What Is Babesiosis?

Babesiosis is very similar to malaria and is caused by microscopic parasites (intraerythrocytic) Babesia. Babesia not only is similar to the malaria parasite, Plasmodium falciparum in morphology, but infects red blood cells in the same way as P. falciparum. Although more than 100 species have been reported, only a few have been identified as causing human infections. At present, four main species of Babesia are known to infect humans. They are Babesia microti, B. divergens, Babesia MO1, and B. duncani (formally WA-1). In the United States, three species of Babesia, B. microti, B. duncani and B. MOI (B.divergens-like) are the causative agents of human babesiosis. In Europe, most reported cases are due to B. divergens and occur in splenectomized patients. Isolated cases due to B. microti-like parasites have been reported over a wide geographic range in Europe, Australia, Asia, Africa, and South America.

How Is Babesia Transmitted?

The intermediate host for Babesia is the same hard-bodied tick that transmits B. burgdorferi, the causative agent of Lyme disease. Babesia spp. is transmitted through a bite of an infected tick, primarily of the genus Ixodes. The intermediate vector host is white-footed mice and other small mammals. Humans are dead-end hosts. [See Babesia life cycle below.]


In the US, the blacklegged ticks are responsible for spreading the disease. On the East Coast and in several mid-western states, it is the Eastern blacklegged tick. This tick is also known as the “deer tick,” or Ixodes scapularis, and on the West Cost it is known as the Western blacklegged tick, or I. pacificus. In Europe I. ricinus is responsible for spreading the disease. (Note: Although white-tailed deer are the most important food source for the adult stage of the tick, deer are not infected with Babesia).

The parasite typically is spread by the young nymph stage of the tick. Nymphs are mostly found during warm months (spring and summer) in areas with woods, brush, or grass.

Infected people might not recall a tick bite because the nymphs are very small (about the size of a poppy seed).

Other possible ways of becoming infected with Babesia include:

  • Receipt of a contaminated blood transfusion (no tests have been licensed yet for donor screening); or
  • Transmission from an infected mother to her baby during pregnancy or delivery.

What Are the Infected Regions?

Babeisa is endemic in the same regions as Lyme disease since the intermediate host is the same tick that transmits B. burgdorferi, the causative agent of Lyme disease. B. microti was thought to be present only in the Northeast and Midwest parts of US, but recently cases have been reported from Western Coastal regions of the US and Switzerland. B. duncani is most frequently identified on the West Coast. B. MO1 (B. divergens-like) organisms have been identified in three cases, two from the Midwest (Missouri and Kentucky) and one from Washington State. B. divergens is found in Europe.

What Are the Symptoms?

Clinically, Babesiosis appears to have a wide spectrum of disease severity depending on the immune status of the patient. Flu-like symptoms usually begin 1-9 weeks after inoculation and are non-specific. Symptoms include fever, fatigue, malaise, headache, chills, and myalgia, nausea, vomiting, reduced appetite, and depression. In most patients (immunocompetent) the symptoms last for a few weeks to several months. Co-infection with Lyme disease or Anaplasmosis may complicate the clinical presentation and predispose the patient to more severe disease. A small group of patients are asymptomatic. In severe cases, Babesia can cause hemolytic anemia (from the destruction of red blood cells).

Babesiosis can be a severe, life-threatening disease, particularly in people who:

  • Do not have a spleen
  • Have a weak immune system for other reasons (such as cancer, lymphoma, or AIDS)
  • Have other serious health conditions (such as liver or kidney disease)
  • Are elderly

How Is Babesiosis Diagnosed?

The diagnosis of Babesiosis should be considered in patients who live or travel to areas that are endemic for Babesiosis and experience a viral-like illness or have been bitten by Ixodes ticks or recently had a blood transfusion. As the symptoms and signs are relatively nonspecific, laboratory testing is required for diagnosis.

Click here on how to order a test kit from IGeneX.

FOR HEALTHCARE PROFESSIONAL: Click here for more information on the type of diagnostic tests for Babesiosis. In addition, physicians may want to consider having blood tested for other possible causes of their patient’s symptoms, including other tick-borne infections, such as Lyme disease.

Can A Tick Be Tested?

At IGeneX, ticks can be tested for presence of Babesia by the Babesia PCR test. Any tick including one that has been removed from a patient or found in the yard, can be sent to IGeneX for testing.

Click here for more information on how to test a tick.