A 117-year retrospective analysis of Pennsylvania tick community dynamics

Abstract

Background: Tick-borne diseases have been increasing at the local, national, and global levels. Researchers studying ticks and tick-borne diseases need a thorough knowledge of the pathogens, vectors, and epidemiology of disease spread. Both active and passive surveillance approaches are typically used to estimate tick population size and risk of tick encounter. Our data consists of a composite of active and long-term passive surveillance, which has provided insight into spatial variability and temporal dynamics of ectoparasite communities and identified rarer tick species. We present a retrospective analysis on compiled data of ticks from Pennsylvania over the last 117 years.

Methods: We compiled data from ticks collected during tick surveillance research, and from citizen-based submissions. The majority of the specimens were submitted by citizens. However, a subset of the data was collected through active methods (flagging or dragging, or removal of ticks from wildlife). We analyzed all data from 1900-2017 for tick community composition, host associations, and spatio-temporal dynamics.

Results: In total there were 4491 submission lots consisting of 7132 tick specimens. Twenty-four different species were identified, with the large proportion of submissions represented by five tick species. We observed a shift in tick community composition in which the dominant species of tick (Ixodes cookei) was overtaken in abundance by Dermacentor variabilis in the early 1990s and then replaced in abundance by I. scapularis. We analyzed host data and identified overlaps in host range amongst tick species.

Conclusions: We highlight the importance of long-term passive tick surveillance in investigating the ecology of both common and rare tick species. Information on the geographical distribution, host-association, and seasonality of the tick community can help researchers and health-officials to identify high-risk areas.

Keywords: Community composition; Museum collections; Passive surveillance; Ticks.

Fig. 1

Annual reported cases of Lyme disease. By state from 2006–2017 (left) and by counties in Pennsylvania from 2006–2017 (right). Public data from the Centers for Disease Control and Prevention

Fig. 2

Distribution of the five most abundant tick species across Pennsylvania over time. Prevalence rates (tick counts per 100,000 population, left) represent tick abundance adjusted by county population for time periods 1960–1969, 1990–1999, 2000–2009 and 2010–2018. Cumulative counts of ticks by species shown on the right

Fig. 3

Annual submissions of tick specimens by year. On the left is the annual sum of all tick counts (log-transformed) from 1900 to 2017. On the right are the proportional contributions of the five major tick species to the total tick counts (1900–2017). The grey shaded area represents periods where there were few or no tick submissions from the top five most abundant taxa

Fig. 4

Seasonal distribution of tick submissions over time. On the left is the total proportion of tick specimens received at different months of the years from 1900 to 2017. On the right are the proportional seasonal abundances of each of the five major tick species (1900–2017)

Fig. 5

The seasonal distribution of D. variabilis, I. cookei and I. scapularis specimens by life stages from 1900 to 2017. The proportion was calculated by comparing the monthly abundance of each life stage (larvae, nymphs and adults) to the cumulative sum of all stages by species

Fig. 6

Chord diagram representing associations between tick species and vertebrate hosts parasitized. Submissions (not counts) were used to quantify host association. We chose submissions over counts to avoid skews in abundance by hosts. The wider the chord, the more submissions exist for any given tick species-to-host