Babesiosis · What Is It, Transmission, Treatment, and More

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 bacterial tick-borne diseases, such as anaplasmosis or ehrlichiosis. Since ticks like warm and humid environments, they are usually found in forested areas with wood, bushes, and tall grass. 

In the United States, nearly all cases of human babesiosis are caused by Babesia microti (B. microti), which is widely distributed across the northeast and upper midwest regions of the United States, including Connecticut, Massachusets, New Jersey, and New York state. Other species, like Babesia duncani (B. duncani), have also been reported in the west coast. On the other hand, in Europe, the majority of cases are caused by Babesia divergens (B. divergens). In countries where malaria is endemic, Babesia is often mistaken for Plasmodium, the parasite responsible for malaria, so its true prevalence in these regions remains unknown.    

Individuals living 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 (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. 

Babesiosis 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. 

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 1-6 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, as a result of hemolytic anemia, caused by the destruction of red blood cells. Hemolytic anemia can lead to jaundice, which refers to 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.  

Clinicians may suspect babesiosis after reviewing an individual’s detailed medical history, particularly in 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 if clinical suspicion is high but confirmation through blood smears has not been possible.  

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 azithromycin or, alternatively,  clindamycin and quinine. 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. 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.