Babesia are single-celled parasites that infect the red blood cells of vertebrates and cause the disease called babesiosis. They are transmitted to humans through tick bites.

Babesia is widespread throughout the world. The geographic distribution of the tick Ixodes ricinus (common wood tick, the most common tick species in Europe) suggests that babesiosis could occur in humans in any part of Europe.

Known hosts are cattle, deer and rodents

Ticks can pick up babesia from an infected host (e.g. rodent) during the blood meal and transmit it to humans when bitten. The most common vector in Europe is the common wood tick(Ixodes ricinus). The wood tick (Dermacentor reticulatus) is a vector of Babesia canis canis, the causative agent of canine babesiosis ("canine malaria").

In humans, the disease is rarely diagnosed. Infection is often clinically inconspicuous, i.e. without symptoms. The disease is more severe in immunocompromised individuals, patients who have had their spleen removed, and the elderly. When symptoms do occur, they are similar in some respects to malaria: fever, anemia, and jaundice may occur. In acute cases, coffee-brown to red-colored urine is common. Severe cases that damage the kidneys and liver can also lead to death.

Treatment depends on the severity of the symptoms. Babesiosis infection in humans is usually treated with antibiotics (clindamycin) and quinine.

A vaccine is currently not available. Essential for prevention is the avoidance of tick bites or the rapid removal of ticks from the body. This allows early detection, removal and thus minimizes the risk of transmission of the bacteria.

In Austria, Babesia divergens, Babesia venatorum and Babesia microti have been detected in ticks so far. Babesiosis cases in humans with a source of infection in Austria have also been described. Most reported cases of babesiosis in Europe are due to B. divergens and occur in immunocompromised individuals, as well as in individuals who have had their spleen removed (the spleen plays an essential role in the immune system, especially in defending against germs in the blood).

The following criteria, are associated with high morbidity and mortality: Parasitemia >10%; hemoglobin <10 g/dL; previous splenectomy; adult respiratory distress syndrome; and acute renal failure.

Typical treatment of human babesiosis in Europe is based on clindamycin and quinine in conjunction with blood transfusion. Clindamycin can sometimes be used alone, and exchange transfusion can be avoided in mild cases. Atovaquone in combination with azithromycin is also effective.

Infections caused by B. divergens are usually more severe (often fatal if not treated appropriately) than infections caused by B. microti, in which clinical recovery usually occurs.