Tick-borne condition • Parasitic

Babesiosis

Babesiosis is an infection of the red blood cells caused by microscopic Babesia parasites, spread to people mainly through the bite of an infected blacklegged (deer) tick — not the Lone Star tick. Many people never feel sick, but it can cause a flu-like illness and, in certain higher-risk groups, become serious. This page is educational only and is not medical advice; only a licensed clinician can diagnose or treat babesiosis.

Last reviewed: 2026-07-20 · Educational only — not medical advice.

Not transmitted by the Lone Star tick. This illness is spread by Blacklegged tick — a different tick. If a Lone Star tick bit you, this is generally not the concern.

What is babesiosis?

Babesiosis is caused by Babesia parasites — most often Babesia microti in the United States — that infect and destroy red blood cells. People usually get it from the bite of an infected blacklegged tick (Ixodes scapularis), the same tick that spreads Lyme disease and anaplasmosis. Less commonly, it can spread through a contaminated blood transfusion or from a pregnant person to their baby during pregnancy or birth. The Lone Star tick — this site's flagship species — is not a recognized vector of babesiosis.

Symptoms

Often none — many infected people never develop symptoms
Fever, chills, and sweats
Headache, body aches, and fatigue
Loss of appetite and nausea
Hemolytic anemia from the parasite destroying red blood cells, which in more serious cases can cause weakness, dizziness, dark urine, or yellowing of the skin or eyes

How soon do symptoms appear?

Symptoms, when they appear, usually develop within a few weeks to a couple of months of exposure — often about 1 to 4 weeks after a tick bite. They can start around a week after infection and, especially in people with weakened immune systems, may first appear or recur months later (CDC).

Diagnosis

A clinician diagnoses babesiosis using your symptoms, exposure history, and lab tests. The main test is a manual (non-automated) microscope review of a peripheral blood smear to look for parasites inside red blood cells; molecular (such as PCR) and antibody (serologic) tests can provide supporting evidence. Testing and interpretation are clinical decisions — babesiosis cannot be self-diagnosed.

Treatment

People who have no symptoms often need no treatment. When treatment is needed, CDC clinical guidance describes a typical course combining atovaquone plus azithromycin (an alternative is clindamycin plus quinine), generally about 7 to 10 days in otherwise healthy people. Severe cases — for example, very high parasite levels or serious anemia — may need longer treatment, hospitalization, or an exchange transfusion. Specific drug choices and doses are decisions for a licensed clinician, including special considerations during pregnancy and for people who are immunocompromised.

Prevention

There is no vaccine for babesiosis, so preventing tick bites is the best protection. Use an EPA-registered repellent and permethrin-treated clothing, stay toward the center of trails and away from leaf litter and brush, and do a full-body tick check after being outdoors. Remove any attached tick promptly with fine-tipped tweezers — CDC notes that blacklegged tick nymphs usually must stay attached more than 36 to 48 hours to transmit the parasite, so prompt removal lowers the risk.

Where it occurs

Most U.S. babesiosis cases occur in the Northeast and the upper Midwest, matching the range of the blacklegged tick. Cases cluster in the warmer months when nymphal ticks are most active. Because it shares the same vector, babesiosis often overlaps geographically with Lyme disease and anaplasmosis.

When to seek care: Contact a clinician if you develop fever, chills, fatigue, or flu-like symptoms after a tick bite or time outdoors where blacklegged ticks live — especially if you have no spleen, a weakened immune system, liver or kidney disease, or are over 50, since these groups face a higher risk of severe illness. Seek prompt care for warning signs such as dark urine, yellowing of the skin or eyes, severe weakness, or trouble breathing. For emergencies such as trouble breathing or anaphylaxis, call emergency services. Educational information only — not medical advice. See our sources and disclaimer.

Babesiosis — frequently asked questions

Does the Lone Star tick spread babesiosis?+

No. Babesiosis is spread mainly by the blacklegged (deer) tick, not the Lone Star tick. Like Lyme disease, babesiosis is not carried by the Lone Star tick — a common point of confusion for people focused on that species.

How soon after a tick bite would babesiosis symptoms start?+

Symptoms usually begin about 1 to 4 weeks after a bite. They can start around a week after infection or develop over a few weeks to months, and may appear later in people with weakened immune systems.

Is babesiosis dangerous?+

For most healthy people it is mild or causes no symptoms at all. It can become serious, even life-threatening, for people without a spleen, those with weakened immunity, older adults, and people with liver or kidney disease.

Can you get babesiosis without a tick bite?+

Yes, though it is less common. Babesia can also spread through a contaminated blood transfusion or from a pregnant person to their baby during pregnancy or birth.

Sources used

Babesiosis
A parasitic infection of red blood cells spread mainly by the blacklegged tick, concentrated in the Northeast and upper Midwest.
CDC
Where Ticks Live
The Lone Star tick is widely distributed in the Northeast, South, and Midwest U.S.
CDC
Preventing Tick Bites
CDC
Tick Removal
Step-by-step guidance for removing an attached tick.
CDC

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Babesiosis

Babesiosis is a red-blood-cell infection from Babesia parasites spread by the blacklegged (deer) tick — not the Lone Star tick. Learn the symptoms, the roughly 1–4 week onset window, diagnosis, treatment, and prevention. Educational, not medical advice.

Educational • not medical advice

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