5.2 Lyme Disease

State of the Science

Blacklegged tick

In the eastern United States, Lyme disease is transmitted to humans primarily by blacklegged (deer) ticks.

Lyme disease is a tick-borne bacterial disease that is endemic (commonly found) in parts of North America, Europe, and Asia. In the United States, Lyme disease is caused by the bacterium Borrelia burgdorferi sensu stricto (B. burgdorferi; one of the spiral-shaped bacteria known as spirochetes) and is the most commonly reported vector-borne illness. It is primarily transmitted to humans in the eastern United States by the tick species Ixodes scapularis (formerly I. dammini), known as blacklegged ticks or deer ticks, and in the far western United States by I. pacificus, commonly known as western blacklegged ticks.1 Illness in humans typically presents with fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system.2 Since 1991, when standardized surveillance and reporting of Lyme disease began in the United States, case counts have increased steadily.3 Since 2007, more than 25,000 Lyme disease cases have been reported annually.4 The geographic distribution of the disease is limited to specific regions in the United States (Figure 5.2), transmission occurs seasonally, and year-to-year variation in case counts and in seasonal onset is considerable.2,3,5 Each of these observations suggest that geographic location and seasonal climate variability may play a significant role in determining when and where Lyme disease cases are most likely to occur.

Although the reported incidence of Lyme disease is greater in the eastern United States compared with the westernmost United States,2,3 in both geographical regions, nymphs (small immature ticks) are believed to be the life stage that is most significant in pathogen transmission from infected hosts (primarily rodents) to humans (Figure 5.2, Figure 5.3).6,7 Throughout the United States, the majority of human cases report onset of clinical signs of infection during the months of June, July, and August. The summer is a period of parallel increased activity for both blacklegged and western blacklegged ticks in the nymphal life stage (the more infectious stage) and for human recreational activity outdoors.3,7

Infection rates in humans vary significantly from year to year. From 1992 to 2006, variation in case counts of Lyme disease was as high as 57% from one year to the next.3 Likewise, the precise week of onset of Lyme disease cases across states in the eastern United States, where Lyme disease is endemic, differed by as much as 10 weeks from 1992 to 2007. Much of this variation in timing of disease onset can be explained by geographic region (cases occurred earlier in warmer states in the mid-Atlantic region compared with cooler states in the North); however, the annual variation of disease onset within regions was notable and linked to winter and spring climate variability (see “Annual and Seasonal Variation in Lyme Disease” below).5

 

Figure 5.2: Changes in Lyme Disease Case Report Distribution

Figure 5.2: Changes in Lyme Disease Case Report Distribution
Maps show the reported cases of Lyme disease in 2001 and 2014 for the areas of the country where Lyme disease is most common (the Northeast and Upper Midwest). Both the distribution and the numbers of cases have increased. (Figure source: adapted from CDC 2015)47

The geographic and seasonal distributions of Lyme disease case occurrence are driven, in part, by the life cycle of vector ticks (Figure 5.3). Humans are only exposed to Lyme disease spirochetes (B. burgdorferi) in locations where both the vector tick populations and the infection-causing spirochetes are present.8 Within these locations, the potential for contracting Lyme disease depends on three key factors: 1) tick vector abundance (the density of host-seeking nymphs being particularly important), 2) prevalence of B. burgdorferi infection in ticks (the prevalence in nymphs being particularly important), and 3) contact frequency between infected ticks and humans.9 To varying degrees, climate change can affect all three of these factors.

Aside from short periods of time when they are feeding on hosts (less than three weeks of their two- to three-year life cycle), ticks spend most of their lives off of hosts in various natural landscapes (such as woodlands or grasslands) where weather factors including temperature, precipitation, and humidity affect their survival and host-seeking behavior. In general, both low and high temperatures increase tick mortality rates, although increasing humidity can increase their ability to tolerate higher temperatures.10,11,12,13,14,15,16,17,18,19 Within areas where tick vector populations are present, some studies have demonstrated an association among temperature, humidity, and tick abundance.20,21,22 Factors that are less immediately dependent on climate (for example, landscape and the relative proportions within a community of zoonotic hosts that carry or do not carry Lyme disease-causing bacteria) may be more important in smaller geographic areas.23,24 Temperature and humidity also influence the timing of host-seeking activity,13,16,17,25 and can influence which seasons are of highest risk to the public.

In summary, weather-related variables can determine geographic distributions of ticks and seasonal activity patterns. However, the importance of these weather variables in Lyme disease transmission to humans compared with other important predictors is likely scale-dependent. In general, across the entire country, climate-related variables often play a significant role in determining the occurrence of tick vectors and Lyme disease incidence in the United States (for example, Lyme disease vectors are absent in the arid Intermountain West where climate conditions are not suitable for tick survival). However, within areas where conditions are suitable for tick survival, other variables (for example, landscape and the relative proportions within a community of zoonotic hosts that carry or do not carry Lyme disease-causing bacteria) are more important for determining tick abundance, infection rates in ticks, and ultimately human infection rates.20,26,27

Observed Trends and Measures of Human Risk

Geographic Distribution of Ticks

Because the presence of tick vectors is required for B. burgdorferi transmission to humans, information on where vector tick species live provides basic information on where Lyme disease risk occurs. Minimum temperature appears to be a key variable in defining the geographic distribution of blacklegged ticks.20,26,28 Low minimum temperatures in winter may lead to environmental conditions that are unsuitable for tick population survival. The probability of a given geographic area being suitable for tick populations increases as minimum temperature rises.26 In the case of the observed northward range expansion of blacklegged ticks into Canada, higher temperatures appear to be a key factor affecting where, and how fast, ticks are colonizing new localities.29,30,31,32,33

Maximum temperatures also significantly affect where blacklegged ticks live.20,26 Higher temperatures increase tick development and hatching rates, but reduce tick survival and egg-laying (reproduction) success.11

Declines in rainfall amount and humidity are also important in limiting the geographic distribution of blacklegged ticks. Ticks are more likely to reside in moister areas because increased humidity can increase tick survival.16,19,20,26,28,30 

Geographic Distribution of Infected Ticks

Climate variables have been shown to be strong predictors of geographic locations in which blacklegged ticks reside, but less important for determining how many nymphs live in a given area or what proportion of those ticks is infected.20,21 The presence of uninfected nymphs and infected nymphs can vary widely over small geographic areas experiencing similar temperature and humidity conditions, which supports the hypothesis that factors other than weather play a significant role in determining nymph survival and infection rates.18,20,21,22,25 Additional studies that modeled nymphal density within small portions of the blacklegged tick range (north-central states and Hudson River Valley, NY), and modeling studies that include climate and other non-biological variables indicate only a weak relationship to nymphal density.34,35 Nonetheless, climate variables can be used to model nymphal density in some instances. For example, in a single county in northern coastal California with strong climate gradients, warmer areas with less variation between maximum and minimum monthly water vapor in the air were characteristic of areas with elevated concentrations of infected nymphs.22 However, it is likely that differences in animal host community structure, which vary with climatic conditions (for example, relative abundances of hosts that carry or do not carry Lyme disease-causing bacteria), influenced the concentration of infected nymphs.18,36

Geographic Distribution of Lyme Disease

Though there are links between climate and tick distribution, studies that look for links between weather and geographical differences in human infection rates do not show a clear or consistent link between temperature and Lyme disease incidence.27,37,38

 

Annual and Seasonal Variation in Lyme Disease

Temperature and precipitation both influence the host-seeking activity of ticks, which may result in year-to-year variation in the number of new Lyme disease cases and the timing of the season in which Lyme disease infections occur. However, identified associations between precipitation and Lyme disease incidence, or temperature and Lyme disease incidence, are limited or weak.39,40 Overall, the association between summer moisture and Lyme disease infection rates in humans remains inconsistent across studies.

House finch

Birds such as the house finch are the natural host of West Nile virus.

The peak period when ticks are seeking hosts starts earlier in the warmer, more southern, states than in northern states.25 Correspondingly, the onset of human Lyme disease cases occurs earlier as the growing degree days (a measurement of temperature thresholds that must be met for biological processes to occur) increases, yet, the timing of the end of the Lyme disease season does not appear to be determined by weather-related variables.5 Rather, the number of potential carriers (for example, deer, birds, and humans) likely influences the timing of the end of the Lyme disease season.

The effects of temperature and humidity or precipitation on the seasonal activity patterns of nymphal western blacklegged ticks is more certain than the impacts of these factors on the timing of Lyme disease case occurrence.17,18 Peak nymphal activity is generally reached earlier in hotter and drier areas, but lasts for shorter durations. Host-seeking activity ceases earlier in the season in cooler and more humid conditions. The density of nymphal western blacklegged ticks in north-coastal California consistently begins to decline when average daily maximum temperatures are between 70°F (21°C) and 73.5°F (23°C), and when average maximum daily relative humidity decreases below 83%–85%.17,18

Projected Impacts

Warmer winter and spring temperatures are projected to lead to earlier annual onset of Lyme disease cases in the eastern United States (see “Research Highlight” below) and in an earlier onset of nymphal host-seeking behavior.41 Limited research shows that the geographic distribution of blacklegged ticks is expected to expand to higher latitudes and elevations in the future and retract in the southern United States.42 Declines in subfreezing temperatures at higher latitudes may be responsible for improved survival of ticks. In many woodlands, ticks can find refuge from far-subzero winter air temperatures in the surface layers of the soil.43,44 However, a possibly important impact of climate change will be acceleration of the tick life cycles due to higher temperatures during the spring, summer, and autumn, which would increase the likelihood that ticks survive to reproduce.33,45 This prediction is consistent with recent observations of the spread of I. scapularis in Canada.30,46

To project accurately the changes in Lyme disease risk in humans based on climate variability, long-term data collection on tick vector abundance and human infection case counts are needed to better understand the relationships between changing climate conditions, tick vector abundance, and Lyme disease case occurrence.

 

References

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