Babesia Species


  • Malaria-like protozoan infection infects erythrocytes.
  • Greater than 100 species worldwide. Most human infections are due to the rodent strain B. microti in the U.S., while B. divergens and B. bovis cattle strains cause human illness in Europe and elsewhere.
    • Phylum Apicomplexa
    • Eight Babesia species have been described as causing human infections: B. bigemina, B. bovis, B. crassa-like, B. divergens, B. duncani, B. microti, B. odocoilei and B. venatorum.
    • Four clades:
      • B. microti (accounts for most human infections in the U.S.)
      • B. duncani and B. ducani-like (dogs, wildlife, occasional cause of human infection Pacific NW)
      • B. divergens (primarily cattle pathogen but predominant infection of humans in Europe)
        • Primarily seen in asplenic individuals.
      • B. venatorum (deer, most extensive human case series described in China)
  • Transmitted by Ixodes (hard-bodied) ticks, it mainly occurs from May to September.
    • Common, global zoonosis (domestic and wild animals) may rarely infect humans.
    • In the U.S., it is primarily seen in Northeast and upper Midwestern states--mainly due to B. microti transmitted by I. scapularis (deer ticks).
    • Incubation period: 1-6 weeks
  • For many years, Babesia was frequently transmitted by blood transfusion in the U.S., although many blood banks have screening protocols for donors. Still, this is not universal.
    • It may also be transmitted to the fetus during pregnancy.
  • Occasional infections documented by other Babesia species. The importance is that standard Babesia microti serology may not detect these infections.
    • B. duncani and B. duncani-like strains occurring in the Pacific Northwest.
    • B. divergens-like organisms:
      • WA-1 is described in California and Washington states.
      • MO-1 is described in Missouri.
    • Europe: B. divergens > B. venatorum, B. microti.
      • Ixodes ricinus (sheep tick) is the primary vector.
    • Asia: B. microti-like organisms in China, Japan, Taiwan, and S. Korea.

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Last updated: June 8, 2024