Theileria taurotragi infection

Previous Authors: J A LAWRENCE AND S M WILLIAMSON

Current Authors:
J A LAWRENCE - Extraordinary Professor, DPhil, BSc, MRCVS (ret.), DTVM, Department of Paraclinical Veterinary Science, University of Zimbabwe, Harare, Zimbabwe 
B J MANS - Principal Researcher, BSc, BSc (Hons) Biochemistry, MSc (Biochemistry), PhD (Biochemistry), Agricultural Research Council, Onderstepoort Veterinary Research, 100 Old Soutpan Road, Pretoria, Gauteng, 0110, South Africa

GENERAL INTRODUCTION: THEILERIOSES OF CATTLE

Introduction

Theileria taurotragi was first described by Martin and Brocklesby¹⁹ in 1960, under the name Cytauxzoon taurotragi, as a benign parasite of the eland (Taurotragus oryx). It was subsequently demonstrated that the parasite was readily transmissible to the ox, in which it caused a transient febrile reaction,³⁵ and it became clear that T. taurotragi was one of the benign theilerial parasites of cattle which had been known collectively as Theileria mutans since the beginning of the twentieth century.³¹ The disease it causes was first described by Neitz²² in 1957, when it was mistaken for a mild form of T. mutans infection transmitted by Rhipicephalus appendiculatus. The parasite is widely distributed through eastern and southern Africa, and in addition to the ox and the eland it will infect sheep and goats under experimental conditions but does not produce piroplasms.^{22, 33} It  also infects cell lines from a number of other species of Bovidae in vitro.³⁰ The main significance of Theileria taurotragi is the confusion that it causes in the differential diagnosis of mild reactions to Theileria parva, although it can occasionally be responsible for cases of cerebral theileriosis (Turning sickness). Theileria parva and T. taurotragi both utilise the same tick vector, Rhipicephalus appendiculatus, and antibodies to T. taurotragi cross-react in the T. parva immunofluorescent antibody test (IFAT) ¹⁷, which has complicated epidemiological studies on T. parva.

Aetiology and life cycle

The morphological and biological characteristics of T. taurotragi are generally similar to those of T. parva, but the ultrastruture of the merozoites in bovine brain is different ⁶. In the eland, bars and veils found in the erythrocytes are associated with the piroplasms, ³⁶ but these structures are not present in the ox. The parasite is transmitted by R. appendiculatus and Rhipicephalus evertsi evertsi, ³² Rhipicephalus pulchellus⁴ and Rhipicephalus zambeziensis.¹⁴ Recovered cattle remain carriers for at least 11 months.¹³ The schizonts stimulate transformation of host cells to a limited extent, but the target cells have not been identified. The schizonts can be cultured in vitro in lymphoblasts, as with T. parva.²⁹ Infection can be transmitted readily between cattle by intravenous inoculation of 100 ml of blood, which suggests that intra-erythrocytic division and invasion of new erythrocytes occurs.⁹

Epidemiology

Theileria taurotragi has been recorded from eastern, central and southern Africa and has been confirmed with molecular methods in Botswana ^{2, 21, 23}, Kenya, Mozambique ²⁰, Nigeria ¹⁶, Rwanda ¹, South Africa ³⁷, Sudan ²⁸, Tanzania ⁶, Uganda ^{24, 25}, Zambia ¹² and Zimbabwe ¹. This corresponds with the range of R. appendiculatus, but its other vector ticks are also found throughout this region ³⁴. Theileria parva also occurs in most of these areas, indicating an overlap in geographic distribution.  Reverse line blot analysis has allowed estimations of prevalence for T. taurotragi, which may be as high as 76% ²⁴. Older cattle generally have a higher prevalence, reflecting infection over time; indigenous cattle breeds seem to be more resistant to infection than exotic or cross-breeds; and cattle grazing inside wildlife reserves have a higher prevalence compared to cattle grazing outside, as a result of contact with eland, which show higher levels of infection than cattle ²⁴. Theileria taurotragi seems to be cattle and eland specific, although it has also been detected in bushbuck (Tragelaphus sylvaticus) in Uganda ²⁴. Extensive screening of African buffalo (Syncerus caffer) has not detected T. taurotragi to date ^{18, 24, 26}.  A single report of the detection of T. taurotragi in four African buffalo, used a multiplex real-time PCR, could not be confirmed with sequencing ⁷.  While mixed infections of T. parva and T. taurotragi in cattle are common where parasites are sympatric, this is probably due to maintenance of both parasites in cattle or the overlap in geographic distribution of African buffalo and eland ¹⁵.

Clinical signs

Infection of cattle with T. taurotragi results in a mild febrile reaction of 1 to 14 days’ duration commencing after an incubation period of 10 to 21 days.^{5, 13} Slight enlargement of superficial lymph nodes, especially the parotid lymph nodes draining the ears on which infected ticks have fed, is present, but there is no evidence of anorexia or other clinical signs. Toxicosis, resulting from heavy infestation with R. appendiculatus, exhibits similar signs and may occur with and overshadow the reaction to the Theileria. The parasite may occasionally cause nervous signs associated with cerebral infarction (Turning sickness).

Infection in sheep is benign. In eland the parasite sometimes causes severe or fatal disease.¹⁰

Pathogenesis and pathology

Theileria taurotragi stimulates lymphoid proliferation in the same way as T...