Significant Genetic Impacts Accompany an Urban Rat Control Campaign in Salvador, Brazil

Abstract

Rats thrive in human-dominated landscapes and have expanded to a near global distribution. Norway rats (Rattus norvegicus) contaminate food, damage infrastructure, and are reservoirs for zoonotic pathogens that cause human diseases. To limit these negative impacts, entities around the world implement intervention and control strategies designed to quickly and drastically reduce the number of rats in a population. While the primary goal of these interventions is to reduce rat numbers and their detrimental activities, there are important, yet unexplored, population genetic implications for these rapid population declines. Here, we compare the population genetics of R. norvegicus before, immediately after, and several months following a rodenticide-based eradication campaign targeting rats in an urban slum of Salvador, Brazil. This slum has been the focus of long-term research designed to understand and reduce the risk of leptospirosis for people in this area. We also look for a clear source of rats contributing to population recovery, by either rebounding through breeding of local survivors, or by immigration/reinvasion of the site. We found evidence of severe genetic bottlenecks, with the effective population size dropping 85-91% after eradication, consistent with declines in population sizes. These rapid declines also led to a strong shift in the genetic structure of rats before and after the eradication campaign. Relatedness increased in two of the three study areas after eradication, suggesting reduced population sizes and an uneven impact of the campaign across colonies within the population. Lastly, dozens of low-frequency alleles (mean frequency of 0.037) observed before the campaign were undetected after the campaign, potentially lost from the population via drift or selection. We discuss the public health and ecological implications of these rapid genetic impacts of urban control efforts. Our data suggests that targeting the genetic viability of rat populations may be another important component for integrated pest management (IPM) strategies, designed to reduce urban rats.

Hunger Pandemic: Urban Rodents' Boom and Bust During COVID-19

“…Thus, an increase in acceptance of rodenticide baits could be the main cause of a population crash, driven by the reduction of human-derived food resources during lockdown. Similarly to other reported cases, after such a mortality event, the genetic variation within the remaining population could be up to 90% lower than the original population 70 . Australia is currently one of the only countries in the world where rodenticide resistance has not yet been detected.…”

Poverty, sanitation, and Leptospira transmission pathways in residents from four Brazilian slums

“…Teasing apart direct and indirect drivers of transmission revealed the context within which transmission occurs and the relevant pathways governing it., This facilitates defining intervention targets, assessing their effectiveness[ 39 ], and prioritizing their implementation in the most vulnerable parts of the community. Indeed, while rodent control through rodenticide application does not appear to be effective (this study and [ 15 ]), socioeconomic[ 40 ] and sanitary [ 41 ]interventions have improved health outcomes for the urban poor in Brazil. Further, community-based approaches that involve residents and local stakeholders in designing and implementing risk-reduction interventions [ 42 ], have reduced malaria [ 43 ]and diarrheal disease risk in urban areas.…”

“…Further, trash collection and rodent control services, where available, did not mitigate transmission risk. Rodenticide campaigns have scarcely been evaluated in urban slums, but they appear to be less effective here[ 15 ] compared to non-slum areas[ 34 ]. Similarly, efforts to control to Dengue fever risk through controlling Aedes mosquito populations has had limited success, due to the continuous financial and logistic investment required for interventions, which include spraying insecticides and removing mosquito breeding habitats [ 35 ].…”

“…The majority of leptospirosis cases reported annually in Brazil occur in urban slums[ 4 ], where trash and open sewers support high densities of rats and expose humans to sources of contamination[ 6 , 14 ]. Governmental interventions to reduce reservoir abundance, namely trash collection and application of rodenticides by the Center for Control of Zoonoses (CCZ), are absent in many parts of the communities, and where present, their effectiveness has rarely been evaluated[ 15 ].…”

Rats and the COVID-19 pandemic: Early data on the global emergence of rats in response to social distancing

“…Additionally, if large numbers of rats concurrently migrate, then a greater variety of individual genotypes within populations would have moved significant distances while under intense selective pressure 40 . It is reasonable then to assume there is some influence on the genetic profile of future urban rat populations 42,43 . While stressed rat populations may initially decrease due to loss of food and increased competition and muricide, rats breed rapidly, and should rapidly re-establish population equilibrium 44 .…”