What Is It, Transmission, Treatment, and More

Author: Anna Hernández, MD

Editors: Ahaana Singh, Kelsey LaFayette, BAN, RN

Illustrator: Jillian Dunbar

Copyeditor: Joy Mapes

Modified: 27 Dec 2023

What is babesiosis?

Babesiosis is an infectious disease caused by Babesia, microscopic parasites that infect the blood and destroy red blood cells. The Babesia genus infects millions of cattle and small animals throughout the world, as well as ticks in the Ixodes genus, which are responsible for the spread of most human infections. In the last few decades, cases of human babesiosis have been reported with increasing frequency in the United States, especially on the northeastern coast.

How do you pronounce babesiosis?

Babesiosis is pronounced as buh-bee-zee-oh-sis. The word “babesiosis” originates from Victor Babeş, a Romanian microbiologist who identified the parasite in sick cattle in 1888. 

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What causes babesiosis?

Babesia infection is typically spread by the bite of infected Ixodes scapularis ticks, more commonly known as blacklegged ticks or deer ticks. Ixodes ticks are the main vector for babesiosis, meaning they are the living organisms that spread the parasite from one host to another. These are the same ticks that often carry Borrelia burgdorferi, the causative agent of Lyme disease, as well as other tick-borne diseases, such as anaplasmosis or ehrlichiosis. 

In the United States, nearly all cases of human babesiosis are caused by Babesia microti (B. microti), while in Europe, the majority of cases are caused by Babesia divergens (B. divergens). B. microti is widely distributed across the northeast and upper midwest regions of the United States, including Connecticut, Massachusetts, New Jersey, and New York state. Other species, like Babesia duncani, have also been reported in the west coast. Since ticks like warm and humid environments, they are usually found in forested areas with wood, bushes, and tall grass. 

Individuals who live in an endemic area (i.e., a certain place where a disease is regularly found), or those who travel to an endemic area during active tick season (i.e., May to September in the United States) are most likely to develop babesiosis. In addition, individuals with a compromised immune system, who have had their spleen surgically removed (i.e., splenectomy), or who have received a blood transfusion are at higher risk of developing babesiosis

When an infected tick bites a human, the parasites inside the tick’s gut gradually move to the human host’s skin, and then into the blood vessels. In most cases, the tick needs to be attached to a person for 24 to 36 hours or more before they can transmit the parasite. The longer a tick is attached, the more likely transmission will occur. Once in the blood, Babesia parasites reproduce, leading to the destruction, or hemolysis, of red blood cells. This results in the clinical manifestations of the disease.

Is babesiosis contagious?

Babesia is not contagious and can be transmitted from person-to-person only through a contaminated blood transfusion or by congenital transmission from an infected individual to a fetus during pregnancy or delivery. Although bloodborne transmission represents a minority of cases of babesiosis, according to the Centers for Disease Control and Prevention (CDC), B. microti has become one of the most common parasites transmitted by blood transfusion in the United States. 

All other cases of human babesiosis are transmitted through the bite of an infected tick.

What are the symptoms of babesiosis?

Symptoms of babesiosis vary significantly from person-to-person. Some individuals may not experience any symptoms at all, while others may develop a severe, potentially life-threatening disease. 

If symptoms are present, they usually appear one to six weeks after infection. Most symptomatic individuals experience mild, flu-like symptoms characterized by fever, chills, sweats, headache, weakness, and loss of appetite. These symptoms can last for weeks to a few months, but they typically resolve fully. People who contract babesiosis may also experience malaria-like symptoms, such as hemolytic anemia, caused by the destruction of red blood cells. In turn, hemolytic anemia can lead to jaundice, which refers to a yellow discoloration of the skin and mucous membranes, as well as darkening of the urine. 

Certain species of Babesia, like Babesia divergens, are more likely to cause severe infection than others but are responsible for fewer cases of babesiosis. In addition, immunocompromised individuals, individuals without a spleen, and older adults are more likely to develop severe symptoms from the infection. Finally, coinfection with Lyme disease or anaplasmosis can further complicate babesiosis and lead to a more serious case. 

How is babesiosis diagnosed?

Clinicians may suspect babesiosis after reviewing an individual’s detailed medical history, particularly for individuals who have visited an endemic area and present with fever and flu-like symptoms.

If babesiosis is suspected, microscopic analysis of blood smears may be performed to screen for Babesia parasites in the individual’s red blood cells. Depending on the structure of the parasite, microscopic analysis may show a ring form or “maltese cross” shape inside the red blood cells. Careful examination of multiple blood smears may be necessary in certain cases because Babesia parasites can be easily overlooked or confused with malaria

Other laboratory tests, such as detection of antibodies and molecular testing, may be performed in cases of high clinical suspicion in which confirmation through blood smears has not been possible. 

How is babesiosis treated?

Treatment of babesiosis varies according to the severity of the disease. Asymptomatic individuals may not require medical treatment besides supportive measures. In contrast, symptomatic individuals are usually treated with a combination of antimicrobial medications, such as atovaquone and clindamycin or azithromycin. For severe cases of babesiosis, individuals may require an exchange transfusion, a procedure in which a person’s blood is slowly removed and replaced by donor blood or plasma. 

Preventive measures can reduce the risk of contracting tick-borne infections when spending time outdoors. The best way to reduce the risk of becoming infected is to stay clear of tick habitats, such as leaf litter, tall grass, and forested areas with lots of vegetation. However, other effective measures include wearing clothes that minimize exposed skin (e.g., hats, long pants, boots, long-sleeved shirts, etc.), wearing light-colored clothes so ticks can be spotted more easily, and using tick repellents. After being outdoors, it is important to perform a tick check, searching the entire body and clothes for ticks. If a tick is found attached to the skin, the tick can be removed by grabbing it with a pair of tweezers as close to the individual’s skin as possible and pulling it away. If the tick was attached to the individual for only a few hours, the risk of infection is very small.

Are there lingering effects of babesiosis?

Older individuals, individuals without a spleen, and those with a compromised immune system are at an increased risk of developing severe, lingering effects of babesiosis. Some of the potential complications include having a very low and unstable blood pressure, severe hemolytic anemia, coagulation problems, failure of vital organs, which can rapidly progress to shock, and even cardiac arrest.

What are the most important facts to know about babesiosis?

Babesiosis is an infectious disease caused by a parasite from the Babesia species. Human babesiosis is usually transmitted through the bite of an infected blacklegged tick (Ixodes scapularis), also known as a deer tick, in endemic areas, such as the northeastern United States and certain areas of Europe. Less commonly, babesiosis can be transmitted through a contaminated blood transfusion, as well as congenitally from an infected individual to a fetus during pregnancy or delivery. Cases of human babesiosis can range from asymptomatic to severe and life-threatening. Serious cases are more common in older individuals, individuals without a spleen, and those with a decreased immune response. Because the clinical findings are not specific, diagnosis of babesiosis is usually made by microscopic identification of Babesia parasites on one or more blood smears. Treatment of babesiosis includes a combination of antimicrobial agents, such as atovaquone and azithromycin, as well as supportive measures. Risk of transmission can be reduced by limiting time spent in tick habitats, wearing clothes that minimize exposed skin, applying tick repellants, and performing tick checks after being outdoors. 

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Related links

Lyme disease
Plasmodium species (Malaria)
Insect stings and bites: Nursing

Resources for research and reference

Centers for Disease Control and Prevention. Babesiosis. In: DPDx - Laboratory Identification of Parasites of Public Health Concern. Published October 30, 2017. Accessed April 9, 2021.

Jameson JL, Fauci A, Kasper D, Hauser S, Longo D, Loscalzo J. Harrison's Principles of Internal Medicine. 20th ed. McGraw-Hill Education; 2018.

Krause PJ. Human babesiosis. Int J Parasitol. 2019;49(2):165-174. doi:10.1016/j.ijpara.2018.11.007

Leiby D. Transfusion-associated babesiosis: shouldn't we be ticked off? Ann Intern Med. 2011;155(8):556–557. doi:10.7326/0003-4819-155-8-201110180-00363