Weather and climate effects on Lyme disease exposure

In addition to climate, seasonal weather variations have a strong effect on tick populations. Changes in temperature and precipitation from year to year affect the Lyme disease incidence. Multiple studies have shown that there is a correlation between the amount of precipitation and the incidence of Lyme disease. After nymph ticks feed on and infect their host with Lyme disease they lose their ability to effectively control their water content.^[13] During a year with very little precipitation many ticks may die following feeding because of this loss of water regulatory control.^[13] These nymph ticks that have died out will never reach adulthood and lay eggs. Two years later their offspring nymph tick population will be reduced and thus Lyme disease incidence will also decrease.^[13] Schauber et al. (2005) also suggests a similar tie between lagged precipitation as measured by the Palmer Hydrologic Drought Index and Lyme disease incidence in the northeastern United States.^[14]

In addition to direct effects on nymph tick populations summer precipitation may also be a factor for controlling the population of the ticks' primary host – the white-footed mouse.^[13] Decreased rainfall in the summer months may also decrease the amount of vegetation available for mice populations to eat in order to sustain themselves during the winter.^[15] It thus follows that a decrease in rainfall may decrease the population of the ticks' primary host and thus reduce tick populations and Lyme disease incidence.

Subak (2003) proposes that a link between human behavior and precipitation could be another factor in Lyme disease incidence.^[13] In the northeastern United States when the summer weather is especially hot and dry, it may disincline people from outdoor activity.^[13] In fact, Subak (2003) found a link between dry conditions and a decreased Lyme disease incidence during late summer months in the northeast United States.^[13]

Temperature may also play a role in Lyme disease incidence. Schauber et al. (2005) found a positive correlation between the mean summer temperature and the rate of Lyme disease in the northeastern United States.^[14] Additionally, Subak (2003) suggests that there may also be a relationship between warmer winters and increased Lyme disease.^[13] This study discovered a tentative link between warmer winter weather lagged one year and Lyme disease incidence.^[13] A more moderate winter may increase the survival of the white-footed mouse host allowing for increased tick populations in the spring and summer.^[13] These warmer winter temperatures may also allow for more adult tick activity during winter and cause an increase in nymph populations the following year.^[13]

Outdoor activities tend to take place under fair weather conditions, which are those that ticks thrive in.^[16] Populations of the nymphal stage, the stage which most commonly transmits Lyme to humans,^[17] are highest during the late spring and early summer,^[17] directly preceding the months when greatest number of reported cases of Lyme disease occur.^[18]

Typical summer outerwear consists of shorter garments that enable ticks greater exposure to skin. Tick saliva has immunosuppressive properties^[19] that shut off the body's response, which would alert the body to the tick's presence. This allows ticks to remain attached for an extended period of time while feeding, increasing the probability of disease transmission.^[20]

A number of different methods have been shown to mitigate exposure to Lyme disease. Individuals who practice diligence in ticks removal and exposure reduction techniques will be resilient to the risks of contracting Lyme disease,^[21] but further risk reduction can be attained by observance of the daily weather since ticks use a number of host seeking techniques that the weather can affect.^[22] ticks are able to detect hosts by their shadow.^[22] On bright sunny days, shadows contrast greatly with the surrounding brightly lit areas.^[23] Wearing light colored clothing on these days reduces risk of ticks exposure by reducing the contrast. Conducting outdoor activities on overcast or slightly foggy days also reduces shadow detection since contrast with the surroundings is minimized.^[23]

Ticks can detect the odor of a host.^[22] On breezy days remaining downwind from prime ticks habitats will minimize scent detection.^[24] Ticks seek hosts only at temperatures above 7 °C (45 °F).^[7] Risk of exposure at temperatures below these is extremely minimal.

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