Rats have been the subject of considerable public fear and loathing for centuries and the notion that they are carriers of disease is embedded in our culture. Stephen Battersby and Joanne Webster discuss.

While rats have suffered from a poor public image over the years, it is difficult to determine the extent to which this is justified. They have certainly been extremely efficient survivalists and it may partly be this that leads to the scare stories. In truth, little work has actually been undertaken on the epidemiology of zoonoses in wildlife. In the national commensal rodent survey, 1993, local authorities surveyed a random sample of properties.1 This survey found 4.6 per cent of all domestic premises (excluding those associated with commercial activities) to be infested by rats. This compared with 3.3 per cent in the period 1976 to 1979, representing an increase of over 39 per cent. Although proportionately small, this would amount to a substantial number of premises.

It has been estimated that in 1993 there were approximately 21 million dwellings in total in England and Wales.1,2 At the reported rate of infestation this would indicate over 950,000 dwellings infested with rats that same year. However, the commensal rodent survey did not make a detailed assessment of the reason for the increase in infested premises, nor of the public health consequences.

More recently, an assessment of rat infestations in dwellings has been undertaken as part of the 1996 English House Condition Survey (EHCS).4 As the methodology used was different from the 1993 survey, the results are not comparable. The 1993 survey was specific and undertaken by local authority officers with expertise in surveying and identifying rodent infestations.

The 1996 EHCS was part of a much larger exercise primarily focusing on the condition of the housing stock. Greater reliance was placed on information provided by the occupants than in 1993. Results from the 1996 EHCS indicate that only 0.3 per cent of occupied properties had rat infestations inside and 1.6 per cent had infestations outside. This would represent nearly 58,000 occupied premises having internal rat infestations and over 313,000 having rat infestations outside. For all dwellings the total figure for infestations using this basis would still be over 428,000.

It was the findings of the 1993 commensal rodent survey that prompted a further study to be undertaken by the Robens Centre for Public and Environmental Health (RCPEH). The study commenced with a postal survey of local authorities in 1998. It indicated that 45 per cent of local authority officers considered that the number of infestations had increased over recent years.5 This was reinforced by a subsequent survey of pest control companies.6 Approximately 42 per cent of local authority respondents thought that at least 25 per cent of infestation were due to defects in private sewers and 33 per cent of officers thought at least a quarter of infestations was due to defects in public sewers. Only open watercourses and ditches were seen as a greater contributory factor than broken below-ground drainage. If that were the case, it might be reasonable to assume that rats as commensal animals and found in close proximity to our homes would represent a major health hazard.

This view appears to be reinforced when examining the most recent data on rats and disease. Webster and Macdonald found that in a sample of up to 259 rats trapped on farms, largely in the south of England, rats carried a wide variety of zoonotic species.7 They also found that some organisms, that it is assumed rats carry, were not present at all; in particular Salmonella spp. The presence of Toxoplasma gondii in 35 per cent of the rats indicates that wild rats may represent an important intermediate host reservoir for the organism that causes toxoplasmosis. Of particular interest they found Listeria spp. and Yersinia enterocolitica (causing Yersiniosis) in 11 per cent of the sample, Cryptosporidium parvum in 63 per cent and antibodies for Coxiella burnetii (Q fever) in 34 per cent of rats sampled. The presence of Hantavirus (Hantaan fever) in the UK was the most surprising of all, with 4 per cent of rats positive for the antibodies. This was the first record of these organisms in wild rats in the UK. These figures compare, for example, with 14 per cent prevalence for Leptospira spp. (causing Weil's disease) that it is often assumed all rats carry. As these rats were all caught in rural environments a more recent exercise was undertaken as part of the RCPEH study to ascertain the diseases carried by urban rats A sample of 40 rats was caught in London and in Surrey over a 14-month period, all caught in and around occupied housing. Trapping rats in such locations proved more difficult than on farms, in part due to the difference in population densities between the two habitats and also probably due to increased neophobia among urban rats due to the higher control pressures. Perhaps unexpectedly, the parasitic burden appears to be less than for rural brown rats. So far as helminths are concerned, rats in both samples were found to be infected with Capillaria spp. (causing Capillariasis), Toxocara spp. (causing Toxocariasis), Trichuris spp., Hymenolepis spp., and Taenia spp. (all causing inter alia diarrhoeal disease). Only Trichuris spp. were found in greater prevalence in the urban rat sample.

The urban rats were found to be infected with bacteria such as Listeria spp. (causing Listeriosis), Yersinia enterocolitica (which is related to the plague bacterium and causes yersiniosis), Vibrio spp. (causing diarrhoeal disease), the protozoa Toxoplasma gondii (causing Toxoplasmosis), Coccidia and Eimeria spp. (causing Coccidiosis) and Entamoeba spp (amoebic dysentery). However, in most cases the prevalence was at a lower percentage than for rural rats. The only exceptions were Vibrio spp. and Entamoeba spp., but these were found in only five per cent and two per cent of the urban sample respectively. The initial results with respect to urban rats however should not be taken as implying that rats do not pose a risk to public health. It is clear from the study of rural rats that wild brown rats on rural farms can serve as vectors of disease, and may represent a serious risk to the health of humans and domestic animals in the UK. Urban rats may carry lower levels of zoonotic infections, and this is a pattern that has been found previously, but there is no reason for complacency.8,9,10

The reason why urban rats may carry lower levels of disease may be due to population dynamics, largely the result of control measures such as pest control and good sanitation. The rural rat study trapped rats where infestations were of greater densities than in most urban areas. This is characteristic of the rural/urban rat divide.11 Crowding would favour parasite transmission, and thus it is important to maintain the control of rats in towns and cities so that urban populations do not come to resemble their rural counterparts. Rat control in rural areas also needs to be improved.

There were also found to be greater parasitic burdens in rats from livestock farms than arable farms. There is a high potential for soil and water contamination by infected excreta from the livestock, which can pass to the rats. The livestock indirectly top up the reservoir of infection in the rat population. Furthermore, it has been suspected that rats play a part in contaminating livestock with Cryptosporidium via contamination of animal feed. It can be seen how the cycle of zoonotic infection could be thus maintained in such circumstances. This can lead to recontamination of water even after other control measures have been taken. By comparison with livestock, there are low human parasite infection rates in the UK and therefore rats in sewers may not actually be exposed to the level of parasites we might assume. This might not be the case in developing countries.

In the past, local authorities have with the water authorities controlled rat infestations on a more coherent basis than is currently the case. It is known that the majority of infestations are now going untreated, local authorities are putting fewer resources into rodent control and in some cases charging for a previously free service leading to lack of reporting.4 If we fail to have in place effective control measures, while allowing drains and sewers to deteriorate permitting rats to escape and food sources to increase via litter, we may have localised increased rodent population densities. If they increase to levels comparable to that in the rural environment, there will be a concomitant increased risk of maintaining zoonotic infection. There is also some evidence that directors of pubic health are unaware of the extent to which rats carry zoonotic species and assume that local authority rodent control activities will prevent public health problems.12 These are only initial results and the sampling of rat blood and faeces in the urban environment is only one part of the project. To determine whether the picture painted by this work is true nationally for the UK, a more substantial and national study of the prevalence of parasites carried by urban rats should be undertaken in a range of towns and cities.

The study at Surrey University is also seeking to make some assessment as to the economic damage from rat infestations. This is proving difficult, but will continue. In the USA Pimentel et al have estimated that the annual economic costs of rat damage as $19bn worth of losses and damage. 3It is estimated that the US has become the land of a billion rats, most of them introduced. In the urban and suburban areas of the US, Pimentel has suggested there is roughly one rat for every human, causing fires by gnawing electric cables and polluting foodstuffs. In this country there is greater secrecy and the insurance industry cannot make an assessment, as such damage is not normally covered by insurance policies. Nevertheless, it has been acknowledged by different staff from Railtrack regional offices that rats gnaw cables and that delays and other problems have been caused as the result of rat damage. In one area two delays in 1999 amounted to 2,500 minutes which with the penalties imposed equated to a cost of £100,000.14

If the public health risks from rat infestations are not sufficient to ensure that effective investment is made in rat control at both national and local levels, one might think the economic costs might prompt a review of the legislative and administrative systems in place.

S Battersby, Robens Centre for Public and Environmental Health (RCPEH), University of Surrey, Guildford and J Webster, Wellcome Trust Centre for the Epidemiology of Infectious Disease (WTCEID), University of Oxford.
REFERENCES
1. Meyer, A N, Shankster, A, Langton, S D & Jukes, G, National Commensal Rodent Survey 1993. Environmental Health Journal, London, Vol 103/6, June 1995, pp 127- 135
2. Department of the Environment Transport and the Regions, English House Condition Survey 1996. The Stationery Office, London, 1998.
3. Welsh Office, Welsh Housing Statistics 1998, The Stationery Office, 1998.
4. Ministry of Agriculture Fisheries and Food, Rodent infestations in domestic properties in England - a report arising from the 1996 English House Condition Survey, MAFF, London 1999.
5. Battersby, Stephen, Urban Rat Control, Underground Drainage and Public Health: Report of a postal survey of local authorities in England and Wales. RCPEH, University of Surrey, Guildford, July 1998.
6. Battersby, Stephen, Urban Rat Control, Underground Drainage and Public Health: Report of a postal survey of pest control companies in England and Wales. RCPEH, University of Surrey, Guildford, November 1998.
7. Webster, J P, and Macdonald, D W, Parasites of wild brown rats (Rattus norvegicus) on UK farms, Parasitology (1995) 111, 247-253.
8. Hathaway, S C, (1981) Leptospirosis in New Zealand: and ecological view NZ Vet. J.29: 109-112.
9. Blakelock, J H & Allen R E (1956) A survey of rats trapped in the Wellington area for ectoparasites and organisms of the Salmonella, Pasteurella and Leptospira groups. NZ. Vet. J. 4: 154-156.
10. Gordon-Smith, C E, Turner, L H, Harnson, J L, Room, J C, (1961) Animal Leptospirosis in Malaya. Bull WHO. 24: 807-816.
11. Twigg, G. (1975) The Brown Rat. (1st edn) Devon: David & Charles (Holdings) Limited
12. Battersby, Stephen (1999), Urban Rat Control, Underground Drainage and Public Health: Report of a postal survey of Directors of Public Health. RCPEH, University of Surrey, Guildford, October 1999.
13. Pimentel, D, Lach, L, Zuniga, R, and Morrison, D (1999) Environmental and economic costs associated with introduced non-native species in the United States. Paper at American Association for the Advancement of Science (AAAS) Anaheim, Ca, 24 January 1999.
14. Battersby, S A, (2000) Rat infestations, Underground Drainage and Public Health, RCPEH University of Surrey, unpublished.

Readers may be interested in purchasing the WHO Environmental Health Pamphlet on "Rodents" available from the publications department for £7.50 each. Tel: 020 7827 5882.