Bull. Org. mOnd. Santd 48, 469-477 Bull. Wld Hlth Org.f 19739 A critical review of currently used single-dose rodenticides NORMAN G. GRATZ1 The introduction of the anticoagulants in the early 1950s, with their much greater safety to nontarget animals, resulted in a general decline in the use of single-dose rodenticides. However, the appearance of rodent resistance to the anticoagulants, first in the United Kingdom, later elsewhere in Europe, and still more recently in the USA, has revived interest in the use of single-dose rodenticides. Unfortunately, owing to their danger to nontarget mammals, the use of several of these compounds must be restricted; others, despite their long use, are now recognized to be unsatisfactory because of their poor accep- tance or reacceptance by rats and mice. Thus, only very few compounds of this type are available for unrestricted use and there is an urgent needfor the development of effective alternatives. Virtually any mammalian poison can be considered duals in their response to the single-dose rodenticides. as a potential rodenticide; there are, however, certain In addition, the effectiveness of even the best rodenti- highly selective criteria that must be applied to any cides will be negated if they are offered in an un- candidate compound that quickly eliminate all but attractive or repellent bait, or if they are presented a few of the enormous number of available chemical in a manner that does not allow full expression of and biological products. The number of single-dose the efficacy of the compound-e.g., at too low a rodenticides in common use, and the length of time dosage. that these compounds have been in use, shows that With the introduction of the anticoagulant roden- the situation is surprisingly stable, especially com- ticides in the early 1950s, the use of single-dose pared with that for other pesticides, particularly rodenticides declined to some extent. Since most ofthe insecticides. This does not mean that the available latter are toxic to a broad spectrum of warm- rodenticides are completely satisfactory; each of blooded animals, there is almost always a risk of them has certain shortcomings and, in fact, none accidental poisoning of man, his domestic animals, will give satisfactory control of even a single species or desirable wild species. In addition, the frequent in all circumstances. development of bait shyness to several single-dose The ideal characteristics of an acute rodenticide toxicants also favoured the use of anticoagulants in would be a high degree of toxicity to rodents, a very many circumstances where either group of com- ready acceptability, the failure to induce " bait pounds could be utilized. There are however, many shyness" when sublethal quantities are consumed situations where rodenticides applied in a single dose or, in other words, a high degree of reacceptance, (as opposed to the multiple dosing required with and as high a degree as possible of specificity to anticoagulants) may be used to advantage. Many rodents. The relative importance of other character- large-scale rodent control campaigns against field istics, such as persistence in baits, solubility, cost, or domestic rodent species are expensive in labour availability, ease of use, etc. would vary from one and baiting materials and the costs of multiple set of circumstances to another. rebaiting may prove excessive, especially in the There are considerable variations among rodent developing countries. In the case of outbreaks of populations, between sexes, and often among indivi- disease, where immediate rat control is required, the use of single-dose toxicants will usually provide 1 Scientist/Entomologist, Vector Biology and Control, a more rapid reduction in the rodent population. World Health Organization, 1211 Geneva 27, Switzerland. In epidemics of plague, rodent control should, of 3032 - 469- 470 N. G. GRATZ C w 0 d) U) 0 co r i Z.o C Z Wg'Z E E c o E .0- E o =CD k- 0 E m0. E ,. 0- 0 0 00 0C 0. E ~~ 0 0 C 0C ~~~~~ ~~~3 ~~= ~~4 00C (a U) 0 oc , C 0mC .- ._ c 0co W ~ 4, N *= N 0 .0 C'). U)0 rD=. w E0 0 0.Uc co j<-° coE ' co°> 0 U. >0 0~ ._0 r- .0 ~0W.. 0 0e o E- je 0 <t E C o 0. E CDO ~0C 0. D_ @ 00 E E ~ 0 00 CD0 E E EED E E C - w &D0 ° °5c _ 0) E .6 0 0 .0 .0 . 0- C 31 3. 0 co 0 0 c)0 0 C C 0 0 0 0 co 0 0 0 0 >. >. .)d _)' c 00 C C 0 0CU o-0 o0 0 16 C CD0 0 .' 0 0. 0- - -*0U 0 0 0 a 0 0 0 . 0. 0 g 0 0.0 0 0 ._ ._ CL CL 0) 0) U) cm 0 *. _ E 00 0 0 C0. 0. '. 'a 0 . 0 0 0 ra6 ra i6~ ~~ 0 -cE ~~~~~~~~~~~~~~~~~~0 0 cm 0 C0 0. o-.- o *!= *5L 0 0 0 0 0 0 0 0 o0 0 0 0 C) -* * 0 0 4) 00 co Co 0O U) cm 0. CDDOU) 0 -~~~~ 0 Co 0)C a) 0 0 cD 0 N N E co --° C0 N N CD 1 0 Cl) X a, CUCD Ea Coc N IT CO N L) U) CD C0 N N . Ch w coO cyi 0 4) 0 0 0 N 00 Us O C _ C.) 0 >4)- 0 0- N O Q0 .c oo4 N OD .0 0 U Co0 000 OC 0.C C ,0 , o EDN Lo | oD -UCJ> o q0 I 1~ ~ ~ ~ ~~~~~~1 0 CY) 0 >.CD 0 r, co mE o -U .0 0 E N1 u 0 0 0 0) 0 0._.2 Cfl<<Z-Ju.c U (D co 0-C~ ~~ N3 SINGLE-DOSE RODENTICIDES 471 course, follow the use of insecticides to control the been prepared for red squill powder by WHO ectoparasites. (Specification No. WHO/SRT/4) reading as follows: The appearance of rats resistant to the entire gamut of anticoagulant rodenticides in areas of The material shall consist essentially of the dry, powdered, fleshy inner-bulb scales of the red variety Denmark and the United Kingdom, however, has of Urginea maritima, fortified when necessary with the virtually excluded the use of this group ofcompounds alcohol-soluble extract of the same... in these places. Anticoagulant resistance has also appeared in the Netherlands but the resistant popu- A description then follows of the chemical, physical, lation appears to have been eliminated by the rapid and biological requirements.2 use of a single-dose rodenticide-fluoroacetamide- Perhaps the main reason for the continued use of in the town in which the resistance had appeared red squill in Norway rat control has been its limited (Ophof & Langeveld 1). More recently, resistance to acceptance by animals other than rodents. It will the anticoagulant rodenticides has appeared in cause vomiting in many animals but it is effective Rattus rattus in the Liverpool dock area of England against rodents since they cannot vomit and thus (Wkly epidem. Rec., 1971) and has been detected eliminate the material. However, cases have been for the first time in the USA, in North Carolina, reported of poisoning of cattle, sheep, chickens, and in R. norvegicus (Jackson et al., 1971). There is, dogs. Red squill is extremely irritating to the skin therefore, most decidedly a place for the single-dose and rubber gloves should be worn when preparing rodenticide in modern rodent control campaigns. baits from this material. In this paper only those single-dose rodenticides A serious limitation to the use of red squill is its whose use is widespread or that are likely to be comparative ineffectiveness against R. rattus; much available to anyone considering the use of one of higher doses are necessary to achieve a kill of this this group of compounds have been reviewed; some species and these high concentrations are not readily have almost completely fallen out of use or are accepted by the roof rat or the mouse. Its use is used only in limited areas and, therefore, have not therefore restricted to Norway rat populations. An been considered. Chemosterilants have not been additional limitation is the fact that individual rats discussed since they have been reviewed by Howard that have ingested a sublethal dose of red squill will & Marsh (1972). The characteristics of the most develop an aversion or bait shyness that is likely to important of the compounds reviewed are sum- last for several weeks. marized in Table 1. Koren & Good (1964) have described a community programme in Philadelphia, USA, where red squill rat RED SQUILL was widely used for control: 10% by weight of fortified red squill powder was mixed with 27% Red squill is the oldest of the rodenticides in of rolled oats and 63 % of mixed cracked corn and current use. Its rodenticidal properties were known cornmeal; corn oil was added as a binder instead in the Mediterranean area in mediaeval times and of water and it was found that this prevented baits it was recommended for rodenticidal use in a number from becoming mouldy for 2-4 weeks. In a test of eighteenth and early nineteenth century publica- comparing red squill with an anticoagulant, pindone, tions. Despite this long history, it did not come into the authors concluded that the red squill formulation, large-scale use until the late nineteenth century which required 1 visit per station only as opposed (Chitty, 1954). The slowness with which it was to 3 for the anticoagulant, was considerably cheaper adopted may well have been due to the very consider- in material and labour and even more effective than able variations in the potency of the different the anticoagulant.