Assessing the efficacy of oral intake of insecticides on mortality of fleas and ticks on commensal Rattus species

Many rodent-borne pathogens can be transmitted via their ectoparasites to humans and can cause severe zoonotic diseases (e.g. plague, tick-borne encephalitis, typhus). Managing relevant ectoparasites in rodents may reduce human infection risk. The purpose of this laboratory study was to screen potential insecticides for their palatability to commensal rat species and their efficacy against fleas and ticks feeding on rats. The preferences shown by Norway rats (Rattus norvegicus) for four insecticides (fipronil, cythioate, thiamethoxam and ivermectin) were assessed when presented as a choice in cereal-based pellets. Subsequently, specific doses of insecticide-containing cereal pellets were fed to rats to determine the ability of each insecticide, over time, to kill cat fleas (Ctenocephalides felis), stickfast fleas (Echidnophaga gallinacea) feeding on Norway rats, and possum ticks (Ixodes trichosuri) feeding on Norway rats and black rats (R. rattus). Fipronil, cythioate and thiamethoxam were similarly palatable to rats, but ivermectin was avoided at high concentration. Fipronil was the most effective in killing cat fleas, stickfast fleas and ticks feeding on rats. Cythioate and ivermectin performed similarly against two flea species, while the effects of ivermectin and fipronil were comparable against ticks. Cythioate was not efficacious against ticks at the doses tested. Fipronil should be tested as a systemic compound to minimise arthropod infestation in small mammals to reduce adverse effects of arthropod-borne disease on humans. This insecticide should also be tested in combination with a rodenticide to prevent migration of arthropods with zoonotic relevance from rodent carcasses to humans.