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Theileria parva

I. General Information

1. NCBI Taxonomy ID:

2. Disease:

East Coast Fever

3. Introduction

Theileria parva, a tick transmitted protozoan parasite, causes a severe lymphoproliferative disease in cattle variously known as East Coast fever, January disease or corridor disease. The main vector of the parasite is a three-host tick Rhipicephalus appendiculatus. The disease is present in 10 countries in Eastern and Central Africa, however the vector is more widely distributed than the parasite and there is a potential danger of the disease spreading to other areas. Theileria parva is highly pathogenic to cattle and mortality in Bos taurus cattle and their crosses can approach 100%. The parasite is considered to be the major factor in inhibiting the introduction of highly productive taurine breeds of cattle in sub-Saharan Africa. Where such cattle are maintained successfully in Africa, it is through heavy cost to the farmer, involving the use of toxic and expensive chemicals to kill the vector. Estimated annual losses are around US$190 million. The most common form of the disease in cattle due to T. parva is East Coast fever (ECF). It is characterized by high mortality in exotic cattle and large numbers of schizonts and piroplasms. A mild form of the disease, known as January disease, occurs predominantly in Zimbabwe. This disease occurs seasonally, and the outbreaks, coinciding with the emergence from diapause of adult ticks in January, are usually characterized by high morbidity and low mortality with low numbers of parasites. The main vector of January disease is Rhipicephalus zambesiensis (Morzaria et al., 2000).

4. Microbial Pathogenesis

The sporozoite develops in the salivary glands of the vector and is introduced into the mammalian host during tick feeding. Sporozoites rapidly enter lymphocytes by a receptor-mediated process and differentiate into schizonts. After several cycles of multiplication, a proportion of schizonts undergo merogony to produce merozoites, which invade erythrocytes and develop into piroplasms, the infective stage for ticks. The disease is characterized by pyrexia, generalized lymphadenopathy, and leukopenia (Musoke et al., 1992).

5. Host Ranges and Animal Models

The African buffalo plays an important role in the epidemiology of the disease since it is a natural host that does not suffer from the disease but remains a constant source of infection for ticks. The disease that occurs in cattle after the introduction of T. parva from buffalo is referred to as corridor disease, and is characterized by high mortality and low parasitosis (Morzaria et al., 2000).

6. Host Protective Immunity

Cattle that recover from ECF develop solid, strain-specific immunity that lasts for several years. Experiments have shown that passive transfer of antibodies to the schizont and piroplasm stages from immune to naïve cattle is not protective. There is now strong evidence that protective immunity is mediated by the generation of MHC class I restricted cytotoxic T cells (CTLs) specific for schizont-infected lymphocytes (Morzaria et al., 2000).

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