Assessing trends in rat populations in urban and non-urban environments in the Netherlands

Rats in urban areas pose health risks as they can transmit various zoonotic pathogens. Monitoring rat populations in urban areas is therefore a key determinant in risk assessments for taking adequate control and preventive measures. However, large-scale and long-term monitoring of rat populations is labor-intensive and time consuming. The aim of this study was to develop a low-cost and low-time- consuming method to gain insight into the trends of rat populations in urban and non-urban environments in the Netherlands, and to identify potential drivers of these trends. From 2014 to 2018, local municipalities or their pest control organizations voluntarily submitted quarterly overviews of rat nuisance reports in urban areas. For non-urban areas, a nationwide record of reported bycatch species from the muskrat control was used to assess a potential trend. To identify potential drivers of observed trends, employees of nine municipalities were interviewed. Rat nuisance reports from 25 municipalities were analyzed. An increasing trend in rat nuisance reports was observed in 12, a decreasing trend in 3 and no trend in 10 municipalities. In non-urban areas, no trend in the bycatch of rats was detected. The increase in rat nuisance reports was associated with a large municipality resident size. No consistent drivers could be identified, but potential drivers were discussed in the interviews. Although it was not possible to quantify their influence on the rat population trends seen, they provide direction for future studies on drivers of rat populations.