East Coast Fever _ Theileria parva

Introduction

Bovine theileriosis is a tick-borne protozoal disease caused by six species of Theileria. Two of these species, Theileria parva and Theileria annulata, are pathogenic and responsible for significant economic and production losses in affected ungulates. T. parva, the causative agent of East Coast fever, occurs in Africa between southern Sudan, South Africa, and Zaire. T. parva can infect cattle, Indian water buffalo, African buffalo and waterbucks, however cattle and Indian water buffalo are the only species that develop symptomatic infections. Those breeds not native to areas where East Coast fever occurs are particularly susceptible.

Life Cycle and Transmission

The primary vector for East Coast fever is Rhipicephalus appendiculatus but R. zembeziensis and R. duttoni also are capable of transmitting the disease. Protozoal sporozoites are produced in the salivary glands of nymph or adult ticks, and subsequently inoculated into a susceptible animal during feeding. Typically, a tick must be attached for 48-72 hours before it transmits the sporozoites. However, if environmental temperatures are high, the organism can be transmitted in only a few hours. Once inside the host, the organisms target lymphocytes. Schizonts may be found in regional lymph nodes within 5-8 days after inoculation. The schizont, also referred to as a macroschizont or Koch’s blue body, causes blast-transformation of the infected lymphocyte (Figure 1) which produces two daughter cells, each of which contains a schizont. Every 3 days, the number of schizonts increases ten-fold. Approximately 10-25 days post-infection (average 14 days), schizonts develop into merozoites which are also known as microschizonts. These merozoites are initially found in the cytoplasm of lymphocytes, reticular cells, and macrophages. They subsequently invade erythrocytes where they become piroplasms. The piroplasm-containing erythrocytes are ingested by larval or nymphal ticks during feeding. Once inside the gut of the tick, the protozoa undergo a sexual cycle which results in the formation of a motile stage of the organism which allows it to reach the salivary glands of the tick so the entire life cycle can begin