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Article Review: Spatial Movement and Lyme Disease

Understanding how a disease moves, or will move, over time is vital. Pathogens have their own unique ways of spreading to new places. Some common examples include hosts/reservoirs moving the pathogen into new places or changing environmental variables allowing for a geographic spread of both the pathogen and its hosts, reservoirs, and/or vectors. In the case of Lyme disease, the pathogen is highly dependent on the presence of its vector (Ixodes scapularis ticks) and suitable reservoirs such as the white-footed mouse (Peromyscus leucopus). This blog will review an article that found changes in environmental factors due to climate change will lead to a spread in the range of both I. scapularis and Borrelia burgdorferi, the causative agent of Lyme disease.

Ogden et al (2006) explored how the spatial distribution of Lyme disease may change due to climate change using an I. scapularis population model from Ogden et al (2005). They were specifically interested in studying the northern spread of the disease into Canada as the current northern limit is near the United States/Canada border. As I. scapularis requires temperatures above 0°C in order to establish in a new location (Ogden et al. 2004), they mapped annual degree-days > 0°C limits using temperatures projected for the 2020s, 2050s, and 2080s. These temperature projections were generated from two Global Climate Models (Canadian CGCM2 and UK HadCM3) using two different greenhouse gas emission scenarios (A2 and B2). While the A2 emission scenario assumes no change in the current emission trends, the B2 scenario does assume a reduction in greenhouse gas emissions. The A2 scenario found that I. scapularis establishment would move northwards about 200 km by 2020s and 1,000 km by 2080s. While the A2 and B2 scenarios were pretty similar for the initial years, there was a noticeable decrease in the tick’s range expansion between 2050s and 2080s under scenario B2. The researchers also explored how the abundance of I. scapularis would change over time with climate change. They found that tick abundance almost doubled by the 2020s at the current northern limit under scenario A2. They concluded that the increased spatial spread combined with the tick’s increased survival at already established locations will lead to a detectable range expansion of I. scapularis within the next two decades. Lyme disease is therefore predicted to expand significantly northwards due to climate change within this century.

Overall, this study exposed two important findings: (1) there will be a northwards spatial movement of I. scapularis due to climate change and (2) we should expect to see an increased abundance of I. scapularis ticks at existing locations. Together, these discoveries suggest that Lyme disease is very likely to expand its range in the upcoming years. Canada already has white-footed mice, which will further ease the spread of Lyme disease into its new northern territory. Although they found no significant differences between the expected northward spread of I. scapularis between now and the 2050s under scenarios A2 and B2, they did find that reducing greenhouse gas emissions (scenario B2) will help reduce its distribution between the 2050s and 2080s. We should therefore see this as a warning sign that the continued high release of greenhouse gasses will further aid this pathogen in expanding its range, thereby exposing more individuals to Lyme disease.

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Ogden, N.H., Lindsay, L.R., Beauchamp, G., Charron, D., Maarouf, A., O’Callaghan, C.J., Waltner-Toews, D., Barker, I.K. 2004. Investigation of relationships between temperature and developmental rates of tick Ixodes scapularis (Acari: Ixodidae) in the laboratory and field. Journal of Medical Entomology 41: 622-633.

Ogden, N.H., Bigras-Poulin, M., O’Callaghan, C.J., Barker, I.K., Lindsay, L.R., Maarouf, A., Smoyer-Tomic, K.E., Waltner-Toews, D., Charron, D. 2005. A dynamic population model to investigate effects of climate on geographic range and seasonality of the tick Ixodes scapularis. International Journal for Parasitology 35: 375-389.

Ogden, N.H., Maarouf, A., Barker, I.K., Bigras-Poulin, M., Lindsay, L.R., Morhsed, M.G., O’Callaghan, C.J., Ramay, F., Waltner-Toews, D., Charron, D.F. 2006. Climate change and the potential for range expansion of the Lyme disease vector Ixodes scapularis in Canada. International Journal for Parasitology 36: 63-70.