Stephen A Battersby1, Robin Parsons2, Joanne P Webster3
1 Robens Centre for Public and Environmental Health, University of Surrey, Guildford, Surrey, GU2 5XH.
2 Department of Veterinary Services, University of Oxford, Parks Road, Oxford, OX1 3PT.
3 The Peter Medawar Building for Pathogen Research, University of Oxford, South Parks Road, Oxford, OX1 3FY.
Correspondence: Stephen Battersby, Robens Centre for Public and Environmental Health,
University of Surrey, Guildford, Surrey, England.
Telephone: +44 (0) 1483 259209,
The control of commensal brown rat (Rattus norvegicus) infestations by local authorities has been largely seen as a matter of protecting public health. In recent years there has been a reported increase in domestic premises which have been the subject of complaint about rat infestations, but no assessment has been made of the potential risks to public health.
This study examined the possible sources of rat infestations in England and Wales. At the same time there is evidence of substantial under-reporting of rat infestations. The parasitic burden of wild brown rats in urban environments was also examined, compared to that found among rural wild rats, and the public health implications considered. The range and prevalence of zoonotic infection was, in general, lower among urban rats than their rural counterparts. This may be attributable to domestic livestock driving zoonotic cycles of infection and differences in population densities.
However it has been found that domestic premises with rat infestations are more likely in the older parts of urban areas with poorer environments and where, it is argued, it is also likely that there will be lower levels of complaint due to greater tolerance of rats even when they are known to be present. The study reported here has examined the administrative structures involved in the control of rats, including the attitudes of local authorities and Directors of Public Health and the relationship between local authorities and the water and sewerage undertakers, on the control of rats in sewers. It has been concluded that where the control of rats depends solely on complaint, and rat control strategies are not fully developed, there will be increased risks to public health.
The result of ineffective strategies and lack of co-operation from other agencies, as may be occurring in some parts of England and Wales, will be that the rat population and colony densities could increase in urban areas. This is most likely in those areas where there are other social stresses, thereby enhancing the potential for increased parasitic loads within the rat population. This has implications for the health of those communities and could further contribute to inequalities in health.
Key Words: Brown rats (Rattus norvegicus), dwellings, inequalities, infestations, public health, risk, urban
In recent years there have been reports of increased levels of infestation of domestic premises by commensal brown rats (Rattus norvegicus) (Meyer et al, 1995) and increased complaints to local authorities of rat infestations (NPTA, 2001). The level of complaints about rats to local authorities does not in itself mean an increased rate of infestation, nor an increase in the total rat population. It could reflect more frequent sightings of rats in daylight resulting from changes in rat behaviour or reduced tolerance by some members of the public. Whilst the rodent survey undertaken as part of the English House Condition Survey 1996 (MAFF, 1999; Langton et al, 2001) indicated a lower rate of infestation around domestic premises than Meyer et al (1995), it used a different methodology. It did however, point to a number of key factors influencing the risk of rat infestations in and around dwellings.
Several diseases likely to be transmitted by animals are emerging as serious threats to public health. However, the epidemiology of many zoonotic diseases is poorly understood, and even in the UK, baseline data on the prevalence of important zoonoses in wildlife are scarce.
Although fear of the wild rat as a carrier of disease is embedded in our culture, and immortalised in our literature, the scientific literature relating to ratborne infection is scant. There is a question whether the assumption that rodent control is essential for public health in England and Wales remains valid as changes to the operation of local government and sewerage service in recent years have failed to take serious account of this.
A survey of a wide range of parasites of wild brown rats on UK farms to rectify the lack of baseline data on rat-borne infection (Webster & Macdonald, 1995), found them to be infected with 13 zoonotic species with a range of 2-9 simultaneously per rat, and up to 10 non-zoonotic species. The results suggested rural wild rats could be a serious risk to the health of humans and domestic animals in the UK.
The situation in urban environments, in contrast, remains largely unknown. It is documented that certain parts of the sewerage system contain rats (Bentley, 1960; Twigg, 1975); even with a continuous baiting programme the population can be reduced but never completely eliminated (Channon et al., 2000). Furthermore, with reports of a deterioration in the integrity of the sewerage infrastructure (Battersby 1998, 1999) coupled with less sewer baiting (Battersby 1998, 1999) rats may have increased opportunity for direct and indirect contact with humans and their companion animals in an urban environment (Bradshaw, 1999). This present study as part of a larger consideration of the issues, aimed to make, we believe for the first time, a preliminary investigation of the parasites infecting urban rats and thereby help assess the public health risks.
This study has utilised a number of methods to assess the risk factors that influence the existence of rats in and around homes. A literature review, a postal survey of local authorities and three regional research seminars attended by representatives of local authorities, pest control companies and water and sewerage undertakers, were undertaken.
To make this initial assessment of the parasites carried by Rattus norvegicus in urban areas, forty wild brown rats were live-trapped using Blederberry traps, and subsequently humanely killed with a rising concentration of CO2. The rats were trapped at thirteen urban and suburban sites in and around London between May 1999 and May 2000. All sites were identified in liaison with local authority officers following complaints to environmental health departments. Consultation with the Home Office confirmed that no licence was required as all samples were taken immediately after death. The main criteria for site selection were: proximity to housing; obscuration from public view; agreement of the occupier where relevant; no treatment yet in progress; and if possible, association with drainage defects. Between one and six traps were used per site depending upon the scale of infestation and nature of the site. All rats were trapped in the open between May 1999 and June 2000, with a pause in the trapping between early December 1999 and March 2000. Only six rats were trapped on land that was not within the curtilage of a dwelling or part of a housing estate. These were at two sites adjacent to major railway lines, one of which was heavily flytipped, and on which four rats were trapped.
Methodological and diagnostic details were, wherever possible, matched to those of Webster & Macdonald’s (1995) study. In brief, rats were categorised by weight as juveniles (< 100g), sub adult (100-200g) or adult (>200g) following Calhoun (1962), and an approximately matched distribution of rat age/weight ranges were sampled to that of Webster & Macdonald’s (1995) study (here 20% juveniles: 32% sub-adults: 47% adults). Serum was collected by cardiac puncture post asphyxiation and used to test for antibodies against Toxoplasma gondii by the IgG indirect latex agglutination test (ILAT: Toxoreagent; Eiken Ltd.); Titres of >1:16 were considered positive (Webster, 1994). Individual fresh faecal samples were inoculated directly onto each of: MacConkey agar, XLD agar, Hektoen agar, Yersinia selective agar and Campylobatcer agar (Oxoid Ltd), and Selenite broth, Phosphate Buffered Saline (PBS) and Listeria selective broth (Biomerieux Ltd). The last six samples were also cultured onto CT SMAC agar for E.coli 0157 (Biomerieux Ltd). All were incubated at 37oC overnight except the PBS which was incubated at 4oC for two to three weeks. Suspect colonies were obtained in pure culture and identified using API identification strips (bio Merieux UK Ltd). The enrichment broths were then sub-cultured onto the following media: PBS onto Yersinia selective agar; Listeria broth onto Listeria selective agar; and Selinite F onto Rambach agar and XLD agar (Oxoid Ltd). These were then incubated at 37oC overnight and examined for suspect colonies.
The remaining faecal samples from each rat were processed for enteric parasites using faecal concentrator (Evergreen Scientific) and examined microscopically.
Although it is difficult to assess the actual population, attempts have been made to assess the number of domestic properties infested by way of survey rather than by records of complaints, which is probably the least accurate method. Meyer et al. (1995) argued that the high media profile afforded to commensal rats reflected public concern at the potential damage and the continued risk of disease transmission, but a high public profile leads to misrepresentation of localised events. The 1993 commensal rodent survey (Meyer et al., 1995) specifically sought to determine whether levels of infestation of both the brown rat and the house mouse (Mus domesticus) had changed since the mid to late 1970s and work undertaken by Rennison and Drummond (1984). The 1993 survey found 4.6% of all domestic premises (excluding those associated with commercial activities) to be infested by rats compared with 3.3% in 1976-79 as found by Rennison and Drummond (1984). This represented an increase of over 39%. Although still proportionately small, it amounts to a substantial number of premises.
In 1991 there were 19.7 million dwellings in England (DoE, 1993). It is estimated there were approximately 1.25 million dwellings in 1997 in Wales with 10,000 new dwellings added each year (Welsh Office, 1998). It is assessed that the total number of dwellings in England and Wales in 1993 was approximately 20.95 million. At the rate of infestation reported by Meyer et al. (1995) over 963,000 dwellings in England and Wales would have been infested with rats in 1993. At the same rate of infestation in 1996 in England and Wales, 995,900 dwellings would have been infested from a total housing stock of 21.65 million (DETR, 1998; Welsh Office, 1998).
The assessment of rat infestations in dwellings undertaken as part of the English House Condition Survey (EHCS) 1996, (MAFF, 1999) used a different methodology from that of Meyer et al. (1995) and the results are not directly comparable. Apart from differences in surveyor expertise, greater reliance was placed on information provided by the occupants. Results from the EHCS 1996 indicate that 0.3% of occupied properties had rat infestations inside and 1.6% had infestations outside. Using the EHCS 1996 (DETR, 1998) estimate of 20.4 million dwellings, there would thus be approximately 61,000 domestic premises with internal infestations and 326,4000 dwellings with infestations outside in England. Using the figure of 21.65 million for dwelling stock for England and Wales and assuming the same rate of infestation for both countries as found in the EHCS 1996, 65,000 dwellings would have had internal infestations and 346,400 with infestations outside, a total of 411,400 infested premises. This is less than half the estimate based on the 1993 figures of Meyer et al. (1995) for the 1993 rodent survey.
Reliance in the EHCS 1996 on information from occupiers about the level of infestations, and the lack of information about vacant dwellings, means the 1996 figures are likely to be an underestimate (MAFF, 1999). Concern about over-reliance on occupiers’ views and attitudes, and their sightings of rats, is supported by the findings of Meyer et al. (1995) and Bradshaw (1999). For example occupiers were found to be far less willing to take action on rats than for mice. This may partly be due to ignorance of, or an unwillingness to accept the existence of the infestation, and is a possible explanation for the lower figures in the 1996 survey. The 1993 survey found a quarter of rat infested domestic properties were not subject to any control. It was found that in about 10% of premises the occupier exercised control measures (Meyer et al., 1995).
A more detailed analysis of data from the EHCS 1996, (Langton et al., 2001) suggests slightly different figures for infested premises. Some 0.23% of domestic premises had brown rats living indoors and 1.6% had rats living outdoors. For England this would equate to about 47,000 dwellings with rats indoors and 326,400 outdoors, a total of 373,400 infested premise, and implies even fewer infested premises than originally estimated (MAFF, 1999). Using the mean number of rats per infestation of 2.2 as used by Harris et al. (1995), and the estimated figure of infestation rates of Langton et al (2001) produces a figure of 821,500 as the lowest estimate for the number of rats living in close proximity to humans in and around dwellings in England. Taking the infestation rates identified by Meyer, et al. (1995) there would be over two million rats living in and around the immediate vicinity of our houses. These figures take no account of rats living elsewhere in the urban environment.
Dwellings that were more susceptible to rodent infestations were identified in the EHCS 1996 (MAFF, 1999; Langton, et al., 2001). The prevalence of rats (and mice) was found to be significantly greater for dwellings where pets or livestock were kept in the garden. This may be of some public health significance given the affect of Toxoplasma gondii on the behaviour of rats, which act as an intermediate host, until passed to the cat as primary host (Webster, 1994). Rats with positive Toxoplasma titres exhibit reduced neophobia (Webster et al, 1994).
The relationship found in the EHCS 1996 between dwellings with pets and livestock and the presence of rat infestations might be explained partly by the higher proportion of rural properties with domesticated animals. There is also a greater population of rats in rural areas by comparison with urban areas (MAFF, 1999), with Harris et al (1995) suggesting a mean of 8.8% of rural domestic premises infested compared with 3.25% in urban areas. The association with indoor infestations may be a result of external infestations invading the dwelling. In urban environments, companion animals, particularly dogs, appear to be associated with rat infestations, and there is also a positive relationship between the presence of free-ranging cats and rats, perhaps because of a common benefit derived from access to waste food (Childs et al, 1991, cited by Langton et al, 2001).
Of further concern to environmental health professionals is that dwelling condition is also a factor. It was found in the EHCS 1996, that unfit properties were more likely to have an infestation than other properties (MAFF, 1999; Langton et al., 2001). A trend was identified of increasing infestation rate with increasing poor condition (MAFF, 1999). The standard of fitness in s.604 of the Housing Act 1985 (by which dwellings are currently judged) includes a number of requirements including satisfactory drainage. Properties unfit due to a failure to meet that requirement often failed other requirements and so it was difficult to identify any particular aspects of a property’s condition that led to an increased risk of rodent infestations. Nevertheless, Langton et al (2001) found a trend to link unfitness of housing with rat infestations. The only variable relating to general disrepair that Langton et al (2001) found linked to rat infestations was blocked drains.
Langton et al (2001) found the issue of housing density to be more important in urban than rural areas. It seems likely that the higher the density of dwellings, the more likely it is that a nearby dwelling can be a source of infestation, especially as the home range of rats may well encompass more than one dwelling at a time and dispersal by both rats and mice is more likely to be successful over short distances. Rat infestations were found to be significantly more common in older properties, even after adjustment for the presence of pets. It was concluded by Langton et al (2001) that infestations by commensal rodents have two general forms. Firstly, those in properties that are less than satisfactory in respect of fitness for human habitation, situated in areas with multiple problems including neglected or derelict buildings, commonly in urban areas with high densities of dwellings. The second form is infestation of older properties, on large plots in rural areas with low-density housing, reflecting reservoir populations of commensal rodents present in agricultural habitats.
The presence of rats in urban areas is a common indicator of a degraded environment (Colvin, 2001). In the EHCS 1996 (DETR, 1998; MAFF, 1999), surveyors were also asked to score problems in the vicinity of the dwelling under survey. A high correlation was found between problem areas, with widespread litter, vandalism, scruffy gardens and neglected and vacant buildings, and rat infestations. Langton et al (2001) confirm that dwellings in areas with substantial prob-lems such as dereliction and litter had a significantly higher prevalence of rats. The DETR (1998) constructed an indicator of poor living conditions from a range of housing and environmental information obtained in the survey. This found 552,000 disadvantaged households out of a total of 1.3 million living in such areas. The disadvantaged groups most likely to be present are certain ethnic minority groups (30% of Pakistani and Bangladeshi households are housed in poor living conditions), unemployed households (19%) and the long-term sick or disabled (under 60 years old) who are also more likely than average to be housed in poor living conditions.
The postal survey of local authorities in England and Wales as part of the present study indicated that defects associated with under-ground drainage were an important factor in above-ground infestations, with almost three-quarters of respondents considering that up to 40% of surface infestations were attributable to defects in the sewerage infrastructure (public and private drainage and sewerage) (Battersby, 1998). Table I indicates the assessment by local authority officers of the importance of certain factors that contribute to surface rat infestations. It shows that local authority officers considered broken private sewerage to be the second most common contributory factor to an infestation, with open watercourse and ditches being the most common factor. Concern was expressed in the research seminars and postal surveys in the present study about the termination of agency agreements with the sewerage undertakers and the inadequacy of sewer baiting. Of the 67 respondents to the postal survey, 70 (42%) said that there was an agency agreement with the sewerage undertaker and 96 (57%) said there was not. The picture is slightly complicated by those authorities with more than one sewerage undertaker operating in the district, and three respondents were unable to answer this question. Of those authorities without an agency agreement, 52% said there had been such an agreement within the previous five years. There were clear regional variations on agency agreements but over 10% of responding authorities to this study also said there was no sewer baiting in their areas. It was found that liaison with the water and sewerage companies was often minimal even where an agency agreement existed or the local authority was undertaking sewer baiting. There was no coherent or consistent approach to the control of rats in sewers with almost three-quarters of respondents indicating they met with the sewerage undertaker to discuss rodent control only “as necessary” or “never”.
The significance of the presence of rats in and around homes depends upon the range and prevalence of parasitic species among urban rats. This study found that the range and prevalence of parasitic species detected tended to be lower than that previously obtained from rural rats (see Table II). The prevalence of certain species were significantly lower in urban rats, for Capillaria spp., Toxocara cati, Hymenolepis nana, H. diminuta, Taenia taeniaeformis and Toxoplasma gondii. The prevalence of Listeria spp., and Yersinia enterocolitica was also lower in urban rats. Pasteurella spp., and Pseudomonas spp. were not detected at all amongst urban rats even though present in rural rats. The only species that showed significantly higher prevalence levels amongst urban rats was Trichuris spp. (see Table II).
There may be at least two potential explanations for the discrepancy between the high parasite loads previously identified amongst rural rats compared to the low rats in urban rats reported here. The first relates to the competitive impact of wildlife and domestic livestock on driving cycles of zoonotic infection, and the second relates to the different population densities between the two habitat types.
The survey of Directors of Public Health (DPHs) in this study indicated that there is little liaison with local authorities on rat control and public health. Out of 61 responses 21 (34%) said there was no liaison with the local authority with regard to rat infestations and their control. Forty (66%) said that there was some liaison but 20 of these typified this as “rare” with 19 typifying it only “as necessary”. Only one described the liaison as “regular” and in no instances was it thought of as “frequent”. Local authority officers in the regional seminars rarely raised the issue of liaison with other public health professionals, such as the DPH, yet most also suggested that protection of public health was the primary reason for rodent control. It was also found that DPHs are often not fully aware of the range of zoonotic agents that can be carried by rats.
Whilst concern has often been expressed about the total number of rats in the country, it may be that not only are there fluctuations in the total rat population, but there will be local ‘hotspots’. There are indications that in urban areas these will be linked to areas of general deprivation (Mortimer, 1989; DETR 1998a, MAFF 1999; Langton et al, 2001) and areas of older housing with similarly aging drains and sewers.
Table I: Contributory factors to rat infestations as assessed by the responding local authority officers in England and Wales
Residents of rundown areas are likely have a lower health status and could be more susceptible to the risks the close proximity of rats may pose. It has been acknowledged in the Black Report (DHSS, 1980) that health inequalities exist in this country and this is likely also to be a reflection of economic status. At the end of the 1990s these inequalities still existed, whether measured in terms of mortality, life expectancy or health status, whether categorised by socio-economic measures or by ethnic group or gender (Acheson, 1998). The welfare state was established following the Beveridge Report of 1942, which set out a national programme of policies and services to combat the “five giants of Want, Disease, Ignorance, Squalor and Idleness” (Beveridge, 1942). If under-reporting of rat infestations is a major problem, then it is most likely in those areas where there is already social exclusion, disengagement and lower health status.
There is also a higher rat infestation rate for houses in multiple occupation (HMOs) compared with singly occupied dwellings, and the number of HMOs will affect the level at which an area is infested (Meyer et al., 1995). Again with a high turnover of residents, and the transitory nature of occupation there is likely to be a lower level of complaint.
Table II: Parasite prevalence among wild brown rats (Rattus norvegicus)
When a pest control service has been contracted out this may be to provide a service for dealing with complaints or requests for treatment, and the level and location of complaints may be a reflection of the level of concern or tolerance by members of the public. Local authorities may wish to assess how the terms of the contract contribute to the development or implementation of a rat control strategy. For example, will contractors feed back information on particular environmental factors that are contributing to infestations such as drainage defects, litter or harbourage, and the proportion of complaints that are confirmed infestations? Or can the contractors provide information to indicate whether the risk areas as identified in national surveys are similar locally, and what level of treatment is being undertaken there? Complaints alone are not an adequate basis for assessing the problem. Results from a random sample of premises in London in 1972 indicated that rat-infested premises might be three times more prevalent than that which notification indicated (Rennison & Shenker, 1976) and the absence of sightings on which complaints are based should not be taken to mean an absence of rats (Bradshaw, 1999). Local authorities and any rat control strategy should recognise that infestations are less likely to be professionally treated where pest control is contracted out (Meyer et al., 1995).
At first sight this study indicates that overall the risks to public health in urban areas may not be as great as often feared, as the prevalence of parasites in urban areas is lower than in the rural environment. However that is no ground for complacency. Examination of the possible reasons for this difference demonstrates the need for greater vigilance. In a rural environment, soil and water contamination by infected excreta from domestic livestock may potentially spread infection to the sympatric rat population, and thereby maintain or even initiate rodent reservoirs of infection. Indeed, a previous study on another zoonose, Coxiella burnettii, found consistently higher infection rats among rodent population on livestock farms, but low or zero for rats on arable farms where commercial livestock were absent (Webster et al, 1995). Although urban areas do contain numbers of companion animals, zoonotic rates tend to be low amongst such species. For instance Nichol and Snow (1981) studying domestic cats in an urban environment found only 14.9% to be infected with any parasitic species with Toxocara cati the most common (at 11.5%). Thus cats at least may play a lesser role than domestic livestock in driving zoonotic cycles. Similarly even though it is known that rats inhabit the sewerage infrastructure, in direct contact with human waste and a substantial proportion of above-ground defects are attributable to defects in the sewerage infrastructure, the prevalence and range of parasites carried or excreted by humans in the UK urban environment is currently likely to be low whilst standards of hygiene are properly maintained. This would mean a low transfer to the wild rodent population. If the proportion of the human population carrying parasites increased then this could be reflected in the parasites carried by rats.
However, the role of domestic pets in the risks is still of relevance however. Cats may not drive the zoonotic cycles generally but are the definitive host of Toxoplasma gondii. Infection of rats has been found to lead to behavioural changes in the rat that would benefit the parasite by making the infected rats more susceptible to predation by domestic cats. T. gondii appears to alter the rat’s perception of the risk from cats (rats have evolved anti-predator avoidance of areas which show signs of cats’ presence), and in some cases the innate aversion changes to ‘imprudent attraction’ (Berdoy et al, 2000). This benefits the parasite as the life cycle can only be completed in cats. T gondii causes toxoplasmosis in humans, and infection across the placenta also occurs where a pregnant mother acquires the primary infection.
The second explanation for the lower parasitic prevalence in urban rats, and relevant where control is inadequate, relates to differences in rodent population densities between the two habitats. There is evidence that rat population densities are often very high within rural environments, but generally very low, with restricted inter-group social interaction within the modern developed urban environment (Twigg, 1975). The latter has been achieved primarily through pest control programmes and enhanced sanitation (Twigg, 1975). Such low population densities as well as trap shyness (neophobia) in response to control pressure imposed may be reflected in the extended time taken to trap the urban rats in this present study. High rat population densities and overcrowding, on the other hand, favour the transmission of parasites, particularly those spread by direct contact or short distance aerolization (Anderson, 1993). Webster and Macdonald’s (1995) study demonstrated that the zoonotic prevalence range and intensity was highest amongst the most densely rat-populated farms. Other studies of Leptospira spp. have reported a similar pattern with moderately high infection rates in rural brown rates but low or zero rats from urban and suburban sites (Hathaway, 1981; Blakelock and Allen, 1956 in New Zealand; Gordon-Smith et al, 1961 in Malaya).
However urban rats do carry parasites that can cause ill-health and must pose a risk to the health of communities where standards of environmental and personal hygiene are not maintained. It seems that there is a lack of awareness amongst medical professionals, which could also lead to misdiagnosis, and a failure to recognise the need for intervention. It is unlikely that the situation will improve whilst there is such a low level of liaison between environmental health professionals in local authorities and the DPHs, a lack of liaison that is often reflected in the relationship with the sewerage undertakers, other local authority services and servicing companies.
This study included an initial assessment of the range of parasites carried by commensal brown rats. At this time the prevalence of parasites generally seems greater in rural rats than in those found in urban areas, perhaps because of predation from pest control activities in the past leading to lower population densities. The sampling of urban rats was undertaken as part of a wide-ranging study and further work in this area is required.
The presence of rats in urban areas is linked to poor areas of housing, often older, but not exclusively areas of older housing, where there is generally a degraded environment. These are areas of multiple deprivation and social exclusion, areas suffering from a combination of linked problems such as unemployment, poor housing, bad health and poor quality environments (Social Exclusion Unit, 2001). Thus where people whose health status is already likely to be compromised are living, the presence of rats in numbers could pose an additional pressure on their health and well-being. At the same time, social inertia, a result of social exclusion, may lead to underreporting of infestations.
Effective rat control strategies should be seen in the context of the development of comprehensive community strategies, and should address those environmental factors that encourage rat infestations, which in themselves are a reflection of poorer environmental quality. This community strategy as required by Local Government Act 2002 provides the over-arching framework for the activities of the council and its partners to address local needs and to utilise the powers provide by the Act to promote or improve the economic, social or environmental wellbeing of their area. Where rat control is contracted out then the contractor is another of the partners, along with the sewerage undertaker in all cases.
The number of domestic premises infested by rats may be increasing (Meyer et al, 1995). There has been an increase in complaints to local authorities (Battersby, 1998; NPTA, 2001) possibly due to a greater awareness by some members of the public, but a substantial proportion of infestations actually go unreported and untreated (Meyer et al, 1995, MAFF, 2000). More local authorities now charge for what has previously been a free rodent control service (NPTA, 2001), which may hinder reporting and proper treatment, especially in those older parts of inner cities where income levels are low.
Furthermore, many local authorities lack any strategic approach, often contracting out the rodent control service, which responds only to complaints. Increasingly fewer local authorities act as agents for the privatised sewerage undertakers on sewer baiting (Battersby, 1998 and 1999). Liaison between local authorities and the sewerage undertakers is variable across the country and non-existent in some areas. As the rat population in the sewers increases containment within the sewerage infrastructure will be prejudiced by inadequate maintenance, both public and private. There is often inadequate repair of defects in private drainage and poor control over the quality of work. Coupled with litter and poor environmental quality above-ground, the potential exists for the rat population densities in urban areas, at least on a localised basis, to approach those found on farms with an increased risk to public health.
The situation for the last fifty years when there has been a decline in the rat population should not therefore be assumed to be that which will apply generally in the future. Furthermore, medical practitioners need to be made aware of the range of parasites carried by rats, as there is every chance that illhealth currently caused by rat infestations may be misdiagnosed, and the public health evidence to justify investment in more effective control of rats may be being lost.
We are grateful for the help and assistance of several pest control servicing companies and local authorities, as well as that of landowners for access, and to Dr M Berdoy for comments on the manuscript. The work was funded by grants from Hepworth Building Products Ltd, Killgerm Chemicals Ltd and the Royal Society.
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