Fig 1.
Weekly confirmed case data for Lassa fever in Nigeria between the weeks ending 7th January 2018 until 12th July 2020.
Fig 2.
Model flowchart of the transmission and population dynamics of the system of Eq 3.
Blue solid arrows denote recruitment. Black solid arrows denote progression of the disease. Red dashed arrows denote disease transmission. Purple solid arrows denote mortalities. Parameters are detailed in full in Table 2 where λh and λr are defined in Eq 2 (i) and (ii) respectively, and B(t) is defined in Eq 1.
Table 1.
Description of compartments of the model in (3).
Table 2.
Description of parameters of the model in 3.
Table 3.
Fixed and fitted parameters to be estimated in the model.
The parameters of interest were inferred using algorithm 1 in section 2.4.
Fig 3.
The epidemiological model captured 3 consecutive Lf epidemics in Nigeria.
The simulated cases compared with the observed data. In orange is the 90% range of values Ih takes in the final generation at each time point; the median value in blue. Confirmed case data for Nigeria are in black. The model replicates the sharp increase in case incidences occurring at the start of the year for 3 years.
Fig 4.
The marginal posterior distributions of the final set of accepted particles from fitting.
Fig 4 top left the shape parameter of the rodent recruitment function, s. Fig 4 top right the rodent-to-rodent transmission rate βrr. Fig 4 mid left the human-to-human transmission rate βhh. Fig 4 mid right the rodent-to-human transmission rate βrh. Fig 4 bottom left the date of minimum rat recruitment ϕ.
Fig 5.
Underlying vector dynamics reveal high-risk period of spill-over transmission for Nigerians.
The figure showcases the evolution of M. natalensis compartments throughout the observed period, simulated using the parameters derived from the final generation of accepted values. The median value is represented by the dashed line, while the colored area illustrates the range. Susceptible rats are depicted in red, infected rats in green, and recovered rats in blue. Notably, the recruitment of susceptible rats progressively rises, providing impetus for the growth of infected rats, reaching its peak in late December. Consequently, this surge in infected rats leads to spillover infections in humans.
Fig 6.
The range and median of the effective reproduction rate for rat-to-rat transmission Rrr(t) and when the threshold for Rrr(t) ≥ 1 is met.
In Fig 6 (a) Rrr(t), median dashed-line and range in coloured block, exhibits a sharp increase towards the end of the year, foreshadowing the subsequent outbreaks in the following months. To maintain clarity, the data is limited to the years 2019 and 2020, as no complete earlier records are available. Fig 6 (b) showcases a box diagram illustrating the time of year when Rrr(t) exceeds the threshold of 1, denoting high transmission. The bottom panel of Fig 6 (b) captures the onset of the high transmission period, while the upper panel displays its conclusion. The intermediate phase witnesses a rapid shift in reservoir dynamics, leading to an escalation in the number of infected vectors.