Prevalence and risk factors of Lassa seropositivity in inhabitants of the forest region of Guinea: a cross-sectional study

Abstract

Background: Lassa fever is a viral hemorrhagic fever endemic in West Africa. The reservoir host of the virus is a multimammate rat, Mastomys natalensis. Prevalence estimates of Lassa virus antibodies in humans vary greatly between studies, and the main modes of transmission of the virus from rodents to humans remain unclear. We aimed to (i) estimate the prevalence of Lassa virus-specific IgG antibodies (LV IgG) in the human population of a rural area of Guinea, and (ii) identify risk factors for positive LV IgG.

Methods and findings: A population-based cross-sectional study design was used. In April 2000, all individuals one year of age and older living in three prefectures located in the tropical secondary forest area of Guinea (Gueckedou, Lola and Yomou) were sampled using two-stage cluster sampling. For each individual identified by the sampling procedure and who agreed to participate, a standardized questionnaire was completed to collect data on personal exposure to potential risk factors for Lassa fever (mainly contact with rodents), and a blood sample was tested for LV IgG. A multiple logistic regression model was used to determine risk factors for positive LV IgG. A total of 1424 subjects were interviewed and 977 sera were tested. Prevalence of positive LV Ig was of 12.9% [10.8%-15.0%] and 10.0% [8.1%-11.9%] in rural and urban areas, respectively. Two risk factors of positive LV IgG were identified: to have, in the past twelve months, undergone an injection (odds ratio [OR] = 1.8 [1.1-3.1]), or lived with someone displaying a haemorrhage (OR = 1.7 [1.1-2.9]). No factors related to contacts with rats and/or mice remained statistically significant in the multivariate analysis.

Conclusions: Our study underlines the potential importance of person-to-person transmission of Lassa fever, via close contact in the same household or nosocomial exposure.

Figure 1. Location of 30 villages in Lassa fever serosurvey, Guinea, 2000.

Figure 2. Prevalence of Lassa virus-specific IgG by age and sex, Guinea, 2000 (N = 975).

Figure 3. History of collecting rats and/or mice, Lassa fever serosurvey, Guinea, 2000 (N = 1404).

Only a small proportion of subjects declared habits of collecting dead rats and/or mice in their everyday life. This practice seems to increase with age: for children under 10 years of age, no subject declared rare or occasional collection of rats and/or mice compared to 13% among those aged 70 and above.

Figure 4. History of cutting up rats and/or mice, Lassa fever serosurvey, Guinea, 2000 (N = 1405).

Habits of cutting up rats and/or mice were rare among children, and tended to increase with age to be relatively constant between age classes in adults.

Figure 5. History of eating rats and/or mice, Lassa fever serosurvey, Guinea, 2000 (N = 1396).

Habits of eating rats and/or mice were common among persons included in the study, and comparable in all age groups, 70–80% of persons declaring rare or occasional consumption.