Closeup of a blacklegged tick nymph. (Credit: Graham Hickling)
Baltimore, Delmarva Peninsula Among Hotspots; Most Doctors Don’t Think To Test For Disease
In A Nutshell
- A new study finds blacklegged ticks carrying Babesia microti, the parasite that causes babesiosis, have expanded into Delaware, Maryland, Virginia, Washington D.C., and West Virginia.
- Researchers detected the parasite in 2.7% of ticks collected from 2010 to 2024, with some hotspots showing infection rates as high as 12.5%.
- Half of infected ticks also carried Lyme disease bacteria, and one was triple-infected with three dangerous pathogens.
- Doctors are advised to include babesiosis in differential diagnoses, especially since it doesn’t respond to standard antibiotics and may be underdiagnosed.
EDGEWOOD, Md. — A parasite once confined to New England is now showing up in places it wasn’t expected. Research reveals that Babesia microti, which causes the tick-borne illness babesiosis, has spread southward into the mid-Atlantic states. The disease destroys red blood cells, often mimics the flu in its early stages, and, unlike Lyme disease, it doesn’t respond to common antibiotics.
As babesiosis cases climb, scientists and public health officials warn that most doctors in these areas may be unprepared to recognize or treat the infection.
“In the 21st century, the tick that carries the babesiosis parasite has expanded its range from New England, and down into Maryland and Virginia,” lead researcher Ellen Stromdahl tells StudyFinds. Stromdahl is a retired entomologist from the Vector-Borne Disease Laboratory, Defense Centers for Public Health-Aberdeen.
“Likely a combination of factors contributes to this range expansion,” she explains. “Warmer temperatures caused by climate change, especially during the winter, have allowed ticks and their host rodents to proliferate. Wetter weather is probably contributing to their success, too. Additionally, reforestation efforts have supported an overpopulation of white-tailed deer. The animals don’t carry the parasite themselves, but they do transport ticks across long distances.”
Babesiosis Cases Appear in New States
Research by Stromdahl and team of 19 colleagues, published in the Journal of Medical Entomology, shows that blacklegged ticks carrying the B. microti parasite causing babesiosis, have expanded into Delaware, Maryland, Virginia, West Virginia, and Washington D.C. Human cases followed a clear timeline: Maryland reported its first locally-acquired case in 2009, followed by Washington D.C. (2013), Virginia (2016), and West Virginia (2017).
This expansion creates serious problems for healthcare systems. Many doctors outside New England don’t recognize babesiosis symptoms. Unlike Lyme disease and other bacterial tick illnesses that respond to common antibiotics, babesiosis requires specific antiparasitic drugs. Delaying proper treatment by just one week can lead to severe complications or death, especially in elderly patients or those with compromised immune systems.
Plain, un-ornamented scutum (the hard shield on its back) with a darker, almost black central area and a reddish-brown skirt; long, slender mouthparts (palps and hypostome) protruding forward; lack of white “spot” on the dorsal shield (rules out the lone star tick, Amblyomma americanum); and no ornate dorsal patterning (rules out the American dog tick, Dermacentor variabilis). Females have a reddish-brown body and scutum, and under bright light their dark brown legs can look lighter. (Credit: Graham Snodgrass)
Scientists Find Infected Ticks Across Three States
Researchers collected ticks from forests, fields, and grasslands across Delaware, Maryland, and Virginia between 2010 and 2024. They discovered blacklegged ticks (Ixodes scapularis) infected with B. microti in all three states. Out of 1,310 ticks tested, 2.7% carried the parasite. Infection rates varied dramatically by location; some areas had less than 1% infected ticks while others reached 12.5%.
Virginia’s Northampton County yielded the first B. microti-positive blacklegged tick in 2012. Half of infected ticks also carried Borrelia burgdorferi, the Lyme disease bacterium. One tick was a triple threat, harboring B. microti, B. burgdorferi, and Anaplasma phagocytophilum (anaplasmosis).
Scientists also found B. microti in Ixodes keiransi ticks. These rarely bite humans but may spread the parasite among wildlife.
Two Babesiosis Hotspots Identified
Two regions show concentrated outbreaks. The Delmarva Peninsula (spanning parts of Delaware, Maryland, and Virginia) accounted for 11 of the 30 locally-acquired human cases across the region. Maryland’s Baltimore Metro Health District formed the second hotspot, with five cases between 2018 and 2022.
Stromdahl says the pattern likely stems from long-established blacklegged tick populations in these areas. “Research has suggested that the longer the ticks are established, the more likely they are to be infected with the babesia parasite,” she says.
The numbers tell a concerning story. Both 2022 and 2023 saw seven cases each, a dramatic jump from earlier years when annual cases stayed between zero and four.
Stromdahl tells StudyFinds that after publication of the study, Virginia health officials confirmed that there have been five new cases of locally-acquired babesiosis in 2025 reported from the eastern shores of Virginia.
Healthcare Workers Must Prepare for More Cases
Blacklegged ticks’ northern population, which bites humans far more often than southern ticks, keeps expanding into new areas. These ticks usually carry Lyme disease bacteria first, setting the stage for babesiosis to follow. When ticks harbor multiple disease-causing organisms, as happened with half the infected ticks in this study, patients suffer more severe symptoms that last longer.
Study authors issued a clear warning in their paper: “Jurisdictions in the southern mid-Atlantic region should expect babesiosis cases, and Lyme disease and anaplasmosis coinfections, and healthcare providers should consider these tick-borne infections as part of the differential diagnosis.”
Public health officials need expanded surveillance programs and better education about prevention, recognition, and treatment. Millions of Americans in these growing tick habitats face new risks when venturing outdoors. If you’ve been bitten by a tick or think you may have been, contact your doctor right away.
“A few weeks after the bite of an infected tick, babesia symptoms might include a sudden onset of fever, chills, sweats, headaches, body aches, loss of appetite, nausea and fatigue,” says Stromdahl. “The parasite destroys red blood cells, therefore babesiosis causes a couple of symptoms not commonly associated with other tick-borne diseases [such as] jaundice and fainting.”
She adds that many, if not most, doctors may not think to test for babesiosis, especially in areas where the disease is not officially considered endemic. It’s important to advocate for yourself and ask to be tested for the condition even if they don’t bring it up. “The disease is likely underdiagnosed,” says Stromdahl, “and can be severe if missed, especially in the elderly or immunocompromised.”
Why is babesiosis, which used to be mostly found in New England, now showing up in places like Maryland and Virginia?
ES: In the 21st century, the tick (Ixodes scapularis, the blacklegged or deer tick) that carries the babesiosis parasite has expanded its range from New England, and down into Maryland and Virginia.
Likely a combination of factors contributes to this range expansion. Warmer temperatures caused by climate change, especially during the winter, have
allowed ticks and their host rodents to proliferate. Wetter weather is probably
contributing to their success, too.
Additionally, reforestation efforts have supported an overpopulation of white-tailed deer. The animals don’t carry the parasite themselves, but they do transport ticks across long distances.
How big of a concern is this for people living or vacationing in the Mid-Atlantic?
ES: Most of the ticks you’ll encounter in the Mid-Atlantic are lone star or American dog ticks, but there are still plenty of blacklegged ticks. Most are not infected with any human pathogen—but some are. If you’re bitten and develop symptoms a couple weeks later, babesiosis is now something to consider.
How do you actually find and test a tick for disease?
ES: Ticks are relatively easy to collect because all they want in the world is a blood meal. Blood is their only food and drink – they don’t eat pollen, plants, other insects, they don’t drink water – only blood. So they come to us! We test those that have bitten people, and those from animals. We collect unattached ticks questing for a blood meal by dragging a cloth through the leaf litter.
Sometimes we lure them to a trap. One example of a trap is a chunk of dry ice set in the woods on a cloth with a border of sticky tape. The carbon dioxide from the dry ice attracts the ticks and they are collected from the cloth with tweezers, sticky tape, etc.
In this study, we first crushed the tick and performed a chemical reaction to extract its DNA. Then we tested tick DNA using PCR (polymerase chain reaction) assays that looked for the specific organisms that cause human disease (microbes causing Lyme disease, babesiosis, Rocky Mountain spotted fever, etc.) They have many closely related organisms that are not linked to human disease. We were careful to target the specific disease-causing microbe. If we got a positive sample, we used another process (a second PCR, DNA sequencing, etc.) to confirm the positive test.
What does it mean if a tick is carrying both Lyme disease and babesiosis pathogens?
ES: If you find a blacklegged tick attached to you, especially if it has fed long enough to look engorged, it could transmit both Lyme and babesiosis. In our studies, and in those from other labs, we found a much higher percentage of the ticks infected with the Lyme bacteria than the babesia parasite, but, in these studies, more than half of the ticks carrying the agent of babesiosis were carrying the agent of Lyme.
Understanding why and how this occurs is an active area of research. Be sure to ask for a test for both. Babesiosis can be more severe when patients have
concurrent Lyme disease.
Diagnosis can be difficult, as the disease is rare and early symptoms of babesiosis resemble conditions more likely to be expected in elderly
populations, or associated with other more common tick-borne diseases, which might be treated empirically with antibiotics typically prescribed for Lyme disease or anaplasmosis, such as doxycycline. Antibiotics alone are not effective against babesiosis.
What symptoms should people watch for?
ES: A few weeks after the bite of an infected tick, babesia symptoms might include a sudden onset of fever, chills, sweats, headaches, body aches, loss of appetite, nausea and fatigue. The parasite (Babesia microti) destroys red blood cells, therefore babesiosis causes a couple of symptoms not commonly associated with other tick-borne diseases – jaundice and fainting.
Are most doctors even considering babesiosis yet?
ES: No, we are trying to spread awareness of babesiosis here in the mid-Atlantic. All the medical guidance, online and in print, for providers and patients, advises that babesiosis is endemic only in the Northeast and Upper Midwest. Babesiosis is likely missed and underdiagnosed, but if left untreated can be fatal. Babesiosis is especially severe in the immunocompromised and the immunosenescent (elderly) and because early symptoms of babesiosis resemble conditions more likely to be expected in elderly populations, for example, cardiac failure – it can be missed.
Why are places like the Delmarva Peninsula and Baltimore seeing more cases?
ES: The ecology of the hotspots infected ticks is an area of active research. Tick surveillance is spotty and lacking in many parts of the mid-Atlantic, as is
surveillance of the animals that carry babesiosis (and spread it to the ticks). There could be other hotspots.
Similarly, medical experts agree that human cases of babesiosis are underreported in the mid-Atlantic – there could be additional hotspots of undiagnosed human cases.
However, we did find hotspots [in] the Delmarva Peninsula and eastern Maryland. The blacklegged tick that spreads babesiosis has been established longer in these areas, and research has suggested that the longer the ticks are established, the more likely they are to be infected with the babesia parasite.
Were there any surprises in the study?
ES: When we went to search for blacklegged ticks around the house
of the index case of locally acquired babesiosis in Maryland, we were not
sure if we would catch any ticks at all. Just to find the vector tick species would have been a win – 16 was actually a lot! In Lyme-endemic areas, 30-50% of adult blacklegged ticks are infected with the Lyme bacteria, so we would expect a few of these to be carrying that. We did not expect to find the agents of anaplasmosis or babesiosis!
What do you want everyday people to take away from this research?
ES: In woodsy brushy areas, use repellent and check for ticks. Remove ticks immediately, save the tick. If it attached to you or your pet, especially if it is engorged, save it, make a note of the date you were bitten. Try to identify it. If symptoms develop a couple of weeks after tick bite, tell your provider you were bitten by a tick.
Paper Summary
Methodology
Researchers collected questing ticks using flagging and dragging techniques in various habitats (old fields, ecotones, forests) across multiple sites: Queen Anne’s County, Maryland (2013); southeastern Virginia and the Virginia Eastern Shore (2010-2017); Aberdeen Proving Ground, Maryland (2017-2023); and three Delaware counties (2019-2024). They also trapped small mammals using Sherman live traps to collect ticks feeding on them. After morphological identification, the ticks underwent DNA extraction and PCR testing for Babesia microti, Anaplasma phagocytophilum, and Borrelia burgdorferi. B. microti-positive samples received additional confirmation through secondary PCR tests targeting different genetic regions, with some samples sent to CDC and Mayo Clinic for verification. A subset of ticks was tested for Babesia duncani, a tick-borne pathogen from western North America.
Results
Of 1,310 Ixodes scapularis ticks tested, 2.7% carried Babesia microti, with site-specific infection rates ranging from under 1% to 12.5%. The first B. microti-positive blacklegged tick was identified in Northampton County, Virginia in 2012. Half (18/36) of the infected ticks also carried Borrelia burgdorferi, and one tick had a triple infection including Anaplasma phagocytophilum. The researchers also found B. microti in Ixodes keiransi ticks from Virginia—the first documented occurrence in this species from the state. No ticks tested positive for Babesia duncani. The research team documented the progression of human babesiosis cases across the region, with Maryland reporting the first locally-acquired case in 2009, followed by Washington D.C. (2013), Virginia (2016), and West Virginia (2017). Two areas emerged as hotspots: the Delmarva Peninsula and the Baltimore Metro region.
Limitations
The study relied on sampling from specific collection sites rather than systematic surveillance across the entire region, which may limit the generalizability of findings. Not all counties were sampled, creating potential geographic gaps in the data. The researchers note that babesiosis may be underdiagnosed and underreported due to limited awareness among healthcare providers and difficulties in diagnosis. In Delaware specifically, no interviews were conducted to determine if reported babesiosis cases were locally acquired, limiting the ability to confirm autochthonous transmission in that state.
Funding and Disclosures
The research received funding from several sources: CDC Epidemiology and Laboratory Capacity Program, Delaware Department of Natural Resources and Environmental Control, National Capital Lyme Disease Association, National Institute of Allergy and Infectious Diseases (Grant K25AI067791), and Old Dominion University Honors College. The authors declared no conflicts of interest in the study.
Publication Information
The paper “Emerging babesiosis in the mid-Atlantic: autochthonous human babesiosis cases and Babesia microti (Piroplasmida: Babesiidae) in Ixodes scapularis (Acari: Ixodidae) and Ixodes keiransi (Acari: Ixodidae) ticks from Delaware, Maryland, Virginia, West Virginia, and the District of Columbia, 2009 to 2024” was published in the Journal of Medical Entomology on April 22, 2025. Ellen Y. Stromdahl served as lead author, with 19 co-authors from various institutions including the Defense Centers for Public Health-Aberdeen, state health departments, universities, and federal agencies.