Hotspots: are some areas of sewer network prone to re-infestation by rats ( Rattus norvegicus ) year after year?

The records of sewer baiting work for three London boroughs were examined to see whether there were locations that exhibited 'rat' (Rattus norvegicus) activity more often than would be expected by chance, a feature we dubbed 'hotspots'. More than 100000 baiting records were checked covering 15 years of the London Borough of Enfield (Enfield) and 5 years each of the London Borough of Barnet (Barnet), and the Royal Borough of Kensington and Chelsea (RBKC). The additional boroughs were included for comparison to see whether any effect observed was confined to Enfield or was a feature that could be found in both inner and outer city locations. Each borough was divided into kilometre squares corresponding with those found both on Ordnance Survey maps and also Thames Water Utilities Asset maps. The number of records per square were logged and then the number of positive records for all the manholes in that square on a year-on-year basis. We examined 350 km2 in Enfield, 377 km2 in Barnet and 66 km2 in RBKC. The data were subjected to a weighted analysis (i.e. allowing for the number of manholes per square and the varying total rat population from year to year). Some areas were shown to exhibit significantly higher amounts of activity than others suggesting that their distribution is not random and that there must be environmental and other factors, which make certain locations attractive to rats. Squares with very high activity, repeating year-on-year, 'hotspots', were found in all three boroughs, suggesting that the phenomena are widespread.