Ticks capable of carrying the bacteria that cause Lyme disease and other pathogens will show earlier seasonal activity and a generally northward expansion in response to increasing temperatures associated with climate change [Likely, High Confidence]. Longer seasonal activity and expanding geographic range of these ticks will increase the risk of human exposure to ticks [Likely, Medium Confidence].

This finding is from chapter 5 of The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.

Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. The authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 2 and 3.

The approach and organization of this chapter was decided after conducting a comprehensive literature review. Two case studies, Lyme disease and West Nile virus, were chosen as representative examples of vector-borne diseases in the United States for this chapter because of their high incidence rates and the body of literature available on the association between climatic and meteorological variables and occurrence of these diseases.

Regarding human outcomes related to vector-borne diseases, there is a much greater volume of published literature available on meteorological and climatic influences on vectors. As a result, our certainty in how climate change is likely to influence the vectors far exceeds our certainty in how changing climatic conditions are likely to affect when, where, and how many cases of vector-borne diseases are likely to occur.

Although the topic of zoonotic diseases was included in the original prospectus, it was later removed due to space constraints. Additionally, since both West Nile virus infection and Lyme disease are zoonotic diseases, these case studies address concepts that are common to both vector-borne and zoonotic diseases.

Description of evidence base: There is strong evidence that temperature affects the geographical distribution of ticks,77f948ec-3f41-4367-a120-6096a78706f5 2471c8e7-348f-40c2-9a28-0d46d3d1f1df eb0e35fc-5e5e-4df4-900c-b85fc4f26d28 c50c2ea8-a6e8-4d12-80cc-272598748b84 the timing of host-seeking activity of ticks,4902bb7e-2914-469b-b0ee-06dc728f00c5 d9419ba6-ddc5-40dc-bc5d-ec5e2aa49fd3 3089e09f-56d0-4d4b-88ec-0449550a27c5 and even the timing of Lyme disease case occurrence.0360d0f9-db3c-40a7-841c-a286027e0e7b However, the abundance of ticks infected with Lyme disease spirochetes, which is considered a better predictor of human risk for Lyme disease compared with nymphal density alone, has rarely been found to be strongly associated with meteorological variables.94cb8d14-af54-4ab8-9c54-d502f5e0ea45 Studies aimed at identifying meteorological variables associated with the geographical distribution of human Lyme disease vary in their support for demonstrating positive associations between temperature and Lyme disease.d35c84bd-e490-4662-8da2-241de3090e3f 197d65cd-c05e-4ddb-8a9d-5a9aed134974 bc29c835-8d1d-4608-abe9-6320f34a1b16

New information and remaining uncertainties: While the effects of temperature, precipitation, and humidity on the spatial distribution of ticks and the timing of their host-seeking activity have been clearly established in both the eastern and western regions of the United States, where Lyme disease is common, the degree to which climate change will alter Lyme disease incidence remains uncertain. The observation that meteorological variables play a lesser role than other variables in predicting the density of nymphs infected with Lyme disease bacteria raises uncertainty in how climate change will affect the distribution and magnitude of Lyme disease incidence. This uncertainty is reflected in results from models aiming to associate meteorological variables with Lyme disease incidence that yielded inconsistent findings.d35c84bd-e490-4662-8da2-241de3090e3f 197d65cd-c05e-4ddb-8a9d-5a9aed134974 bc29c835-8d1d-4608-abe9-6320f34a1b16

Assessment of confidence based on evidence: Based on the evidence, there is high confidence that climate change, especially temperature change, is likely to cause shifts in the geographical distribution of ticks capable of carrying B. burgdorferi to more northern latitudes, the timing of host-seeking activity of ticks, and the timing of Lyme disease case occurrence. While these changes are likely to influence human disease, due to the few sources with limited consistency, incomplete models with methods still emerging, and some competing schools of thought, there is medium confidence surrounding how, and how much, climate change will influence the risk of human exposure to ticks carrying B. burgdorferi.

References :

• Meteorological influences on the seasonality of Lyme disease in the United States (0360d0f9)
• Climatic analysis of Lyme disease in the United States (197d65cd)
• A climate-based model predicts the spatial distribution of the Lyme disease vector Ixodes scapularis in the United States (2471c8e7)
• Spatiotemporal Patterns of Host-Seeking Ixodes scapularis Nymphs (Acari: Ixodidae) in the United States (3089e09f)
• Seasonal activity patterns of Ixodes pacificus nymphs in relation to climatic conditions (4902bb7e)
• Field and climate-based model for predicting the density of host-seeking nymphal Ixodes scapularis , an important vector of tick-borne disease agents in the eastern United States (77f948ec)
• A spatially-explicit model of acarological risk of exposure to Borrelia burgdorferi-infected Ixodes pacificus nymphs in northwestern California based on woodland type, temperature, and water vapor (94cb8d14)
• Effect of latitude on the rate of change in incidence of Lyme disease in the United States (bc29c835)
• Effect of Climate Change on Lyme Disease Risk in North America (c50c2ea8)
• Effects of landscape fragmentation and climate on Lyme disease incidence in the northeastern United States (d35c84bd)
• Environmentally related variability in risk of exposure to Lyme disease spirochetes in northern California: Effect of climatic conditions and habitat type (d9419ba6)
• Increasing habitat suitability in the United States for the tick that transmits Lyme disease: A remote sensing approach (eb0e35fc)