Geographic and Seasonal Characterization of Tick Populations in Maryland Using Tick Identification Data Collected by the Maryland Department of Agriculture

BACKGROUND:   Maryland law mandates human tickborne disease reporting and thousands of reports are received annually by Maryland health departments. However, there is no tick surveillance program and geographic and seasonal tick distributions in Maryland are incompletely understood.  The Maryland Department of Agriculture (MDA) maintains paper records of ticks submitted for identification.  We characterized these reports to provide insights into tick presence and potential effects of climactic conditions.

METHODS:   We entered identification data for 2005-2012 into a Microsoft ® Access ® 2010 database.  The frequency and seasonality of tick species by life stage and jurisdiction were characterized using Microsoft ® Excel ® 2010.  Information provided included submitter demographics, tick species, degree of tick engorgement, and date and location of collection.  TickMap, a geospatially referenced clearinghouse for tick collection records, was explored to further disseminate the data.

RESULTS:   During 2005-2012, there were 1,011 submissions, with an annual average of 126.4 (range 53-263); numbers of submissions increased over time.  Of all submissions, 977 (96.6%) were ticks; the remainder comprised dirt specks, brown marmorated stink bugs and mites.  For 2009-2011, the most common submitted ticks were Amblyomma americanum (56%), Dermacentor variabilis (17%) and Ixodes scapularis (16%). The majority of I. scapularis were submitted by residents of counties west of the Chesapeake Bay and the distribution was similar for all years. In 2009, A. americanum were submitted from residents of counties on both sides of the Bay south of Baltimore City; however, in 2010 and 2011, A. americanum were also submitted from counties farther north, suggesting range expansion.  On average during 2009-2011, I. scapularis nymphs were submitted starting in March whereas A. americanum nymphs were submitted starting in April.  I. scapularis adults were submitted in the fall whereas A. americanumadults were submitted March-August.  The database was transferred to MDA to facilitate ongoing electronic data collection and analysis.  Ticks were displayed on TickMap based on nearest town of collection. 

CONCLUSIONS: The convenience sample of submitted ticks is not representative of all ticks throughout Maryland.  Despite this, we documented northerly expansion of A. americanum, consistent with anecdotal reports.  This reinforces the need for systematic tick surveillance to understand disease risk for targeted interventions.   Statewide collaborations were enhanced and TickMap allowed broader dissemination of findings.  The MDA tick identification service provides insights into areas of tick expansion and potential effects of climate change.  Wider advertising of this service will allow us to better leverage these data.