Characteristics of the urban sewer system and rat presence in Seattle

Rats are abundant and ubiquitous in urban environments. There has been
increasing attention to the need for evidence-based, integrated rat
management and surveillance approaches because rats can compromise public
health and impose economic costs. Yet there are few studies that
characterize rat distributions in sewers and there are no studies that
incorporate the complexity of sewer networks that encompass multiple sewer
lines, all comprised of their own unique characteristics. To address this
knowledge gap, this study identifies sewer characteristics that are
associated with rat presence in the city of Seattle’s urban sewer system.
We obtained sewer baiting data from 1752 geotagged manholes to monitor rat
presence and constructed generalized additive models to account for
spatial autocorrelation. Sewer rats were unevenly distributed across
sampled manholes with clusters of higher rat presence at upper elevations,
within sanitary pipes, narrower pipes, pipes at a shallower depth, and
older pipes. These findings are important because identifying features of
urban sewers that promote rat presence may allow municipalities to target
areas for rat control activities and sewer maintenance. These findings
suggest the need to evaluate additional characteristics of the surface
environment and identify the factors driving rat movement within sewers,
across the surface, and between the surface and the sewers.