Rodent Resistance to the Anticoagulant Rodenticides, with Particular Reference to Denmark

Bull. Org. mond. Sant 1972, 47, 611-618 Bull. Wld Hlth Org.

Rodent resistance to the anticoagulant rodenticides, with particular reference to Denmark

M. LUND 1 Inherited resistance to anticoagulant rodenticides was discovered in populations of Rattus norvegicus about 14 years ago. Similar resistance has now been reported from several countries in north-western Europe andfrom the USA. In order to detect resistance and to control it effectively, basic data on the susceptibility of rat populations are required for each country, and trapping surveys should be made in any area where resistance is suspected. Acute poisons are needed to control resistant rats although the shift from anticoagulants to acutepoisons is a retrograde step asfar as efficiency is concerned, andincreases the hazard ofcontrol operations to man and other animals. Resistance to anticoagulants in Mus musculus has been reported from England, and resistant mice are probably to be found in other countries also in view of the great individual variation in susceptibility of this species to these rodenticides.

Apart from a single case of apparent resistance to nism of the blood. When the anticoagulant replaces endrin in a wild population of Pitymys pinetorum vitamin K this protein is rendered inactive, and gra- (pine mouse) after treatment for 11 years (Webb & dually the clotting capacity of the blood decreases, Horsfall, 1967, only two species of rodent-Rattus until fatal spontaneous internal bleeding occurs. Since norvegicus and Mus musculus-are reported to have this process is rather slow the rats normally feed for developed an inherited resistance to rodenticides. 2-4 days before warning symptoms occur, and by These two species, however, are not only responsible that time the lethal dose has usually been exceeded. for great economic losses but also represent a serious The effect ofthe non-anticoagulant poisons is gene- potential threat to public health in many parts of rally much more acute, and the rats often experience the world. symptoms of poisoning a few minutes after ingesting From about 1950 a " golden era " of rat control, the bait. Brown or Norway rats (R. norvegicus) are based on the use of anticoagulant rodenticides, was very suspicious of unfamiliar baits and are likely to experienced in many countries, particularly in Wes- eat only a sublethal dose. After having recovered, the tern Europe, where it lasted for almost 10 years. Dur- rats cannot readily be induced to feed again on the ing this period the anticoagulants-first dicoumarol same bait for a considerable time. and its derivatives, especially warfarin, and later the As the anticoagulants are cumulative in their effect, indandiones also-more or less replaced the " acute " the concentration in the bait can be very low, e.g., poisons and led to decreases in the level of rat infesta- 0.005-0.05 %, and the baits are therefore more readi- tion in many countries. Control operators soon came ly accepted by the rats. On account of the chronic to realize that rats could virtually be exterminated effects of these poisons, the risks to man and most over large areas; at the same time, however, prebait- domestic animals are rather small. Furthermore, as ing and other techniques previously used were practi- vitamin K is an effective antidote and since secondary cally forgotten. poisoning is very unlikely to occur, the anticoagulants The marked difference between the success rates for can be considered to be almost ideal rodenticides. the acute poisons and the anticoagulants depends on In large-scale operations prebaiting with unpoison- the following factors. All anticoagulant rodenticides ed bait, which as a rule is necessary in order to induce compete with vitamin K in the body for the synthesis the rats to consume a lethal dose of an acute poison, of a protein involved in the complex clotting mecha- is very laborious and expensive. The introduction of the readily accepted anticoagulant rodenticides saved 1 Scientist, Danish Pest Infestation Laboratory, Lyngby, Denmark. both time and money without the risk of inducing

2940 -611- 4612 M. LUND bait-shy behaviour in the rats. Attempts to create SCREENING TESTS " rat-free " areas and towns, based primarily on permanent baitboxes containing anticoagulants, now Until 1966 trapped rats from the area of resistance seem to be successful in England, the Federal Repub- and suspected areas of resistance in Denmark were lic of Germany, and Denmark, although at one time tested by feeding them on 0.05 % warfarin bait for 10 they would have seemed utopian. days; those surviving were termed " resistant ". On Thus, the detection in 1958 of rat populations in the basis of this definition, a clear separation of sus- Scotland that were resistant to warfarin (Boyle, 1960) ceptible and resistant individuals was obtained, with followed by similar discoveries in Wales in 1960 very few intermediates. At a conference of the Euro- (Drummond, 1966), Denmark in 1962 (Lund, 1964), pean and Mediterranean Plant Protection Organiza- the Netherlands in 1966 (Ophof & Langeveld, 1969), tion (1967) a standard test was proposed for identify- the Federal Republic of Germany in the period 1968- ing warfarin-resistant individuals of R. norvegicus, 71 (H. J. Telle, unpublished report to WHO, 1971), based on investigations carried out by Drummond & and the USA in 1972 (Jackson & Kaukainen, 1972; Bentley (1967). According to this test a brown rat is Brothers, 1972; Brooks, personal communication, resistant if it survives for 6 days on a diet of 0.025 % 1972) were not merely reports of resistance to a warfarin bait. A more elaborate procedure for rodenticide, but represented a threat to rat control in measuring the susceptibility level of a particular those countries. rodent species to a given anticoagulant has been suggested by Drummond (unpublished report to WHO, 1966). DETECTION OF WARFARIN RESISTANCE The standard screening test for resistance in rats Apart from the Netherlands, where the authorities was soon adopted in Denmark, and in the following were on the alert after receiving reports from the years it was shown that various degrees of resistance United Kingdom and Denmark, the discovery of re- could be found in the rat populations concerned sistance to anticoagulants followed the same course (Lund, 1969). While some rats from the area of in all the countries concerned. Up to the time when resistance continued to feed at the same level through- resistance was detected no regular and systematic out the test period and survived with no signs of trapping of rat populations had been carried out to poisoning, others stopped feeding on days 5 or 6 check the effectiveness of the anticoagulants and to and were often characterized by pale feet and small obtain basic data for estimating the susceptibility of external haemorrhages. However, the latter animals rats to these compounds. Complaints about failures recovered and survived for months. A third group of of control operations were from time to time received rats fed at the same level during the whole period of by the local authorities, but when the complaints were the test died from internal bleeding a few days closely investigated, they were always found to be due later. The typical feeding behaviour of " normal " to faulty techniques, the poor quality of the bait, or susceptible rats from other areas is a marked decrease the availability to the rats of food containing extra- in, or a complete cessation of, the consumption of ordinary amounts of vitamin K. This was also the bait on the fifth or sixth day at the latest, and death view taken when cases of resistance were reported by generally occurs 3-10 days after the start of the test. local operators. In Denmark, for example, it was In the United Kingdom similar differences are claimed in 1962 that on a few farms in eastern found between Scottish and Welsh rats, and between Jutland, rats could not be killed with warfarin, and rats from different areas of resistance in England precious time was wasted before rats were trapped on (Drummond & Wilson, 1968). those farms and tested in the laboratory. It was then A rapid method of distinguishing between resistant established that a certain degree of tolerance had and susceptible rats is used in the United Kingdom developed; two-thirds of the rats survived a routine (Greaves & Ayres, 1967). A single dose of 1 mg of test with 1 g of bait containing 0.025 % warfarin for 5 warfarin per kg of body weight is injected in rats, and successive days. The consequences, however, were not blood samples for prothrombin time estimations are realized until further tests revealed that a considerable taken immediately before, and 24 hours after, the proportion of the rats were able to survive for months injection. A good correlation is generally found be- on 0.05 % warfarin bait as the only food; some preg- tween the results of this test and those of the standard nant females even gave birth to live young during this feeding test. Resistant rats that survive the feeding period. test show little or no change in the prothrombin time, RESISTANCE IN RODENTS TO ANTICOAGULANTS 613 whereas rats that die as a result of the feeding test cestershire and four in Nottinghamshire showed re- show a marked change. sistance, but the difference between susceptible and resistant individuals was not as marked as in those AREAS OF RESISTANCE from the Scottish and Welsh populations. It was suggested by Drummond (1970) that resistance in popu- The conditions under which resistant rat popula- lations from various sites in England may be of differ- tions have arisen are similar in all the countries con- ent types. The resistant rats from Gloucestershire and cerned. The areas are typically rural with small vil- Nottinghamshire seem in any case to have been com- lages and scattered, often old, farm buildings with pletely exterminated. A larger area of resistance was poor sanitary conditions that cannot be made rat- found in Kent in 1968, but it has not been examined proof. Very often large rat populations are found in in detail (Greaves, 1971). Since 1969 a few more cases deep-litter poultry houses and in old barns with ready have occurred in Carmarthenshire, Suffolk, and access to grain stores, silage, chicken food, and Berkshire, but apparently they are unconnected with garbage in piggeries. In the United Kingdom, corn- the cases reported earlier (Drummond, personal ricks situated near barns often provide good shelter communication). and food for rats. Outside the United Kingdom the first area of re- The first reported case of resistance to anticoagu- sistance to be reported was on the peninsula between lants in rats was established on a farm near Glasgow, Vejle and Horsens in Jutland, Denmark. When it was Scotland, but since about 90 % of the rats trapped on detected in 1962 five or six old farms, close to each this farm were resistant, and since other resistant other, were involved, but, as trapping later indicated, populations were detected up to 20 km away during it seems likely that resistance had by that time already the next 2 years, its seems unlikely that resistance ori- spread to a larger area, making countermeasures al- ginated on the farm. In 1962 the known area of resis- most hopeless. In the spring of 1972 the presence of tance was about 24 x 8 km and comprised 7 different resistant rats was confirmed in more than 60 sites on sites, but no systematic trapping could be carried the peninsula, and trapping has revealed cases of re- out and the true extent of the area is unknown. sistance in 15 municipalities, some of them up to Resistant rat populations are now found in a fairly 80 km from the original sites. The total area in- extensive region between Edinburgh and Glasgow. It volved now covers roughly 9 000 km2. All the same, seems obvious that some spreading has occurred, but it seems likely that resistance has not arisen indepen- it has certainly not exceeded 5-10 km in the past 5 dently in the various new sites; this assumption is years (Boyle, personal communication). based on the observations that rats trapped in the The second area of resistance to be detected is " new " areas included a much lower proportion of situated on the border between Wales and England resistant individuals than did those trapped on the in the Welsh county of Montgomeryshire, where two peninsula, and that rats from the "new" sites adjacent farms were found to have resistant rats in showed a lower level of resistance. 1960. After the original infestation had been treated The next area outside the United Kingdom where no new cases were reported in the area for about a resistance in the brown rat was proved by laboratory year, but several new sites then appeared and the investigations, was the Province of Drenthe in the number increased during the following years, the north-eastern part of the Netherlands (Ophof & area expanding into the English county of Shropshire Langeveld, 1969). Two populations tested in 1966 at a rate of about 4.6 km radially per year (Drum- showed some degree ofresistance. They were immedi- mond & Bentley, 1967). This rate of spread is simi- ately controlled by means of an acute poison (fluoro- lar to that of a rat population in unoccupied land acetamide), and apparently the operations were (Drummond, 1970). successful since no new cases have so far been found At about the same time two areas in England in in the Netherlands. the counties of Somerset and Nottinghamshire seem During the period 1968-71 rats trapped in different to have had resistant rat populations; they were, sites in Lower Saxony, Federal Republic of Germany, however, successfully controlled with acute poisons were tested for resistance (H. J. Telle, unpublished before the resistance was fully confirmed. These areas report to WHO, 1971). The preliminary results indi- differ from all other known areas in that they are not cate that various degrees of resistance are present in rural, one being a city block and the other a municipal rats on the islands of Norderney and Borkum, as well refuse tip. In 1966 rats caught on two farms in Glou- as in the cities of Oldenburg and Nordhorn. 614 M. LUND

Recent reports from the USA (Jackson & Kau- in the level of resistance in the Welsh and Scottish kainen, 1972; Brothers, 1972; Brooks, personal com- populations may indicate that the genes are not iden- munication, 1972) reveal that, as had been expected tical. The results obtained from genetic crosses of for several years, resistance has become established in resistant rats in Denmark do not fit the theory that American rat populations. In North Carolina 25 rats a single dominant gene is responsible for the resist- trapped on farms near Raleigh in 1972 were all resis- ance. A marked difference was found in the offspring tant and survived the 6-day standard test. This high of resistant male and female rats crossed with sus- level of resistance may indicate that the mutation had ceptible wild rats (23.3 % and 42.6 % of the offspring been present in the rat populations for several years resistant, respectively). After 4 generations, not more before it was detected. than about 50% of the offspring from resistant A national survey to determine whether resistance parents were resistant, compared with 44 % in the first may exist undetected was undertaken in 30 cities generation. Recently, however, the offspring of a in the USA in the spring of 1972. The first report from Danish resistant male crossed with a female from the the New York State Department ofHealth shows that English susceptible strain have been tested for resist- that 1 of 17 rats from Newark, N.J., and 1 of 35 rats ance and the results are very similar to those obtained from Norfolk, Va., survived the screening test. These with Welsh rats (Greaves, personal communication). figures are quite similar to those found in " new " areas of resistance in Denmark, and seem to repre- The effect ofother anticoagulants sent an early stage of resistance. When resistance to warfarin was detected in the United Kingdom and Denmark, other anticoagulant LABORATORY INVESTIGATIONS ON RESISTANCE rodenticides were examined in the hope of finding a Genetics substitute. So far, coumachlor, dicoumarol, diphaci- none, chlorophacinone, phenylindanione, and pin- It was soon established in the United Kingdom as done have been tested and resistance has been found well as in Denmark that resistance to anticoagulants to all of them, including the distantly related indan- in rats is an inheritable character, and crosses of diones, which had never been used in Denmark up resistant rats from Wales with a domesticated albino to that time. strain showed that resistance, at any rate in this area, In 1967, however, coumatetralyl, which is closely is due to a single dominant autosomal gene (Greaves related to warfarin, turned out to be more effective & Ayres, 1967). The assumption of a single gene than the other anticoagulants against resistant rats in mechanism is based not only on back-crosses of sur- Scotland and Wales (Great Britain, Ministry of Agri- vivors from the 6-day test to the susceptible albino culture, Fisheries, and Food, 1968), and preliminary strain for 5 generations, but also on the finding of a tests in Denmark produced similar results. between coat colour and resistance genetic linkage that & This of the re- It was soon established, however, some degree (Greaves Ayres, 1969a). mapping to was also in sistance gene is of particular interest when the kind of resistance coumatetralyl present of resistance in other and some warfarin-resistant rat populations in the United occurring areas, possible & Ayres, and in Denmark differences, are to be evaluated. Kingdom (Greaves 1969b) The genetic nature of resistance was also studied in (Lund, 1969). Since a large proportion of the war- rats were in field couma- the USA on Welsh resistant rats and their descen- farin-resistant killed the by and since a rat has never been known to sur- dants (Pool et al., 1968), the English findings being tetralyl, test 14 com- confirmed. Hermodson et al. crossed hetero- vive a prolonged feeding of days, this (1969) is still used in areas of resistance. The zygous Welsh rats with a susceptible albino strain pound feeding rats offered bait and obtained a ratio of 0.93: 1 in the offspring, which behaviour of coumatetralyl may help is very close to the 1: 1 ratio expected. From a few them to survive, since they tend to feed irregularly rats producing only resistant offspring in back- and intermittently; such feeding behaviour is very sel- in Danish tests when warfarin is used crosses, Hermodson et al. developed a homozygous dom seen strain resistant to warfarin. (Lund, 1969). a is also It seems likely that single dominant gene Other on resistance responsible for anticoagulant resistance in the Scot- laboratory investigations tish rats (D. A. Price-Evans & P. M. Sheppard, Resistance to anticoagulants was at first thought unpublished report to WHO, 1966), but the difference to be the result of an increased ability of the rats to RESISTANCE IN RODENTS TO ANTICOAGULANTS 615 utilize vitamin K in the food; mineral oil was there- both have a supply of the altered protein, which is fore added to the warfarin bait in order to reduce the relatively unaffected by high levels of warfarin. The absorption of vitamin K. This procedure had only a homozygous rats, however, having only the altered slight effect on Scottish and Danish rats but caused protein with a lowered affinity for vitamin K, have a high rate ofmortality among resistant Welsh rats in much higher requirements for the vitamin. the laboratory. As the mineral oil seems to affect only This theory is supported by the discovery of the the uptake of vitamin K from the bait and not that protein concerned in the ribosomes in the liver cells from other sources, the advantage of the method with (Hermodson et al., 1969). Using 14C-warfarin, Her- resistant rats in the field is negligible. The addition of modson et al. (op. cit.) showed that the ribosomes antibacterial agents such as sulfaquinoxaline to the from normal rats were able to bind 2 or 3 times as bait in order to reduce the amount of vitamin K much warfarin as were those from resistant rats. produced by intestinal bacteria was not more effec- Some other phenomena previously encountered in tive. The administration of phenylbutazone in an the United Kingdom and Denmark are also explained attempt to prevent a possible binding of warfarin to by the hypothesis. Rats from the areas of resist- serum proteins, and of ethionine to inhibit abnormal ance, or offspring of crosses of these rats, may die enzyme activity that might be responsible for the suddenly with internal bleedings, although they have rapid breakdown of warfarin, was also unsuccess- never, or not for several months, been exposed to ful. an anticoagulant. This phenomenon can readily be No significant difference has been found in the explained by the theory of decreased ability to utilize response ofresistant rats to injected and oral doses of vitamin K in the food. Vitamin K deficiency may warfarin; it therefore seems unlikely that the resist- also be responsible for a decrease in the proportion ance is due to the rapid excretion of warfarin or to of resistant individuals in a mixed population of rats its conversion to a nontoxic substance (Price- kept for 3 years in an outdoor pen (Lund, 1968). This Evans & Sheppard, unpublished data, 1966). may indicate that resistance would disappear if the Investigations carried out in the USA on descen- effect of the mutation were not counteracted by se- dants ofresistant Welsh rats (Hermodson et al., 1969) lection with anticoagulants. have thrown much light on the mechanism of resistance. Studies with 14C-warfarin showed no differ- COUNTERMEASURES ence in the rate and manner of warfarin metabolism in normal and resistant rats. On the other hand, vita- Resistant rats seem to have been completely eradi- min-K-depleted resistant rats fed for'3 days on a diet cated in the Netherlands by the immediate applica- low in vitamin K, and thereafter fed a measured dose tion of acute poisons, and the sites in England where of vitamin K3, required much larger amounts of the resistance seems to have been controlled were charac- vitamin to restore the prothrombin level of the blood terized by an early stage in the development of resist- than did normal rats. Moreover, while heterozygous ance. It appears, therefore, that resistant rat popula- resistant rats required about twice as much vitamin tions should be discovered as early as possible if K3 as susceptible rats, homozygous rats needed 20 there is to be any hope of eliminating them. In the times as much as normal rats. three original areas in Scotland, England, and Den- mark, the reaction may have been too slow, the neces- THE MECHANISM OF RESISTANCE sary financial and labour resources may not have been available, or the area of resistance may have been The results obtained by Hermodson et al. (1969) too large for effective control measures to be applied are explained by those authors on the basis of a by the time the resistance was discovered. rather simple hypothesis. It is assumed that vitamin Between the years 1962 and 1965 an attempt was K and the anticoagulants compete for a site on a made to estimate the area of anticoagulant resistance protein necessary for the synthesis of vitamin-K-de- in Scotland and to eliminate the rat population in a pendent clotting factors. A mutation in the resistant small part of the area (4 x 3 km, by means ofwarfarin rat may have lowered to some degree the affinity of followed up with acute poisons (C. M. Boyle & J. H. that site for vitamin K, and at the same time lowered Cuthbert, unpublished report to WHO, 1966). In its affinity for anticoagulants to a much greater spite of the efforts that were made, complete clear- extent. Resistance in the Welsh rats is claimed to be ance was not obtained. Similar results were obtained dominant since heterozygous and homozygous rats in another trial and it was concluded that even if the 616 M. LUND spread of resistance could be considerably retarded, land, coumatetralyl being the only anticoagulant per- there was always the possibility that a few resistant mitted, even in areas where resistance has not yet rats would survive even the most intensive treatment been found. with acute poisons. Coumatetralyl has now replaced It is remarkable that so few of the countries where warfarin and apparently very few practical difficulties anticoagulants have been used continuously for rat with resistance to this compound have been en- control for more than 15 years have reported cases of countered in the field. Recent reports seem to indi- resistance-possibly owing to a failure to detect re- cate that the level of rat infestation is now consider- sistant rat populations (since no country carries out ably lower than it was in 1965 (Boyle, personal com- systematic trapping in suspected areas) or because munication). This may be partly the result of using other rodenticides are used at once if the effect of coumatetralyl, but partly also the result of improve- anticoagulants is unsatisfactory in some areas. It is ments in environmental standards and of a general not surprising therefore that resistant rat populations awareness by the public of resistance in rats. were detected in the Federal Republic of Germany In the Montgomeryshire/Shropshire area of Eng- and the USA as soon as surveys were made. Much land and Wales resistance was investigated from 1962 can be learned by other countries from the Nether- by means of trapping surveys, and in 1966 a contain- lands of the importance of early detection, and from ment zone 5 km wide was established around the experience in the United Kingdom, Denmark, the outer limits of the known area. Within this zone only Federal Republic of Germany, and the USA of the acute poisons (mostly zinc phosphide) were to be disastrous effect of late discovery and hesitating used. It was hoped in this way to stop the emigration acceptance of the fact of resistance. However, resis- of resistant rats to other parts of the country. After tance is not easily discovered if basic data on sus- the containment zone had been enlarged to include a ceptibility levels of rats to anticoagulants are not pocket of resistance a few km to the south-east, it available for each country and if resistance in rat surrounded an area of about 3 100 km2. Even if the populations is not periodically checked against spread of resistance was delayed to some extent, by these data. 1971 it was clear that resistant rats had spread through and beyond the containment zone and the work was RESISTANCE IN THE HOUSEMOUSE therefore discontinued (Drummond, 1971). In Denmark, the situation is similar to that in The susceptibility of normal brown rats to anti- Scotland. It had been hoped to prevent the resistant coagulants does not vary much but there is great rats from spreading outside the peninsula where re- individual variation in the response of housemice sistance originated by establishing a dilution zone (Mus musculus) to the same compounds. While a on the western side of the peninsula with permanent 6-day feeding test with 0.025% warfarin bait will stations containing bait poisoned with thallium sul- produce almost 100% mortality in brown rats, 9-10 % fate. Before this could be arranged financially, how- of housemice from previously untreated populations ever, trapping revealed cases of resistance several km may survive a 21-day test with the same concentration beyond the absolute limit for the proposed control of warfarin (Rowe & Redfern, 1964). Obviously, an zone. As in the United Kingdom, coumatetralyl was even larger proportion of mice may survive in the substituted for warfarin in the area of known resist- field, where other sources of food are always avail- ance and in sites suspected to contain resistant rats. able. This treatment was followed up with thallium sulfate The use of anticoagulants for the control of house- and zinc phosphide poison-baiting whenever resist- mice in Denmark has been discontinued in favour of ance to coumatetralyl was encountered. acute poisons, particularly crimidine. It is evident that Unfortunately, the level of rat infestation on the anticoagulant resistance may develop relatively quick- peninsula does not seem to be much lower than it is ly in a species with a wide range of susceptibility, in other parts of the country, in spite of the great and in the Federal Republic of Germany a certain efforts made by the control operators in the area. degree of resistance could be produced in the labo- The spread of resistance has certainly not been great- ratory by the selection of mice with warfarin in rela- ly delayed, and it is expected that resistance to anti- tively few generations (Roll, 1966). Laboratory in- coagulents will be a problem in most parts of Jutland vestigations in the United Kingdom revealed de- in the next few years. Since October 1972 the use of creased susceptibility to warfarin in some populations warfarin for rat control has been prohibited in Jut- of housemice from urban areas where warfarin had RESISTANCE IN RODENTS TO ANTICOAGULANTS 617 been used in previous years (Dodsworth, 1961). any sign of haemorrhages. This suggests a more com- More than one gene seems to be involved in anti- plex resistance mechanism than in the rat. coagulant resistance in the housemouse (Rowe & An international standard test for detecting resist- Redfern, 1965; Roll, 1966). Laboratory investigations ance to anticoagulants in housemice cannot be pro- on the vitamin-K-dependent clotting factors have posed on accdunt of the great individual variation shown that after 1 day on a 0.025 % warfarin bait in response and the obvious possibility of differences diet the level of these factors in the blood of resist- in the susceptibility of geographical races of house- ant mice is considerably reduced, but during the fol- mice, as suggested by Roll (1966). Bait-shyness is a lowing days on the same diet the level returns more minor problem in the housemouse, and this species or less to normal, indicating that resistance may be seems less suspicious of poisons than the brown rat; partly the result of an inheritable ability to develop therefore, acute rodenticides can be used more suc- tolerance (Rowe & Redfern, 1968). The same investi- cessfully. Thus, resistance to anticoagulants in the gations showed that some mice were able to live for housemouse does not appear to be such a serious 3 weeks with practically unclottable blood without problem as resistance in the brown rat.

RESUME

RtSISTANCE DES RONGEURS AUX RODENTICIDES ANTICOAGULANTS, EN PARTICULIER AU DANEMARK

Les rodenticides anticoagulants ont fait preuve d'une Des recherches sur les aspects gen6tiques de la resis- efficacite remarquable dans la lutte contre le rat brun tance aux anticoagulants ont ete faites sur des rats du (Rattus norvegicus) depuis le debut des annees 1950, et ils Royaume-Uni et du Danemark. Chez les premiers, la ont permis de detruire une grande partie des populations resistance semble dependre d'un gene unique dominant, de ce rongeur aux Etats-Unis d'Amerique et dans de mais les donn6es obtenues chez les seconds lors d'epreuves nombreux pays d'Europe. Cependant, la resistance du de croisement n'ont pas confirne le fait. rat brun A la warfarine et A d'autres anticoagulants, dece- En dehors de la resistance, la seule difference physio, lee en 1958 en Ecosse, en 1960 au Pays de Galles, en 1962 logique entre les rats resistants et les rats sensibles est que au Danemark, en 1966 aux Pays-Bas, en 1968/71 en les premiers ont besoin d'un apport beaucoup plus 61ev6 Republique f6derale d'Allemagne et en 1972 aux Etats- de vitamine K (deux fois plus eleve pour les rats r6sistants Unis, pose de serieux problemes. heterozygotes; vingt fois plus eleve pour les homo- On considere que le rat brun est resistant lorsqu'il zygotes). Dans certains cas, les rats resistants ne peuvent survit apres 6 jours d'un regime contenant 0,025 % de survivre que dans un milieu ou la vitamine K est particu- warfarine. En Ecosse, au Pays de Galles et au Danemark, lierement abondante. les tentatives visant a empecher la dissemination des La r6sistance s'exerce A l'egard de tous les autres anti- populations de rats resistants ont echoue; seuls les Pays- coagulants, y compris les indandiones. Cependant, le Bas ont apparemment reussi A eliminer la resistance en coumatetralyl fait preuve d'une efficacite superieure recourant tres pr6cocement A 1'emploi de poisons a contre les rats resistants; c'est actuellement le seul anti- toxicite aigue comme le fluoroacetamide. coagulant utilise dans la plus grande partie du territoire Au Danemark, la presence de rats resistants a et danois (Jutland). constatee sur un territoire d'environ 9000 km2. I1 est Chez la souris domestique (Mus musculus), les varia- probable que le caractere s'est repandu a partir des pre- tions individuelles de la sensibilit6 A la warfarine sont miers foyers detectes car la resistance se manifeste avec beaucoup plus marquees que chez le rat brun et le risque une acuite et une ampleur moiadres dans les regions d'apparition rapide d'une resistance aux anticoagulants recemment atteintes. En Republique federale d'Alle- est plus grand. Des populations resistantes ont et6 signa- magne et aux Etats-Unis, des investigations approfondies lees au Royaume-Uni. II semble que le caractere soit ont imm6diatement montre l'existence de populations de transmis hereditairement par l'intermediaire de plusieurs rats resistants A des degres variables. Dans certains cas, genes. Chez la souris, les cons6quences de la resistance l'intensite du phenomene apporte la preuve de son aux anticoagulants sont moins graves que chez le rat, car anciennet6 bien qu'il soit rest6 ignor6 pendant des l'espece peut etre efficacement combattue par les poisons annees. A toxicite aigue. 618 M. LUND

REFERENCES

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