Environmental and behavioral factors impacting risk

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On the local level, risk for Lyme disease is not homogeneous. Even in areas considered endemic, nymphal tick abundance can vary significantly. For example, in New York State, which has reported more Lyme disease cases over the

12-year period from 1993 to 2004 than any other state in the US (CDC, 2006a), tick abundance can vary significantly not only within the state (Daniels et al, 1998) but also within a county (Falco and Fish, 1992; Falco et al., 1995), and even between neighborhoods (Maupin et al, 1991). Such disparity in the distribution of ticks is likely due to several factors, including host activity patterns and changes in microhabitat that impact tick survival, both of which are strongly influenced by the local landscape features. In the northeastern US, risk for Lyme disease based on landscape features can be largely divided into two categories: residential and recreational.

The peridomestic nature of Lyme disease was first described in 1988, when studies in southern New York State of both Lyme disease cases and I. scapularis ticks removed from humans suggested that tick bites and B. burgdorferi infections often occurred around the home as a result of activities on the lawn (Falco and Fish, 1988a, 1988b). Subsequent investigations have provided additional evidence that residential exposure to tick bites is a major factor contributing to the development of Lyme disease. For example, in a study of lawns and adjacent woodlots in residential areas of southeastern Connecticut, 26.5 percent of I. scapularis nymphs and 36.4 percent of I. scapularis adults were collected directly from lawns, although the risk of exposure to infected nymphs varied spatially with the type of landscape and with each individual residence (Stafford and Magnarelli, 1993). In a residential subdivision in Westchester County, an attempt was made to determine the spatial distribution of I. scapularis in all habitats within the residential environment (Maupin et al., 1991). Most nymphs were collected from the woods (67.3 percent) and ecotone (unmaintained edge) (21.6 percent), with smaller numbers found on ornamental plants (9.1 percent) and lawns (2.0 percent) (Figure 5.7). However, the presence of ticks on lawns and ornamental plants was of concern because most

Habitat type

Figure 5.7 Abundance of Ixodes scapularis ticks (all stages) collected by drag sampling in four habitat types on 67 properties, Armonk, NY. Data from Maupin et al. (1991).

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Habitat type

Figure 5.7 Abundance of Ixodes scapularis ticks (all stages) collected by drag sampling in four habitat types on 67 properties, Armonk, NY. Data from Maupin et al. (1991).

Lawn Ornamentals Ecotone Woods

Lawn Ornamentals Ecotone Woods residents spent the majority of their outdoor time involved in recreational and lawn-maintenance activities in these two habitats, with lawns that are adjacent to woodlots of special concern (Maupin et al, 1991; Duffy et al, 1994). Perhaps the most important reason that exposure to infected ticks in Westchester County, New York, is largely peridomestic is the amount of edge that typifies many properties. Mowed backyard lawns frequently abut woodland along a strip of unmaintained edge habitat harboring ticks. The extremely high level of residential risk for Lyme disease may also explain the propensity for young children to acquire tick bites, since children under 10 years of age are most likely to spend much of their outdoor recreational time in their own back yard (Falco and Fish, 1988b).

While epidemiologically less important than residential risk, recreational activities also provide an important opportunity for tick exposure in the northeastern US. In particular, recreational parks may present a high risk for human exposure to tick bites, as tick abundance can be quite high (Falco and Fish, 1989). Additionally, serologic studies have shown that outdoor workers in parks and other areas of largely undisturbed woodland have an elevated Lyme disease risk when compared to control groups without such exposure (Bowen et al, 1984; Schwartz and Goldstein, 1990). It should be noted that outside of the northeastern US, where there tends to be less development and a more rural landscape, recreational areas may play an even more important epidemiologic role in the risk of exposure to I. scapularis ticks by humans.

Parks are also likely to play a role in human exposure to Lyme disease in urban areas. Although the suburban and rural landscapes are far more important contributors to the overall burden of Lyme disease cases in the US, evidence suggests there is at least some risk in more developed and populated urban areas. For example, in Westchester County, New York, studies utilizing a serosurvey of canine exposure to B. burgdorferi and remote sensing technology to analyze land-cover composition demonstrated that there was a higher than expected risk of exposure in the southern, urban region (Dister et al, 1993; Falco et al, 1993). While exposure was lower in the urbanized area compared to the suburban/rural northern region (67.3 percent vs 17.3 percent seroprevalence in dogs, respectively), there was at least some local exposure to ticks in the urban landscape (Falco et al, 1993).

Additional studies have demonstrated that I. scapularis ticks infected with B. burgdorferi can be found in and near urban settings. For example, adult I scapularis infected with B. burgdorferi were collected within 3 km of Philadelphia, Pennsylvania (Anderson et al, 1990), and in Bridgeport, Connecticut, collections of ticks and blood from white-tailed deer revealed both I. scapularis infected with B. burgdorferi, as well as seropositive deer. In the latter study, the authors concluded that foci for Lyme disease can occur in forested urban settings as well as in rural areas if there are ticks, rodents, birds, and large mammals present (Magnarelli et al., 1995). In northern New York City (Bronx borough), mouse trapping and drag sampling conducted in Van Cortlandt Park resulted in the collection of I. scapularis infected with B. burgdorferi or with A. phagocy-tophilum (Daniels et al, 1997). Although tick populations were low, the presence of infected ticks suggested that I. scapularis can exist at low populations in urban areas when deer are present even on an intermittent, seasonal basis, as apparently occurs in this park. Unlike the peridomestic exposure to ticks infected with B. burgdorferi that is typical of suburban and rural areas, tick populations in urban areas likely result in more focal exposure, restricted to woodland habitat "islands" which exist primarily as parkland (Daniels et al., 1997).

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