Surge in Anaplasmosis Cases in Maine, USA, 2013–2017

Susan P. Elias; Jessica Bonthius; Sara Robinson; Rebecca M. Robich; Charles B. Lubelczyk; Robert P. Smith, Jr.


Emerging Infectious Diseases. 2020;26(2):327-331. 

In This Article


We conclude that the surge in anaplasmosis incidence in Maine, an increase of 602% from 2013 to 2017, was a combination of increased transmission and testing effort, although we cannot partition the relative contribution of each. The 231% rise in hospitalized patients with HGA and the geographic expansion of HGA incidence and hospitalization indicate increased transmission. Range expansion of I. scapularis ticks in Maine likely has contributed to the rise in HGA cases in areas where this tick species is emergent (i.e., a recent colonizer). In addition, zoonotic amplification of A. phagocytophilum is likely occurring where I. scapularis ticks are established. Because of less efficient enzootic transmission, human infection with Babesia microti, the agent of babesiosis, lags behind B. burgdorferi transmission over time and space.[11] Less efficient enzootic transmission of A. phagocytophilum also may be the case, but we know of no confirmatory studies.

Concurrent to increased transmission was the 1,085% increase in tickborne disease panel testing performed by the 2 major providers of testing results to Maine during 2013–2017. Increased testing effort may reflect increased clinician and patient awareness and ready availability of tickborne disease panels that detect multiple pathogens. These panels may lead to detection of mild A. phagocytophilum infections or co-infections in persons with nonspecific febrile illness, as suggested by increased detection of less severely ill persons, such as children. Thirty-eight of 39 pediatric HGA cases were reported after 2013, but there were no pediatric hospitalizations. Before the use of panels, pediatric HGA cases may have been ascribed to another illness with similar symptoms.

Studies relying on diagnostic tests are subject to test sensitivity and specificity. PCR is the most effective diagnostic test during early-stage A. phagocytophilum infection with high sensitivity and specificity.[12,13] In this study, false positive PCR results were unlikely, based on test specificities reported by Mayo and NorDx.

Collaboration among all state health departments and testing laboratories across New England could help extend our findings. Vermont cases increased 1,078%, from 37 in 2013 to 399 in 2017,[14] and New Hampshire cases increased 260%, from 88 in 2013 to 317 in 2017.[15] Correlation between incidence and testing effort at the county level would corroborate a relationship between rising tickborne diseases and testing effort, if panel data included patient county of residence and travel history. Corroborating datasets on density of A. phagocytophilum–infected I. scapularis ticks would also help clarify the risks posed to human health.