Sewers and drains Association with urban infrastructure 1. Rodents in housing
Public Health Significance of Urban Pests
397
Urban sewers are the perfect man-made rat habitat. They minimize temperature fluc- tuations, with cooler conditions in the summer and warmer conditions in the winter; they
provide a steady influx of food, as wastes of all types are flushed through the system; and they greatly reduce or eliminate predation. Due to the more stable climate, breeding
continues year-round without seasonal fluctuations. Heavily infested sewers and drains can act as reservoirs of rats, which restock surface areas where control efforts have been
undertaken. Thus, sewer systems are very important harbourages for urban rat colonies and should be considered in conjunction with surface control efforts Twigg, 1975.
In urban areas of the United Kingdom, at least a quarter of the surface infestations in over a half of the local authority districts are due to defects in the sewer system. In some
local authorities, over half the surface infestations are attributable to defects below ground Battersby, 2002.
Rats do not normally live in active drains and sewers, but instead they live in disused pipes, in excavations adjacent to cracks or bad joints in pipelines, or in the dry parts of the
network, such as benching raised ledges at manholes and inspection chambers Hall Griggs, 1990. Colonies normally are located in one area and sorties of limited extent are
made in search of food Bentley, 1960.
The home range of rats in sewers may be extremely limited. This limited range may be unique to sewers that are particularly favourable to rats or to situations where there is
regular traffic between sewer and surface. These favourable conditions and restricted ranges can inhibit control efforts. Bentley 1960 reported that rat colonies that expe-
rienced the aforementioned conditions may subsist between but not include two man- holes. These rats do not depend on the sewage flowing from upstream for a food supply;
rather, the waste is discharged from a source between the manholes. Bait placed at these manholes will therefore not be effective. Nevertheless, the distance moved may depend
on the availability of food. When circumstances require them to do so, rats in sewers may move great distances 140–200m Bentley, 1960.
12.4. Commensal rodents and human health concerns 12.4.1. Zoonoses of rats
Commensal rodents have been associated with a variety of zoonoses. This is of particu- lar concern because of their close association with people. Over the last 10 centuries, rat-
borne diseases may have taken more lives than all of the wars ever fought Nowak, 1999.
Gratz 1984 included schistosomiasis as one of about 40 diseases rats carry, and Nowak 1999 reported that as many as 200 million people worldwide are infected with this dis-
ease. Rats can also spread murine typhus, plague, salmonellosis, leptospirosis, trichinel- losis and rat-bite fever Nowak, 1999. Webster MacDonald 1995 found that brown
rats in the United Kingdom were infected with 13 different endoparasitic organisms and zoonotic agents, with some having infections of up to 9 of these simultaneously see Table
Commensal rodents
396
12.3. Association with urban infrastructure 12.3.1. Rodents in housing
Dwellings that were more susceptible to rodent infestations were identified in the English House Condition Survey EHCS of 1996 Langton, Cowan Meyer, 2001. A high cor-
relation was found between rat infestations and areas where problems of litter, vanda- lism, dishevelled gardens, neglect and vacant buildings were widespread Langton,
Cowan Meyer, 2001; Murphy Oldbury, 2002. Domestic mouse infestations were most likely to occur where there was poor structural maintenance, poor hygiene and ample
internal harbourage Murphy Oldbury, 2002. The 2001 EHCS DEFRA, 2005 revea- led that 2.9 of 16 676 occupied homes in England had rat infestations outside, repre-
senting a 70 increase over results reported in 1996. The EHCS also revealed that infes- tations inside the home in 2001 were 1.4 and 0.3 for mice and rats, respectively. This
is a slight decrease from the 1.8 and 0.4 for mice and rats reported for 1996. It should be noted, however, that a previous rodent survey within an inner-city area, using a dif-
ferent methodology, found considerably higher levels of infestations Meyer et al., 1995, indicating that within urban areas, there will be hot spots and that national surveys will
not identify the true level of infestations in specific areas. Such a hot spot was described in Manchester, England, where 50 of the terraced properties had mouse infestations
Murphy, Lindley Marshall, 2003. This was substantially higher than infestation rates reported in the EHCS and supports the notion that infestations of commensal rodents
may be clumped.
H ousing density is one important factor that influences urban rodent infestations. Because the home range of rats can encompass more than one dwelling, the higher the
density of homes in an area, the more likely it is that rodents infesting one home will disperse and colonize the surrounding dwellings. Dispersal by both rats and mice is also
more likely to be successful over short distances.
The age of housing is another factor that influences rodent infestations in urban areas, with rat infestations significantly more common in older properties Langton, Cowan
Meyer, 2001 and with infestation rates higher in dilapidated structures. In the United States, the presence of rats in urban areas is taken to be a common indicator of a degra-
ded environment Colvin in Martindale, 2001. There is also some evidence that defec- tive drains, an artefact of an ageing community infrastructure, are linked to outdoor rat
infestations Langton, Cowan Meyer, 2001; Battersby, 2002.
Other factors associated with the urban environment have a significant effect on the size of the populations of rats and mice. Pest control practitioners have raised concerns that
excess litter, carelessly discarded food waste and inadequate sewer baiting contribute to the existence of above-ground rat infestations Battersby, 2002.