[Rodenticide resistance and consequences]. / Rodentizidresistenz und Konsequenzen.

Resistance to anticoagulant rodenticides, such as warfarin was first described in 1958. Polymorphisms in the vitamin K epoxide reductase complex subunit 1 (VKORC1) gene and respective substitutions of amino acids in the VKOR enzyme are the major cause for rodenticide resistance. Resistant Norway rats in Germany are characterized by the Tyr139Cys genotype, which is spread throughout the northwest of the country. Resistant house mice with the VKOR variants Tyr139Cys, Leu128Ser and Arg12Trp/Ala26Ser/Ala48Thr/Arg61Leu (spretus type) are distributed over a number of locations in Germany. Resistance can reduce management attempts with consequences for stored product protection, hygiene and animal health. Anticoagulants of the first generation (warfarin, chlorophacinone, coumatetralyl) as well as bromadiolone and difenacoum are not an option for the control of resistant Norway rats. The same applies for house mice whereby the tolerance to compounds can be different between local incidences. Due to the higher toxicity and tendency to persist, the most potent anticoagulant rodenticides brodifacoum, flocoumafen and difethialone should be applied but only where resistance is known. In other cases less toxic anticoagulants should be preferred for rodent management in order to mitigate environmental risks. Resistance effects of further VKOR polymorphisms and their combinations, the spread of resistant rats and conditions supporting and reducing resistance should be investigated in order to improve resistance management strategies.

A scheme for the placement of rodenticide baits for rat eradication on confinement livestock farms.

We investigated whether the allocation of rodenticide baiting points to specific structural elements would result in complete rat eradication on livestock farms, as opposed to assigning the baiting points only to places where there were obvious signs of rat activity. The goal was to establish an effective rodent-control program that is easy for untrained persons to conduct.Rat-control strategies were examined on 25 farms in Velen (Muensterland), Germany, where an average of 20% of trapped rats were resistant for bromadiolone according to a blood-clotting response (BCR) test. All farms were investigated for signs of rat activity prior to and after the control measure. Differences in the percentage level of farmer compliance in setting up the baiting points as prescribed were analysed for each type of baiting point and in total, and were compared between the group of farms which achieved complete rat eradication and those which did not. Farms achieving complete eradication had an average of 81% compliance with prescribed control plans, whereas a significantly lower compliance level of only 51% was recorded on farms that did not achieve eradication. A >/=75% level of implementation of the control plan always resulted in complete control success. The new method of bait-point allocation was incorporated into a self-explanatory computer program, which was verified to be effective during a rat-control campaign in the restricted area after an outbreak of classical swine fever near Soltau in northern Germany, in July 2001. This program, which is available on the Internet, enables the creation of individualised rat-control plans, including complete documentation of the control measure.

Biological control of rodents using Sarcocystis singaporensis.

Parasites have been identified as an important factor in regulating vertebrate populations. In replicated field experiments (plots up to 4 ha) performed in Thailand we tested whether commensal and field rodents could be artificially infected and controlled with the host-restricted apicomplexan protozoon Sarcocystis singaporensis which is endemic in Southeast Asia. When bait-pellets containing high numbers of these parasites were consumed by rodents of three species (Rattus norvegicus, Rattus tiomanicus, Bandicota indica) in different agricultural habitats (chicken farm, oil palm plantation, ricefield), we observed a parasite-induced mortality ranging from 58% to 92%. Detection of merozoites of S. singaporensis in lung tissue samples of rats collected dead at the experimental sites using a species-specific monoclonal antibody confirmed that S. singaporensis was the causative agent of mortality. As observed with brown rats, the parasite's effect on the host was not related to sex. These experiments demonstrate for the first time that artificial infection of rodents with an endemic protozoon has the potential for effective population control.

Genospecies diversity of Lyme disease spirochetes in rodent reservoirs.

To determine whether particular Borrelia burgdorferi s.l. genospecies associate solely with rodent reservoir hosts, we compared the genospecies prevalence in questing nymphal Ixodes ticks with that in xenodiagnostic ticks that had fed as larvae on rodents captured in the same site. No genospecies was more prevalent in rodent-fed ticks than in questing ticks. The three main spirochete genospecies, therefore, share common rodent hosts.

Competence of urban rats as reservoir hosts for Lyme disease spirochetes.

To determine whether urban rats serve efficiently as reservoir hosts for the agent of Lyme disease, we recorded the frequency of infection in nymphal Ixodes ricinus (L.) ticks that had fed as larvae on experimentally infected Norway rats, Rattus norvegicus (Berkenhout), or on black rats, R. rattus (L.), and evaluated the nidicolous venue of transmission. Subadult vector ticks attached readily to Norway rats as well as black rats and virtually all became infected in the course of feeding. Larval ticks detached when these nocturnally active hosts were at rest. Rats appeared to be competent reservoir hosts of Lyme disease spirochetes in a transmission cycle in urban sites.

Risk of urban Lyme disease enhanced by the presence of rats.

To determine whether Norway rats contribute to the risk of human Lyme disease in a central European city park, densities of endemic rodents were compared as were feeding densities of vector ticks and prevalence of infection by the Lyme disease spirochete. Only Norway rats and yellow-necked mice were abundant, and three times as many mice as rats were present. More larval ticks fed on rats than on mice, and far more nymphs engorged on the rats. All rats but only about half of the mice infected ticks. Each rat was more infectious than each infectious mouse. Infected rats were distributed throughout the city. Spirochetes infected about a quarter of the questing nymphal ticks. The capacity of rats to serve as reservoir hosts for the Lyme disease spirochete, therefore, increases risk of infection among visitors to this and other urban parks.