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

L yme 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).The Study MECDC provided the number of confirmed and probable cases for Maine residents during 2008-2017, available by county of residence and age of onset.For 2013-2017, we obtained the annual number of hospitalizations for Lyme disease and HGA for Maine residents, with age and county of residence at admission, from the Maine Health Data Organization.We obtained the annual number of multipathogen (including HGA) PCR tick panel orders during 2013-2017 from NorDx and Mayo Medical Laboratories (MML).NorDx (Scarborough, ME, USA) started using its panel for the agents of HGA and babesiosis in 2015 (H.Webber, NorDx, pers.comm., 2018 Sep 12).All orders were for Maine patients (travel history not specified).MML provided data from 2 branch laboratories, Mayo Clinic Rochester (MCR; Rochester, MN, USA) and Mayo Medical Laboratories, New England (MMLNE; Andover, MA, USA).MML has offered the panel for ≈10 years; the panel contains a PCR test for the agents of human monocytic ehrlichiosis, HGA, babesiosis, and Borrelia miyamotoi infections (B.Pritt, MML, pers.comm., 2018 Aug 3).MML data comprised specimens sent to MML from Maine clients, without patient residence or travel history (B.Pritt, A. Boerger, MML, pers.comm., 2018 Oct 8).We were unable to obtain data from other laboratories; however, MML and NorDx combined accounted for 72% of HGA test reports sent to MECDC during 2013-2017.The MML panel had sensitivity and specificity of 1 for detection of A. phagocytophilum compared with standard PCR (7).The NorDx panel had sensitivity and specificity of 1 compared Surge in Anaplasmosis Casesin Maine, USA, 2013-2017