Babesiosis [ba-be-si-o-sis] and Theileriosis [thei-le-ri-o-sis] are caused by Babesia and Theileria species, which belong to the group known as piroplasms. These are pear-shaped organisms that parasitize the red blood cells of vertebrate hosts. They produce a malaria-like disease in livestock, pets and humans. Babesia species can be transmitted transovarially (from a female tick through the egg to the next generation), while Theileria species appear to only be transmitted transstadially (through each stage of the tick’s life cycle). In splenectomised individuals, both Babesiosis and Theileriosis can be fatal.

Babesia divergens is the cause of Red Water Fever in cattle. This disease gets its name from the colour of the animal’s urine, which becomes dark red due to haemoglobin produced by the rupture of red blood cells. Blood in the urine (haemoglobinuria) occurs in most species with Babesiosis. B. divergens has been implicated as the most common agent of human Babesiosis in Europe.

B. major, which also affects cattle, occurs in coastal regions of Britain (Lewis and Herbert 1980).

B. motasi is generally transmitted by Haemaphysalis ticks. It causes severe Babesiosis in sheep and was first identified in Britain from sheep in Wales.

Theileria species do present in Europe but it is generally in tropical regions that large-scale problems occur. T. ovis and T. recondite both occur in the UK (Lewis et al., 1981, and Alani and Herbert, 1988)