Infectious Disease Compendium



(2012 Review)

Protozoans that infect red cells of humans and animals. Includes B. microti in the US (the NE), B. divergins (Europe) and B. bovis in Europe. Over 100 others have been reported, esp in animals. Tick spread, after feeding on mice (the ticks, not the patient).

Epidemiologic Risks

Tick bite. Found all over the NE and SE of the US. Sometimes from transfusions (159 reported cases)(PubMed), and do not assume the blood is local, it can come from all over the country (PubMed)


It has also been found in France, and I would worry this is a marker for disease in all of Western Europe (PubMed). Diagnosis has increased 20 fold this decade in the New York Lower Hudson Valley (PubMed) and it is creeping into Eastern Pennsylvania (PubMed).  Also increasing in Maine (PubMed). And "The reported incidence of confirmed babesiosis in Wisconsin increased 26-fold from 2001 to 2015" (PubMed). As the climate changes so will the range of all environmental infections.

Blood transfusions and screening does decrease disease.

There is one case of transplacental spread and it is the most common transfusion infection in the US. and a pair of cases from transplanted kidneys.

B. microti. Such cases occur in the Northeast and upper Midwest, primarily from May through October.

B. duncani and B. duncani–type organisms. Pacific Coast from northern California to Washington.

B. divergens–like organisms Kentucky, Missouri, and Washington.

Europe. B. divergens, B. venatorum and B. microti.

Asia. B. microti–like organisms in Japan and Taiwan, and KO1 strain in South Korea.

Sporadic cases of babesiosis all over the world.


Most are subclinical or mild. In the asplenic or immunoincompetent can be fevers, "flu" like symptoms and hemolytic anemia, thrombocytopenia and a transaminitis, often severe. Can present as fever, splenomegaly, and splenic infarcts (PubMed) as well as splenic rupture (PubMed).

Asplenic, elderly, or immunocompromised have increased risk for symptomatic infection and complications, such as MOSF and death.

It causes a post-infectious warm-antibody autoimmune hemolytic anemia. The alleged mechanism(s) are in the (NEJM) report.


Serology PLUS looking for the organisms on smear. There are some, shall we say, wackaloon labs, often beloved by naturopaths, that diagnosis Babesia (often with Lyme) with a promiscuity and lack of rigor that is most curious. If you are in ID, you probably know who they are.


Clindamycin 600 mg q 8h PLUS quinine 650 mg q 8h for 10 d


atovaquone 750 mg every 12 hours plus azithromycin 500 mg on day 1 and 250 mg per day thereafter for 7 days (PubMed).

DO NOT give monotherapy as it leads to resistance.

Resistance can occur to azithromycin-atovaquone during the treatment in highly immunocompromised patients (PubMed).

Other drugs to try are doxycycline OR pentamidine. An AIDS patient with refractory disease was treated with atovaquone-proguanil (250 mg/100 mg) for prolonged courses in addition to red cell exchange (PubMed). In patients with poor immune systems, especially lymphoma, can be difficult to treat (PubMed).

Smear positivity can persist for three weeks after successful treatment and PCR can persist for at least a month. The PCR should be negative after three months.

If > 10% parasitemia, consider exchange transfusion esp if really sick.

Resistance has occured on therapy, albeit in a very immunocompromised patient (PubMed).


Co-infection with Borrelia (Lyme) and/ or Anaplasma occurs.

There seems to be labs that will diagnose this disease when it really isn't there. Beware. It is often a misdiagnosed infection of those who have "chronic" lyme.