An outbreak investigation of a highly fatal Lassa Fever in Bo District, Sierra Leone, 2023

Ibrahim Conteh^1,2,&, Hassan Swarray^1,2, Gbessay Saffa¹, Amara Alhaji Sheriff^1,2,3, Solomon Aiah Sogbeh^1,2,3, Umaru Sesay^1,2,3, Kassim Kamara^1,2,3, Anna Jammeh^2,3, Adel Hussein Elduma^2,3, Gebrekrstos Negash Gebru^2,3

 

¹Ministry of Health, Freetown, Sierra Leone, ²Sierra Leone Field Epidemiology Training Program, Freetown, Sierra Leone, ³African Field Epidemiology Network, Freetown, Sierra Leone

 

 

^&Corresponding author
Ibrahim Conteh, Ministry of Health, Freetown, Sierra Leone.

 

 

 

 

Lassa fever is a zoonotic acute viral disease caused by the Lassa fever virus. Lassa fever is endemic in many West African countries, including Sierra Leone, Liberia, and Guinea. The main reservoirs of the Lassa fever virus are the Mastomys natalensis rodent. Lassa Fever is an acute and severe disease, marked by a range of symptoms similar to many other diseases that can easily be mistaken, making early detection and containment difficult. Lassa fever transmission to humans occurs through contact with items contaminated with infected rodent droppings. In healthcare settings with inadequate infection control measures, person-to-person transmission is also common [1]. The incubation period for Lassa fever infection ranges from 2 to 21 days, with an overall case fatality rate of 1%, but 15% among severe or hospitalized cases. Fatal outcomes typically occur within 14 days of symptom onset [2]. Lassa fever symptoms may include one or more of the following: fever, general weakness, malaise, headache, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough, and abdominal pain. In the severe Lassa fever state, patients may present with facial swelling, fluid in the lung cavity, bleeding, and low blood pressure [2]. However, about 80% of Lassa fever cases are asymptomatic, with a death rate of 1% [3].

 

In Africa, Lassa fever accounts for 100,000 to 300,000 infections and approximately 5,000 deaths annually [4]. However, these estimates might be crude and inaccurate due to weak and non-standardized Lassa fever surveillance systems among the Lassa fever endemic countries in the region. In some areas of Sierra Leone and Liberia, Lassa fever infections account for 10% to 16% of all hospital admissions [1]. In Sierra Leone, Lassa Fever is one of the immediately reportable diseases under surveillance. It is endemic in the Kenema District and poses a substantial threat, with 428 confirmed cases recorded at the Kenema Lassa Fever Reference Laboratory between 2012 and 2019 [5]. Lassa fever ranks among the leading causes of mortality in the country, demonstrating a high fatality rate. Six of Sierra Leone's 16 districts reported confirmed Lassa fever cases in the last three years [6].

 

On the 2nd of February 2023, the Bo District Health Management Team (DHMT) notified the National Disease Surveillance Programme of a cluster of deaths suspected of Lassa fever. The deaths were reported from Damballa village, Selenga Chiefdom, Bo District. Although Damballa has a well-established Lassa fever surveillance system, it was not sensitive enough to detect the cases early. Upon receipt of the notification, the National Surveillance Program requested the African Field Epidemiology Network through the Sierra Leone Field Epidemiology Training Program (SLFETP) to support the Bo District Health Management Team (DHMT) in the outbreak investigation and response. From February 2 to March 14, 2023, a team of the SLFETP intermediate trainees and National surveillance officers were deployed to investigate, confirm diagnoses, identify sources of infection, interrupt transmission, and establish the epidemiological characteristics of the outbreak. This investigation sought to inform public health decision-makers to guide future control and response efforts.

 

 

Study Design and Period

 

We conducted a descriptive outbreak investigation of cases and contacts in Bo District from February 2 to March 10, 2023.

 

Study Area

 

This investigation was conducted in Selenga Chiefdom, Bo District. Selenga Chiefdom is among the sixteen chiefdoms in the Bo district. It has only two functional health facilities (a community health center and maternal and child health post) with a population of 9,786 according to the 2022 projected population using 2021 mid-term census data. Bo district is among the five districts that share a border with Kenema District, considered the Lassa Fever endemic district in Sierra Leone. Lassa Fever cases have been reported in Bo District and several other parts of Sierra Leone. However, since 2015, when the Bo district recorded its last outbreak of Lassa Fever, Selenga's chiefdom has not reported any outbreak.

 

Case definition

 

Lassa fever cases were categorized into suspected, confirmed, and probable cases, adapting the Lassa Fever case definition in the third edition of the integrated disease surveillance and response technical guidelines.

 

A suspected case: was any person who presented with gradual onset of one or more of the following: malaise, fever, headache, sore throat, cough, nausea, vomiting, diarrhoea, myalgia (muscle pain), central chest pain, hearing loss, and either history of contact with excreta or urine of rodents or history of contact with a confirmed Lassa fever case or with unexplained bleeding from Bo District since January 1 to March 14, 2023.

 

A probable case: was any suspected case who died before the sample was collected for laboratory testing or any suspected case with an epidemiological link to a confirmed case.

 

A confirmed case: was any suspected case with laboratory confirmation (positive IgM antibody, PCR, or virus isolation) from January 1 to March 14, 2023.

 

Contact: We defined contact as any person who had direct or indirect contact with a case or surfaces contaminated by body secretions of cases within Selenga Chiefdom in Bo district from January 1 to March 14, 2023.

 

Data collection

 

We adopted the Viral Hemorrhagic Fever (VHF) Case Investigation Form of the third edition of the Integrated Diseases Surveillance and Response (IDSR) technical guideline. From February 2 to March 14, 2023, a comprehensive investigation was conducted in eight health facilities and their respective catchment communities in three chiefdoms: Selenga, Niawa Lenga, and Kakua. We collected demographic and clinical data, including age, sex, occupation, date of onset, exposure, and travel history within 21 days before the onset of symptoms. Information was collected through interviews and reviews of health facility records. We interviewed patients, contacts, and family members and reviewed facility records and case-patient notes from these facilities.

 

Laboratory diagnosis

 

We collected blood samples from six suspected cases and sent them to the Kenema Reference Laboratory for confirmation of Lassa fever. The samples were tested using Reverse transcription polymerase chain reaction (RT-PCR) to confirm Lassa fever infection.

 

Active case search and contact tracing

 

To determine the scope and magnitude of the outbreak, we searched for additional cases in the hospital and the cases´ community that met the suspect or probable case definition. For cases in nearby healthcare facilities, we obtained medical records for patients admitted between January 01, 2023, and March 14, 2023, with fever above 38oC and who did not respond to appropriate antimalarial and antibiotic treatment within 72 hours. We also interviewed clinicians and nurses to identify suspected cases. For cases in the community, we went to the cases´ houses and assessed all 169 households. In addition, we interviewed 11 key informants in the village (i.e., village and religious leaders) and community health workers to find additional cases and identify contacts. The key informants were selected purposefully. Thirty contacts were line-listed, followed up, and monitored daily for 21 days to document any signs and symptoms of Lassa fever.

 

Environmental assessment and rodent trapping

 

The investigation team assessed all 169 households and their surrounding areas in the Damballa Community, Selenga Chiefdom. All households were assessed since the community is very small and uses communal amenities, including sharing toilets, water, and food, the team assessed the case-patient residence, food and water storage, household waste management, and evidence of rodent infestation. A total of 50 Sharman traps were laid down in and outside of the affected households, and in 10 nearby households, seven rodents were trapped using Sharman traps and bait, three from the case-patient residence and four from the surrounding households. The samples of the trapped rodents were tested by RT-PCR. The nearby households were selected randomly.

 

Data analysis

 

We described the demographic, clinical, and exposure data for each case identified during the investigation.

 

Ethical considerations

 

This investigation was performed as a part of the Lassa fever outbreak investigation and public health response activities; thus, it did not require ethical clearance from the Sierra Leone Ethics and Scientific Review Committee. The District Health Management Team provided administrative clearance to conduct the investigation, and verbal informed consent was sought before interviewing the cases or their family members. Data collected from cases were stored on a password-protected computer to maintain privacy and confidentiality and shared only among the investigators. No identifiable information was disclosed publicly.

 

 

Descriptive epidemiology, clinical, and laboratory findings

 

Eight suspected Lassa fever cases were recorded in Selenga chiefdom, Bo District. However, no cases were detected from Niawa Lenga, and Kakua chiefdoms. Of the 8 suspected cases, two were probable cases, a male adult and a female child, both of them died before being notified to the district surveillance unit. Of the remaining six suspected cases, two were confirmed for Lassa fever, and both died, making the case fatality rate 100% (2/2). Both confirmed cases were females (Table 1). The population at risk of Lassa fever was 9786 residents in Selenga chiefdom in Bo District, with an attack rate of 20 per 100,000 population (2/9786*100,000).

 

The median age of both probable and confirmed cases was 22 years with Inter Quartile Range (IQR) of 24 years to 30 years. All the cases (suspected, probable, and confirmed) presented with fever (8/8). While bleeding was reported by the two (100%) probable cases and two (100%) confirmed cases. Similarly, the two (100%) probable cases and two (100%) confirmed cases reported generalized pain (Table 1).

 

The two confirmed cases and two probable cases lived in the same household, implying a propagated, person-to-person outbreak which is confirmed by the epi curve (Figure 1).

 

Case 1: A 10-month-old female developed symptoms of fever and cough on January 23, 2023. She was administered drugs at home which were bought from over the counter, but she did not respond to treatment. She developed seizures on January 25th, 2023, and was then taken to Damballa Community Health Centre. She tested negative for malaria and was diagnosed with Acute Pneumonia. Antibiotic treatment was prescribed and commenced at Damballa Community Health Centre before referral to Bo Government Hospital on the same day, January 25, 2023, for additional medical care. The child died while the family was getting ready to leave for Bo and was buried without further investigation. This case was later classified as a probable case.

 

Case 2: A male bike rider, aged 24 years, father of case 1, developed symptoms of fever, headache, and body aches on January 19, 2023, shortly after arriving in Damballa, Selenga Chiefdom, Bo District from Kikifaranah community, Diang Chiefdom, in Koinadugu District. He developed generalized weakness and vomiting the following day and sought treatment from an unlicensed community quack nurse. With no improvement observed, he then visited Damballa Community Health Centre on January 25, 2023, where he was suspected of having pneumonia and tuberculosis. He was treated for pneumonia and kept on observation overnight. He tested negative for malaria. The case died at the Damballa Community Health Centre on January 26, 2023. This case was also later classified as a probable case. On January 27, 2023, six people, comprising relatives and community neighbors, were involved in the washing and handling of the corpse up to the burial. However, none of them developed signs and symptoms after follow-up for 21 days.

 

Case 3: On January 28, 2023, a 21-year-old woman, mother of Case 1 and girlfriend of Case 2, developed similar signs and symptoms as cases 1 and 2. The following day, January 29, 2023, she presented at the Damballa Community Health Center. Following a clinical assessment at the Community Health Center, the attending clinician referred the patient to Bo Government Hospital on the same day for further investigation and treatment. Against medical advice, the family chose to take her to a nurse in Bo, where she received home treatment. Later, on January 31, 2023, her condition worsened, and she began vomiting blood and bleeding from the nose. She was taken to Mercy Hospital in Bo. However, due to the severity of her condition, staff at the Mercy Hospital referred her to Bo Government Hospital, where she died on arrival. After taking her corpse to Dambala, the community notified the staff at Dambala Community Health Center on February 1, 2023. The staff then notified the Bo District Health Management Team for further investigation. On February 2, 2023, a blood sample was taken and delivered to the Kenema Lassa fever unit for testing. The sample tested positive for Lassa fever.

 

Case 4: On January 29, 2023, a 57-year-old woman developed symptoms similar to her daughter (Case 3). On the same day, she was taken to a traditional healer for treatment. Her condition continued to worsen; thus, on February 1, 2023, she was then taken to Bo Government Hospital, where she was suspected of Lassa fever and died on the same day. The laboratory and surveillance units were immediately notified. An investigation commenced on the same day, and the sample was collected and delivered to the Kenema Lassa fever unit the following day. The laboratory confirmed Lassa fever on the same day the sample was delivered to the laboratory.

 

Contact tracing and active case search

 

On February 2nd, 2023, the investigation team started contact tracing, and thirty contacts, all from the community, were line-listed, followed up, and monitored for 21 days each. The contact tracing and active case search were done till March 14, 2023. Two of the contacts complained of fever, headache, and body pains, for which they were classified as suspected cases. However, their blood samples tested negative for Lassa fever (Table 1). Two additional cases were identified among the contacts during the active case finding.

 

Environmental assessment and rodent trapping

 

All the confirmed and probable cases were from residents of Damballa Community and Selenga Chiefdom. Damballa is divided by a swamp and situated close to a river. The neighborhood has overgrown weeds, inadequate water supply services, and poor sanitation facilities. Of all the 169 assessed households, 64 (38%%) complained of high rodent infestation and visible rodent droppings in their homes. Inspection of the house where the four deaths occurred revealed visible rodent holes and droppings, poor hygiene, and no kitchen facility. The house is very close to the bush, which provides easy access of rodents to the house. The house doors were broken and lacked sealing, making it easy for rodents to enter and exit. Food storage containers in the house lacked lids and were open at the time of visits. Three rats samples that were trapped from the case patient´s residence tested positive for Lassa fever. However, the remaining four samples trapped from the surrounding households tested negative for Lassa fever.

 

Following the investigation, we sensitized clinicians on early diagnosis of Lassa fever and raised awareness among affected communities on environmental sanitation and hygiene and early facility-based healthcare service-seeking behavior.

 

 

This investigation confirmed a Lassa fever outbreak with two confirmed and two probable cases in Dambala Community, Bo District. Our finding showed that three of these cases visited the health facility at Dambala, and the healthcare workers could not suspect or detect any of them, which should be worrisome. The three first cases were suspected of malaria or pneumonia and treated for these diseases, which delayed accessing the proper treatment. This indicates a low index of suspicion among the healthcare workers, thus signaling a weakness in the Lassa fever surveillance system. Clinicians at primary healthcare facilities usually have difficulty detecting Lassa fever cases because the disease has similar clinical symptoms to other illnesses like pneumonia, typhoid fever, and malaria. [7,8]. Hence, all cases seen at the health facility were treated for diseases with no suspicion of Lassa fever. Factors that might have led to the late detection of the Lassa fever cases include limited knowledge of the health facility staff on Lassa fever case detection and diagnosis [8]. This investigation found that the all the confirmed and probable Lassa fever cases lived in the same household, indicating a possible propagated or Person-to-person transmission. Moreover, another piece of evidence for the sources of infection for this outbreak is the rodents trapped and tested positive for Lassa fever from the residence of the case patients. However, sequencing was not done to establish the genetic link between the viruses detected from the rodents and the case-patients.

 

The environmental assessment revealed poor hygiene practices and rodent infestation, which has been an additional clue that the rodents have been a possible source of infection since 2015.

 

Despite these clues for the source of the infection, confirming human-to-human transmission was challenging because the first person who got sick in the affected household did not provide enough precise information. However, all four cases belonged to the same household, and all developed clinical symptoms within the Lassa fever incubation period; this suggests a human-to-human transmission route.

 

A study conducted by Madueme et al. to understand the pathway of Lassa fever transmission found that the burden of infection increases when transmission routes are interconnected or combined with other transmission routes [9]. The two confirmed cases being females indicate that the risk of the disease is high among females. This could be due to the fact that females are more involved in household activities, which exposes them to droppings of infected rats. A study conducted in the Tonkolili District of Sierra Leone reported that two cases of pregnant women died from symptoms similar to Lassa fever, and they were classified as probable cases of Lassa fever [10]. Similarly, a study by Kassim et al. found that more than half (59%) of the confirmed Lassa fever cases were females [11]. In this current study, the CFR was high, probably due to the late detection and reporting of the cases. A similar finding was reported in a study conducted in Margibi County, Liberia, in 2018 on lessons learned from the investigation and response to the Lassa Fever outbreak, which also reported a high CFR (63.2%) due to late detection of cases [12]. One main limitation of this study is that even though the trapped rodents tested positive for Lassa fever, no sequencing was done to link the rodents' infection with the confirmed cases.

 

 

A Lassa fever outbreak was confirmed in Damballa Community, Selenga Chiefdom, Bo District. The district's weak Lassa fever surveillance system led to late identification of/missing cases and a high case fatality rate.

 

We recommend that the Ministry of Health support the Bo District Health Management Team in conducting refresher training for healthcare staff on Lassa fever case detection to increase their index of suspicion. The Bo DHMT should continue community sensitization on Lassa fever and train healthcare workers on case detection and reporting.

 

 

The authors declare no competing interest.

 

Funding

 

The United States Centers for Disease Control and Prevention (US CDC) funded the outbreak investigation through its Cooperative Agreement (CoAg) with the African Field Epidemiology Network. However. contents in this paper are solely the authors' responsibility and do not necessarily represent the official views of the U.S. Centers for Disease Control and Prevention.

 

 

Ibrahim Conteh Led the investigation and response to the outbreak and drafted the manuscript. Hassan Swaray and Gbessay Saffa provided technical support during the outbreak investigation, data analysis, and manuscript writing. Amara Alhaji Sheriff, Solomon Aiah Sogbeh, Umaru Sesay, and Anna Jammeh provided technical guidance during the outbreak investigation and edited the manuscript. Adel Hussein Elduma edited and reviewed the manuscript. Gebrekrstos Negash Gebru supervised the outbreak investigation and response and edited and reviewed the manuscript. All the authors read and approved the manuscript for publication.

 

 

Table 1: Summary of Lassa fever cases, Bo District, 1th January - 14th March 2023

Figure 1: Epidemiological curve of Lassa fever investigation, Bo district, 2043

 

 

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