1. Introduction
American black bear (Ursus americanus) populations are stable or increasing in many regions of the United States and populations in the northeastern United States have been increasing substantially since the 1980’s (Hristienko and McDonald, 2007). Pennsylvania has seen a dramatic increase in its bear population, with the estimated population doubling between the late 1980’s and 2001 and continuing to increase in the intervening twenty years (Hristienko and McDonald, 2007; Ternent, 2006).
Black bears have a broad geographic range across North America and can travel long distances, especially dispersing male bears (Lee and Vaughan, 2003; Liley and Walker, 2015), and this behavior may influence distribution of parasites that use this animal as a host. Ectoparasite surveys of black bears have been limited (Addison et al., 1978), but this species has been found to host several species of fleas (Rogers and Rogers, 1976; Addison et al., 1978), lice (Hopkins, 1954; Scholten, 1962; Rogers and Rogers, 1976; Addison et al., 1978), and ticks (Rogers and Rogers, 1976; Furman and Loomis, 1984; Yabsley et al., 2009; Al-Warid et al., 2016). However, many of these ectoparasite surveys were conducted before the etiological agents of various tick-borne diseases were known and tick species distribution and abundance in many areas of the northeastern United States has changed in recent years (Pak et al., 2019; Sonenshine, 2018).
Reported cases of tick-borne disease in humans have increased substantially in recent decades, with the number of cases reported to the CDC nearly tripling between 2004 and 2017, and are likely vastly under-reported (Schwartz et al., 2017; Rodino et al., 2020). Lyme disease, caused by the bacterium Borrelia burgdorferi and primarily transmitted by the blacklegged tick, Ixodes scapularis, is the most frequently reported vector-borne disease and one of the most frequently reported infectious diseases in the United States (Schwartz et al., 2017). With the increased reports of tick-borne disease in humans and domestic animals and geographic expansion of several ticks of high medical importance, particularly I. scapularis (Pak et al., 2019; Sonenshine, 2018) and the lone star tick (Amblyomma americanum) (Sonenshine, 2018; Springer et al., 2015), it is probable that wildlife have increased contact with these medically-relevant tick species as well (Halsey et al., 2018; Springer et al., 2015).
The blacklegged tick and the American dog tick (Dermacentor variabilis) are common throughout the Northeast and can transmit a variety of pathogens to humans and animals (Eisen et al., 2017; Pak et al., 2019). Both tick species are host generalists and have been documented parasitizing a wide range of species (Bishopp and Trembley, 1945; Halsey et al., 2018; Keirans et al., 1996; Sonenshine, 2018). As immatures, both tick species have demonstrated higher host-specificity and in the Northeast most frequently feed on small mammals and mesocarnivores (Bishopp and Trembley, 1945; Eisen et al., 2017; Sonenshine, 2018). Common hosts for adult ticks are medium- and large-bodied mammals (Bouchard et al., 2013; Eisen et al., 2017).
To date, most wildlife studies of I. scapularis and B. burgdorferi in North America consist of research on two species – the white-tailed deer (Odocoileus virginianus) and the white-footed mouse (Peromyscus leucopus) (Halsey et al., 2018). This has led to a dearth of critical literature on the role of other species in tick dispersal, host suitability, and host reservoir status (Halsey et al., 2018). Wild animals, especially large-bodied, habitat generalist species like black bears, can also serve as sentinels for tick detection as on-host tick surveillance is less influenced by short-term weather and environmental conditions and vegetation that can disrupt host-seeking tick surveillance strategies (Merrill et al., 2018). Black bears could be important in tick dispersal and ecology (Al-Warid et al., 2016) as well as useful sentinel hosts for tick surveillance (Merrill et al., 2018).
Standardized tick surveillance methods have not been developed or consistently deployed for host species infrequently evaluated for tick burden and parasitism. Studies of ticks on wild mammals other than white-tailed deer and white-footed mice most frequently consist of opportunistic tick collections (e.g. Al-Warid et al., 2016; Skinner et al., 2017) or timed surveys (e.g Zolnik et al., 2015). Opportunistic surveys have revealed that ticks are regularly found on black bears and have been documented parasitizing nearly all black bear populations evaluated in North America (Skinner et al., 2017). However, as a large mammal capable of hosting high tick burdens (Al-Warid et al., 2016), black bears present a particular challenge for accurate tick burden surveys, especially for live animals when time is a critical constraint for sampling efforts. A consistent and standardized method for assessing tick burden has not yet been developed for this large mammal.
The role of black bears as hosts for different tick species, tick life-stages, and movement of ticks over the landscape requires further evaluation to better understand the role these large mammals serve in tick ecology (Al-Warid et al., 2016; Zolnik et al., 2015). The purpose of this study was to evaluate tick species, abundance, and life-stages present on black bears in Pennsylvania and to evaluate tick spatial distribution on black bears in order to improve surveillance techniques. Additionally, as part of a concurrent project on sarcoptic mange in black bear populations, co-parasitism of ticks with Sarcoptes scabiei, the etiological agent of sarcoptic mange, was also investigated.
