Last updated:
10/20/2025
Years published: 1986, 1989, 1990, 1994, 2001, 2003, 2009, 2025
NORD gratefully acknowledges Gioconda Alyea, MD (FMG), MS, National Organization for Rare Disorders, for the preparation of this report.
Babesiosis is an infection caused by microscopic parasites from the Babesia genus. These parasites are usually carried and transmitted by ticks (vectors). Babesiosis primarily affects animals, but in rare cases, it can infect humans.1,2,3,4
Babesiosis often causes no symptoms or causes only mild symptoms, but in some people, it can lead to serious complications. Symptoms typically appear 1 to 4 weeks after infection and may include fever, fatigue, malaise, loss of appetite, joint and muscle pain, chills, sweats and headaches. Some people also experience nausea, vomiting, abdominal pain and enlarged liver or spleen (hepatosplenomegaly). Severe cases can involve jaundice, impaired kidney function and blood abnormalities like hemolytic anemia, low number of platelets (thrombocytopenia) and low number of leukocytes (leukopenia). Babesiosis may occur together (co-infection) with Lyme disease (20-25% of cases) and/or ehrlichiosis. In rare cases, babesiosis may trigger acute respiratory distress syndrome (ARDS).4,5
Babesiosis is usually treated with a combination of two prescription drugs for 7 to 10 days, though longer treatment may be needed for people with a weakened immune system. The preferred regimen is atovaquone with azithromycin. If these drugs can’t be tolerated, clindamycin with quinine is used instead, though this option has more side effects and is typically reserved for severe cases.4
Most people with babesiosis do not have any symptoms (asymptomatic) or experience only mild symptoms. However, in some people, babesiosis can cause severe complications. These severe cases usually only occur in people who are older than 50 years of age, have an impaired immune system (immunocompromised), or who have had their spleen removed (splenectomized).
The symptoms associated with babesiosis usually develop approximately one to four weeks (incubation period) after exposure to the parasite. Symptoms vary greatly from person to person. The initial symptoms may include:
Additional early symptoms include joint pain (arthralgia), muscle pain (myalgia), chills, sweats and headaches.
Other symptoms may include:
Affected individuals may also have additional symptoms including nausea, vomiting, and/or abdominal pain. In some people, an abnormally large liver and or spleen (hepatosplenomegaly) may be present. Immune compromised individuals with severe babesiosis may have impaired kidney function and an abnormal yellow discoloration to their skin, mucous membranes and whites of the eyes (jaundice).
Laboratory examination of blood samples from affected individuals may reveal abnormally low levels of red blood cells (hemolytic anemia) due to their destruction by the parasite. Additional laboratory findings may include abnormally low levels of platelets (thrombocytopenia) and white blood cells (leukopenia).
In about 25% of human babesiosis cases, affected individuals also have Lyme disease.4,12 Individuals who have infection with both diseases at the same time usually have more severe symptoms and a longer duration of those symptoms. Babesiosis can also present together with other diseases transmitted by ticks (tick borne disorders) such as Ehrlichiosis/Anaplasmosis, tularemia and Rickettsia.
Babesiosis is an infection caused by microscopic parasites from the Babesia genus. Although over 100 species of Babesia have been identified, only a few are known to cause disease in humans. These include Babesia microti, Babesia divergens, Babesia duncani, and a strain currently referred to as MO-1. The species involved varies depending upon specific geographic location.4
In the U.S., most human cases of babesiosis are caused by the rodent strain Babesia microti, mainly in the Northeast and Upper Midwest. Rare cases in Missouri, California and Washington are linked to local strains named MO-1, CA-1 and WA-1 (also known as B. duncani).
In Europe, Babesia divergens, a cattle strain, is the main cause, though B. microti and similar strains also appear. B. bovis, another cattle strain, occasionally infects humans. B. venatorum has been reported in both Europe and China. Other species are still being studied. 4,6,10
The life cycle of Babesia microti involves two main hosts, a rodent (mainly the white-footed mouse, Peromyscus leucopus or deer tick, and a specific type of tick from the Ixodes genus.4,6
When an infected tick takes a blood meal from a mouse, it introduces sporozoites, which are the parasite’s infectious “seed” forms. These sporozoites enter the mouse’s erythrocytes (red blood cells) and multiply by asexual budding, a process where the parasite copies itself without needing a mate. Within the bloodstream, some parasites develop into male and female gametes, the reproductive forms, although these cannot be distinguished using a standard light microscope.
The tick serves as the definitive host, meaning it is the host in which the parasite completes its sexual phase. When a suitable tick feeds on an infected mouse, it ingests the gametes; inside the tick, the gametes unite and begin a sporogonic cycle, ultimately producing new sporozoites that move to the tick’s salivary glands, ready to infect the next host. Unlike some larger Babesia species, transovarial transmission, passing the parasite from a female tick into her eggs, has not been demonstrated for the smaller babesiae such as B. microti, so the infection does not appear to be inherited by tick offspring.
People usually become infected through the bite of an infected tick which is known as the vector. Vector is a term for any organism that is infected with and later transmits a particular disease agent (i.e. a bacterium or virus) to another organism, which may then become infected. As commented above, the deer tick (Ixodes dammini or scapularis) is the most common vector that transmits babesiosis. When this happens:4,6,10
Humans are considered “dead-end” hosts, meaning the infection generally does not spread from person to person through tick bites. However, Babesia, in rare cases, can be transmitted through blood transfusions.6,7,8,9 Most people get infected from tick bites during outdoor activities in areas known for babesiosis. Babesia parasites do not spread from person to person like the flu or the common cold. There are some reports suggesting that if a mother has babesiosis, she might pass it to her baby during pregnancy or when the baby is born.6,10
Human babesiosis is a tick-borne disease caused by Babesia species, an intraerythrocytic parasite belonging to the Apicomplexan group. Six species have been confirmed to infect humans, three of which are found in the United States: Babesia microti, Babesia duncani, and a Babesia crassa–like agent. The most prevalent strain in the U.S. is B. microti, which is endemic in the Northeast and Upper Midwest and transmitted by the blacklegged tick (Ixodes scapularis) also known as the deer tick.5
From 2011 to 2019, reported tickborne illnesses in the U.S. increased by 25%, and babesiosis cases rose significantly in several states. A total of 16,456 cases were reported nationwide, with 98% coming from 10 states: Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont and Wisconsin. The largest increases occurred in Vermont (1,602%), Maine (1,422%) and New Hampshire (372%). These three states, which were not previously considered endemic areas by the CDC, now show consistent yearly case reports and evidence of Babesia microti in local tick populations. They should be reclassified as endemic regions.5,11
Climate change has contributed to the rise in babesiosis by expanding tick habitats and populations. People with a weakened immune system, such as people without a spleen or the elderly, are at higher risk for severe illness or death. Survivors may suffer long-term complications, including chronic fatigue, kidney failure and heart problems. Symptoms can vary widely, from no symptoms at all to life-threatening multi-organ failure, depending on the individual’s immune status. It affects males and females in equal numbers.
In response to the increased risk, the U.S. Food and Drug Administration (FDA) now recommends screening blood donations for Babesia in 15 high-risk states and Washington, D.C. These include all previously mentioned endemic states as well as Delaware, Maryland, Pennsylvania and Virginia.15
Due to the expanding geographic range and increased incidence of babesiosis, public health officials are emphasizing the need for tick prevention education, especially for residents and travelers in high-risk areas. The CDC recommends healthcare providers consider babesiosis in patients with compatible symptoms who have been in affected areas.
Due to the ability of Ixodes scapularis to carry multiple pathogens, individuals diagnosed with babesiosis are often tested for other tick-borne diseases such as Lyme disease, anaplasmosis, ehrlichiosis, relapsing fever and Powassan virus. In a nationwide study of over 16,000 ticks, Borrelia burgdorferi (the cause of Lyme disease) was the most commonly detected pathogen, while Babesia microti was found in 2% of ticks. Coinfections are relatively rare but more common in the Northeast, where up to 28% of ticks carry multiple pathogens. Studies also show high rates of co-exposure in humans; for example, 54% of people with babesiosis had antibodies against Lyme disease, and 24% of hospitalizations for babesiosis listed Lyme as a co-diagnosis. While coinfection worsens symptoms and prolong illness, especially in cases involving Lyme disease, the overall impact on disease severity and mortality requires further study.12
A U.S. Map with the reported cases of Babesiosis can be found at the CDC link.
Babesiosis is usually diagnosed by looking directly for the parasite in the blood. This is done with a thin blood smear, a test where a drop of blood is spread on a glass slide, stained with special dyes such as Giemsa or Wright stain, and then examined under a microscope. This allows doctors to see the parasite itself and estimate how severe the infection is by measuring how many red blood cells are affected. Because the parasite can be present in very small numbers early in the illness, several blood smears may need to be examined before it is detected. Under the microscope, the most common finding is a ring-shaped form of the parasite inside red blood cells. Sometimes the parasite forms a cluster of four, known as a tetrad or “Maltese cross,” which is a distinctive feature of babesiosis.4,6,15
Other laboratory methods are also used. Polymerase chain reaction (PCR) testing, which detects genetic material from the parasite, is more sensitive than blood smears and is usually performed in specialized laboratories. Antibody testing, performed by indirect immunofluorescent antibody (IFA) testing, can confirm whether the immune system has produced a response to Babesia. A single positive antibody test shows that a person has been exposed, but it cannot distinguish between a past infection and an active one. If antibody levels rise significantly between an early blood sample and a later one, this provides clear evidence of a recent infection.15
Babesiosis may also cause certain changes in routine blood and urine tests. Some people develop anemia, which is a low red blood cell count, along with elevated lactate dehydrogenase (LDH), a marker of red blood cell breakdown. Platelet counts may drop, a condition called thrombocytopenia. Liver enzymes may become elevated, reflecting irritation or stress on the liver, a change referred to as transaminitis. Protein can sometimes be detected in the urine, and blood tests may show elevated levels of blood urea nitrogen (BUN) and creatinine, which indicate that the kidneys are affected.4,15
Treatment
In most healthy people, babesiosis usually resolves spontaneously and causes few or no symptoms. Treatment of people who have symptoms is usually done with a combination of two prescription medications for at least 7 to 10 days. In people with weakened immune systems, a longer course of treatment is often necessary.16
Depending on the symptoms and severity, affected people can be treated at home or may need to be treated at a hospital with intravenous medication. Once a person’s symptoms begin to improve, the treatment should be transitioned to the oral form of the medications used in the outpatient setting. Treatment should continue until a full 7 to 10-day course is completed.
Atovaquone should always be taken with food, preferably a high-fat meal, to improve absorption. Higher doses of azithromycin (such as 1,000 mg on the first day, followed by 500 mg daily) may be used in people with severe disease or those who have low immunity (immunocompromised). The intravenous form of the medication quinidine gluconate, although effective, is no longer available in the United States.16
In people with a significantly weakened immune system, treatment is often more complex. These individuals may require at least six continuous weeks of medication. Treatment may begin at the hospital (inpatient regimen) and then it can continue at home (outpatient regimen) once the condition stabilizes.16
Ongoing monitoring is essential. Blood smears should be done daily until the level of parasites in the blood drops below 4%, and then weekly afterward. Treatment should continue until no parasites are seen on blood smears for at least two consecutive weeks.
For people who relapse or do not respond well to standard regimens, some infectious disease specialists may recommend alternative combinations. These may include three- or four-drug regimens involving atovaquone, azithromycin, clindamycin, quinine, or atovaquone/proguanil (also known by the brand name Malarone).16
In cases of severe babesiosis, affected people may require hospitalization and supportive treatments. These may include medications to reduce fever, drugs to stabilize blood pressure (vasopressors) and blood transfusions if anemia becomes severe.
Some people may benefit from an exchange transfusion, a procedure in which a portion of the patient’s blood is replaced with blood from a donor. This is usually done when the level of parasites is higher than 10%, or in people where there are only moderate to high levels of parasites but there are also serious complications, such as organ dysfunction or severe anemia.16
In critical cases, mechanical ventilation may be needed if the lungs are affected, and dialysis may be required if the kidneys stop functioning properly. Mechanical ventilation is a medical procedure where a ventilator helps patients breathe by pushing air into their lungs, typically used for respiratory failure. Dialysis is a treatment that cleans the blood of waste products and excess fluid when the kidneys are not functioning properly, removing these impurities by diverting blood through a machine or using the body’s own abdominal lining.
For updated information about babesiosis please visit the Center for Disease Control and Prevention (CDC).
Information on current clinical trials is posted on the Internet at https://clinicaltrials.gov/ All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
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For information about clinical trials sponsored by private sources, contact: http://www.centerwatch.com/
For information about clinical trials conducted in Europe, contact: https://www.clinicaltrialsregister.eu/
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