Lassa fever is a disease endemic in West Africa caused by the Lassa virus (LASV) [1]. LASV is transmitted through the contamination of foods and other items by the urine and faeces of the infected multi-mammate rat, Mastomys natalensis [1,2]. Lassa fever was first discovered in Nigeria in 1969 [3], and there have been thousands of confirmed cases since. In moderate to mild cases, Lassa fever may cause fever, general body weakness, malaise, headache, sore throat, muscle pain, and chest pain [4].
Lassa fever is endemic in Nigeria and cases peak annually during the dry season (December - April) and decrease in May [5]. The incubation period for Lassa fever is between 1 - 3 weeks, which can lead to infection spread from region to region [6]. The rodent host is the primary driver of the Lassa fever seasonal trends, with reproduction cycles of the multimammate rat increasing during the rainy season [7]. Lassa fever is endemic in West African countries such as Nigeria, Liberia. Guinea, Mali, Benin, Togo and Sierra Leone, with Nigeria s the most affected country. Lassa fever cases have also been observed in Germany, Netherlands, Sweden and USA following travel to West Africa. In Nigeria, LASV-endemic areas cover approximately 40% of country [5].
There are seven distinct linages of the LASV recorded, and lineage I, II, and III have been identified in north-eastern, southern and north-central Nigeria [7]. There has not been any other detection of Lineage I since 1969, when the first LF case was reported [7]. Lassa fever cases in Nigeria have increased since 2016, with 633, 833 and 1189 confirmed cases in 2018, 2019 and 2020, respectively (Table 1) [8].
The emergence of the Coronavirus disease (COVID-19) has generated public health concerns in the Nigeria due to the similarities in symptoms, such as fever, cough and shortness of breath [9]. The COVID-19 pandemic has proven difficult to tackle in Nigeria since the initial case on 27 February 2020 [13]. In December 2020, Nigeria entered the second wave of the pandemic, prompting the establishment of an Emergency Operation Centre by the Nigeria Centre for Disease Control (NCDC) to control the rate of spread and limit case fatalities [14, 15].
The number of Lassa fever confirmed cases in Nigeria from 2017 to 2018, 2018 to 2019 and 2019 to 2020 increased by 296, 86 and 451, respectively, during the peak epidemiological period, usually the first 15 weeks of each year (Table 1). In 2021, the confirmed cases markedly decreased by 679, indicating plausible underreporting of Lassa fever owing to COVID-19 [10]. The mild and asymptomatic nature of COVID-19 and Lassa fever [11], coupled with noncompliance with testing, is a significant factor contributing to the underreporting of Lassa fever cases, as people may be hesitant to attend testing sites due to COVID-19. Further misdiagnosis may be seen due to shared clinical features of COVID-19 and Lassa fever, which may be difficult to distinguish in the early stages of disease [12]. This temporal coincidence may have led to increased public health and socioeconomic setbacks owing to Nigeria's limited health care resources for intensive public health interventions [10]. This study describes the current state of Lassa fever in Nigeria amidst the COVID-19 pandemic.
The NCDC, in January 2020, declared Lassa fever endemic in Nigeria [16]. In response to this, the National Emergency Operation Centre (EOC) was activated in 8 states, including the Federal Capital Territory (FCT). The EOC consisted of an interdisciplinary and multi-partner team to subdue the spreading of the disease [17]. Managing Lassa fever in some of these states has proved difficult due to the simultaneous outbreak of COVID-19 [18]. As Lassa fever peaks in the dry season (December-April), different emergency phases of the virus have been experienced in the country.
Nigeria has recorded a decrease in the number of cases of the LASV since the first quarter of 2020. After a three-month emergency phase (24 January 2020 – 28 April 2020), the Lassa fever outbreak in 2020 was declared over, due to a considerable decrease in the number of confirmed cases [19]. Despite this, the Lassa Fever Technical Working Group has continued coordinating response and surveillance efforts throughout the country [19].
According to the NCDC, in 2020, there were 6791 suspected cases and 1189 confirmed cases, while there were only 4654 suspected cases and 510 confirmed cases in 2021 [8], indicating a large drop in the epidemiological trend in 2021 (Table 1) [8]. Between weeks 1-35, 909 confirmed cases and 170 deaths have been recorded (Figure 1), indicating a spike compared to the same period in 2021 (365 confirmed cases, 83 deaths) [8]. Between weeks 1-35 in 2022, 70% of all confirmed cases reported were from three states: Ondo (31%), Edo (26%), and Bauchi (13%) States (Figure 1) [8]. The age group affected predominantly is 21-30 years, while the male-to-female ratio is 1:0.8 for confirmed cases [8].
The increase in the epidemiological trend of Lassa fever since the beginning of 2022 has prompted the activation of the National Emergency Operation Centre response to curtail the spread of Lassa fever disease [16]. This has involved the deployment of the National Rapid Response Teams (NRRT), multi-sectoral, multidisciplinary coordination of outbreak response activities in Nigeria [8].
Number of confirmed cases and deaths in Nigeria by state, (week 1-35), 2022 [8]
The disease prevention measures for Lassa fever recommended to the community by the NCDC include: ensuring proper environmental sanitation such as the blockage of holes in the house that could serve as entry points for rats, covering of dustbins, appropriate refuse disposal such as setting up dumpsites at locations far from homes, appropriate storage of foodstuffs such as rice, beans, maize and garri in well-covered containers with tight-fitting lids, avoiding the drying of foodstuffs by spreading them outside on the floor or roadsides where they are exposed to contamination,and avoiding bush burning which could lead to the displacement of rats from bushes to human dwellings. The NCDC also recommended eliminating rats in homes and communities through methods such as setting rat traps, and practising good personal hygiene by washing hands frequently with soap and water or using of hand sanitisers [20].
As of 4 September 2022, the measures that have been put in place to curtail the Lassa virus include: the activation of the EOC response mode level 2 in January 2022 to facilitate an effective multi-sectoral and multidisciplinary coordination of the 2022 Lassa fever outbreak response and subsequent de-escalation in May 2022. Eight molecular laboratories have been set up to ensure that all samples are tested, and results are available within a short period. A Lassa fever preparedness assessment was carried out across the 36 states and FCT, and State Public Health Emergency Operation Centres (PHEOCs) were activated in affected states. The NCDC also deployed National Rapid Response Teams (NRRT) to the FCT, Nasarawa, Edo, Ondo, Bauchi, Ebonyi, Oyo, Taraba, and Benue states [8]. The NRRT, through the PHEOCs, strengthen preparedness and response activities such as outbreak investigation, contact tracing, response coordination, case management, psycho-social care for infected people, risk communication, and infection prevention and control activities [21]. In addition to implementing an environmental response campaign in high-burden states by the Federal Ministry of Environment, risk communication and community engagement activities using the media and Nigeria Lassa fever epidemiological study were also implemented [8]. Lastly, the NCDC uses the One Health approach to strengthen the capacity of states to effectively manage LASV, COVID-19 and other zoonotic diseases [21].
There is currently no vaccine licensed for LASV, despite several vaccine candidates in development [22]. The International AIDS Vaccine Initiative (IAVI)’s LASV vaccine candidate is currently in phase I clinical trial stage for evaluation, and it is expected to undergo phase II trial once the phase I is completed [23].
Despite these measures, similarities in the signs and symptoms of COVID-19 and Lassa fever diseases, leading to potential misdiagnosis, is a great challenge towards curbing Lassa fever in Nigeria during the COVID-19 pandemic. Most Lassa fever cases are asymptomatic, and limited testing capacity leaves patients unaware of their condition, leading to communal transmission [17, 18]. Furthermore, inadequacy in the healthcare system poses a challenge to curbing Lassa fever in Nigeria. With the current doctor-to-patient ratio of 1:5000, a burden is placed on the health system to fight more than one epidemic at a time [17, 18].
Nigeria has eight centres for LASV testing across 36 states and Federal Capital Territory (FCT). An increased number of test centres will enhance surveillance and monitoring activities and improve active case detection, thereby limiting the rate of disease spread. As there is currently no licensed LASV vaccine, it is best to continue to intensify the implementation of the LF Environmental response campaign in high-burden states and other areas. Although, the predominant age group reported between weeks 1-35 in 2022 is 21-30 years, all age groups are advised to take the necessary precautions against LASV.
In this work, we described the current situation of Lassa fever in Nigeria in the context of the COVID-19 global pandemic. The recent epidemiological trends indicate that Lassa fever remains a concern. Therefore, it is important to continue surveillance, monitoring, and active testing across all regions to prevent further transmission and health complications. The co-existence of the COVID-19 pandemic has made the Lassa fever situation more difficult to handle. Eliminating these viral infections will necessitate a concerted effort from the government and the general public, the increased uptake of COVID-19 vaccine is vital, and a licensed LASV vaccine is needed soon.
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