Babesiosis

Basics

Description

  • Predominately tick-borne infectious disease caused by Babesia Protozoa
  • Transmitted to humans:
    • Primarily through the bite of an infected Ixodes tick
    • Solid organ transplantation rarely
    • Transplacental transmission
  • Babesiosis is associated with a dysfunctional immune response and red blood cells (RBCs) lysis:
    • Parasites at the merozite stage invades the RBCs
    • These mature into trophozoites, which feeds on the RBCs
      • Often seen as ring forms under microscopy:
    • The trophozoite undergoes asexual replication within the RBC dividing into multiple merozoites
    • Merozoites are released into the blood stream when the RBC ruptures
    • Merozoites infect new RBCs continuing the cycle
  • Increasing rates of infection:
    • Expanding deer population
    • Increased tick density
    • Increased human habitat in wooded areas
    • Increased community awareness and testing
  • An acute infection can also be relapsing
  • Babesiosis can be classified by severity
  • Asymptomatic:
    • 20% adults
    • 40% children
  • Mild:
    • Flu-like illness
    • Slight hemolysis with low parasitic load
  • Moderate:
    • Higher fevers, fatigue, and possibly jaundice
    • Hemolysis, elevated liver enzymes, mild thrombocytopenia
  • Severe disease:
    • Severe hemolysis, high fevers, shortness of breath, dark urine (due to hemoglobinuria), organ failure (such as renal failure), or cardiovascular collapse
    • Parasitemia >4%
  • Risk factors for severe disease:
    • Asplenia or hyposplenism
    • Immunosuppression (eg, HIV/AIDS, chemotherapy, immunosuppressive therapy)
    • Age >50 yr
    • Neonates
  • Complications
    • High rates of coinfection with other tick-borne pathogens (50% of Babesia-infected patients have Lyme, 10% of Lyme-infected patients have Babesia)
    • Complications seen in nearly half of hospitalized cases ARDS (most common), anemia, CHF, renal failure, DIC, shock, coma, splenic rupture (rare), warm autoimmune hemolytic anemia
    • Fatality rate: 3–9% in hospitalized patients, 20% in highly immunosuppressed patients and transfusion cases

Etiology

  • Species:
    • US: Babesia microti (reservoir: white-footed mice, vector: Ixodes scapularis tick)
    • Europe: Babesia divergens (more severe, often in asplenic patients)
    • NE China: Babesia venatorum and Babesia crassa like
  • Transmission:
    • Ticks are primary method of transmission: mainly nymph stage, >75% cases June–August
    • Blood transfusions: more severe due to recipient comorbidities, FDA recommends blood donor screening, Red Cross implemented testing in 2020 in high-risk states
    • Solid organ transplantation (rare)
    • Transplacental transmission (rare)
  • Pathogenesis:
    • Incubation: (tick bite: 1–4 wk, transfusion: 3–7 wk but up to 6 mo)
    • Hemolysis through release of merozoites from RBC
    • Host inflammatory response (pyrogenic cytokines)

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