Babesiosis is a malaria-like parasitic disease caused by infection with Babesia, a genus of Apicomplexa. Human babesiosis is an uncommon but emerging disease in the Northeastern and Midwestern United States and parts of Europe, and sporadic throughout the rest of the world. It occurs in warm weather. 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, and they can have a major impact on health of domestic animals in areas without severe winters. In cattle, a major host, the disease is known as Texas cattle fever, redwater, or piroplasmosis. Babesia species are in the phylum Apicomplexa, which also has 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, related to babesia of dogs 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 K01 strain (an isolated case observed in South Korea, see isolated cases)

Half of all children and a quarter of previously healthy adults are asymptomatic with Babesia infection. For those who develop symptoms, they are similar to malaria, because both cause fever and hemolytic anemia. 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. Less common symptoms and physical exam findings of mild-to-moderate babesiosis: Headache Muscle pain Anorexia Nonproductive cough (mucus is not coughed up) Arthralgias (noninflammatory joint pain, unlike arthritis, which is inflammatory) Nausea Vomiting Sore throat Abdominal pain Pink eye Photophobia (abnormal intolerance to visual perception of light) Weight loss Emotional lability Depression Hyperesthesia (more sensitive to stimuli) Enlarged spleen Pharyngeal erythema Enlarged liver Jaundice (yellowing of the skin and of the sclera) Retinopathy with splinter hemorrhages Retinal infarcts Neutropenia 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. Severe cases occur mostly in people who have had a splenectomy. 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. Babesia parasites reproduce in red blood cells, where they can be seen as cross-shaped inclusions (four merozoites asexually budding, but attached together forming a structure looking like a Maltese cross)[7] and cause hemolytic anemia, quite similar to malaria. Unlike the Plasmodium parasites that cause malaria, Babesia species lack an exoerythrocytic phase, so the liver is usually not affected. In nonhuman animals, Babesia canis rossi, Babesia bigemina, and Babesia bovis cause particularly severe forms of the disease, including a 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 in color (icterus) due to the failure of the liver to metabolize the excess bilirubin. Hemoglobinuria is seen due to excretion of red-blood-cell lysis byproducts via the kidneys. Fever of 40.5°C (105°F) develops due to release of inflammatory byproducts. Veterinary treatment of babesiosis does not normally use antibiotics. In nonhuman animals, diminazen (Berenil), imidocarb, or trypan blue would be the drugs of choice for treatment of B. canis rossi (dogs in Africa), B. bovis, and B. bigemina (cattle in Southern Africa). In acute cases in cattle, blood transfusion may be carried out. A vaccine is effective against B. canis canis (dogs in the Mediterranean region), but is ineffective against B. c. rossi. B. imitans causes a mild form of the disease that frequently resolves without treatment (dogs in Southeast Asia).

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