Rise in Babesiosis Cases, Pennsylvania, USA, 2005–2018

David Ingram; Tonya Crook


Emerging Infectious Diseases. 2020;26(8):1703-1709. 

In This Article

Abstract and Introduction


Babesiosis is an emerging infection in the state of Pennsylvania, and clinicians need to be made aware of its clinical manifestations as well as the risk factors associated with severe disease. Before 2010, our tertiary academic center in central Pennsylvania previously saw zero cases of babesiosis. We saw our first confirmed case of Babesia infection acquired in Pennsylvania in 2011; we recorded 2 confirmed cases in 2017 and 4 confirmed cases in 2018. All 4 cases from 2018 were thought to be acquired in southcentral Pennsylvania counties, whereas prior reports of cases were predominately in the southeast and northeast counties of the state.


Babesiosis, a tickborne infection caused by protozoan parasites that infect erythrocytes, has been identified as an emerging infection of concern in the state of Pennsylvania, USA.[1–4] Infection with Babesia parasites can cause a range of symptoms, including fever, myalgias, and fatigue. Although many patients are asymptomatic, the infection can be severe in some persons, especially those who are >50 years of age, immunosuppressed, or asplenic.[1,5,6]

The most common species known to cause human infection in the United States is Babesia microti, which is transmitted by the Ixodes scapularis tick (blacklegged tick). Transmission can also occur through blood transfusion; B. microti is currently the most common pathogen transmitted through the blood supply in the United States.[7,8] The highest incidence of human infection has been reported in the Northeast and upper Midwest states.[9] Until fairly recently, few cases of B. microti infection have occurred in Pennsylvania. However, new data suggest not only increased prevalence of ticks harboring B. microti within the state[10] but also a rise in the number of cases seen in clinical practice.[2–4]

Babesiosis is not a mandatory reportable infection in Pennsylvania; however, the Pennsylvania Department of Health does receive reports of cases of babesiosis from healthcare providers that elect to do so. Among these recounted cases, the department has seen a 20-fold increase in the past 12 years (E. Negrón, Pennsylvania Department of Health, pers. comm., 2018 Aug 1). A similar trend was documented in a recent study from February 2019 involving a 4-hospital system in southeastern Pennsylvania, where clinicians saw an increase in cases from ≤7 cases annually during 2008–2014 to 26 cases in 2015.[4]

We have suspected a similar increase in the number of cases at our institution, Penn State Milton S. Hershey Medical Center (Hershey, PA, USA), which is a tertiary academic center located in central Pennsylvania. We performed a retrospective review of all of the confirmed cases of babesiosis at our institution for the period 2005–2018 to determine whether we were truly seeing an increased number of cases and to highlight the demographic and clinical characteristics of these patients.