Rodent control strategies and Lassa virus: some unexpected effects in Guinea, West Africa

Abstract

The Natal multimammate mouse ( Mastomys natalensis ) is the host of Lassa mammarenavirus, causing Lassa haemorrhagic fever in West Africa. As there is currently no operational vaccine and therapeutic drugs are limited, we explored rodent control as an alternative to prevent Lassa virus spillover in Upper Guinea, where the disease is highly endemic in rural areas. In a seven-year experiment, we distributed rodenticides for 10–30 days once a year and, in the last year, added intensive snap trapping for three months in all the houses of one village. We also captured rodents both before and after the intervention period to assess their effectiveness by examining alterations in trapping success and infection rates (Lassa virus RNA and IgG antibodies). We found that both interventions reduced the rodent population by 74–92% but swiftly rebounded to pre-treatment levels, even already six months after the last snap-trapping control. Furthermore, while we observed that chemical control modestly decreased Lassa virus infection rates annually (a reduction of 5% in seroprevalence per year), the intensive trapping unexpectedly led to a significantly higher infection rate (from a seroprevalence of 28% before to 67% after snap trapping control). After seven years, we conclude that annual chemical control, alone or with intensive trapping, is ineffective and sometimes counterproductive in preventing Lassa virus spillover in rural villages. These unexpected findings may result from density-dependent breeding compensation following culling and the survival of a small percentage of chronically infected rodents that may spread the virus to a new susceptible generation of mice.

Modelling Lassa virus dynamics in West African Mastomys natalensis and the impact of human activities

“…-How does vertical transmission of LASV in juvenile rodents impact overall virus prevalence and persistence in the population? The model could examine different rates of (pseudo)vertical transmission, which might facilitate persistence and be important for resurgence of infections following large declines in populations, including following control [17]. -What is the impact of seasonal fluctuations in host population density on LASV transmission dynamics?…”

“…In years when rainfall was below average and the wet season was short, M. natalensis population densities were significantly lower [24]. Moreover, these could also incorporate aspects of control to better understand how different control measures might dampen or even increase the viral incidence in populations, as seen in Guinea, West Africa [17]. -What is the effect of rodent reproductive seasonality and birth pulses on LASV prevalence over time?…”

“…A fifth parameter, , the infection recovery rate, also has greater impact on , than the remaining parameters. Notably, carrying capacity and adult mortality are two parameters that can probably be substantially impacted by humans, because, for example, crops might provide resources for M. natalensis [ 25 , 28 ] and increase or decrease mortality, along with human activities such as rodent control and killing [ 17 ]. Changes in density will also probably impact , which itself is dependent on contact rates [ 58 , 59 ].…”

“…Despite this, our work and that of others suggest in West Africa the M. natalensis population dynamic fluctuation depends on seasonally available food, and human activities, habitat or rainfall changes will probably impact these. These studies are important; in Upper Guinea a seven-year rodent control experiment with 10–30 day control periods annually and a single intensive three-month trapping exercise led to a rapid increase in rodents following control efforts with concurrently high LASV infection rates, suggesting density-dependent compensation drove increased viral incidence [ 17 ]. Our model supports these field studies, suggesting continuous control to reduce the carrying capacity ( ) and adult mortality ( ) might be most successful in reducing .…”

“…Prevention of LASV infection is currently effectively limited to hygiene and rodent control. However, recent studies have shown some mouse control approaches might alter the dynamics and even lead to increases in host infection, and therefore human risk, following population recovery [ 17 ], a phenomenon also possibly observed after efforts to cull bats infected with VHF-causing marburgviruses [ 18 ]. Together, these findings suggest rodent behaviours and their responses to the environment may play a key role in determining human infection and disease risk [ 4 , 14 , 16 ].…”