Public Health Significance of Urban Pests
395
Calhoun 1962 reported from observations in urban apartment blocks in the state of Maryland that predation by dogs and cats and sporadic attacks by people did not have
any appreciable effect on the density of the rat population. Indeed, general observations indicated that city blocks with dogs and feral cats also had high-density populations of
rats. The presence of free-ranging cats and rats in urban areas appears to be positively related, perhaps due to a common benefit derived from access to waste food Langton,
Cowan Meyer, 2001. Also, the presence of pets may lead to the provision of food and shelter for rodents Langton, Cowan Meyer, 2001. For example, in England, infesta-
tions of commensal rodents inside and outside homes are higher in those properties where pets or livestock are kept in the garden Langton, Cowan Meyer, 2001; DEFRA, 2005.
Because predation by dogs and cats has no appreciable effect on rodent densities in urban areas, rodent control is likely to be the primary mechanism by which population densi-
ties can be kept low.
12.2.6. Movement
Rats, particularly the brown rat, do not normally move great distances, especially in urban areas where streets act as barriers Twigg, 1975. This may not be the case in rural areas,
where rats have been reported to move as many as 3.3 km at speeds of 0.5–1.1 km an hour in one night Taylor Quy, 1978. Involuntary dispersal may also result when rats are
transported with goods in vehicles. Habitat destruction will also cause movement when rats are forced to seek alternate shelter.
The diameter of the normal home range of the brown rat varies from 25 m to 150 m Grzimek, 1975. MacDonald, Mathews Berdoy 1999 reported that farming activity
caused home ranges of brown rats around farms to fluctuate, with males having larger home ranges 679 linear metres when crops were in the field and smaller home ranges
90 linear metres after the harvest. Farming activity, however, appeared to have no effect on females who, in general, had smaller home ranges.
Resource availability also had an effect on home range. When resources were plentiful, the home range of females was smaller 85 linear metres than when resources were not
as plentiful 428 linear metres; similar effects were noted for males MacDonald, Mathews Berdoy, 1999.
The home range of the roof rat is never more than about 100 m
2
. This species often has smaller territories that surround the food sources it defends Gillespie Myers, 2004.
Commensal house mice have been recorded as travelling over 2 km, but this is exceptio- nal. Typically, they will not move more than 3–10 m in buildings WHO Regional Office
for Europe, 1998. Murphy, Williams Hide 2005 used DNA analysis to assess rela- tedness among house mice that colonized terraced housing in England and found that
mice colonizing adjoining buildings were related, but the mice in infested properties on other streets and terraces were genetically different. Their work indicated that each block
represented individual breeding units and that migration rates between blocks were very low, because mice moved easily between adjoining properties, but moved little between
non-adjacent housing blocks Murphy, Williams Hide, 2005. The focus of control should therefore be the blocks of properties, not individual houses.
Commensal rodents
394
viour is replaced by neophilic behaviour, as rats explore their ever-changing environment.
12.2.5. Population growth and socialization of commensal rodents
It has been suggested that two major factors govern the ultimate size of a rodent popu- lation: the amount of harbourage or cover and food available. In an urban environment,
the situation is probably somewhat more complex than that. For example, social status within a colony and exclusion of inferior males will be a factor Calhoun, 1962.
Nevertheless, it has also been suggested that when food or harbourage are reduced, there will be migration Twigg, 1975. Usually, rodent populations attain equilibrium, when
deaths are balanced by births, until and unless there is a change in habitat or food sup- ply. Furthermore, it has been suggested that rat populations may fluctuate over a 10-year
cycle Swift, 2001. In the assessment of the effectiveness of any control strategy, the affect of any natural fluctuations in population would need to be taken into account.
With respect to commensal rat control, Barnett 2001 concluded that there is no one den- sity-related factor that can be identified as the key to keeping population size down.
People, however, may exercise varying degrees of control over a number of interrelated factors, including the availability or scarcity of food and water, and lack of shelter, pre-
dation, pathogens and social interaction. Changes of the environment caused by people will be addressed in section 12.5, “Control of commensal rodents”.
As noted, the social ranking of individual rats influences behaviour. Rats exhibit varying degrees of aggressive behaviour, and to avoid the possibility of aggression rats of lower
social status avoid contact with higher-status rats by visiting food sources at different times and for very short periods. In a colony, individuals that become social outcasts bet-
ween weaning and sexual maturity show a slow growth rate and lower adult weight, and they are more likely to enter traps Calhoun, 1962. Social outcasts, as a means of avoiding
conflict, exhibit a tendency to shelter in less favourable areas with greater exposure to the weather. They also form non-reproducing male cohorts, which occasionally include
females incapable of breeding or rearing young.
Incomplete extermination of a rat population can lead to increased reproduction within the population. When most members of a rat population have been killed, the remain-
der may breed more quickly, thus increasing the population to its original level Greaves, Hammond Bathard, 1968; Barnett, 1975.
A similar increased growth rate is observed when rats colonize a new and favourable habi- tat. At first, the reproductive rate is high, but it gradually declines as the population rea-
ches its optimal size. Control strategies should recognize this pattern, particularly during urban redevelopment, so that redundant lengths of sewers and drains are removed or sea-
led and potential harbourage sites and food sources are minimized. The aim should be to make the environment less favourable and to monitor the rodent population regularly.
Although predation has an effect on the behaviour of rats MacDonald, Mathews Berdoy, 1999, it does not appear to have a significant effect on population density.
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