Evaluation of rodent control to fight Lassa fever based on field data and mathematical modelling

Abstract

The Natal multimammate mouse (Mastomys natalensis) is the reservoir host of Lassa virus, an arenavirus that causes Lassa haemorrhagic fever in humans in West Africa. Because no vaccine exists and therapeutic options are limited, preventing infection through rodent control and human behavioural measures is currently considered to be the only option. In order to assess the efficacy of rodent control, we performed a 4-year field experiment in rural Upper Guinea and developed a mathematical model to simulate different control strategies (annual density control, continuous density control, and rodent vaccination). For the field study, rodenticide baits were placed each year in three rural villages, while three other villages were used as controls. Rodents were trapped before and after every treatment and their antibody status and age were determined. Data from the field study were used to parameterize the mathematical model. In the field study, we found a significant negative effect of rodent control on seroprevalence, but this effect was small especially given the effort. Furthermore, the rodent populations recovered rapidly after rodenticide application, leading us to conclude that an annual control strategy is unlikely to significantly reduce Lassa virus spillover to humans. In agreement with this finding, the mathematical model suggests that the use of continuous control or rodent vaccination is the only strategy that could lead to Lassa virus elimination. These field and model results can serve as a guide for determining how long and frequent rodent control should be done in order to eliminate Lassa virus in rural villages.

Bayesian estimation of Lassa virus epidemiological parameters: implications for spillover prevention using wildlife vaccination

“…The epidemiological dynamics of Lassa virus are assumed to follow an SIR model where individual rodents are either susceptible to Lassa infection (S), currently infected by Lassa virus and infectious to other rodents (I), or recovered from Lassa virus infection and immune to further infection (R). We do not include the complication of modeling an exposed class (e.g., Marien et al 2019) because experimental studies suggest viral shedding may commence rapidly (2-3 days) after exposure (Rosenke, K. Unpublished data). Our model allows for horizontal transmission of Lassa virus among classes as well as vertical transmission from mother to offspring and transmission of protective maternal antibodies.…”

“…Our model allows for horizontal transmission of Lassa virus among classes as well as vertical transmission from mother to offspring and transmission of protective maternal antibodies. We include the possibility of vertical transmission and maternal antibody transfer because both have been demonstrated in related arenaviruses and included in previous models exploring the efficacy of Lassa control measures (Marien et al 2019). Within a location, x, we assume individuals encounter one another at random and that juveniles and adults move between different locations x and y at per capita rates m J , x , y and m A , x , y , respectively.…”

“…Recent modeling and genetic studies have confirmed the primary importance of zoonotic transmission, providing evidence that human to human transmission is rare outside of hospital settings (Lo Iacono et al 2015; Siddle et al 2018; Kafetzopoulou et al 2019). Because most human infections result from contact with infected rodents, strategies for reducing human infection have focused on reducing human contact with the reservoir species, M. natalensis , reducing the reservoir population as a whole through trapping or poisoning, or reducing the proportion of infected rodents through vaccination (Mendoza et al 2018; Saez et al 2018; Marien et al 2019).…”

“…In contrast to rodent removal, no experimental studies examining the impact of rodent vaccination exist because vaccines targeting Lassa virus in the reservoir population are under development, but not yet available. As a consequence, efforts to predict how well vaccination campaigns might work have relied on computer simulations (e.g., Marien et al 2019). Using individual based simulations to predict the effectiveness of culling and vaccination, Marien et al (2019) found that Lassa virus could be eliminated from its reservoir population only through continuous rodent control or vaccination.…”

“…As a consequence, efforts to predict how well vaccination campaigns might work have relied on computer simulations (e.g., Marien et al 2019). Using individual based simulations to predict the effectiveness of culling and vaccination, Marien et al (2019) found that Lassa virus could be eliminated from its reservoir population only through continuous rodent control or vaccination. Because these conclusions rest on informal model parameterization, do not integrate seasonal variation in reproduction, and investigate only a small range of possible vaccination strategies, their generality remains unclear.…”