The House mouse in London
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Mammal Rev. 1973, Volume 3, No. 2, pages 64-69
The House mouse in London A. M. SHENKER London Pests Unit, M.A.F.F., S.E. Region, 15 West Tenter Street, London E.1
INTRODUCTION Firstly, why talk at all about the House mouse in London? Is it any different from the House mouse in other towns or cities? I do not think that there is any fundamental difference at all, and the only reason for considering it separately is that the available evidence shows that London, by which I mean Central London, has a much larger percentage of properties infested by mice than anywhere else in the country. Or one might consider that the mouse is more successful in London than elsewhere. If this is so there must be reasons to explain it, and my aim therefore is to consider some of the factors that are thought to contribute to the increase in the number of infestations in London, or explain the relative success of the mouse in the London environment.
Although my job is to be concerned with mouse control I do not propose to discuss the organization of control measures in this paper, largely because these are closely associated with economic, administrative, and political considerations. While I will concentrate on what may be described as the environmental and biological factors, it would be misleading if I did not at least mention some of the non-biological ones.
INCREASE IN MICE IN LONDON Whether there is an actual increase in the numbers of mice, or an increase in the number of mouse infestations cannot be proved as there has been no detailed study to find out. However, the evidence, on a comparative basis strongly suggests that there has been an increase in the number of infestations, even though it does not necessarily mean that the numbers of mice have increased. What is quite certain is that the numbers of infestations reported is increasing, and in practical terms this may in some quarters be considered to be the most important point.
Each year the local authorities in England and Wales provide the Ministry of Agriculture, Fisheries and Food with statistics relating to the infestations in their areas (Drummond, 1972). One of these sets of figures gives the numbers of properties found to be infested by mice following notification, and generally speaking notifications can be equated with confirmed complaints.
Table 1 shows that in the 5-year period 1966-1970 there has been a rise in the numbers of properties inspected and found infested, following notification, in all parts of the country; and that in nearly all parts, the rise occurred in annual steps. In particular, the Table shows that Central London has a much higher percentage of properties infested than anywhere else. In 1970 the notification percentage for Central London was 2.36, and the next highest was Manchester with 1.35. It seems almost certain that the trend has continued through 1971. Meehan (1972) states that there has been a considerable increase in the mouse population in London, and bases his statement on the fact that there has been an increased demand for the services of the major commercial pest control firm for which he works. The London Pests Unit of the Ministry of Agriculture, Fisheries & Food has also experienced a rise in the number of mouse infestations in Crown properties in London.
Although this does not prove either that the mouse population has increased or that there are more infestations it would seem to be very strong circumstantial evidence.
It is important, however, to appreciate that there are factors that affect the number of notifications or complaints but are not responsible for an increase in numbers of infestations. These will be discussed before going on to consider the environmental and other factors that are thought to be responsible for the latter process.
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66 A . M . Shenker
One of the most important factors affecting the number of complaints received by a local authority is its rodent control policy, and in particular its policy for mouse control. It is perhaps relevant here to state that under the Prevention of Damage by Pests Act 1949 a local authority is obliged to take such steps as may be necessary to secure as far as practicable that its district is kept free from rats and mice. A local authority is not, however, obliged to provide a rodent control service. In London, every London Borough does in fact provide a control service, and in nearly every case a free service is provided for domestic premises.
However, there is a considerable variation in policy. Some boroughs set out to give as complete a service as possible-dealing with nearly all complaints received, including mice. Other boroughs, however, will give a control service for rats but may normally only give advice on mice (although old age pensioners and incapacitated people will generally be given assistance). In those boroughs where this latter type of policy is pursued the ratepayers, on realizing that the local authority will not deal with their mice, cease to notify infestations. Consequently, the number of notifications received tends to be relatively low.
The effects of policy changes can be seen in Table 2 which illustrates the experience of one London Borough. Because of the steep rise in notifications from 1966 to 1968, which resulted in control difficulties, the local authority decided as a matter of policy to stop giving a control service for mice. As a result the number of notifications in 1969 dropped dramatically. In September 1971, however, there was a reversal to a policy of active mouse control, which immediately led to an increase again in the number of notifications,
Table 2 Data from a London Borough showing the effects ofpolicy changes on notifications of mouse infestations
Year ~ No,ofnotificatioG Remarks
Active mouse control Policy change
Mouse control service generally withdrawn except for exceptional cases. Policy change in September to Active Mouse Control Active mouse control
E] 1966 1967 1968 1524 1969 1970
Another factor that may be partly responsible for an increase in complaints of mice, and also for an actual rise in the numbers of mouse infestations, is a decrease in rat infestations.
Because rats are much more abhorrent than mice to most people, there is a greater readiness to report their presence to the local authorities. Mice on the other hand are often treated with some degree of tolerance (for which Walt Disney must take some responsibility). How- ever, with the decline in rats and an increasing awareness in some people at least, of the neces- sity for maintaining good standards of hygiene, mice are reported when previously they might not have been.
A more controversial reason, which is frequently voiced, is that many people now expect the government, both central and local to perform all sorts of services for them, and to take responsibility for many aspects of their lives that previously they managed themselves. This may well be true, but on the other hand it is because many people have tried to help themselves that there has been an increase in infestation-as will be explained in due course.
These then are some of the factors that may serve to confuse the situation and be responsible for increasing the numbers of notifications without being a true reflection of the degree of infestation. However, in my view, the statistical evidence in Table 1 more than counterbalances them, and there are several other factors that are believed to be largely responsible for a real increase in mouse infestations in London.
The House mouse in London 67
Because the increase in mice appears to be nation-wide it may well be that the same factors that operate in London also affect mouse populations elsewhere, but to a lesser degree. These are the environmental and biological factors.
ENVIRONMENTAL AND BIOLOGICAL FACTORS Warfarin resistance Much publicity has been given to warfarin resistance, particularly that associated with rats, but many populations of mice have been found to be resistant to warfarin and other blood anti-coagulants, since the first cases were reported about 1960.
There are good reasons for considering that warfarin resistance is an important factor responsible for the apparent increase in numbers of mouse infestations. That warfarin resistance is widespread in London is beyond doubt. Following the difficulty experienced by many of the London Boroughs in controlling mouse infestations, and similar experience by the London Pests Unit, mice were live-trapped in various parts of London and sent to the Ministrys Laboratory at Tolworth for testing for warfarin resistance. Resistance was con- firmed in almost all mice sent for testing. It should be stated, however, that in some instances, for a number of reasons, warfarin resistance was being masked by faulty treatment.
In 1969, a decision was taken by the London Pests Unit to cease using warfarin for mouse control because it was apparent that in many cases it was proving ineffective. Shenker & Farrell (1969) obtained data from twenty-four London Boroughs and found that in January 1968 all used warfarin for mouse control to a greater or lesser extent, although four used it for less than half their treatments. However, by 1969 five authorities had ceased to use warfarin for mouse control and six more had used it for less than half their treatments.
The fact of warfarin resistance does not of itself explain why an increase in infestation should result. However, part of the explanation is as follows. Warfarin has continued to be readily available for sale to the public, and until the advent of alphachloralose was really the only rodenticide that could be obtained by them. The acute poisons, yellow phosphorus and red squill, were banned under the Animals (Cruel Poisons) Act 1962, zinc phosphide may be used by trained operators, and arsenious oxide may not be used at all in occupied dwelling houses.
Consequently, many people with mice purchased warfarin, and after using it for some time found that instead of killing the mice they were in fact feeding them. By the time that they had realized this the mice had multiplied so that instead of the odd mouse or two there was quite suddenly a relatively large number of them. At thisstage the local authorities were often called in, and acute poisons had to be used. This is not only more hazardous so far as children and pets are concerned, but also requires more expertise, and the length of time between visits is far less flexible than when warfarin is used. Thus, control is more difficult. In the event therefore, before these large infestations were brought under control some of the mice managed to spread out into new territories.
It may be concluded from this, therefore, that warfarin resistance coupled with the lack of a suitable efficient rodenticide that can be obtained by members of the public has been a major factor leading to an increase in mouse infestations.
Houses in multiple occupation This leads to the next factor to be considered, and in Central London possibly the most important. That is, houses in multiple occupation (HMOs). While I do not have figures available I am quite certain that there has been a great increase in this type of property in recent years. The problems associated with HMOs are many, and largely relate to the habits of the mice and practical difficulties of control. It has been explained how warfarin resistance can contribute to an increase in mice, but when resistance occurs in HMOs then the circum- stances are particularly favourable to the mice for the following reasons.
68 A . M. Shenker
In this type of property there are several areas which provide potential sources of food. Each household has its own food storage preparation, and eating areas. There are also separate refuse bins, which all too frequently are left uncovered. In addition there is generally a communal dustbin area for which no one household feels particularly responsible, and consequently food scraps tend to accumulate. As well as having several food areas, HMOs also have several storage areas for luggage, brooms, etc. so that there is increased potential harbourage.
Because mice tend to travel only short distances, and live as close as possible to their food supply it can be seen that a house in multiple occupation provides a greater number of potential habitats for mice than it did when it was occupied by a single family. Thus, HMOs may sup- port several separate infestations, and if warfarin is used and the mice are resistant it follows that the problem of control has become even more difficult.
The next complication is that in order to carry out thorough treatments it is necessary to obtain access to all the dwellings at more or less the same time, otherwise not all the infesta- tions can be treated, and those untreated will eventually spread into the treated areas from which the mice may have been cleared. Thus, the problem of access is really a major one, and as already indicated, the use of acute poisons is much less easy than using warfarin.
There is therefore an interaction between the environmental factors which favour mice, warfarin resistance, and the difficulties of control which results in an increase and spread of the mouse population.
Demolition of old properties In London, as in many other parts of the country there is a wide-scale demolition of old properties to make way for redevelopment. This in itself should not result in an increase in the numbers of mice, but because their habitats are being suddenly destroyed the mice are dispersed and find their way into new premises, and in this way of course it is likely that the number of infestations will increase. Almost certainly there will be a rise in complaints from properties adjacent to the demolition areas.
Construction of buildings Any factor that serves to increase the difficulties of carrying out control measures against mice is...