What Is Babesia Duncani? Symptoms, Testing, and Treatment

Babesia duncani is a tiny parasite that lives inside red blood cells and causes a fever illness much like a mild form of malaria. It is a less familiar cousin of the parasite behind most babesiosis cases in the US, and it behaves differently enough that testing and treatment aimed at the common type can miss it. Doctors still know surprisingly little about it, including exactly how most people catch it.

Where Babesia Duncani Came From and How It Differs

Babesiosis is a tick-borne infection of red blood cells that resembles malaria.1SpringerLink / Parasitology Research. In vitro cultivation of Babesia duncani (Apicomplexa: Babesiidae), a zoonotic hemoprotozoan, using infected blood from Syrian hamsters (Mesocricetus auratus) In North America it is caused mostly by Babesia microti, which causes more cases each year than all other types combined.2SpringerLink / Parasitology Research. In vitro cultivation of Babesia duncani (Apicomplexa: Babesiidae), a zoonotic hemoprotozoan, using infected blood from Syrian hamsters (Mesocricetus auratus) Babesia duncani is a second North American cause, while Babesia divergens causes most cases in Europe.3SpringerLink / Parasitology Research. In vitro cultivation of Babesia duncani (Apicomplexa: Babesiidae), a zoonotic hemoprotozoan, using infected blood from Syrian hamsters (Mesocricetus auratus) B. duncani was described as its own species using parasites taken from sick patients in Washington and California.4PubMed. Description of Babesia duncani n.sp. (Apicomplexa: Babesiidae) from humans and its differentiation from other piroplasms Earlier reports called it “WA1,” the name of the Washington sample, so older articles may use that label.5PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review

Its genes place it on a separate branch from B. microti and from the Babesia species that infect cattle and sheep.6PubMed Central. Comparative genomic analysis of Babesia duncani responsible for human babesiosis A test or drug tuned to B. microti can’t be assumed to work on B. duncani. That is why the name on a lab report matters. A result saying “Babesia microti negative” says nothing about B. duncani. Some labs test the two separately and report each on its own line. It’s reasonable to ask which species your test actually looked for.

Confirmed cases are also very rare. Only a handful have been described in the medical literature. Antibodies turn up more often than illness, which suggests many infections cause no symptoms. Rare in the records doesn’t mean rare in nature, partly because few people get tested for it.

Symptoms of Babesia Duncani Infection

These symptoms match those of B. microti, so symptoms alone cannot tell the two apart.13PubMed Central. Update on Babesiosis Babesia infection can also cause no symptoms at all, or illness ranging from mild to life-threatening.14PubMed. Transfusion-transmitted babesiosis in an immunocompromised patient: a case report and review In northern California about 3.5% of people tested, and 16% to 18% of higher-risk people, had antibodies to B. duncani.15PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review

Fever, aches and fatigue overlap with flu, Lyme disease and many viral illnesses. Dark urine is the more specific clue and is worth reporting promptly. Symptoms can also appear weeks or months after exposure, as happened in transfusion cases. Only a blood test can confirm the cause.

A high fever with dark urine, yellowing skin or shortness of breath needs same-day care. Anyone without a spleen who develops a fever after a tick bite or a transfusion should be seen quickly.

Who Gets Seriously Ill

Severe babesiosis can lead to organ failure or death.16PubMed. Transfusion-transmitted babesiosis in an immunocompromised patient: a case report and review Older age, having no spleen, and other kinds of immune weakness raise the risk of severe illness.17PubMed. Transfusion-transmitted babesiosis in an immunocompromised patient: a case report and review These patients may need longer treatment to clear the parasite from the blood.18PubMed. Transfusion-transmitted babesiosis in an immunocompromised patient: a case report and review The spleen normally filters infected red blood cells out of the blood, which is why losing it matters so much. People may lack a spleen because it was surgically removed after an injury or for a blood disorder, or because it stopped working, as can happen in sickle cell disease. A weakened immune system includes people in cancer treatment such as chemotherapy, those taking organ transplant medicines that suppress immunity, people with advanced HIV, and those on long-term high-dose steroids.

Transfusion adds its own risk. Many people who receive blood are already seriously ill, so an infection hits them harder. One reported transfusion case was a premature baby, who did not have the usual fever.

How People Catch Babesia Duncani

The tick that spreads B. microti is well known, but how B. duncani spreads in nature was long unknown.19Journal of Parasitology, 104(4):429-432. An Improved Improved PCR Protocol For Detection of Babesia duncani In Wildlife and Vector Samples In California the parasite has been found in winter ticks, a tick that mainly feeds on deer.20PubMed Central. Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus About 7% of young ticks sampled carried it, rising to about 21% in Marin County.21PubMed Central. Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus About three in four mule deer tested were infected.22PubMed Central. Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus Deer appear to be a main animal carrier in the West.

Blood transfusion is the other known route. Of transfusion-spread babesiosis cases where species was known, about 5% were B. duncani and 90% B. microti.23PubMed. Transfusion transmitted babesiosis: A systematic review of reported cases In one case, the donor was a California man still carrying the parasite 10 months after donating.24PubMed. The third described case of transfusion-transmitted Babesia duncani That patient was diagnosed more than 4 months after the transfusion.25PubMed. The third described case of transfusion-transmitted Babesia duncani The infection went unnoticed because tests for B. microti in the donors did not detect it.26PubMed. The third described case of transfusion-transmitted Babesia duncani

Human cases have been reported across Canada.27PubMed Central. Human Babesiosis Caused by Babesia duncani Has Widespread Distribution across Canada Some reports suggest cases in the eastern US as well.28Daniel Cameron MD. Babesia duncani: Symptoms, Treatment, and East Coast Spread

For tick protection, use repellent with DEET or picaridin on skin and permethrin on clothing. Check your body daily after time outdoors, including the scalp and groin, and remove any tick promptly with fine tweezers close to the skin. Winter ticks are active in cooler months, unlike many other ticks, so protection isn’t only a summer matter.

Because how it spreads is not fully known, a lack of a remembered tick bite does not rule it out. People with a past positive result should not donate blood.

How Babesia Duncani Is Tested

There are no government-licensed tests for B. duncani in the US or Canada.29SpringerLink / Parasitology Research. In vitro cultivation of Babesia duncani (Apicomplexa: Babesiidae), a zoonotic hemoprotozoan, using infected blood from Syrian hamsters (Mesocricetus auratus) Testing is therefore done by specialty labs using their own methods. Standard B. microti tests can miss it entirely.30PubMed. The third described case of transfusion-transmitted Babesia duncani Every published confirmed case was proven by a smear, a DNA test or growing the parasite in a lab animal.31PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review

A blood smear looks for parasites inside red blood cells under a microscope. A DNA test, often listed as PCR on lab reports, detects the parasite’s genetic material.32PubMed. Nested qPCR assay to detect Babesia duncani infection in hamsters and humans A refined DNA test can find as little as one infected cell in a small blood sample and is more accurate than the microscope.33PubMed. Nested qPCR assay to detect Babesia duncani infection in hamsters and humans An antibody test (often reported as IFA, with a titer) shows the body has met the parasite, but antibodies are common in people with no illness.34PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review Newer tests that detect parasite proteins can show active infection within 2 to 3 days in animal studies.35PubMed Central. A set of diagnostic tests for detection of active Babesia duncani infection

A smear depends on a technician spotting parasites in a thin film of blood, so early or low-level infections can be missed. A negative smear doesn’t rule out infection, especially when symptoms are mild or just beginning. A smear is most useful when many red cells are infected, as in more severe illness.

A positive DNA test means the parasite is likely in the blood right now. It can be designed to distinguish B. duncani from B. microti, but only if the lab built it to look for B. duncani. A negative DNA test is reassuring but not absolute, because a very small amount of parasite may fall below what the test can detect.

A titer like 1:256 is a dilution, and higher numbers mean more antibodies. A positive antibody test with a negative DNA test may mean past exposure, and a titer alone doesn’t show whether infection is active.

Useful questions for your doctor: which lab ran the test and what method it used, whether a DNA test should be added to an antibody result, and whether to retest after treatment to confirm the parasite is gone.

Treatment and Why It Can Be Difficult

The four recommended babesiosis drugs are atovaquone, azithromycin, clindamycin and quinine.36PubMed Central. Establishment of a continuous in vitro culture of Babesia duncani in human erythrocytes reveals unusually high tolerance to recommended therapies Doctors who treat it report that B. duncani is harder to treat than B. microti and usually needs longer courses.37PubMed Central. Human Babesiosis Caused by Babesia duncani Has Widespread Distribution across Canada

Atovaquone and azithromycin are usually given together by mouth, and this pairing is generally preferred for milder cases because it tends to cause fewer side effects. Clindamycin and quinine are typically reserved for more severe illness, often given through a vein in the hospital, because they cause more side effects. Doctors choose among these based on how sick the person is, and no regimen has been proven best for B. duncani specifically. Severe cases may need hospital care and sometimes blood exchange.

The aim is to clear the parasite from the blood, not just ease symptoms. Symptoms can fade before the parasite is fully gone, so stopping on the day someone feels better risks relapse. People who are older, lack a spleen or have weakened immunity often take weeks longer than usual, and repeat blood tests guide when to stop.

The lab evidence is discouraging. In human red blood cells grown in the lab, B. duncani was poorly controlled by all four drugs.38PubMed Central. Establishment of a continuous in vitro culture of Babesia duncani in human erythrocytes reveals unusually high tolerance to recommended therapies In mice, the recommended drug combinations did not clear infection or prevent death.39bioRxiv. Babesia duncani as a model organism to study the development, virulence and drug susceptibility of intraerythrocytic parasites in vitro and in vivo But in another mouse study, atovaquone reduced parasites and 80% of mice survived.40PubMed Central. Treatment of Human Babesiosis: Then and Now Animal findings are mixed and may not match how people respond, and poor results in the lab don’t necessarily mean treatment will fail in people.

Other drugs are under study. In the lab the parasite is very sensitive to pyrimethamine and a related experimental drug.41PubMed Central. Babesia duncani multi-omics identifies virulence factors and drug targets Cancer-related drugs such as panobinostat also killed it in lab screens.42PubMed Central. A propidium iodide-based in vitro screen of the “Bug Box” against Babesia duncani reveals potent inhibitors None of these are proven treatments for people yet.

Babesia Duncani and Lyme Disease

B. duncani often comes up in reviews of chronic Lyme coinfections.43PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review Only 8 confirmed cases had been described in that literature, three from transfusion.44PubMed Central. Chronic coinfections in patients diagnosed with chronic Lyme disease: a systematic literature review B. microti, not B. duncani, is the Babesia known to share ticks with Lyme in the Northeast.45Daniel Cameron MD. Babesia duncani: Symptoms, Treatment, and East Coast Spread

The two get discussed together because both can be spread by ticks, so one bite may expose a person to more than one germ. Their symptoms overlap, including fatigue, aches and fever, so people with ongoing illness and a Lyme diagnosis often wonder about a second cause. Lyme is caused by a bacterium and Babesia by a parasite, so they need different tests and different drugs.

The tick link is weaker for B. duncani, which has been found mainly in a different tick that feeds largely on deer in the West. Assuming a Lyme patient also has it is not supported by what is known about where the two circulate.

If you’ve been told you have a coinfection, remember that a positive antibody test is not the same as active infection. A second opinion from an infectious disease specialist can help, and confirming active infection with a smear or DNA test before any long treatment is wise.