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Item Lassa Fever in post-Ebola Sierra Leone. Sociodemographics and case fatality rates of in-hospital patients admitted at the Kenema Government Hospital Lassa Fever Ward between 2016-2018(Springer Science and Business Media LLC, 2019-11-06) Kangbai, Jia BaingaBackground Lassa fever (LF) is a zoonotic acute viral illness mainly found in West Africa. The disease is endemic in some parts of West Africa including Sierra Leone, Liberia, Guinea and Nigeria; while other neighboring countries at high risk of its outbreak since the animal vectors are distributed throughout the region.Methods This is a retrospective mixed cohort study that analysed the treatment history containing the sociodemographic and clinical characteristics of 52 laboratory-confirmed LF cases that were admitted to the Kenema Government Hospital Lassa Fever Ward (KGHLFW) during 2016 to 2018; i.e. during the post Ebola outbreak in Sierra Leone. The LF patients whose treatment history we analysed came from either within or outside Kenema district were the KGHLFW is located.Results Majority (59.6%, n = 31/52) of the LF cases recorded during the period under review were adults; females (65.4%, n = 34/52). 2016 recorded more (40.4%, n = 21/52) LF cases; 2017 (28.8%, n = 15/52) and 2018(30.8%, n = 16/52).Conclusions We highlighted the significance of LF preventive and control measures that can target its seasonal epidemics. These measures could include strategies that can reduce human contact with the rodent vector as well as raise sensitization and awareness about LF among local residents especially those residing along the LF belt in eastern Sierra Leone.Item Endotheliopathy and Platelet Dysfunction as Hallmarks of Fatal Lassa Fever(Centers for Disease Control and Prevention, 2020-09-29) Horton, Lucy E.L assa fever (LF) is an acute viral hemorrhagic fever endemic to West Africa, where ≈300,000-500,000 cases/year occur and mortality rates are high (1).Humans primarily are infected from exposure to excreta from the rodent host, Natal multimammate mouse (Mastomys natalensis).Pregnant women especially are susceptible to severe disease; infection during pregnancy usually leads to spontaneous abortion (2).The high rates illness and death, lack of vaccines or approved treatments, and potential to cause a public health emergency led to the US Centers for Disease Control and Prevention to classify LF as a category A bioterrorism agent (https://emergency.cdc.gov/agent/agentlist-category.asp) and the World Health Organization to classify LF as a priority disease (https://www.who.int/dg/priorities).Early clinical manifestations and symptoms of LF often are nonspecific and easily confused with other diseases prevalent in the endemic region, such as malaria and typhoid fever.Consequently, patients often are not admitted for LF treatment until symptoms are severe and they have failed other therapies.Bleeding at mucous membranes and edema generally are seen in the most severe cases (3,4), but bleeding diathesis reportedly was common during a recent outbreak in Nigeria (4).Overt hemorrhage is rare, mostly limited to the mucosal surfaces, and not severe enough to cause shock.Pathologic changes seen on autopsy lack major cell and tissue injury but include signs of pleural effusion, pulmonary edema, ascites, and gastrointestinal mucosa bleeding (5,6), all indications of systemic vascular leakage.Severely ill LF patients often have mild to moderate thrombocytopenia, but rarely have platelet counts <100,000/μL (7).Thrombocytopenia is a common feature of hemorrhagic fevers and vascular permeability disorders (8), but the decrease in platelet counts in acute LF is not low enough to cause spontaneous hemorrhage.LF could be characterized as a disease of enhanced vascular permeability but the underlying Endotheliopathy and Platelet Dysfunction as Hallmarks of Fatal Lassa FeverItem Epidemiological insight into the possible drivers of Lassa fever in an endemic area of Southwestern Nigeria from 2017 and 2021(2023-09-08) Cadmus, SimeonBackground Lassa fever (LF) is a viral disease transmitted between animals and humans, commonly found in West Africa, including Nigeria. The region experiences an estimated annual total of about 2 million LF cases in humans, leading to 5,000 to 10,000 deaths. Strikingly, up to 80% of LF-infected individuals show no symptoms, making its true incidence hard to determine in endemic populations. We investigated LF distribution, mortality, survival patterns, and contributing factors during a local outbreak in Nigeria, from 2017 to 2021. Method Data from the Integrated Disease Surveillance and Response weekly line list for 2017 to 2021 were extracted. The survival pattern of LF patients was visualized with the Kaplan-Meier curve, binary logistic regression model was employed to explore LF-associated factors and level of statistical significance (α) was set at 5%. Result Overall, 4,554 participants were recruited between 2017 and 2021. Their average age varied from 31.82 ± 20.0 to 37.85 ± 17.89. LF-positive patients decreased from 26.9% in 2017 to 17.7% in 2021, paralleling the mortality trend. In 2021, patient survival ranged from 5 to 30 days. Male patients had lower survival odds in the initial 10 days of hospitalization, improved chances from days 10 to 20, and reduced probabilities beyond day 20. Residence location and age were significant factors (p<0.05) associated with LF in Ondo State. Conclusion The decline in LF cases in 2021 could be attributed to the ongoing intervention by Nigerian Centre for Disease Control or the disruption caused by the COVID-19 pandemic in 2020. To address LF challenges in hotspot areas, we propose Community Action Networks that would operate using the One Health approach involving local stakeholders sustainably to promote Early Warning/Early Response system in high-risk settings and mitigate LF-related issues. SUMMARY Lassa fever (LF) is an important disease of global public health concern that is endemic in West Africa. In Nigeria, the disease constitutes a major health challenge with outbreaks being recorded on an annual basis despite efforts channeled towards combating it by the government at various levels. This study analysed a five years data of LF in Ondo State southwestern Nigeria. The results identified age and location were identified as important factors associated with infection and mortality among LF patients as the incidence and case fatality rates were highest among adults (≥ 45 years), while the highest number of suspected, confirmed and dead cases was recorded in Owo Local Government Area. Furthermore, we identified drying of food items by the roadside where rodent vectors can access them, presence of a local market, poor and unsafe sewage disposal, and proximity of refuse dumps to residential areas as possible socio-ecological factors/practices fueling the endemicity and seasonal outbreak of LF. These findings emphasize the need for active involvement of community members in the already established national LF surveillance network to facilitate prompt case identification, and early reporting and response in the LF-endemic areas of the country.Item Ecological correlates and predictors of Lassa fever incidence in Ondo State, Nigeria 2017–2021: an emerging urban trend(Nature Portfolio, 2023-11-27) Cadmus, SimeonLassa fever (LF) is prevalent in many West African countries, including Nigeria. Efforts to combat LF have primarily focused on rural areas where interactions between rodents and humans are common. However, recent studies indicate a shift in its occurrence from rural to urban areas. We analysed secondary data of reported LF outbreaks from 2017 to 2021 in Ondo State, Nigeria to identify the distribution pattern, ecological variations, and other determinants of disease spread from the ward level using nearest neighbour statistics and regression analysis. Data utilised include LF incidence, ecological variables involving population, nighttime light intensity, vegetation, temperature, market presence, road length, and building area coverage. ArcGIS Pro 3.0 software was employed for spatial analysis. Results revealed spatio-temporal clustering of LF incidents between 2017 and 2021, with an increasing trend followed by a decline in 2021. All wards in Owo Local Government Area were identified as LF hotspots. The ecological variables exhibited significant correlations with the number of LF cases in the wards, except for maximum temperature. Notably, these variables varied significantly between wards with confirmed LF and those without. Therefore, it is important to prioritise strategies for mitigating LF outbreaks in urban areas of Nigeria and other LF-endemic countries.Item Current sampling and sequencing biases of Lassa mammarenavirus limit inference from phylogeography and molecular epidemiology in Lassa fever endemic regions(Public Library of Science, 2023-11-08) Arruda, Liã BárbaraLassa fever (LF) is a potentially lethal viral haemorrhagic infection of humans caused by Lassa mammarenavirus (LASV). It is an important endemic zoonotic disease in West Africa with growing evidence for increasing frequency and sizes of outbreaks. Phylogeographic and molecular epidemiology methods have projected expansion of the Lassa fever endemic zone in the context of future global change. The Natal multimammate mouse (Mastomys natalensis) is the predominant LASV reservoir, with few studies investigating the role of other animal species. To explore host sequencing biases, all LASV nucleotide sequences and associated metadata available on GenBank (n = 2,298) were retrieved. Most data originated from Nigeria (54%), Guinea (20%) and Sierra Leone (14%). Data from non-human hosts (n = 703) were limited and only 69 sequences encompassed complete genes. We found a strong positive correlation between the number of confirmed human cases and sequences at the country level (r = 0.93 (95% Confidence Interval = 0.71-0.98), p < 0.001) but no correlation exists between confirmed cases and the number of available rodent sequences (r = -0.019 (95% C.I. -0.71-0.69), p = 0.96). Spatial modelling of sequencing effort highlighted current biases in locations of available sequences, with increased sequencing effort observed in Southern Guinea and Southern Nigeria. Phylogenetic analyses showed geographic clustering of LASV lineages, suggestive of isolated events of human-to-rodent transmission and the emergence of currently circulating strains of LASV from the year 1498 in Nigeria. Overall, the current study highlights significant geographic limitations in LASV surveillance, particularly, in non-human hosts. Further investigation of the non-human reservoir of LASV, alongside expanded surveillance, are required for precise characterisation of the emergence and dispersal of LASV. Accurate surveillance of LASV circulation in non-human hosts is vital to guide early detection and initiation of public health interventions for future Lassa fever outbreaks.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.