Babesia: An Emerging Infectious Threat in Transfusion Medicine

Babesiosis is an emerging zoonosis caused by protozoan parasites of the genus Babesia. The disease is endemic primarily in the Northeast and upper Midwestern United States. The genus Babesia comprises multiple species of apicomplexan parasites that infect red cells of many vertebrate hosts. Babesia divide and replicate in the hosts’ red blood cells and are called piroplasms due to their pear-shaped appearance within the RBCs (Figure 1). They are transmitted by ixodid tick vectors as they feed on a blood meal from the host [1]. Babesiosis has long been recognized as an economically important disease of cattle, but only in the last 40 years has Babesia been recognized as an important pathogen in man. The majority of cases in the United States are caused by B. microti and occur in the Northeast and upper Midwest [2]. A small number of infections caused by B. duncani and B. duncani–like organisms have been reported on the West Coast from California to Washington State [3]. Additionally, B. divergens–like organisms have been reported in Kentucky, Missouri, and Washington State [4]. In Europe, almost all reported cases have been due to B. divergens, and a few have been caused by the EU1 species, now called B. venatorum [5,6]. Sporadic cases of babesiosis have also been reported in Asia, Africa, Australia, and South America [7]. The symptomatic spectrum of human babesiosis is broad, ranging from clinically silent infections to intense malaria-like episodes resulting occasionally in death. When present, symptoms typically are nonspecific (fever, headache, and myalgia) [8]. Human babesiosis is a zoonosis, and the natural acquisition of human disease is the result of interactions with established zoonotic cycles. Emerging diseases are defined as ‘‘novel, re-emerging, or drugresistant infections whose incidence in humans has increased within the past two decades or whose incidence threatens to increase in the near future.’’ Because of the increasing number of human infections since 1990, human babesiosis can be considered an emerging disease [7]. A number of factors have contributed to the ‘‘emergence’’ of human babesiosis, including a heightened awareness among physicians, a changing ecology, and a larger population of immunocompromised individuals, where fatalities have occurred. This led the Centers for Disease Control and Prevention to add babesiosis to the list of nationally notifiable diseases in 2011.