What Is Babesiosis? Symptoms, Causes, and Treatment
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick Answer: What is babesiosis?
Babesiosis is a disease caused by Babesia parasites that enter the bloodstream through a tick bite and destroy red blood cells. Flu-like symptoms such as high fever, chills and fatigue start one to four weeks after exposure. [1] Blood tests confirm it, and effective treatment is available. [1] [6] Most people recover, but it is fatal in about 1 in 10 people hospitalized. [1]
Key Takeaways
Babesiosis is caused by Babesia parasites, transmitted mainly by blacklegged tick bites in the U.S. (also from blood transfusions). [1]
It usually causes flu-like symptoms starting one to four weeks after exposure — and about 1 in 4 people have no symptoms at all. [1]
The parasites infect and destroy red blood cells, which can cause hemolytic anemia and other life-threatening complications in high-risk people. [1]
People at higher risk of severe illness include those over 60, those with weakened immune systems, and those without a functioning spleen. [1]
Fewer than 3,000 U.S. cases are reported each year, mostly in Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin. [5] [1]
Blood tests looking for parasites in the blood confirm the diagnosis. [1]
Combinations of antiprotozoal, antibiotic, and antimalarial medications treat babesiosis effectively — quick treatment lowers the risk of serious complications — and most people recover fully. [1] [6]
What the research says
Two standard drug combinations work equally well, with fewer side effects on one. In a randomized trial of 58 people with non-life-threatening babesiosis, atovaquone plus azithromycin caused side effects in 15% of patients versus 72% with clindamycin plus quinine, and symptoms had resolved at three months in 65% versus 73% (P=0.66, no significant difference). (Krause et al., N Engl J Med, 2000) [2]
Death is uncommon overall but can reach up to 20% in higher-risk groups. A review reports that death from babesiosis occurs in up to 20 percent of people who are immunocompromised or who acquire the infection through blood transfusion. (Krause, Int J Parasitol, 2019) [3]
Red cell exchange has not been proven to improve survival. In a review of 108 patients treated with red cell exchange for severe babesiosis, overall mortality was 20%, and the authors concluded it cannot be said that the therapy favorably influences clinical outcome. (Tannous et al., Transfus Med Rev, 2021) [4]
What Is Babesiosis?
Babesiosis (pronounced "buh-BEE-zee-oh-sis") is an illness caused by many species of the parasite Babesia. You can get it through the bite of infected blacklegged ticks or, less commonly, from an organ transplant or blood transfusion. [1]
Babesiosis usually causes flu-like symptoms. But the parasite's destruction of red blood cells can lead to life-threatening complications — especially if you have a weakened immune system. A common complication is hemolytic anemia, a condition where your red blood cells are destroyed faster than your body can make new ones. [1]
How Common Is Babesiosis?
Babesiosis is rare, with fewer than 3,000 cases reported in the U.S. each year. However, it is considered an emerging infectious disease, which means the number of cases has increased in recent years. [1] [3]
Region | Where babesiosis is found |
Global | Areas where Babesia parasites and their carrier ticks coexist — the U.S., Europe, and Asia |
U.S. regions | Mostly the Upper Midwest and Northeast, with a few cases on the West Coast |
Highest U.S. risk states | Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin |
What Are the Symptoms of Babesiosis?
Symptoms of babesiosis start one to four weeks after exposure. [1] They look a lot like the flu:
Symptom | Description |
High fever | Elevated body temperature |
Fatigue | Tiredness that can be significant |
Chills | Feeling cold and shivering |
Sweating | Often accompanying fever and chills |
Headache | Ranging from mild to severe |
Muscle or joint aches | Generalized body aching |
Loss of appetite | Reduced desire to eat |
Cough | Respiratory irritation |
About 1 in 4 people have no symptoms of babesiosis. And unlike other tick-borne diseases, it usually does not cause a rash — so you cannot rely on a rash to warn you. [1]
Symptoms of Severe Babesiosis
Severe babesiosis may cause additional symptoms:
Severe symptom | What it involves |
Yellowish skin or eyes (jaundice) | A warning sign of red blood cell destruction |
Pale skin (pallor) | From anemia |
Dark urine | A sign of hemolysis |
Shortness of breath | Difficulty breathing |
Nausea and vomiting | Digestive distress |
Abdominal pain | Stomach-area pain |
Neck stiffness | Tension in the neck |
Sudden mood changes | Extreme emotional reactions (emotional lability) |
Weight loss is not a common sign of babesiosis — but if you cannot eat or are vomiting, you might lose weight.
What Causes Babesiosis?
Several types of Babesia parasites cause babesiosis. Babesia are protozoa — one-celled organisms too small to see without a microscope. They get into your blood through a tick bite, where they infect and destroy your red blood cells. In mild cases, this can make you tired. In severe cases — such as when you have a compromised immune system — the infection can cause life-threatening complications. [1]
Babesia species | Role |
B. microti | The most common cause of symptomatic babesiosis in the U.S. |
B. duncani | Sometimes causes illness |
B. divergens | Sometimes causes illness |
B. venatorum | Sometimes causes illness |
B. crassa | Sometimes causes illness |
How Do You Get Babesiosis?
Babesiosis spreads from animals to humans when a tick bites an infected animal — usually a small rodent, like a mouse — and then bites you. The parasite infects your blood cells and reproduces there. [1]
In the U.S., the blacklegged tick (Ixodes scapularis) is the most common carrier. Blacklegged ticks also carry Lyme disease and anaplasmosis. They are often called "deer ticks," though deer usually do not carry babesiosis. [1] In Europe, castor bean ticks (I. ricinus) transmit Babesia. [1]
You can also get babesiosis from a blood transfusion when a donor is infected without knowing it. [1]
Transmission question | Answer |
Is it contagious person to person? | No — babesiosis does not spread between people |
Can it pass from mother to baby? | Rarely, from a pregnant woman to the fetus |
Can you get it from transfusions? | Yes, if the donor is unknowingly infected |
When is risk highest? | Spring and summer, when tiny nymph ticks are most active |
Babesiosis is most common in spring and summer because young ticks (nymphs) emerge in the spring and are more likely to spread Babesia than adults. [1] Nymphs are about the size of a poppy seed — very hard to see — and grow into adults during the summer. You can still get babesiosis from an adult tick, but adults are easier to spot and remove before they bite.
What Are the Risk Factors for Severe Illness?
Anyone can get babesiosis, but you are at higher risk of becoming severely ill if you fall into one of these groups: [1]
Risk factor | Why it raises risk |
Compromised immune system | HIV/AIDS, cancer, immune disorders, and certain immunosuppressive medications weaken defenses |
Missing or non-functioning spleen | The spleen helps clear infected red blood cells |
Age over 60 | Older adults face higher odds of severe disease |
What Are the Complications of Babesiosis?
Babesiosis can cause life-threatening complications, especially with a weakened immune system: [1]
Complication | What it involves |
Hemolytic anemia | Parasites destroy so many blood cells that your body cannot replace them fast enough — can lead to heart conditions and heart failure |
Enlarged spleen or liver | Splenomegaly or hepatomegaly; an enlarged spleen can rupture |
Pulmonary edema | Fluid buildup in the lungs makes breathing difficult and can lead to heart or respiratory failure |
DIC (disseminated intravascular coagulation) | A blood-clotting problem that can cause uncontrollable bleeding |
Organ damage and failure | Acute kidney injury (renal failure) and liver failure |
How Is Babesiosis Diagnosed?
Blood tests diagnose babesiosis. Your provider will give you a physical exam and ask about your symptoms and health history. Tell them if you have recently (within the past three weeks) been in an area where you could have been bitten by a tick — even if you do not remember a bite. It is also important to mention any blood transfusion in the past six months. [1]
If your provider suspects babesiosis, they will order tests to look for parasites in your blood and rule out other illnesses with similar symptoms. [1]
How Is Babesiosis Treated?
Your provider can treat babesiosis with a combination of medications — antiprotozoals, antibiotics, and antimalarial drugs — delivered through an IV or in pill or liquid form. If you are very sick, you may need additional procedures to get better. [1]
Treatment | When it is used |
Atovaquone | One of the antiprotozoal medications used in combination therapy |
Azithromycin | An antibiotic used in combination therapy |
Clindamycin | An antibiotic used in combination therapy |
Quinine | An antimalarial drug used in combination therapy |
Blood transfusion | For patients with too few red blood cells — donor blood increases the count |
Exchange transfusion | For patients with a high number of parasites — blood is slowly removed and replaced with donor blood |
Mechanical ventilation | For patients who cannot breathe well enough on their own |
Dialysis | When kidneys are not working properly |
What Can I Expect If I Have Babesiosis?
What to expect depends on how sick you are when diagnosed and whether you are at risk for severe illness. If you have no symptoms, you usually do not need treatment, but current guidelines recommend treating anyone with symptoms with medication, even when the illness is mild. [1] If you are at higher risk for severe illness, your provider will treat you with medication. If you are severely ill, you will be admitted to the hospital, where you will be treated for the infection and any complications, with frequent blood tests to monitor your response. [1]
Scenario | What to expect |
Healthy immune system, mild case | Treated with medication; no treatment if there are no symptoms |
At higher risk | Treated with medication |
Severely ill | Hospital admission with frequent blood tests |
Duration (healthy immune system) | One to two weeks; medication may bring improvement within a day or two |
Duration (weakened immune system) | Can last months or even years |
Can Babesiosis Come Back?
Sometimes symptoms get better but return later — a relapse. If that happens, your provider will treat you again. You are most at risk for relapse if you have a weakened immune system. [1]
Even after your other symptoms have gone away, you may still feel tired for several months. You should recover fully once your red blood cell counts return to normal — ask your provider how long recovery may take for you. [1]
What Is the Outlook?
Most people with babesiosis make a full recovery. However, babesiosis is fatal in about 1 in 10 people who are hospitalized for it. [1]
Can Babesiosis Be Prevented?
Avoiding tick bites is the best way to prevent babesiosis if you live in or travel to an area where it exists: [1]
Prevention step | Why it helps |
Keep grass shorter than 5 inches | Ticks wait on long grass to find a host |
Stay on cleared paths in wooded areas | Keeps you away from tick habitat |
Use tick-approved bug sprays (usually DEET) | Repels ticks from your skin |
Cover skin with clothing in woods and tall grass | Fewer exposed spots for ticks to reach; pre-treated tick-repellent clothing is available |
Check yourself for ticks after warm-month outings | Have someone check your scalp and back where you cannot see |
Protect your pets | Ask your veterinarian about tick protection and check pets after outdoor time |
Remove ticks safely and promptly | Follow safe-removal directions if you find one on you |
When Should I See a Healthcare Provider?
See a healthcare provider if you have symptoms of babesiosis and could have been bitten by a tick. If you have a weakened immune system, seek medical attention for any signs of infection.
Go to the nearest emergency room if you have signs of serious illness:
Emergency sign | Threshold |
High fever | Over 103°F (39.44°C) |
Breathing difficulty | Severe shortness of breath |
Jaundice | Yellow skin or eyes |
Mental changes | Confusion or other mental changes |
Severe pain | Severe stomach (abdominal) pain |
What Questions Should I Ask My Provider?
Helpful questions to ask your provider include: How do I take my medication? How long do I need treatment? What can I take at home for symptoms? What new or worsening symptoms should I watch for? How soon should I feel better? When should I follow up?
Babesiosis vs. Malaria: How Are They Different?
Babesiosis and malaria share a lot of similarities, but they are different illnesses — and because they are treated with different medications, proper diagnosis matters.
Feature | Babesiosis | Malaria |
Blood cell damage | Infects and damages blood cells | Infects and damages blood cells |
Initial symptoms | Starts flu-like; can progress to serious illness | Starts flu-like; progresses to serious illness if untreated |
Cause | Protozoan parasite (Babesia) | Protozoan parasite (Plasmodium) |
Transmission | Ticks | Mosquitoes |
Where most common | The U.S. | Africa |
Microscope appearance | Looks like rings; sometimes forms a "Maltese cross" shape with four ovals | Looks like rings |
Is Babesiosis the Same as Lyme Disease?
The same kind of tick (I. scapularis) transmits babesiosis and Lyme disease, but they are different illnesses. A parasite causes babesiosis, while bacteria cause Lyme disease. [1]
Conclusion
Babesiosis is a tick-borne illness in which Babesia parasites enter your bloodstream through a bite — most often from the small, poppy-seed-sized nymph ticks active in spring and summer — and destroy your red blood cells. [1] It typically begins with flu-like symptoms one to four weeks after exposure, and roughly a quarter of infected people never feel ill at all. [1] That combination of a subtle bite and a flu-like onset makes awareness critical, especially in the ten high-risk Northeast and Upper Midwest states. [5]
For most people with healthy immune systems, babesiosis is manageable and often resolves with medication or even on its own. For people over 60, those without a functioning spleen, and those with weakened immune systems, it can become life-threatening [1] — which is why early diagnosis and early treatment matter so much. If you have flu-like symptoms after spending time in tick country, tell your provider about the exposure. And if you create health education materials, Rinnit's infographic templates can help you turn tick-borne illness topics like this one into clear, visual guides for your audience.
Frequently Asked Questions
What is babesiosis in simple terms?
Babesiosis is an illness caused by Babesia parasites — one-celled organisms too small to see without a microscope. They enter your bloodstream through a tick bite, where they infect and destroy your red blood cells, usually producing flu-like symptoms. [1]
How do you get babesiosis?
Most often, an infected tick bites an animal carrying the parasite — usually a small rodent like a mouse — and then bites you. In the U.S., the blacklegged (deer) tick is the main carrier. You can also get it from a blood transfusion if the donor is unknowingly infected, but it does not spread from person to person. [1]
Where is babesiosis most common?
It is found in the U.S., Europe, and Asia. In the U.S., cases are mostly in the Upper Midwest and Northeast, concentrated in Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin. [5] [1]
What does babesiosis feel like?
Symptoms start one to four weeks after exposure and include high fever, fatigue, chills, sweating, headache, muscle or joint aches, loss of appetite, and cough. Unlike many other tick-borne illnesses, it usually does not cause a rash. [1]
Can you have babesiosis with no symptoms?
Yes. About 1 in 4 people infected with Babesia never develop symptoms at all. [1]
Is babesiosis the same as Lyme disease?
No. The same tick transmits both, but they are different illnesses — babesiosis is caused by a parasite and Lyme disease is caused by bacteria [1], and they require different treatments. [7]
How is babesiosis different from malaria?
Both are protozoan parasites that damage blood cells and start with flu-like symptoms, but babesiosis is spread by ticks and is most common in the U.S., while malaria is spread by mosquitoes and is most common in Africa. Under a microscope, babesiosis parasites sometimes form a distinctive "Maltese cross" shape. [1]
Who is at risk of severe babesiosis?
People over 60, people with weakened immune systems (from HIV/AIDS, cancer, immune disorders, or certain medications), and people without a functioning spleen face the highest risk of severe, potentially life-threatening illness. [1]
What complications can babesiosis cause?
Severe cases can cause hemolytic anemia, an enlarged spleen or liver, fluid buildup in the lungs (pulmonary edema), a dangerous blood-clotting problem called DIC, and organ damage including kidney and liver failure. [1]
How is babesiosis diagnosed?
Through a physical exam, your symptom and exposure history, and blood tests that look for the parasites in your blood while ruling out other similar illnesses. Tell your provider about any time in tick areas within the past three weeks or any transfusion in the past six months. [1]
How is babesiosis treated?
With combinations of medications — antiprotozoals (atovaquone), antibiotics (azithromycin, clindamycin), and antimalarial drugs (quinine) — given by IV or by mouth. Severe cases may also require blood transfusions, exchange transfusions, mechanical ventilation, or dialysis. [1] [3] Effective treatment is available, and quick treatment can reduce the risk of serious complications. [1] [6]
How long does babesiosis last, and can it come back?
With a healthy immune system, one to two weeks — and medication may bring improvement in a day or two. With a weakened immune system, it can last months or years. Symptoms can also relapse, requiring treatment again, most often in people with weakened immune systems. Fatigue may linger for several months after other symptoms resolve. [1]
Related Reading
References
Cleveland Clinic. Babesiosis. Cleveland Clinic. Last updated March 7, 2023. https://my.clevelandclinic.org/health/diseases/24809-babesiosis
Krause PJ, Lepore T, Sikand VK, et al. Atovaquone and azithromycin for the treatment of babesiosis. N Engl J Med. 2000;343(20):1454-1458. PMID 11078770. https://doi.org/10.1056/NEJM200011163432004
Krause PJ. Human babesiosis. Int J Parasitol. 2019;49(2):165-174. PMID 30690090. https://doi.org/10.1016/j.ijpara.2018.11.007
Tannous T, Cheves TA, Sweeney JD. Red cell exchange as adjunctive therapy for babesiosis: is it really effective? Transfus Med Rev. 2021;35(3):16-21. PMID 34334285. https://doi.org/10.1016/j.tmrv.2021.06.004
Swanson M, Pickrel A, Williamson J, Montgomery S. Trends in reported babesiosis cases — United States, 2011–2019. MMWR Morb Mortal Wkly Rep. 2023;72(11):273-277. PMID 36928071. https://doi.org/10.15585/mmwr.mm7211a1
Centers for Disease Control and Prevention. About Babesiosis. CDC. Last reviewed February 12, 2024. https://www.cdc.gov/babesiosis/about/index.html
Hoversten K, Bartlett MA. Diagnosis of a tick-borne coinfection in a patient with persistent symptoms following treatment for Lyme disease. BMJ Case Rep. 2018;2018:bcr2018225342. PMID 30262525. https://doi.org/10.1136/bcr-2018-225342
Medical disclaimer
This article is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider for diagnosis and treatment decisions.
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