Linking landscape connectivity and the spread of tick-borne Lyme disease
A recent article led by the University of Toronto and co-authored by the JRC examines how wildlife and landscape characteristics interact to influence the spread of Lyme disease.
It finds that the rate at which Lyme disease (a tick-borne disease that poses a risk to human health) is spread by vertebrate animal species depends on 1) the population size, dispersal capacities and immunology of the vertebrate species, and 2) the spatial arrangement of suitable habitats across the landscape.
These findings provide insights into the ecological processes behind the spread of tick-borne disease, and on how landscapes and wildlife populations may need to be managed to buffer and reduce the risk of human exposure to Lyme-disease-bearing ticks.
How ticks spread disease
Whether a tick bite poses a risk to human health depends on many factors, including whether the tick is carrying any serious disease that it can transmit while feeding on blood. One of the most serious tick-borne diseases is Lyme disease.
Lyme disease is caused by a pathogen (a bacteria) carried by ticks, and commonly occurs in temperate regions of the Northern Hemisphere, including large parts of Europe. If untreated, Lyme disease can cause fatigue, pain, joint and muscle aches, memory problems and even complications in the nervous system.
Infected ticks can spread to new locations by attaching themselves to larger vertebrate animals, such as mice, birds or deer. Infected vertebrates can also spread the disease independently, as uninfected ticks in the new locations that feed on their blood become infected, thereby further transmitting the disease to other animals, including humans.
Findings of the article
To study how animals contribute to the spread of Lyme disease and the influence of the landscape and land-cover types on the rate of spread, the authors built a demographic dispersal model of several animal species involved in the spread of the bacteria (from ticks to deer), and tested it in a study area for which they already had data on the abundance of animals and the prevalence of Lyme disease bacteria.
Mice were found to be responsible for the greatest spread of the disease, given their population density and their high levels of tick infestation. The other species considered (birds and deer) contributed to a lesser extent to the pathogen spread due to their smaller numbers, the lower tick-infestation rates of birds and the immunological capacity of deer to clear the disease infection.
Deer can, however, carry large numbers of ticks over greater distances than other vertebrate hosts. Even uninfected ticks help increase the populations of ticks in other locations, which can then feed on infected animals and thereby contribute to the complex processes and interactions that ultimately help to spread the disease.
The spatial configuration of the landscape was also found to play a major role in the spread of Lyme disease. Strategically positioned 'stepping stone' patches (connectors) located between infected and uninfected areas help infected ticks reach areas that would be otherwise isolated or more weakly connected and hence beyond the reach of the disease.
The spread potential of the disease by animal hosts was significantly higher when the hosts could travel using these stepping stones.
The methods applied in this study can suggest ways to manage the isolation and functional connectivity of the different patches and potential sources of Lyme disease, for instance by pinpointing locations where interventions to control the disease prevalence and spread would be more efficient.
In summary, the study throws light on the multiple and complex functional interactions between the pathogen (the Lyme disease bacteria) and the ticks, between the ticks and the vertebrate hosts, between the pathogen and the vertebrate hosts, and between all of these and the spatial configuration of the landscapes in which the pathogen is found and through which it may spread.
The findings show that the structural elements of the landscape and the wildlife populations can be managed so as to reduce the rate of spread and the risk of human exposure to this vector-borne disease.