What to Know About Babesiosis, a Tick-Borne Disease That's on the Rise

Many of the illnesses spread by ticks, including Lyme disease, ehrlichiosis, and anaplasmosis, are bacterial. Ticks can also spread viral illnesses, including Powassan virus, Heartland virus, and Colorado tick fever.

After Lyme, babesiosis or a bacterial disease called anaplasmosis tends to be the next most common tick-borne illness, depending on the state, Diuk-Wasser says. 

But while babesiosis is on the rise, documented cases are still not common. Between 2011 and 2019, 16,456 cases were reported to the CDC from 37 states, according to the 2023 CDC study, with the vast majority of cases coming from 10 states. New York reported the most cases, about 526 per year, followed by Massachusetts and Connecticut. As with most tick-borne illnesses, many cases go unreported, Auwaerter says. But reports can show trends, including when a disease is becoming more common, he says. 

CDC surveillance data from 2011 to 2019 indicates that babesiosis is now endemic in 10 states: Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin. That’s up from seven states in previous years. To prevent the illness from spreading through the blood supply, the Food and Drug Administration recommends that donated blood be screened for the parasites in those 10 states plus Delaware, Maryland, Pennsylvania, Virginia, and Washington, D.C.

Babesiosis is likely spreading for many of the same reasons that Lyme spreads, Auwaerter says, including the large populations of deer and mice that help spread blacklegged ticks across neighborhoods and state lines. 

Infected mice can transmit the parasites to their pups, so pups can be born infected, which provides opportunities for the illness to spread, especially because more mice may be surviving warmer winters, Diuk-Wasser says. Plus, while more research is needed to understand this interaction, co-infection with Lyme disease seems to increase the likelihood that babesiosis spreads. One potential explanation for this could be that the immune system may be less able to fight off a babesia infection if it’s simultaneously trying to respond to the borrelia bacteria that transmit Lyme, she says.

Babesiosis infections can range from asymptomatic to severe. Many people don’t feel any symptoms, but those who do may have some flulike symptoms, including fever, chills, body aches, nausea, and fatigue, according to the CDC. The destruction of red blood cells by parasites can also cause anemia.

These symptoms may appear in the weeks or months after a tick bite. Even without treatment, most adults clear the infection on their own within a year, according to Evan Bloch, an associate professor of pathology and associate director of transfusion medicine at the Johns Hopkins University School of Medicine. 

But babesiosis can be life-threatening for people who don’t have a spleen, have a weakened immune system, have other serious health conditions, or are elderly, according to the CDC.

If you feel sick and there’s reason to suspect babesiosis or another tick-borne disease, a doctor can order a tick-borne disease panel that looks for all possible infections, Auwaerter says. Some infections can be missed, especially if a test is run early in the course of an illness—and be wary of tests not ordered by your doctor, because some might not be credible indicators of a current infection. However, babesiosis, in particular, is easy to identify because the parasites are visible in blood samples when examined under a microscope.

Late spring and early summer are typically when these types of infections are most common, Auwaerter says, though they can happen anytime ticks are active.

Fortunately, there are effective treatments for babesiosis. 

Asymptomatic cases don’t necessarily need to be treated, but doctors will order treatment for asymptomatic people who have evidence of parasites in their blood for over a month, according to Wolters Kluwer’s UpToDate, a decision-making tool for doctors. 

People with mild to moderate disease can be treated with a seven to 10-day oral course of azithromycin, an antibiotic; and atovaquone, an antifungal and anti-parasite drug.

Severe cases require hospitalization. Patients are treated with the same medications but may also require blood transfusions.

Immunocompromised patients with mild to moderate disease can still receive outpatient treatment but with regular monitoring of their blood counts every two to three days.

In a study that began in June, researchers are now testing whether adding the anti-malaria drug tafenoquine to the standard treatment regimen helps hospitalized patients clear the parasite from their systems more effectively and safely.

Historically, babesiosis was one of the leading risks of blood transfusions, with hundreds of cases transmitted that way, according to Bloch. But in 2019, the FDA instituted mandatory babesiosis screening for donated blood in 14 states and Washington, D.C., the first such screening strategy that was specific to certain regions.

Since then, there have been only a couple of breakthrough cases, Bloch says. That could happen if, for example, someone donates blood in their home state after traveling to a state where babesiosis is endemic.

To protect yourself against babesiosis, you’ll want to use the same basic tick prevention strategies you’d use to protect yourself against Lyme or any other tick-borne disease.

Wear long sleeves and long pants. Tuck your shirt into your pants, and tuck your pants into your socks. Consider treating your clothes and shoes with permethrin to keep ticks away, and apply an effective insect repellent to exposed skin. Take a shower after you’ve spent time outside, and perform a thorough tick check when you do.

It takes time for a tick to transmit babesia parasites—usually a tick has to be embedded for 36 to 48 hours to do so, according to the CDC. So if you do find a tick on you, use fine-tipped tweezers to grasp the tick as close to your skin as you can, and then pull it steadily out, trying not to jerk or twist the tweezers.

Diuk-Wasser says that people who want to help researchers better understand how ticks are spreading can use The Tick App, a citizen science project coordinated by researchers from four universities (including Diuk-Wasser’s team at Columbia) to report tick encounters.