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

Abstract and Introduction


Incidence of human granulocytic anaplasmosis is rising in Maine, USA. This increase may be explained in part by adoption of tick panels as a frequent diagnostic test in persons with febrile illness and in part by range expansion of Ixodes scapularis ticks and zoonotic amplification of Anaplasma phagocytophilum.


Lyme disease is the most common vectorborne disease in Maine. Borrelia burgdorferi, the agent of Lyme disease, is transmitted through the bite of infected blacklegged ticks (Ixodes scapularis). Lyme disease cases in Maine increased from a single case in 1986 to 1,844 in 2017, reflecting the northward range expansion of I. scapularis ticks.[1]Anaplasma phagocytophilum, the cause of human granulocytic anaplasmosis (HGA), is also transmitted by I. scapularis ticks and is the second most common tickborne illness in Maine. Only 45 HGA cases were reported during 2000–2008,[2] but case reports rose dramatically during 2013–2017, generating media attention.[3,4] The Maine Center for Disease Control and Prevention (MECDC) reported 663 cases of anaplasmosis in 2017, a 605% increase from 94 cases in 2013, in contrast with Lyme disease cases, which increased by only 33% (1,384 in 2013 to 1,844 in 2017).[5]

We sought to determine whether the increase in anaplasmosis cases reflected broader geographic transmission of A. phagocytophilum from ticks to humans through range expansion of I. scapularis ticks, increased testing effort through increased use of tick panels that detect multiple pathogens by PCR, or both. Evidence for increased transmission would include geographic range expansion of HGA incidence and hospitalizations. Evidence of increased testing effort would be increased use of tick panels, which could lead to discovery of mild A. phagocytophilum infections, especially pediatric cases, because HGA in children is generally a mild illness.[6]