Babesiosis

DEFINITION: A parasitic disease that is transmitted to humans by the bite of an infected tick

• ALSO KNOWN AS: Human piroplasmosis, Nantucket fever
• ANATOMY OR SYSTEM AFFECTED: Blood, kidneys, liver, respiratory system
• CAUSES: Parasitic infection
• SYMPTOMS: Malaise, fatigue, chills, fever, sweating, muscle and joint pain, jaundice, anemia, shortness of breath
• DURATION: Days to months
• TREATMENTS: Combined antiparasitic and antibiotic therapy, supportive care

Causes and Symptoms

Babesiosis is caused by a protozoan parasite that infects livestock and domestic animals and, more rarely, humans. Babesia microti (in the United States) and Babesia divergens (in certain parts of Europe) are the species more often associated with disease in humans. The parasite, which destroys red blood cells, is spread by the bite of infected deer ticks (in the United States), which also transmit Lyme disease. Ticks become infected with the Babesia organism while feeding on infected deer, mice, or voles. The organism can also be transmitted by blood transfusions, although this mode is uncommon.

Many people infected with Babesia have no symptoms of illness, even though infection may persist for months to years. Others experience flu-like symptoms, such as fatigue, poor appetite, fever, chills, sweating, headaches, body aches, or even nausea. Symptoms usually manifest themselves after a one- to nine-week incubation period and may last for weeks. For people with a weak immune system or who lack a functioning spleen, life-threatening, malaria-like symptoms may develop that include high fevers, severe anemia—as a result of the abnormal breakdown of red blood cells—and jaundice.

Treatment and Therapy

People with no symptoms or only mild symptoms usually recover on their own. Otherwise, the standard treatment has consisted of the antibiotic clindamycin and the antiparasitic drug quinine. More recently, therapy has included the antibiotic azithromycin and the antiparasitic drug atovaquone, which are more easily tolerated.

Transfusions may be given to replace the infected red blood cells in severe cases associated with high levels of parasites in the blood. For complications such as very low blood pressure, breathing difficulties, and kidney failure, supportive care consists of vasopressors—to increase the blood pressure—mechanical ventilation, and dialysis.

Perspective and Prospects

The Romanian scientist Victor Babes was the first to document the symptoms of babesiosis in cattle in 1888. The American scientist Theobald Smith and his colleagues identified a protozoan as the cause of disease and the tick as the agent of transmission in 1893. The first case of babesiosis in humans was reported in 1957 in a person whose spleen had been removed.

In the United States, babesiosis is considered an emerging infection, especially in coastal areas of the northeastern United States and its offshore islands, where the first case was reported in 1969 from Nantucket Island, Massachusetts. According to the Centers for Disease Control and Prevention (CDC), cases have been reported in New England, New York, New Jersey, Wisconsin, and Minnesota. According to Penn State Health, the rates of babesiosis increased 9 percent per year between 2015 and 2022. During that period, 3,521 documented individuals were infected with babesiosis. No vaccine is available to date. The best prevention is to wear protective clothing to cover the skin and to use insecticides to repel ticks when journeying through wooded, tick-infested areas.

Bibliography

"About Babesiosis." Centers for Disease Control and Prevention, 12 Feb. 2024, www.cdc.gov/babesiosis/about/index.html. Accessed 12 Sept. 2025.

Considine, Glenn D., editor. Van Nostrand’s Scientific Encyclopedia. 10th ed., 3 vols., Wiley-Interscience, 2008.

Gelfand, Jeffrey A., and Edouard Vannier. “Babesiosis.” In Harrison’s Principles of Internal Medicine, edited by Anthony Fauci et al. 18th ed., McGraw-Hill, 2012.

Montero, E. et al. "Retrospective Study of the Epidemiological Risk and Serological Diagnosis of Human Babesiosis in Asturias, Northwestern Spain." Parasites Vectors, vol. 16, no. 195, June 2023, doi:10.1186/s13071-023-05817-x. Accessed 12 Sept. 2025.

Mylonakis, Eleftherios. “When to Suspect and How to Monitor Babesiosis.” American Family Physician, vol. 63, May 2001, pp. 1969–1974.

"Rates of a Tick-Borne Parasitic Disease Are on the Rise." Penn State Health, 8 Oct. 2024, pennstatehealthnews.org/2024/10/rates-of-a-tick-borne-parasitic-disease-are-on-the-rise/. Accessed 12 Sept. 2025.

"Tick Bites." MedlinePlus, 24 June 2024, medlineplus.gov/tickbites.html. Accessed 12 Sept. 2025.