A project undertaken at The University of Sydney, and supervised by Prof Peter Banks.
Ticks are ectoparasites that can affect cause health problems in wildlife, humans and pets, particularly in urban and peri-urban areas.
The medical impact of tick-borne diseases is well known but there is a gap in understanding how ecological factors, such as the behaviour and abundance of hosts, affect tick populations, especially in urban ecosystems. The major tick of health concern in Australia is the Australian paralysis tick (Ixodes holocyclus). This tick is a generalist, meaning it can feed on a wide range of animals. Our recent work pointed to black rats (Rattus rattus), an introduced species, as potentially important to tick populations in urban “hot spots”. These rats are abundant in areas where ticks thrive, and they host high numbers of juvenile ticks, suggesting a key role in driving urban tick dynamics.
Extensive research into tick ecology in the Northern Hemisphere has unlocked a detailed understanding of how different wildlife hosts contribute to tick life cycles. This knowledge has led to successful tick management strategies, such as host population control and bait boxes that administer repellent, targeting specific wildlife hosts at different stages of the tick’s life cycle (larva, nymph, and adult). However, in Australia, we lack the knowledge required to make evidence-informed decisions around managing ticks, and native bandicoots are mistakenly considered the “main” tick host to become a scapegoat for urban tick problems.
In this project, we studied the role of black rats in urban tick dynamics to understand how this abundant invasive species shapes parasite ecology. Our project tested three key hypotheses:
1) That black rats amplify local tick populations (or alternatively act as ecological traps through grooming)
2) That black rats move regularly between bushland and backyards
3) That a local rat removal program will lead to a reduction in tick abundance
By testing these hypotheses, the project aimed to provide more accurate information on how black rats influence tick populations, offering insights into potential management strategies for controlling ticks in urban areas.
Key Findings:
- Tick attachment and rat grooming behaviour We first examined where ticks attach on the rat’s body. We found that ticks attempt attachment close to the point of first contact on the rat’s body. Thus, only finding ticks on rat’s heads is likely the result of rat’s grooming ticks from elsewhere on their body. Rats may therefore play a role a reducing tick numbers.
- Black rat movement between urban and bushland areas We then tracked the movements of black rats across the boundary between urban and bushland areas. We found that while some rats occasionally crossed this boundary, most spent the majority of their time in either urban or bushland areas. This suggests that urban and bushland rat population are largely seperate with limited opportunities to move ticks between environments.
- Impact of black rat removal on tick populations In the final part of the study, we conducted a year-long experiment removing black rats from urban bushland areas and observed the effects on local tick populations. Despite removing 208 rats from these areas, tick populations did not decrease significantly in the removal areas compared to control sites. This could be because of the presence of untrapped rats, alternate hosts or local factors as ticks numbers were highly variable in space. However, the results did suggest that rat removal alone may not be enough to substanianlly reduce tick numbers in these areas.
Figure 2. A cluster of engorged nymphs (in black circle) on the neck of an anaesthetised black rat (left) and a larval Ixodes tick under the microscope (~0.8mm) (right).
Conclusion and Next Steps
This study underscores the complex role of invasive black rats in urban tick populations. While black rats are abundant in tick-prone areas, their role in amplifying (as hosts) and controlling (through grooming) ticks may create a neutral impact. Hence rat removal alone may not be an effective strategy for a substantial reduction in tick numbers, but further research is needed to explore other management techniques.
Understanding the behaviour of black rats and their impact on tick dynamics is crucial for developing effective tick management strategies in urban areas. By continuing to study the relationship between invasive species and parasites, we can improve our ability to manage tick-borne health risks in urban environments.