Babesiosis Surveillance — Wisconsin, 2001–2015
Elizabeth Stein, MD1; Lina I Elbadawi, MD2,3; James Kazmierczak, DVM3; Jeffrey P. Davis, MD3
Summary
Babesiosis is an emerging tickborne disease endemic to the northeastern United States and the upper Midwest. Many infected persons are asymptomatic but the disease can be life-threatening, especially among older and immunocompromised persons. Prompt diagnosis and treatment in patients with severe infection can prevent serious complications and death.
What is added by this report?
Analysis of Wisconsin babesiosis surveillance data during 2001–2015 indicates expansion of the geographic range and increased incidence. Routine use of polymerase chain reaction testing and automatic electronic laboratory reporting likely contributed to the increased reported incidence of confirmed babesiosis in Wisconsin; however, evidence of blacklegged tick expansion suggests an actual increase in infection rates.
What are the implications for public health practice?
Babesiosis cases in Wisconsin are increasing in number and geographic range. These trends might be occurring in other states with endemic disease, similar suburbanization and forest fragmentation patterns, and warming average temperatures. Accurate surveillance in states where babesiosis is endemic is necessary to estimate the increasing burden of babesiosis and other tickborne diseases and develop appropriate public health interventions for prevention and practice.
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Babesiosis is an emerging zoonotic disease caused primarily by Babesia microti, an intraerythocytic protozoan. Babesia microti, like the causal agents for Lyme disease and anaplasmosis, is endemic to the northeastern and upper midwestern United States where it is usually transmitted by the blacklegged tick, Ixodes scapularis. Although babesiosis is usually a mild to moderate illness, older or immunocompromised persons can develop a serious malaria-like illness that can be fatal without prompt treatment. The most common initial clinical signs and symptoms of babesiosis (fever, fatigue, chills, and diaphoresis) are nonspecific and present diagnostic challenges that can contribute to delays in diagnosis and effective treatment with atovaquone and azithromycin (1). Results of one study revealed a mean delay of 12–14 days from symptom onset to treatment (2). Knowledge of the incidence and geographic distribution of babesiosis can raise the index of clinical suspicion and facilitate more prompt diagnosis and lifesaving treatment (1). The first known case of babesiosis in Wisconsin was detected in 1985 (3), and babesiosis became officially reportable in the state in 2001. Wisconsin babesiosis surveillance data for 2001–2015 were analyzed in 3-year intervals to compare demographic, epidemiologic, and laboratory features among patients with cases of reported babesiosis. To determine possible reasons for an increase in reported Babesia infection, trends in electronic laboratory reporting and diagnosis by polymerase chain reaction testing (PCR) were examined. Between the first and last 3-year analysis intervals, there was a 26-fold increase in the incidence of confirmed babesiosis, in addition to geographic expansion. These trends might be generalizable to other states with endemic disease, similar suburbanization and forest fragmentation patterns, and warming average temperatures (4). Accurate surveillance in states where babesiosis is endemic is necessary to estimate the increasing burden of babesiosis and other tickborne diseases and to develop appropriate public health interventions for prevention and practice.
White-tailed deer are the primary hosts for adult blacklegged ticks, and white-footed mice and other small mammals are reservoirs of B. microti. Most human cases of babesiosis result from tick bites that occur during the spring and summer months, but blood transfusion–related transmission and perinatal transmission have also been reported (1–3,5). Blacklegged ticks were first recognized in Wisconsin in 1968, and during the subsequent decade, their range expanded rapidly, particularly in northwestern Wisconsin (6). Surveys of blacklegged ticks on hunter-harvested deer conducted since 1979 have demonstrated larger numbers of the blacklegged tick population and expansion in geographic range toward northeastern and southeastern Wisconsin (6,7). The concurrent geographic expansion of blacklegged ticks in Wisconsin during recent decades, coupled with observed increases in reported incidence of other tickborne diseases such as Lyme disease and human anaplasmosis in these regions, highlights the need for accurate surveillance for other serious tickborne diseases, including babesiosis (8). Predictive modeling of spatial and temporal trends in tickborne disease in neighboring Minnesota suggests that babesiosis will continue to increase under conditions of warming climate and continued forest fragmentation (4).
In 2001, the Wisconsin Department of Health Services, Division of Public Health defined a confirmed case of babesiosis as the occurrence of fever, anemia, or thrombocytopenia in a patient with confirmatory laboratory findings (i.e., identification of either intraerythrocytic Babesia organisms by blood smear or a fourfold increase or greater in B. microti immunoglobulin G [IgG] antibody titers). A probable case was defined as the occurrence of fever, anemia, or thrombocytopenia in a patient with supportive positive tests (B. microti indirect fluorescent antibody total Ig or IgG antibody titer of ≥1:256 or positive B. microti PCR assay). In 2007, the Division of Public Health expanded the confirmed case definition to include a positive PCR result as confirmatory laboratory evidence, which is consistent with the current Council of State and Territorial Epidemiology babesiosis case definition.* For all reported cases, local health departments interviewed health care providers and patients to assess tick exposure and to document the county of exposure and ascertain the possibility of transfusion-associated transmission.
In 2007, the Wisconsin Electronic Disease Surveillance System (WEDSS) was implemented by the Division of Public Health, and electronic laboratory reporting of babesiosis became possible. During the first 3 years of WEDSS implementation, only 17% of confirmed babesiosis cases were initially reported electronically. However, since 2013, approximately 80% of Wisconsin clinical laboratories use electronic laboratory reporting. All cases with either direct or electronic reporting were included in the analysis. Geographic distribution of reported cases by county of residence was compared during five consecutive 3-year intervals to examine geographic expansion of reported babesiosis cases. Annual incidence rates for county and state were calculated using mid-year population estimates provided by the Wisconsin Division of Public Health, Office of Health Informatics. Mean annual incidence was then calculated for successive 3-year intervals.
During 2001–2015, a total of 430 babesiosis cases were reported to the Division of Public Health, including 294 (68%) confirmed and 136 (32%) probable cases. Among confirmed cases, 189 (64%) occurred in males and 199 (68%) in persons aged >60 years (median age = 66 years; range = 10–100 years). Onset of illness occurred during April–October in 283 (96%) reported confirmed cases. Among 242 (82%) patients with confirmed babesiosis for whom sufficient information was available, 158 (65%) were hospitalized. Three deaths occurred, one in a woman aged 88 years, and two in men aged 64 and 72 years; information on comorbid conditions was unavailable. Three confirmed cases of transfusion-associated transmission were detected in 2008 and one in 2011, before implementation of routine screening for babesiosis by Wisconsin blood banks in 2016. Among probable babesiosis cases, 82 (60%) patients were male, 51 (38%) were aged >60 years (median age = 55 years; range = 6–93 years) and 120 (88%) had illness onset during April–October. Among 108 (79%) patients with probable babesiosis for whom information is available, 26 (24%) were hospitalized and none died. The proportion of all cases reported electronically increased to 51% during 2010–2012 and 67% during 2013–2015, compared with 2007–2009 (Figure 1).
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1University of Wisconsin-Madison School of Medicine and Public Health, Preventive Medicine Department; 2Career Epidemiology Field Officer, Office of Public Health Preparedness and Response, CDC; 3Bureau of Communicable Diseases, Wisconsin Division of Public Health.