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The different species of Babesia vary in their pathogenicity and distribution but they share many similarities, so we shall discuss only one of them, Babesia bigemina, in detail. This species was once a major cause of disease in North America, where it caused ‘Texas fever’ in infected cattle. Following successful eradication campaigns, B. bigemina is of much less economic importance in the USA today, but it remains a serious problem in Central and South America, North and South Africa, Australia, and Asia. It is principally a disease of cattle, although it also infects water buffalo, zebu, and deer. Ticks belonging to the genus Rhipicephalus are responsible for most of the transmissions, and these have a widespread distribution in tropical and subtropical countries.

Cattle normally acquire their infection from others that have recovered from the disease but continue to harbour a subclinical infection. The pathology associated with B. bigemina is unusual in that adult cattle tend to be more severely affected than the young are. This is particularly the case in cattle not previously exposed to the parasite or the local strain – for example those transported to a new region or country or following exposure to infected ticks newly introduced into the area. High‐performance milking cows imported into Africa from temperate regions are at particular risk of succumbing to severe disease symptoms. In Mexico, white‐tailed deer (Odocoileus virginianus texanus) have high seroprevalence for both B. bigemina and Babesia bovis but how important these (and other wildlife) might be as reservoirs of infection is uncertain.

A host of variables, including the age and immune status of the host (e.g., previous exposure and vaccination status) and the strain of the parasite influence the pathogenicity. The damage is primarily associated with the loss of function and destruction of the red blood cells. Lysing of infected erythrocytes releases haemoglobin into the blood stream. The destruction of small numbers of red blood cells has little effect and many infections are sub‐clinical. However, the rapid destruction of numerous cells in a short period can overload the body’s ability to remove the waste material. Consequently, haemoglobin and its breakdown products accumulate resulting in jaundice and their appearance in the urine – and hence the common name for the infection of ‘red water fever’ – or haemoglobinuria. The loss of functioning red blood cells also gives rise to severe haemolytic anaemia. The infected animal develops a fever and cerebral involvement is possible. The precise mechanism by which Babesia induces brain pathology is uncertain although Schetters and Eling (1999) suggest that it might provide a useful animal model for human cerebral malaria.

Human cases of babesiosis are uncommon, and severe disease is usually associated with splenectomised patients or those already suffering from immunodeficiency or AIDS. The importance of retaining the spleen wherever possible means that far fewer splenectomies take place than was once the case. Surgeons perform the operation for a variety of reasons ranging from injury following a car crash to cancer.