small-mammal communities vary across land use gradients, potentially shaping LASV transmission risk in human populations. However, the extent to which anthropogenic environments facilitate small-mammal interactions conducive to viral transmission remains poorly understood. Methods We conducted small-mammal sampling over 43,266 trap nights, detecting 684 rodents and shrews in the LASV-endemic Eastern Province of Sierra Leone. To assess potential for within- and between-species transmission, we constructed space-sharing networks based on co-trapping within species-specific radii informed by home range estimates. These networks approximated shared space use rather than direct interaction, allowing comparison of potential encounter patterns across habitats. Results Small-mammal communities in agricultural settings had greater species richness and were more connected than those in villages and forests, although overall space-sharing rates did not differ substantially. Network topology varied by land use, with greater modularity in village networks. Notably, space sharing among M. natalensis individuals was more frequent in agricultural settings than in villages, suggesting that land use may modulate opportunities for intra-specific viral transmission. LASV seroprevalence across the small-mammal community was 5.7%, with antibodies detected in nine species. We found no major differences in seroprevalence by land use or network complexity. Conclusion These findings underscore the importance of cross-habitat surveillance to understand host ecology and LASV transmission. Accounting for species-specific space use and habitat-dependent interaction structures is crucial for identifying key hosts and spillover settings.Item Contact Networks of Small Mammals Highlight Potential Transmission Foci of Mammarenavirus lassaense(American Society of Tropical Medicine and Hygiene, 2026-04-28) Simons, DavidLassa fever (LASV; Mammarenavirus lassaense) is an endemic zoonosis in West Africa. Human infections arise from rodent-to-human transmission, primarily from the synanthropic reservoir Mastomys natalensis (M. natalensis). In Sierra Leone, small-mammal communities vary across land-use gradients, shaping LASV transmission risk. How anthropogenic environments facilitate the rodent-rodent interactions remains poorly understood. Small mammals were sampled over 43,266 trap nights in Sierra Leone's LASV-endemic Eastern Province, detecting 684 rodents and shrews. To assess potential transmission, space-sharing networks were constructed from co-trapping events within species-specific home range radii. Shared space use was approximated in these networks, enabling the comparison of encounter patterns across habitats. Network topology varied significantly by land use. Village networks were the most connected (highest average degree), whereas agricultural communities supported the most species (higher rarefied richness) and were the most fragmented (higher modularity). Notably, the probability of intraspecific space sharing among M. natalensis was highest in agricultural settings, suggesting that land use modulates key intraspecific transmission pathways. Lassa fever seroprevalence was 5.7% community-wide, with antibodies detected in nine species. No statistically significant association was found between overall seroprevalence and land use or aggregate network structure (mean degree). However, predictive modeling for M. natalensis indicated that a higher individual degree is associated with seropositivity, suggesting complex, scale-dependent relationships. These findings reveal that simple ecological drivers do not fully explain LASV exposure, highlighting the importance of species-specific behaviors (i.e., M. natalensis clustering in agriculture) and the multihost serological landscape in assessing transmission risk.