6 Risk of secondary poisoning 

AC has been approved as a biocidal active substance in the European Union under BPD. In the competent authority assessment report from 2008 (12), it was concluded that "There is unlikely to be an issue of secondary poisoning since a limited exposure to the environment is expected. Chloralose is for indoor use only and immobilisation of mice occurs shortly after bait consumption. Reference should also be made to ESD (2003) (13), which states that the target animal, the mouse, will not eat large portions of the poison bait due to its rapid narcotic effect. Mammal predators may catch a poisoned mouse but with LD50 values no less than 100 mg/kg for cats and dogs, a secondary poisoning risk is considered negligible". However, data from the recent studies referred to in the present report contradicts several aspects of these assumptions.

In almost all (96%) of the 59 included cases in the Windahl et al. study (1), the cat was either reported to be a known rodent hunter or was known to have consumed mice prior to development of symptoms of intoxication (table 3). Consumption of mice was known in 10% of the cases through the owner's observation on the day of consumption or through the cats vomiting mice containing AC during the veterinary visit (in the Windahl et al. study (1), two of the cats included in the study vomited during the veterinary visit with the contents including one mouse each. AC was detected in these mice by analysis). As concluded in the study, these results do not directly prove that the majority of cases included in the study are cases of secondary poisoning through ingestion of mice. However, the collected data on exposure to AC together with the study of bait intake with calculation of the contents of AC in mice show the possibility of such secondary poisoning occurring. Notably, even if intoxication through direct ingestion of bait instead of mice may also occur, the availability of either bait or intoxicated mice outside the traps is a problem for susceptible animal non-target mammals and birds. The knowledge of either poisoning of other cats in the neighbourhood, or of presence of dead or somnolent mice in the cats ́ outdoor living environment reported by approximately a fifth (22%) of the responders, indicate that non–domesticated animals were also at risk of AC-poisoning during the study period (table 3).

Table 3: An overview of reported anamnestic data for the 59 included cases in the Windahl et al. study (2022). Reproduced by permission from the publisher.

¹ Including use of rodenticides in buildings to which the cat had access, such as stables for large animals.

As pointed out by Tegner et al. (2), cats are known dietary neophobes, i.e., they tend to reject unknown or new foods. A pilot study on the use of AC as a poison for feral cat control showed that the AC-containing bait palatability was low, and the cats were very reluctant to eat it, even when it contained low concentrations of AC (14). Cornwell reported that cats did not eat AC bait, even when fasted for 36h (15). This may support that the poisoned cats in the studies presented were secondarily poisoned by eating mice, rather than having eaten bait directly. However, Bernhoft et al. have reported that there also are known cases of AC poisoning where cats have eaten the bait (6).

Additionally, the observations made on bait intake in mice (table 2) by Windahl et al. (1) do not support a theory that mice will not eat large portions of the poison bait due to its rapid narcotic effect. The amount of AC–containing bait consumed by the wild mice after being trapped in the study, is close to the expected daily amount feed intake at up to 12% of the body weight (1). In the same study, they also observed that the timespan from start to end of feeding was ten to fifteen minutes. Thus, even if AC is for indoor use only, there could be enough time after bait consumption for the mice to escape the building and be caught by a carnivore or bird of prey. Additionally, sedation will make the mice an easier prey.

We consider the statement from the EU assessment that "mammal predators may catch a poisoned mouse but with LD50 values no less than 100 mg/kg for cats and dogs, a secondary poisoning risk is considered negligible" to be incorrect. Poisoning symptoms can appear at considerably lower doses than LD50 (1). The calculated intake of bait by mice is below the LD50 for cats observed under animal test conditions, but such lower doses can as it has been shown in the Windahl et al. study (1) still cause significant poisoning symptoms in the cat and cause harm, due to for example heat loss, disorientation, and possible anaesthesia (6). The outcome of AC poisoning can be influenced by the ambient temperature, and there will also be individual variations in sensitivity. The prognosis is regarded as good if the cats receive supportive treatment (1,2). For animals that remain in a poisoned state outdoors in cold environments, the prognosis has been characterised as poor (6). In the report from the Norwegian Veterinary Institute, eight out of 37 cats with AC-poisoning had been euthanised or found dead (6). Some of these were euthanised even though it might have been medically possible to save them. Nevertheless, the report concludes that the calculated AC intake indicates that cats may be poisoned and die from eating mice exposed to AC. Also, even if the animals are treated successfully, there is no doubt that in these numerous poisoning cases the use of AC has led to an unacceptable suffering and undesirable effect on the health of animals. The poisoning cases have also caused major concern and financial costs for the cat owners. Finally, the risk of secondary poisoning of wildlife including birds of prey also has to be considered.