Our current remedies for Lyme disease do not appear to be working, with deer culls, mass spraying, immunization of animals, and advice on preventing tick bites simply no match for these acarids. Is biodiversity the key to controlling Lyme disease?
Forests and Lyme Disease
Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York, and colleagues carried out a study looking at the effectiveness of certain animal species as hosts for Lyme disease bacteria. White-footed mice proved better than chipmunks, deer, and four bird species for housing Borrelia, as did shrews, giving town planners and public health officials something to chew on.
Why Does this Matter?
The application of such work might not appear obvious at first but when you start looking at the habit of these preferred host animals for Lyme disease and then at the destruction of swathes of forest the importance of research like this begins to become clearer. Removal of slices of habitat (to make way for roads, houses, small satellite towns and strip malls, for example) affect some animal species more than others, often disproportionately affecting larger mammals. This leads to a loss of species from an area and, thereby, the favoring of other species who had previously been prey or who had vied for food resources or other kinds of resource.
Tick Hosts and Human Exposure
What Ostfeld and other researchers have noted is that the white-footed mouse is particularly adept at coping with forest fragmentation, leading to a boost in their numbers and increasing exposure to Lyme disease bacteria as humans and their companion animals come into more regular contact with these rodents. Comparing different habitats such as Block Island, where only deer, white-footed mice, and birds are present as tick hosts, and areas of Connecticut where forests remain intact, are being defragmented, or have been so fragmented as to be almost inconsequential as green spaces, can give researchers an insight into the effects of biodiversity loss on Lyme disease.
The Dilution Effect and Lyme Disease
The dilution effect, a term coined by Ostfeld and Keesing in 2000, describes the situation where a number of hosts are present for a species of tick, thus diluting the chances of a tick biting a host infected with Borrelia and, thus, decreasing the overall risk of Lyme disease in an area. Computer models of this scenario, and the effects of reducing biodiversity have found that disease risks does indeed increase as less species are present. Such simulations have been supported by fieldwork such as that by LoGiudice et al., 2003).
The Anti-Goldilocks Effect and Lyme Disease
However, the correlation may not be as simple as it appears, with other researchers like Durland Fish, arguing that raccoons and opossums, two key dilution hosts for Lyme disease bacteria, are frequently found in urban areas, actually increasing their numbers as forests are broken up. It may be that rapid deforestation, fragmentation and loss of habitat for these animals results in fewer Lyme disease cases in subsequent years because fewer people ever go anywhere near a forest. The middle road does not, in this case, appear very advantageous.
Keeping Forests Intact and Using Biodiversity Against Lyme Disease
Certainly, with there being many more advantages to having urban greenspaces and keeping forests intact, the ‘nothing’ option in this seeming ‘all or nothing’ vision appears decidedly unattractive. This is especially true when considering that deforestation and increasing encroachment of concrete landscaping into rural areas has the effect of increasing greenhouse gas emissions and precipitating the spread of ticks that carry Lyme disease. There may not be a direct correlation between the loss of a square foot of forest and an additional person infected but ignoring the impact of forest fragmentation on Lyme disease risk seems foolhardy.