Babesiosis or piroplasmosis is a malaria-like vector-borne parasitic disease caused by infection with a eukaryotic parasite in the order Piroplasmida, typically a Babesia or Theileria, in the phylum Apicomplexa. Human babesiosis transmission via tick bite is most common in the Northeastern and Midwestern United States and parts of Europe, and sporadic throughout the rest of the world. It occurs in warm weather. People can get infected with Babesia parasites by the bite of an infected tick, by getting a blood transfusion from an infected donor of blood products, or by congenital transmission (an infected mother to her baby). Ticks transmit the human strain of babesiosis, so it often presents with other tick-borne illnesses such as Lyme disease. After trypanosomes, Babesia is thought to be the second-most common blood parasite of mammals. It can have major adverse effects on the health of domestic animals in areas without severe winters. In cattle, the disease is known as Texas cattle fever or redwater.
Signs and symptoms Half of all children and a quarter of previously healthy adults with Babesia infection are asymptomatic. When people do develop symptoms, the most common are fever and hemolytic anemia, symptoms that are similar to those of malaria. People with symptoms usually become ill 1 to 4 weeks after the bite, or 1 to 9 weeks after transfusion of contaminated blood products. A person infected with babesiosis gradually develops malaise and fatigue, followed by a fever. Hemolytic anemia, in which red blood cells are destroyed and removed from the blood, also develops. Chills, sweats, and thrombocytopenia are also common symptoms. Symptoms may last from several days to several months. Less common symptoms and physical exam findings of mild-to-moderate babesiosis:
In more severe cases, symptoms similar to malaria occur, with fevers up to 40.5 °C (105 °F), shaking chills, and severe anemia (hemolytic anemia). Organ failure may follow, including adult respiratory distress syndrome. Sepsis in people who have had a splenectomy can occur rapidly, consistent with overwhelming post-splenectomy infection. Severe cases are also more likely to occur in the very young, very old, and persons with immunodeficiency, such as HIV/AIDS patients. A reported increase in human babesiosis diagnoses in the 2000s is thought to be caused by more widespread testing and higher numbers of people with immunodeficiencies coming in contact with ticks, the disease vector. Little is known about the occurrence of Babesia species in malaria-endemic areas, where Babesia can easily be misdiagnosed as Plasmodium. Human patients with repeat babesiosis infection may exhibit premunity.
Cause Babesia species are in the phylum Apicomplexa (the same phylum as the protozoan parasites that cause malaria, toxoplasmosis, and cryptosporidiosis). Four clades of Babesia species infect humans. The main species in each clade are:
B. microti (< 3 μm) B. duncani B. divergens (cattle parasite seen mostly in Europe) and B. venatorum (roe deer parasite, formerly called EU1), most closely related to the large Babesia clade Large Babesia (> 3 μm) mostly infects ungulates, but also includes the K01 strain (an isolated case observed in South Korea)
Pathophysiology
Babesia parasites reproduce in red blood cells, where they can be seen as plus-shaped inclusions (four merozoites asexually budding, but attached forming a structure looking like a "Maltese cross") and cause hemolytic anemia, quite similar to malaria. Unlike the Plasmodium parasites that cause malaria, Babesia species rarely exhibit an exoerythrocytic phase with trophozoite forms. In nonhuman animals, Babesia canis rossi, Babesia bigemina, and Babesia bovis cause particularly severe forms of the disease, including severe haemolytic anaemia, with positive erythrocyte-in-saline-agglutination test indicating an immune-mediated component to the haemolysis. Common sequelae include haemoglobinuria "red-water", disseminated intravascular coagulation, and "cerebral babesiosis" caused by sludging of erythrocytes in cerebral capillaries. In bovine species, the organism causes hemolytic anemia, so an infected animal shows pale mucous membranes initially. As the levels of bilirubin (a byproduct of red blood cell lysis) continue to increase, the visible mucous membranes become yellow (icterus) due to the failure of the liver to metabolize the excess bilirubin. Hemoglobinuria is seen due to the excretion of red-blood-cell lysis byproducts via the kidneys. A fever of 40.5 °C (105 °F) develops due to the release of inflammatory byproducts.
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