Theileria annulata is a protozoan closely related to Babesia. In cattle, it causes the disease tropical theileriosis throughout a long arc of countries from Morocco across to China. Theileria parva is the causative microbe of East Coast fever of cattle in Eastern, Central, and Southern Africa. Theileria species infect monocytic white blood cells of their hosts. The infected cells are induced to divide by the Theileria, which then proliferates within each daughter cell, in a rapidly expanding infection. This causes multiple inflammatory crises, of which pulmonary edema is a predominant cause of death. Theileria annulata is transmitted by Hy. anatolicum, Hy. detritum, and other hyalommas. Theileria parva is transmitted predominantly by Rhipicephalus appendiculatus. Because this is a three-host feeding tick, the opportunities for transmission are from infected cow to feeding larva then through the molt into the nymph which feeds and transmits. Similarly, transmission can also be from feeding nymphs to infected adult. This is known as transstadial transmission, and no transovarian transmission occurs in this case. The development of Theileria in ticks includes sexual reproduction which enables generation of new variants that can evade the immune mechanisms of cattle.
