Lyme disease risk and tick activity

Human risk for Lyme disease is a function of two important environmental factors: seasonal weather and light conditions, which dictate the time of year each stage in the lifecycle is most active; and the abundance of ticks in a given location. Although there are three active stages in the deer tick lifecycle, it is the nymphal tick that has the greatest impact on public health in terms of risk of Lyme disease. This is demonstrated by comparing the seasonal activity of nymphal and adult I. scapularis with the temporal distribution of Lyme disease cases. For example, in a study in Westchester County, New York, an endemic area for Lyme disease, there was a significant correlation between seasonal activity of nymphal I. scapularis and the onset of EM, the hallmark of early Lyme disease (Falco et al., 1999; see also Figure 5.5). In this study, 74.2 percent of all EM's

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Figure 5.5 Monthly abundance of nymphal and adult I. scapularis measured in central Westchester County, NY, and the monthly distribution of EM diagnosed at WCMC, 1991-1996. Taken from Falco etal. (1999).

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Figure 5.5 Monthly abundance of nymphal and adult I. scapularis measured in central Westchester County, NY, and the monthly distribution of EM diagnosed at WCMC, 1991-1996. Taken from Falco etal. (1999).

over a six-year period were diagnosed during the months of June and July, when nymphal ticks are most active. In contrast, EMs were rarely diagnosed during times of the year when just adult ticks are active, and there was no significant correlation between the incidence of EM and adult tick abundance.

The importance of nymphal I. scapularis in the epidemiology of Lyme disease is further supported by studying temporal changes in tick numbers and Lyme disease case reports. In Westchester County, New York, annual fluctuations in nymphal tick abundance, as measured by mark-release-recapture studies of ticks, were shown to be directly correlated with the numbers of patients with EM diagnosed at a local Lyme disease clinic (Falco et al., 1999; Figure 5.6). Similar results were obtained in Connecticut comparing field-collected tick numbers and Lyme disease cases reported to the state health department (Stafford et al, 1998). These studies demonstrate that nymphal I. scapularis serve to drive the seasonal and annual patterns of Lyme disease incidence.

Why are nymphal I. scapularis more important vectors of B. burgdorferi than adult ticks, when the nymphal infection rate is roughly one-half of the adult tick infection rate? This is likely due to three factors:

1. The seasonal activity pattern of the nymphal stage, which peaks during the spring and summer months (Figure 5.4), coincides with the time of year in which outdoor human activity is typically high in the northeastern and midwestern US. This affords greater opportunity for contact between people and nymphal ticks. Additionally, I. scapularis nymphs are active during the warmer months, when people often dress more casually with clothing that is

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1991 1992 1993 1994 1995 1996

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Figure 5.6 Annual nymphal I. scapularis density measured in central Westchester County, NY, and annual numbers of EM cases diagnosed at WCMC, 1991-1996. Taken from Falco etal. (1999).

less likely to act as a barrier to host-seeking ticks than clothing worn during colder seasons.

2. Due to interstadial (stage-to-stage) mortality, nymphal abundance is much higher than that of adults, with one study in Westchester County, New York, showing the number of host-seeking nymphs to be almost 300 percent higher than the number of adult ticks (Daniels et al, 2000). Thus, there are far more nymphs available to parasitize humans and transmit B. burgdorferi than adults.

3. The tiny size of the nymphal tick (Figure 5.3) makes it more difficult to detect and remove them from the body than the larger adult tick (Yeh et al, 1995; Falco et al, 1996). This size disparity becomes even more important when considering the fact that transmission of B. burgdorferi usually occurs a minimum of 48 hours after the tick attaches to the human host (Piesman et al, 1987, 1991; Piesman, 1989). The larger adult tick, therefore, is easier to find and remove before transmission occurs, while the smaller nymph has a greater chance of staying undetected and remaining attached for more than the critical 48-hour period (Yeh et al, 1995; Falco et al, 1996).

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