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Babesiosis A Tick-Borne Illness. Babesia microti.

Babesiosis is a potentially fatal tick-borne illness.

Exploring the life and feeding cycle of Babesia and the symptoms, testing, and treatments of Babesiosis.

Diagnosis of Babesiosis can be challenging, as the tick bite may not have been noticed, clinical symptoms may be non-specific, and laboratory confirmation of infection can be problematic, especially early in the course of these diseases. At times clinicians may need to treat on clinical suspicion alone especially if you live in the Northeast or the upper Midwest.


map of USA where babesiosis has been found

Babesiosis is typically caused by a microscopic parasite Babesia microti that infects red blood cells. There are other species of Babesia that can cause infection, but B. microti is the most common. It is transmitted by bites from infected Ixodes scapularis ticks (also called blacklegged ticks or deer ticks), but occasionally can be transmitted by blood transfusion, or congenitally from mother to infant.[37] It is typically seen in the Northeast and upper Midwest. In 2018, 2,161 cases of babesiosis were reported to the CDC.[38]

Babesia Life Cycle

Babesia life cycle
Babesia Life and Feeding Cycle [39]

The Babesia microti life cycle involves two hosts, most commonly the white-footed mouse, Peromyscus leucopus, and a tick in the genus Ixodes. During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host. Sporozoites enter erythrocytes and undergo asexual reproduction (budding). In the mouse’s blood, some of the spores differentiate into male and female gametes. These gametes can be ingested by another tick, unite and form more sporozoites. The Babesia-infected tick then can introduce the sporozoites into a human host during a blood meal. The sporozoites enter erythrocytes and undergo asexual replication, (budding) which then is responsible for the clinical manifestations of the disease.[39]

Symptoms of Babesiosis [40]

Many people who are infected with Babesia microti feel fine, and do not have any symptoms. Others develop nonspecific flu-like symptoms, such as fever, chills, sweats, headache, body aches, loss of appetite, nausea, or fatigue. As Babesia parasites infect and destroy red blood cells, a hemolytic anemia leading to jaundice(skin turning yellow from destruction of red blood cells) may occur. Some patients may have an enlarged spleen or liver.

Coinfection should be investigated in patients who have a persistent fever while on antibiotic treatment for Lyme disease. If fever persists despite treatment with doxycycline, concurrent B. microti infection is an important consideration.

Laboratory Findings in Babesiosis [41]

For acutely ill patients, the findings on routine laboratory testing frequently include hemolytic anemia(destruction of red blood cells) and thrombocytopenia(low platelet count). Additional findings may include kidney dysfunction and elevated levels of liver enzymes.

Testing for Babesiosis [42]

Diagnosis can be made by microscopic examination of thick and thin blood smears which are Giemsa stained. Repeated smears may be needed sometimes to detect parasites.

Babesia microti in Giemsa-stained blood smears [42]

Antibody testing may be useful in asymptomatic patients who are donating blood, or if the diagnosis is uncertain after a blood smear is checked. An antibody test using B. microti parasites as antigen detects antibodies in88-96% of patients with a B. microti infection.The extent of cross-reactivity between Babesia species is variable. A negative result with B. microti antigen for a patient exposed on the West Coast of the U.S. may be a false-negative, and the patient should be specifically be tested for antibodies to Babesia duncani.

PCR testing can be used to confirm the diagnosis after a positive blood smear to identify the species and also differentiate it from malarial species, such as Plasmodium falciparum, which can look similar microscopically.

Treatment of Babesiosis [43]

Most asymptomatic persons do not require treatment. Treatment decisions should be individualized, especially for patients who are at risk for severe disease such as patients without a spleen or immunosuppressed individuals.

For ill patients, babesiosis usually is treated for at least 7-10 days with a combination oftwo medications — typically either:

  • Atovaquone PLUS azithromycin; OR

  • Clindamycin PLUS quinine (this combination is the standard of care for severelyill patients).

Atovaquone is pregnancy category C and the risk to the fetus is unknown. Because there is data about the safe administration of quinine plus clindamycin during pregnancy, this drug combination is generally recommended for treatment of symptomatic babesiosis during pregnancy. An infectious disease consult is recommended when treating a pregnant patient. Initiation of antibiotic therapy, especially in children, should be determined only after careful discussion with your physician/provider.


Given that diagnostic testing for these tickborne diseases is not 100% sensitive, and symptoms initially may be non-specific, a clinician in an endemic area should keep tickborne infections in mind when considering diagnoses in an ill patient without a clear cause. Unexplained fever, anemia, thrombocytopenia, leukopenia, new onset heart block, myocarditis, pericarditis, cranial nerve palsies, or arthritis can be seen as potential clues to a tickborne infection. A specific history of tick bites may be helpful, although the patient might not even know they have been bitten. Antibiotic prophylaxisfor Lyme disease, when appropriate, will help reduce the incidence of symptomatic disease. While this article is just about four tickborne infections, keep in mind that ticks can transmit many different infections including: Borrelia miyamotoi disease, Colorado tick fever, Heartland and Bourbon virus diseases, Pacific Coast tick fever, Powassan virus disease, Rocky Mountain spotted fever, Rickettsia parkeri rickettsiosis, Rickettsialpox, Tickborne relapsing fever, and Tularemia.[44,45]

Author’s Note: Recommended medications are based on current recommendations, and are correct to the best of my knowledge. However, treatment recommendations may change, and readers are advised to review the most current dosages, medications and contraindications with their physician/provider prior to treatment.


More TICK-BORNE Disease articles: Lyme | Ehrlichiosis | Anaplasmosis

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[37] Parasites- Babesiosis, General Information, CDC, last reviewed: April 11, 2018. Retrieved from:

[38] Swanson M, Gray EB, Surveillance for Babesiosis — United States, 2018, Annual Summary, CDC, Data current as of: April 22, 2019. Retrieved from:

[39] Babesiosis, DPDx - Laboratory Identification of Parasites of Public Health Concern, CDC, last reviewed: October 30, 2017. Retrieved from:

[40] Parasites – Babesiosis Resources for Health Professionals, CDC, last reviewed: October 30, 2019. Retrieved from:

[41] Babesiosis, Laboratory diagnosis, DPDx - Laboratory Identification of Parasites of Public Health Concern, CDC, last reviewed: October 30, 2017. Retrieved from:

[42] Photo credit- Babesiosis Image Gallery, DPDx - Laboratory Identification of Parasites of Public Health Concern, CDC, last reviewed: October 30, 2017. Retrieved from:

[43] Parasites – Babesiosis, Resources for Health Professionals- Treatment, CDC, last reviewed: October 30, 2019. Retrieved from:

[44] Diseases Transmitted by Ticks, CDC, last reviewed: April 2, 2020. Retrieved from:

[45] Other Spotted Fever Group Rickettsioses, CDC, last reviewed: January 18, 2019. Retrieved from:

initially posted: September 28, 2021


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