Could a naturally occurring disease be used to help control rats?
Courtesy of Nga Manu Images
Rather than causing direct harm, many diseases can have more subtle effects on their hosts. For instance, parasite infection in red grouse in Europe is well known to make the birds more vulnerable to predation due to them being in poorer condition. This phenomenon is believed to be quite common in nature, and has been termed the ‘healthy herd’ hypothesis because the overall population is kept healthier by the removal of the more heavily diseased individuals. More extreme than this are cases where parasites actually cause behavioural changes in their hosts that lead to their increased predation. Such parasite manipulation of host behaviour occurs when the parasite needs to be transmitted to its definitive host to complete its lifecycle. The classic example of such manipulation is where the trematode parasite Dicrocoelium dendriticum, which must be transmitted by ingestion from an ant to a sheep, causes infected ants to climb to the tip of blades of grass and stay there waiting for a grazing sheep.
Another compelling example of parasite manipulation of host behaviour is how rats become less neophobic (literally, a fear of anything new) when infected with the protozoan parasite Toxoplasma gondii. This is believed to be an ‘intentional’ effect of the parasite, since it relies on transmission to its definitive hosts (felids, most notably domestic cats) to complete its lifecycle. Solid experimental evidence has demonstrated how this reduction in neophobia leads to increased predation of rats by cats. This observation led Dan Tompkins (Landcare Research) and Clare Veltman (Department of Conservation) to wonder if, in addition to increasing predation by cats, such an effect could potentially improve kill-trapping efficacy of ship rats in the wild (Photo). Wild rats are among the most innately neophobic mammals known, reacting to novel stimuli (such as traps) with extreme caution and often total avoidance, making them particularly hard to trap. However, laboratory-based experiments have demonstrated that the ‘Toxoplasma effect’ increases their trapability in cage traps.
To explore the potential of this approach, Dan and Clare conducted a mathematical modelling exercise to see whether the improvements in trapping efficacy that Toxoplasma could theoretically make, could make it worth testing as an ‘adjunct’ to standard rat kill-trapping. Although trapping is frequently the preferred approach to control ship rats, as it avoids the potential poisoning of valued non-target species, it is generally less successful than poisoning, and its labour-intensive nature can make the cost prohibitive unless community initiatives are involved. Improvements to the efficiency of rat trapping could thus enable a big step towards our current aspirations for a ‘Predator Free New Zealand’.
The modelling process combined existing data in three steps. First, the prevalence of Toxoplasma that may be established in rats was identified from the parasitological literature. This showed that Toxoplasma prevalence of up to 70% has been documented in wild ship rat populations in other countries. Second, the per night probability that an uninfected ship rat will interact with a trap located within its home range was calculated from the New Zealand pest literature, and extrapolated to the predicted population control efficacy of different trapping events. This identified an average capture probability of 3% per night per trap, and converted into a range of trapping events to achieve 90% control of a ship rat population from an estimated 4 nights trapping with 16 kill-traps per hectare up to an estimated 19 nights trapping with 4 kill-traps per hectare. Third, the likely influence of Toxoplasma on the probability of trap success was drawn from the host manipulation literature, and used to predict how prevalence of Toxoplasma infection up to 70% may influence the efficacy of these simulated trapping events. This showed that at 70% prevalence, Toxoplasma reduced the predicted required length of trapping down to just 2 nights trapping with 16 kill-traps per hectare or 12 nights trapping with 4 kill-traps per hectare.
This exercise shows that Toxoplasma infection has the potential to increase the efficacy of trapping to control wild populations of ship rats for the purpose of biodiversity protection in New Zealand. This might help improve levels of rat control for more resource-limited operations, such as those conducted by community groups. In addition, since cats can drive Toxoplasma persistence, this suggests that cats may be best left until last in multi-predator trapping programmes, to facilitate rat trapping. However, as with any modelling study, these findings are based on a set of assumptions that need testing to generate confidence in predictions. Such testing would involve behavioural studies of infected ship rats and their interactions with kill-traps. Should these studies confirm Dan and Clare’s assumptions, the next step would be to conduct investigations into whether rat populations could be seeded with Toxoplasma infection in a cost-effective and safe manner. The use of such an adjunct to improve the efficacy of rat control is not as far-fetched as it may seem – Singapore already manages rat populations for food security by seeding with bait infected with protozoan parasites.
This work was funded by the Department of Conservation.