Journal of Medicine in the Tropics

: 2015  |  Volume : 17  |  Issue : 2  |  Page : 51--55

Lassa fever in Nigeria: Insights into seroprevalence and risk factors in rural Edo State: A pilot study

Ekaete Alice Tobin1, Danny Asogun1, Nosa Akpede2, Donatus Adomeh1, Ikponwonsa Odia1, Stephan Gunther3,  
1 Institute of Lassa Fever Research and Control, Irrua Specialist Teaching Hospital, Irrua, Nigeria
2 Department of Community Health, Ambrose Ali University, Ekpoma, Edo State, Nigeria
3 Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany

Correspondence Address:
Ekaete Alice Tobin
Institute of Lassa Fever Research and Control, Irrua Specialist Teaching Hospital, Irrua, Edo State


Background: The study aimed to assess the prevalence of risk factors and Lassa seroprevalence in Esan West local government area of Edo State. Methodology: One hundred and sixty-six respondents from 50 households were interviewed using structured questionnaires on prevalence of risk factors for Lassa virus exposure, household heads provided information on household risk factors. Determination of Lassa virus specific antibodies immunoglobulin (Ig) in the blood was by an Enzyme-linked immunosorbent assay technique. Analysis was performed with Statistical Package for Social Sciences (SPSS) version 16. (SPSS Inc, Chicago, IL, USA). Results: Rodents were seen in 49 (96.1%) houses in the previous 6 months, garri was eaten as a soaked meal by 97 (58.4%) respondents. IgM was found in 2 (1.3%) samples, and IgG, in 103 (58.2%). Negative IgG sero status was significantly associated with age <20 years (P < 0.00) and marital status as single (P < 0.00). Conclusion: There is a need for health education to improve food hygiene practices and reduce practices that promote rodent contact with humans.

How to cite this article:
Tobin EA, Asogun D, Akpede N, Adomeh D, Odia I, Gunther S. Lassa fever in Nigeria: Insights into seroprevalence and risk factors in rural Edo State: A pilot study.J Med Trop 2015;17:51-55

How to cite this URL:
Tobin EA, Asogun D, Akpede N, Adomeh D, Odia I, Gunther S. Lassa fever in Nigeria: Insights into seroprevalence and risk factors in rural Edo State: A pilot study. J Med Trop [serial online] 2015 [cited 2019 May 23 ];17:51-55
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Full Text


Lassa fever (LF) is an acute febrile disease caused by the Lassa virus, a member of the family Arenaviridae, whose natural hosts are rodents of the genus Mastomys. [1],[2],[3] The disease is estimated to affect 2 million people and cause an estimated 5000-10,000 deaths annually [4] with outbreaks reported from Sierra Leone, Guinea, Liberia, and Nigeria. [5],[6],[7],[8],[9]

Humans become infected through direct contact with infected rodents or their excreta and urine. Other risk factors for infection include drying of grains on the floor within dwellings and poor environmental sanitation. The steady state dissemination of infection within defined geographical boundaries can be determined by the prevalence of antibodies to the virus in human populations. In Sierra Leone Lassa immunoglobulin (Ig) G seroprevalence ranges from 8% to 52%, [10] 2.6-35% in Guinea, [11],[12] and 21% in Nigeria. [13] Seropositivity has also been found in the Central African Republic, Democratic Republic of the Congo, Mali and Senegal. [14] About 6% of 434 sera collected in Lassa village, Nigeria, in August 1970 were positive by neutralization test for LF virus. A second survey in March 1971 found 15 of 47 complete compounds tested in Lassa, Dille and Yuba villages had at least one person with serologically demonstrable experience with LF virus. [15] An IgG prevalence of 26% was found among male forest workers of Duekoue and Guiglo regions in Ivory Coast. [15]

Key to the control of infection in human populations is an understanding of the dynamics of disease transmission and effective communication for behavioral change. Few studies have focused on the risk factors for LF, all were carried out in Sierra Leone and Guinea, and may not represent the true situation in Nigeria. The exact nature and extent of practice of the factors that promote transmission of the disease in Nigerian communities is yet to be determined or documented, even in Edo State that records the highest number of cases in the country. Moreso, prevalence data for Nigeria date back over 20 years and may not be reflective of the true picture presently.

The study sought to determine the prevalence of individual and household risk factors for virus exposure, and to assess the prevalence of Lassa virus-specific antibodies.


Study Area and Design

The study area was Esan West local government area (LGA) of Edo State. With headquarters as Ekpoma, the LGA occupies a land mass of 502 km 2 and has a population of 147,655. [16] The people are mainly farmers, and Esan speaking. Traditional or Christian religions are predominant with only a few Muslims. Basic amenities, such as piped water, electricity and good roads, are inadequate or nonexistent. The study design was cross-sectional.

Study population included individuals and households in Esan West LGA.

Sampling Technique

Random sampling technique was used to select one political ward in the LGA, and one settlement from the selected ward. Fifty households were selected by the systematic selection of 50 houses, and the use of balloting where there was more than one household in a selected house. In all selected households, eligible persons were invited to participate.

Selection Criteria

Criteria for selection of individuals for the study was to be resident with the household for at least 1-year, and willing to give consent. Household members absent during the time of the study were excluded.The household head was required to provide information on exposure risk factors that pertain to the entire householdAdditional criteria to be eligible for serological survey were to be older than 1-year. [17]

Data Collection Tools

Data on the prevalence of risk factors was collected through the use of pretested interviewer-administered structured questionnaires designed by the researchers. Two types of questionnaires were in use, one to be administered to all participants to assess individual risk of exposure, and a second to household heads only, to assess household risk of exposure. Household risk factors considered included rodent contact, rodent consumption, burial practices and food hygiene practices. Rodent contact was assessed via four questions: Ever seeing rodent or rodent excrement in home, rodent excrement stored foodstuff, particularly grains. Individual risk factors considered included ever being bitten by a rodent, consumption of garri porridge, rodent consumption, rodent hunting, ever having traditional surgical procedures and close contact with a corpse.

Blood specimen collection for serological analysis was done under aseptic conditions and using the standard procedure for phlebotomy. Collected blood was transported within 6 hours of collection in a carrier with ice packs.

Laboratory Analysis

Collected blood was analyzed in the Biosafety level (BSL) - 2 laboratory of the Institute of Lassa Fever Research and Control under aseptic conditions and with universal precautions in place. Reagents and equipment were provided by the Bernard Nocht Institute for Tropical Medicine. Testing was done using enzyme-linked immunosorbent assay technique to determine the presence of IgG and IgM antibodies following guidelines provided by the manufacturers.

Data Analysis

Data from the questionnaires and laboratory data were analyzed using Statistical Package for Social Sciences (SPSS) version 16 (SPSS Inc., Chicago, IL, USA). Descriptive data were presented as charts and in frequency distribution tables. Means and standard deviation or median were calculated for continuous variables. Chi-square test of significance was used to assess relationship between seropositivity and demographic factors. Predictors of seropositivity were determined by logistic regression analysis. Step-wise backward selection was performed, and final models included those factors that retained statistical significance. P < 0.05 was considered as statistically significant.

Ethical approval was granted by the Ethics Research Committee of the Irrua Specialist Teaching Hospital as part of a larger study on Epidemiology and Pathophysiology of Lassa fever, and in each community, verbal consent obtained from local leaders. Written informed consent was obtained from all participants. Parents or guardians were additionally required to give consent for children <18 years to have a blood sample taken. Participation was voluntary, and respondents were told they had the liberty to leave the study whenever they wanted to. Confidentiality was maintained as names were omitted throughout the study, and replaced with codes. Subjects were not exposed to any harm during at any point during the study, with the only discomfort coming from pain at the site of phlebotomy.


Individual Lassa Virus Exposure Risk Factors

One hundred and sixty-six respondents participated in the individual risk factor exposure survey. Median age was 39.0 years (range 10-94 years). Ninety-eight (55.1%) respondents were female, 60 (35.7%) had secondary education, 148 (83.2%) were unskilled. The majority, 100 (56.2%) were married. Median duration of residence in the community was 18.0 years (range 1-94 years).

Nineteen (11.4%) respondents claimed to have experienced a rodent bite in the past 1-year, 97 (58.4%) frequently ate garri porridge, 6 (3.6%) admitted to having close contact (including hugging) with a corpse. Seventeen (10.2%) had been subject to traditional surgical procedures (local autopsy, scarification, abscess incision and drainage). Twenty-two (12.4%) respondents had a Eaten rodents at least once in the past 3 months. Fifty-three (29.9%) respondents made a hobby of rodent hunting.

Household Lassa Virus Exposure Risk Factors

Fifty household representatives were interviewed on presence or absence of household exposure risk factors. Forty-nine (96.1%) respondents admitted to ever seeing a rodent in the house in the past 6 months, 15 (29.4%) had only presence of rodent feces as evidence of rodent infestation. Rodent feces were frequently found in raw food in 4 (7.3%) homes. Rodent control measures (trapping) were used in 45 (88.2%) households and caught rodents were commonly thrown into nearby bushes.

Raw food items were sun-dried in 13 (25.5%) households, food stuff was stored away in rodent-proof containers in 31 (60.8%) households. In 11 (21.6%) households, cooking utensils were stored away from reach of rodents, and in 29 (56.9%) households, washed before use.

Lassa Seroprevalence

One hundred and seventy-seven subjects underwent phlebotomy for antibody detection. IgM was found in 2 (1.1%) respondents, and IgG, in 103 (58.2%) [Figure 1].{Figure 1}

Sex (P = 0.09), educational level (P = 0.23), occupation (P = 0.21), marital status (P = 0.57) and age (P = 0.32) were not found to have any statistically significant association with IgM status. IgG status of respondents had no association with sex (P = 0.46), educational level (P = 0.51) and social class (P = 0.05), but was significantly associated with, age (P < 0.00) and marital status (P < 0.00). Single respondents and those aged <20 years had the lowest proportions of IgG sero positive members [Table 1]. Logistic regression left age in the final equation (odds ratio 1.5, 95% confidence interval 1.06-2.13).{Table 1}


Rodents are a common finding in homes in the study area, they invade houses in search of food remains, especially grains. Studies have found Mastomy to be more abundant indoors than in surrounding fields, thus making houses the important location for transmission of the virus. [18],[19],[20] A large number of respondents practiced rodent trapping as a control measure. Trapping is known to reduce rodent populations and viral transmission can be reduced as much as five-fold when Mastomys population is reduced by >90%. [18],[21] In communities where a viral transmission is high, as in the study area, rodent control measures should be considered on a large scale, with focus as the elimination of rodents indoors. However, rodent trapping has long-term effects on population size when applied continuously, even when rodent densities are low. Control measures applied only when rodent densities are high will not have persistent effects, even at high mortality rates. [22] In a project to develop sustainable strategies for rodent control in rural South Africa, it was found that communities had knowledge on the correct use of rodent control measures were lacking. [23] Similarly, rodent control measures have so far received little attention in the study area, and further research is needed into the knowledge, cultural practices of people for the effective design of culturally acceptable and economically feasible rodent control measures. This will require engaging the Ministry of Agriculture. Health education messages should inform communities on the safe handling of rodent carcasses to avoid infection during disposal. A small proportion of respondents were found to consume rodents, described as a source of infection in areas where rodents are considered a delicacy. [24] Virus enter the body through cuts and abrasions on the skin during preparation. While the virus has only been associated with Mastomys natalensis, the difficulty and risk involved in identifying captured rodents puts the consumers at risk and hence rodent consumption should be discouraged. Sun-drying of food stuff is less commonly practiced than has been believed. Sun-drying may be directly on tar roads, for houses by roadside, on cement floors in the compound, or on spread mats. Sun-drying provides ample opportunity for rodents to run over the food item, depositing their waste as they do so. Not only that, viral ridden dust particles rest on the food, rendering them unsuitable for consumption. [25] Thus for the few that hold onto the habit, the use of culturally acceptable technology for drying food and intensified advocacy to traditional rulers to encourage acceptance, will provide alternatives to what is common practice. This calls for collaborative research between the Ministry of Science and Technology and tertiary institutions in the design and production of functional and affordable drying technologies. Kitchen utensils were not commonly stowed away from reach of rodents. Meanwhile, uncovered utensils provide a media for deposition of infected rodent urine, and contamination of food. Fortunately, a large number washed their plates before use. Health education should advocate safer storage of utensils.

The high prevalence of IgG in the study corroborates the endemicity of the disease in the study area. Similar high prevalence has been reported in highly endemic areas in Guinea. [11],[12] IgG however remains in the blood for long periods, so can only be used as an estimate of the burden of the disease. The low prevalence of IgM showed no active infection among respondents at the time, commensurate with the seasonality of the disease. [17] The finding of lower prevalence of IgG seropositivity observed in the young has been noted [21] may be due to the fact that singles, and those with primary level of education are more likely to be young adults who may not have been sufficiently exposed to the virus, compared with the older generation. The lack of association with sex has similarly been reported. [26]


The authors are grateful to the research assistants who participated in data collection.


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