Large-scale structure of brown rat (Rattus norvegicus) populations in England: effects on rodenticide resistance

The brown rat (Rattus norvegicus) is a relatively recent (<300 years) addition to the British fauna, but by association with negative impacts on public health, animal health and agriculture, it is regarded as one of the most important vertebrate pest species. Anticoagulant rodenticides were introduced for brown rat control in the 1950s and are widely used for rat control in the UK, but long-standing resistance has been linked to control failures in some regions. One thus far ignored aspect of resistance biology is the population structure of the brown rat. This paper investigates the role population structure has on the development of anticoagulant resistance. Using mitochondrial and microsatellite DNA, we examined 186 individuals (from 15 counties in England and one location in Wales near the Wales–England border) to investigate the population structure of rural brown rat populations. We also examined individual rats for variations of the VKORC1 gene previously associated with resistance to anticoagulant rodenticides. We show that the populations were structured to some degree, but that this was only apparent in the microsatellite data and not the mtDNA data. We discuss various reasons why this is the case. We show that the population as a whole appears not to be at equilibrium. The relative lack of diversity in the mtDNA sequences examined can be explained by founder effects and a subsequent spatial expansion of a species introduced to the UK relatively recently. We found there was a geographical distribution of resistance mutations, and relatively low rate of gene flow between populations, which has implications for the development and management of anticoagulant resistance

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the developing male Wistar(Han) rat II: chronic dosing causes developmental delay

We have investigated whether fetal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes defects in the male reproductive system of the rat, using chronically exposed rats to ensure continuous exposure of the fetus. 5-6 week old rats were exposed to control diet, or diet containing TCDD, to attain an average dose of 2.4, 8 and 46 ng TCDD kg-1 day-1 for twelve weeks, whereupon the rats were mated, and allowed to litter; rats were switched to control diet after parturition. Male offspring were allowed to develop until kills on PND70 (25 per group), or PND120 (all remaining animals). Offspring from the high dose group showed an increase in total litter loss, and the number of animals alive on post-natal day (PND) 4 in the high dose group was ~26% less than control. The high and medium dose offspring showed decreased weights at various ages. Balano-preputial separation was significantly delayed in all three dose groups, compared to control. There were no significant effects of maternal treatment when the offspring were subjected to a functional observational battery, or learning tests, with the exception that the high dose group showed a deficit in motor activity. 20 rats per group were mated to females, and there were no significant effects of maternal treatment on the fertility of these rats, nor on the F1 or F2 sex ratio. Sperm parameters at PND70 and 120 showed no significant effect of maternal treatment, with the exception that there was an increase in the proportion of abnormal sperm in the high dose group at PND70; this is associated with the developmental delay in puberty in this dose group. There were no remarkable findings of maternal treatment on organ weights, with the exception that testis weights were reduced by ~10% at PND70 (but not PND120), and although the experiment was sufficiently powered to detect small changes, ventral prostate weight was not reduced. There were no significant effects of maternal treatment upon histopathological comparison of high dose and control group organs. These data confirm that developmental exposure to TCDD shows no potent effect on adult sperm parameters or accessory sexual organs, but show that delay in BPS occurs after exposure to low doses of TCDD, and this is dependent upon whether TCDD is administered acutely or chronically

Novel mutations in the VKORC1 gene of wild rats and mice – a response to 50 years of selection pressure by warfarin?

<p>Abstract</p> <p>Background</p> <p>Coumarin derivatives have been in world-wide use for rodent pest control for more than 50 years. Due to their retarded action as inhibitors of blood coagulation by repression of the vitamin K reductase (VKOR) activity, they are the rodenticides of choice against several species. Resistance to these compounds has been reported for rodent populations from many countries around the world and poses a considerable problem for efficacy of pest control.</p> <p>Results</p> <p>In the present study, we have sequenced the <it>VKORC1 </it>genes of more than 250 rats and mice trapped in anticoagulant-exposed areas from four continents, and identified 18 novel and five published missense mutations, as well as eight neutral sequence variants, in a total of 178 animals. Mutagenesis in <it>VKORC1 </it>cDNA constructs and their recombinant expression revealed that these mutations reduced VKOR activities as compared to the wild-type protein. However, the <it>in vitro </it>enzyme assay used was not suited to convincingly demonstrate the warfarin resistance of all mutant proteins</p> <p>Conclusion</p> <p>Our results corroborate the <it>VKORC1 </it>gene as the main target for spontaneous mutations conferring warfarin resistance. The mechanism(s) of how mutations in the <it>VKORC1 </it>gene mediate insensitivity to coumarins <it>in vivo </it>has still to be elucidated.</p

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the developing male Wistar(Han) rat I: no decrease in epididymal sperm count after a single acute dose

It has been reported that fetal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes defects in the male reproductive system of the rat. We set out to replicate and extend these effects using a robust experimental design. Groups of 75 (control vehicle) or 55 (50, 200 or 1000 ng of TCDD kg-1 bodyweight) female Wistar(Han) rats were exposed to TCDD on Gestational Day (GD) 15, then allowed to litter. The high dose group dams showed no sustained weight loss compared to control, but four animals had total litter loss. Pups in the high dose group showed reduced body weight up till day 21, and pups in the medium dose group showed reduced body weight in the first week post partum. Balano-preputial separation (BPS) was significantly delayed in the high dose group male offspring. There were no significant effects of treatment when the offspring were subjected to a functional observational battery, or mated with females to assess reproductive capability. 25 males per group were killed on post natal day (PND) 70, and ~60 animals per group (~30 for the high dose group) on PND120 to assess seminology and other endpoints. At PND120, the two highest dose groups showed a statistically significant elevation of sperm counts, compared to control; however, this effect was small (~30%), within the normal range of sperm counts for this strain of rat, was not reflected in testicular spermatid counts nor PND70 data, and is therefore postulated to have no biological significance. Although there was an increase in the proportion of abnormal sperm at PND70, seminology parameters were otherwise unremarkable. Testis weights in the high dose group were slightly decreased at PND 70 and 120, and at PND120, brain weights were decreased in the high dose group, liver to body weight ratios were increased for all three dose groups, with an increase in inflammatory cell foci in the epididymis in the high dose group. These data show that TCDD is a potent developmental toxin after exposure of the developing fetus, but that acute developmental exposure to TCDD on GD15 caused no decrease in sperm counts

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the developing male Wistar(Han) rat II: chronic dosing causes developmental delay

We have investigated whether fetal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes defects in the male reproductive system of the rat, using chronically exposed rats to ensure continuous exposure of the fetus. 5-6 week old rats were exposed to control diet, or diet containing TCDD, to attain an average dose of 2.4, 8 and 46 ng TCDD kg-1 day-1 for twelve weeks, whereupon the rats were mated, and allowed to litter; rats were switched to control diet after parturition. Male offspring were allowed to develop until kills on PND70 (25 per group), or PND120 (all remaining animals). Offspring from the high dose group showed an increase in total litter loss, and the number of animals alive on post-natal day (PND) 4 in the high dose group was ~26% less than control. The high and medium dose offspring showed decreased weights at various ages. Balano-preputial separation was significantly delayed in all three dose groups, compared to control. There were no significant effects of maternal treatment when the offspring were subjected to a functional observational battery, or learning tests, with the exception that the high dose group showed a deficit in motor activity. 20 rats per group were mated to females, and there were no significant effects of maternal treatment on the fertility of these rats, nor on the F1 or F2 sex ratio. Sperm parameters at PND70 and 120 showed no significant effect of maternal treatment, with the exception that there was an increase in the proportion of abnormal sperm in the high dose group at PND70; this is associated with the developmental delay in puberty in this dose group. There were no remarkable findings of maternal treatment on organ weights, with the exception that testis weights were reduced by ~10% at PND70 (but not PND120), and although the experiment was sufficiently powered to detect small changes, ventral prostate weight was not reduced. There were no significant effects of maternal treatment upon histopathological comparison of high dose and control group organs. These data confirm that developmental exposure to TCDD shows no potent effect on adult sperm parameters or accessory sexual organs, but show that delay in BPS occurs after exposure to low doses of TCDD, and this is dependent upon whether TCDD is administered acutely or chronically

Recombinant expression of Aryl Hydrocarbon Receptor for quantitative ligand-binding analysis

Recombinant expression of the Aryl Hydrocarbon Receptor (AhR) yields small amounts of ligand-binding competent AhR. Therefore, Spodoptera frugiperda (Sf9) cells and baculovirushave been evaluated for high level and functional expression of AhR. Rat and human AhR wereexpressed as soluble protein in significant amounts. Expression of ligand-binding competentAhR was sensitive to the protein concentration of Sf9 extract, and co-expression of the chaperonep23 failed to affect the yield of functional ligand-binding AhR. The expression systemyielded high levels of functional protein, with the ligand-binding capacity (Bmax) typically 20-fold higher than that obtained with rat liver cytosol. Quantitative estimates of the ligand-bindingaffinity of human and rat AhR were obtained; the Kd for recombinant rat AhR was indistinguishablefrom that of native rat AhR, thereby validating the expression system as a faithfulmodel for native AhR. The human AhR bound TCDD with significantly lower affinity than therat AhR. These findings demonstrate high-level expression of ligand-binding competent AhR,and sufficient AhR for quantitative analysis of ligand-binding

Control Of Rats Resistant To Second-Generation Anticoagulant Rodenticides

Second-generation anticoagulant rodenticides were introduced to control Norway rats that had become resistant to first-generation compounds. Unfortunately, some rats have become resistant to these as well. The lack of alternative rodenticides with the same attributes of ease of use and relative safety is potentially a serious problem should resistance become so widespread that anticoagulants are no longer effective. However, the second-generation anticoagulants difenacoum and bromadiolone can still be effective provided most rats in a population possess only a low degree of resistance to them. Measures that maximize the uptake of bait, such as using the most palatable formulation, baiting burrows and saturation baiting have to be implemented. The low levels of resistance discovered so far mean that the most potent anticoagulants, such as brodifacoum and flocoumafen, should also control most populations if baits containing either of them are properly applied. These two rodenticides are restricted to indoor use in the United Kingdom and are thus not available to control those rats living outdoors that are highly resistant to all other anticoagulants. Those rats can, however, be controlled with either zinc phosphide or calciferol, preferably after prebaiting. Strategies to manage resistance in the long-term should be implemented before high-degree resistance spreads. One potential tactic is to stop using anticoagulants altogether and allow deleterious pleiotropic effects to reduce the prevalence of resistance in a population. Any attempts to manager resistance are only relevant if the intention is to retain anticoagulant rodenticides, with their undoubted advantages, as the main method of controlling rodent pests

The Distribution And Significance Of Anticoagulant-Resistant Norway Rats (\u3ci\u3eRattus Norvegicus\u3c/i\u3e) In England And Wales, 1988-95

Between 1988 and 1995 populations of rats on agricultural premises were sampled to investigate the distribution of anticoagulant-resistant rats in England and Wales. In total, approximately 1,670 rats from 115 locations were tested for resistance to warfarin. Rats that were warfarin-resistant were subsequently tested for resistance to difenacoum, and since 1991 for resistance to bromadiolone. In some cases rats were also tested for resistance to brodifacoum, and in 1995 for resistance to flocoumafen. The results of these tests showed that there was a high prevalence of resistance to the first-generation anticoagulant, warfarin, in several regions of England and Wales. Rats from most populations sampled since 1991 appeared to be more resistant to bromadiolone than difenacoum, but in central southern England there were some populations where the reverse was true. In this same part of the country there was a relatively small focus where the rats had high degrees of resistance to several anticoagulant rodenticides. There was little evidence of resistance to brodifacoum or flocoumafen. The data are discussed with respect to the impact of anticoagulant rodenticide resistance on control of rats in the United Kingdom

Control of rats resistant to second-generation anticoagulant rodenticides

Second-generation anticoagulant rodenticides were introduced to control Norway rats that had become resistant to first-generation compounds. Unfortunately, some rats have become resistant to these as well. The lack of alternative rodenticides with the same attributes of ease of use and relative safety is potentially a serious problem should resistance become so widespread that anticoagulants are no longer effective. However, the second-generation anticoagulants difenacoum and bromadiolone can still be effective provided most rats in a population possess only a low degree of resistance to them. Measures that maximize the uptake of bait, such as using the most palatable formulation, baiting burrows and saturation baiting have to be implemented. The low levels of resistance discovered so far mean that the most potent anticoagulants, such as brodifacoum and flocoumafen, should also control most populations if baits containing either of them are properly applied. These two rodenticides are restricted to indoor use in the United Kingdom and are thus not available to control those rats living outdoors that are highly resistant to all other anticoagulants. Those rats can, however, be controlled with either zinc phosphide or calciferol, preferably after prebaiting. Strategies to manage resistance in the long-term should be implemented before high-degree resistance spreads. One potential tactic is to stop using anticoagulants altogether and allow deleterious pleiotropic effects to reduce the prevalence of resistance in a population. Any attempts to manager resistance are only relevant if the intention is to retain anticoagulant rodenticides, with their undoubted advantages, as the main method of controlling rodent pests