Federal surveillance now puts the number of reported babesiosis cases in the United States at 3,586 for 2023, the highest annual figure in the ten years the Centers for Disease Control and Prevention publishes on its data page, and roughly 70 percent above the 2,111 cases logged the year before.
Those numbers, posted in the CDC's babesiosis data and statistics tables, arrive in the middle of a tick season that is already among the most active in nearly a decade. They matter now for a specific and practical reason: babesiosis peaks in late summer and early fall, and the illness is caused by a parasite that infects red blood cells rather than a bacterium that produces the bullseye rash most people associate with a tick bite.
For a household in the Northeast or Upper Midwest, the consequence is narrow but important. A person who is treated for Lyme disease and does not improve may be carrying a second infection from the same bite, and that second infection requires an entirely different prescription.
Inside the Newest National Numbers
The 2023 burden is heavily concentrated. New York reported 1,169 cases, Massachusetts 841, New Jersey 406, Connecticut 246, Rhode Island 242, Maine 196, Wisconsin 124, Vermont 81, Minnesota 73, and New Hampshire 53.
Raw counts understate the situation in smaller states. Rhode Island's incidence was 21.9 cases per 100,000 residents, the highest in the country. Maine reported 14.0, Vermont 12.5, and Massachusetts 11.9. New York's rate, despite the largest case count, was 5.9.
One surveillance limitation belongs up front rather than buried. Babesiosis is not a reportable condition by law in every state. Pennsylvania, Colorado, Nevada, Oklahoma, Mississippi, Idaho, Hawaii, New Mexico, and Alaska carry no reported figures in the CDC table for that reason, and national incidence rates are calculated only across states where reporting was required. Cases are also counted by state of residence, which is not necessarily where exposure occurred.
The direction of travel is not new. CDC researchers reported in 2023 that incidence rose significantly across eight of ten reporting states between 2011 and 2019, with the steepest increases in Vermont, Maine, New Hampshire, and Connecticut. Epidemiologist Megan Swanson said at the time that the data showed an increase "in parts of the U.S. that previously saw few cases."
Two Infections, Two Different Prescriptions
The clinical distinction is the reason this story exists. Lyme disease is bacterial and is treated with antibiotics such as doxycycline. Babesiosis is parasitic and is treated with a combination of atovaquone and azithromycin. A doxycycline course prescribed for Lyme disease does not clear a Babesia infection.
The presentations differ too. Babesiosis usually produces no rash. Symptoms, when they appear, include fever, chills, sweats, headache, body aches, loss of appetite, nausea, and fatigue. Because the parasite destroys red blood cells, severe cases can produce hemolytic anemia, jaundice, kidney injury, low platelet counts, and, in the most serious presentations, organ failure.
Most infections are mild or silent. CDC and independent researchers have estimated that a substantial share of adult cases and roughly half of pediatric cases produce no symptoms at all. The overall case fatality rate has been described by Yale researcher Peter Krause as approximately 1 to 2 percent.
Risk is not evenly distributed. Adults over 50, people without a functioning spleen, transplant recipients, people receiving chemotherapy, and anyone with a weakened immune system face substantially higher odds of severe disease. Transmission can also occur through blood transfusion, organ transplantation, or from mother to child, which is why a recent transfusion history belongs in the conversation with a clinician.
Ellen Stromdahl, an entomologist with the U.S. Army Public Health Center who has tracked the parasite's expansion into the mid-Atlantic, has urged clinicians to consider babesiosis "particularly during peak tick-activity seasons."
Correcting the Coinfection Math
Coinfection figures circulate widely and are frequently misread, including in earlier coverage of this topic. The direction of the calculation determines the answer.
Measured from the Lyme disease side, CDC states that the most common coinfection with Lyme disease is anaplasmosis, occurring in up to about 10 percent of patients, and that babesiosis and other coinfections occur less frequently. From the babesiosis side, the picture looks different: a retrospective analysis of 3,521 babesiosis patients in the TriNetX database between 2015 and 2023 found that 42 percent had at least one tick-borne coinfection, with Borrelia burgdorferi accounting for 41 percent.
Both figures can be true. Babesiosis is far less common than Lyme disease, so a large fraction of babesiosis patients also having Lyme disease is compatible with only a small fraction of Lyme patients also having babesiosis. Readers who encounter a claim that most Lyme patients are coinfected with Babesia should treat that as unsupported.
Whether coinfection worsens outcomes is genuinely unsettled. The same Penn State research group published one analysis associating Babesia coinfection with mortality and later presented a separate analysis concluding coinfection was not associated with higher mortality. That disagreement has not been resolved.
Tick-level surveillance adds a third view. An ecological study of northeastern forests reported by the Infectious Diseases Society of America found that about 10 percent of ticks sampled carried at least two pathogens, with Borrelia burgdorferi and Babesia microti the most common pairing, and Babesia microti detected frequently toward the end of the study period. CDC's own tickborne pathogen surveillance maps, current through 2025, show where the parasite has been found in host-seeking ticks.
Steps for Anyone Bitten This Summer
Prevention is identical across every pathogen the blacklegged tick carries. Use EPA-registered repellent, treat clothing with permethrin, check the whole body after time outdoors, shower within two hours of coming inside, and remove attached ticks promptly.
The specific action this reporting supports is different. Anyone who develops fever, chills, sweats, or severe fatigue within several weeks of possible tick exposure should mention that exposure to a clinician, and should say so again if a course of antibiotics for Lyme disease does not produce improvement. Babesiosis is diagnosed through blood smear examination or molecular testing, neither of which happens unless someone orders it.
People who are immunocompromised, older, or without a spleen should treat a febrile illness after outdoor exposure as a reason to seek care promptly rather than to wait. Jaundice, dark urine, confusion, shortness of breath, or rapidly worsening illness warrant urgent evaluation.
On cost and access, blood smear and PCR testing are generally covered by commercial insurance and Medicare when clinically indicated, and both drugs used to treat babesiosis are available as generics. Patients facing coverage questions can ask the ordering clinician to document the tick exposure and clinical indication, which is often what determines whether a claim is paid.
Considerable uncertainty remains. Full 2024 and 2025 national figures have not been published, so the current season cannot yet be compared with prior years. How many infections go undiagnosed is unknown, since most produce no symptoms. Whether coinfection independently worsens outcomes is disputed. MedicalDaily will report updated CDC surveillance when it is released.
Frequently Asked Questions
What is babesiosis? A parasitic infection, caused in the United States mainly by Babesia microti, that invades red blood cells. It is transmitted by blacklegged ticks, the same species that spreads Lyme disease.
How many cases are reported each year? CDC surveillance recorded 3,586 reported cases in 2023, the highest figure in the agency's published ten-year table and about 70 percent above 2022.
Does treatment for Lyme disease also treat babesiosis? No. Lyme disease is treated with antibiotics such as doxycycline. Babesiosis requires a different regimen, typically atovaquone plus azithromycin.
What symptoms should people watch for? Fever, chills, sweats, headache, body aches, nausea, and fatigue, usually without a rash. Jaundice, dark urine, confusion, or shortness of breath signal severe illness.
Who faces the highest risk? Adults over 50, people without a spleen, transplant recipients, people on chemotherapy, and others with weakened immune systems.
How often do Lyme and babesiosis occur together? It depends on which direction you measure. About 42 percent of babesiosis patients in one large database had a tickborne coinfection, but babesiosis is an uncommon coinfection among Lyme patients compared with anaplasmosis.
Which states report the most cases? By count, New York, Massachusetts, and New Jersey. By rate per 100,000 residents, Rhode Island, Maine, Vermont, and Massachusetts lead.
