Babesiosis

An intra-erythrocytic protozoan (most commonly Babesia microti) transmitted by Ixodes tick bite — a malaria-like illness, not endemic to Australia. Also acquired by blood transfusion, rarely organ transplant, and congenitally/transplacentally.

Who to suspect it in: a returned traveller or recent immigrant with an unexplained febrile illness who has visited or lived in an endemic area — mainly the northeastern and upper Midwestern United States (B. microti), or parts of Europe (B. divergens) — within the last 2 months, with or without a recalled tick bite; or anyone who has had a blood transfusion in the last 6 months. Co-infection with other tick-borne pathogens (Lyme disease, anaplasmosis) occurs in ~20% of US cases and can complicate or delay diagnosis.

Babesiosis is one of several tick-bite-associated causes of fever in a returned traveller, alongside rickettsial infection, Lyme borreliosis, tick-borne encephalitis, and Q fever — worth including on the differential whenever a returned traveller’s history includes a tick exposure.

Clinical features

• Asymptomatic in ~50% of children and ~20% of adults infected — can persist subclinically for months to years.
• When symptomatic (1-4 weeks after a tick bite, or up to 9 weeks — rarely 6 months — after transfusion): malaise, fatigue, chills, fever, headache, myalgia and arthralgia; hepatosplenomegaly, haemolytic anaemia, thrombocytopenia, mild leukopenia.
• Severe/fatal disease — high fever, severe haemolysis, multi-organ dysfunction (ARDS, renal/hepatic failure) — is concentrated in the asplenic, immunocompromised (HIV, TNF inhibitors, anti-CD20 therapy), neonates, and adults over 50.

Diagnosis

• Thick and thin blood smear — intra-erythrocytic parasites (parasitaemia may be too low to detect early). PCR is more sensitive and useful when smear is negative but suspicion is high; serology supports but doesn’t confirm active infection.
• Bloods often show haemolytic anaemia, thrombocytopenia, raised LDH, low haptoglobin.

Management

• Mild-moderate disease: atovaquone + azithromycin for 7-10 days (first-line — fewer adverse effects); quinine + clindamycin is an equally effective alternative.
• Severe disease (parasitaemia ≥10%, B. divergens infection, or pulmonary/renal/ hepatic dysfunction): IV therapy and consider exchange transfusion; refer to infectious diseases/haematology.
• Immunocompromised patients can have persistent/relapsing disease needing ≥6 weeks of therapy, continued for 2 weeks past clearance on blood smear; retreat if PCR/smear remains positive at 3 months.
• Refer to infectious diseases for any confirmed case, especially asplenic or immunocompromised patients. A history of babesiosis is a permanent blood-donation deferral.
• Prevention — tick-bite avoidance (avoid dense vegetation/tall grass in endemic areas), protective clothing, DEET/permethrin, and prompt tick removal.