“Ticking Bomb”: The Impact of Climate Change on the Incidence of Lyme Disease, Canadian Journal of Infectious Diseases and Medical Microbiology, Sept 2018Lyme disease (LD) is the most common tick-borne disease in North America. It is caused by Borrelia burgdorferi and transmitted to humans by blacklegged ticks, Ixodes scapularis. The life cycle of the LD vector, I. scapularis, usually takes two to three years to complete and goes through three stages, all of which are dependent on environmental factors. Increases in daily average temperatures, a manifestation of climate change, might have contributed to an increase in tick abundance via higher rates of tick survival. Additionally, these environmental changes might have contributed to better host availability, which is necessary for tick feeding and life cycle completion. In fact, it has been shown that both tick activity and survival depend on temperature and humidity. In this study, we have examined the relationship between those climatic variables and the reported incidence of LD in 15 states that contribute to more than 95% of reported cases within the Unites States. Using fixed effects analysis for a panel of 468 U.S. counties from those high-incidence states with annual data available for the period 2000–2016, we have found sizable impacts of temperature on the incidence of LD. Those impacts can be described approximately by an inverted U-shaped relationship, consistent with patterns of tick survival and host-seeking behavior. Assuming a 2°C increase in annual average temperature—in line with mid-century (2036–2065) projections from the latest U.S. National Climate Assessment (NCA4)—we have predicted that the number of LD cases in the United States will increase by over 20 percent in the coming decades. These findings may help improving preparedness and response by clinicians, public health professionals, and policy makers, as well as raising public awareness of the importance of being cautious when engaging in outdoor activities.
The major reservoirs for B. burgdorferi are birds and small mammals such as mice and chipmunks. While deer are not competent hosts for B. burgdorferi, they are essential for the I. scapularis life cycle. The tick I. scapularis has three stages of development: larva, nymph, and adult tick. In North America, the life cycle of I. scapularis takes approximately two years to complete . Egg laying usually begins in May; hence, larvae are the most abundant during the summer. These larvae feed on small mammals such as the white footed mouse during summer, at which point transmission of B. burgdorferi occurs. As the winter approaches, the tick larvae enter a dormant stage in which they stay throughout the winter. In the beginning of the spring of the second year, the larvae that survived the winter mold into the next stage of tick development—nymph. During the spring/summer of the second year those nymphs seek suitable hosts for feeding, including humans. Following a bloody meal, the nymphs mold into adults. If an adult tick survives the winter, it will seek another host (usually a large mammal such as deer) on which it will feed and be able to lay eggs. At that point the two year life cycle is completed.
Change and Wildlife, PART II: SPECIES HIGHLIGHTS, Manomet, March 21, 2016As conditions change, moose and deer may alter their habitat selection, shifting where and when they utilize certain types of habitat. For example, decreases in lake ice in Michigan have led to more lake effect snow that creates harsh winter conditions for deer and increases their reliance on shelter in conifer swamps (although this increased precipitation is expected to shift more toward rain over the next century). White-tailed deer are not expected to decline as a direct result of climate change, but these types of changes in migration patterns and seasonal habitat are likely. As the climate changes, this cold/snow limiting line will move and two things are likely to result: (1) the more southerly deer yards will become less critical to survival and (2) deer populations will increase.
Nowhere to run: Big game wildlife in awarming world, NFS, 2016?Extreme weather, disease and changes in habitat are potential climate-driven stressors on white-tailed deer. In northern areas of their range, deer often seek out winter shelter from wind, extreme cold and deep snows in areas known as “deer yards.” Deer yards are typically in dense stands of hemlock and white pine with a generally southern aspect. Northeastern states where deer “yard-up” include Maine, Massachusetts, New Hampshire, New York and Vermont. In the Upper-Midwest, deer yards are found in Michigan, Wisconsin42and Minnesota. Should climate-driven changes alter the suitability and location of deer yard habitats, deer may have difficulty finding new safe areas to over-winter or be forced into areas difficult to protect.