Human granulocytic anaplasmosis in the United States from 2008 to 2012: a summary of national surveillance data

Abstract

Human granulocytic anaplasmosis is an acute, febrile illness transmitted by the ticks Ixodes scapularis and Ixodes pacificus in the United States. We present a summary of passive surveillance data for cases of anaplasmosis with onset during 2008-2012. The overall reported incidence rate (IR) was 6.3 cases per million person-years. Cases were reported from 38 states and from New York City, with the highest incidence in Minnesota (IR = 97), Wisconsin (IR = 79), and Rhode Island (IR = 51). Thirty-seven percent of cases were classified as confirmed, almost exclusively by polymerase chain reaction. The reported case fatality rate was 0.3% and the reported hospitalization rate was 31%. IRs, hospitalization rates, life-threatening complications, and case fatality rates increased with age group. The IR increased from 2008 to 2012 and the geographic range of reported cases of anaplasmosis appears to have increased since 2000-2007. Our findings are consistent with previous case series and recent reports of the expanding range of the tick vector I. scapularis.

Figure 1.

A chart of the annual incidence rate, the number of incident cases per million persons at risk, of anaplasmosis vs. the year of onset of symptoms, 2008–2012. The number of incident cases is from the Nationally Notifiable Diseases Surveillance System, and the number of person-years at risk is from the U.S. Census Bureau.

Figure 2.

A map of reported incidence rates, the number of incident cases per million persons at risk per year, of anaplasmosis in the counties of the United States, 2008–2012. States where the disease was not notifiable for the duration of 2008–2012 are shaded with the “NN” category. The number of incident cases is from the Nationally Notifiable Diseases Surveillance System, and the number of person-years at risk is from the U.S. Census Bureau.

Figure 3.

A chart of the percent of cases of anaplasmosis vs. month of onset, 2008–2012. The data are from the Nationally Notifiable Diseases Surveillance System.

Figure 4.

A chart of the incidence rate of anaplasmosis vs. age group at onset of symptoms, 2008–2012. The number of incident cases is from the Nationally Notifiable Diseases Surveillance System, and the number of person-years at risk is from the U.S. Census Bureau.

Figure 5.

A chart of the case fatality rate vs. age group at onset, 2008–2012. The data are from case report forms.

Figure 6.

A chart of the hospitalization rate vs. age group at onset, 2008–2012. The data are from case report forms.

Figure 7.

A chart of the proportion of cases reporting a life-threatening complication vs. age group at onset, 2008–2012. The data are from case report forms.

Figure 8.

A Venn diagram of supportive laboratory evidence among the 4,982 probable cases: positive immunoglobulin M (IgM) by indirect immunofluorescence assay (IFA IgM), morulae visualization in granulocytes by microscopy (morulae), positive results from serological assays other than IgM or IgG IFAs (other), and positive IgG by IFA IgG. Data are from case report forms.