Zimbabwe theileriosis

Zimbabwe theileriosis

Previous Authors: J A LAWRENCE, B D PERRY 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 
K P SIBEKO-MATJILA - Senior Lecturer, BSc, PhD, Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, Private Bag X04, University of Pretoria, Gauteng, 0110, South Africa 
B J MANS - Principal Researcher, BSc, BSc (Hons) Biochemistry, MSc (Biochemistry), PhD (Biochemistry), Agricultural Research Council, Onderstepoort Veterinary Research, Old Soutpan Road, Pretoria, Gauteng, 0110, South Africa

Introduction

Zimbabwe theileriosis is an acute, frequently fatal disease of cattle caused by Theileria parva infection which occurs in Zimbabwe (see East Coast fever: Figure 29.1). The disease was first recognized and distinguished from classical East Coast fever in 1936,9 although outbreaks may have occurred previously. It has been variously known as January disease, because of its seasonal occurrence, and Fortuna disease, after the farm on which it was first recognized. It is known officially in Zimbabwe as ‘theileriosis’. The disease was described by Neitz16 under the name Rhodesian malignant bovine gonderiosis, and attributed to a new species, Theileria (Gonderia) bovis, but the parasite was declared in a footnote to that paper to be synonymous with Theileria lawrencei, the causal organism of Corridor disease. The name T. parva bovis was subsequently proposed for the parasite by Lawrence10 to differentiate the disease which it causes from Corridor disease. The disease resembles East Coast fever but the seasonal occurrence of theileriosis in Zimbabwe and the fact that infections can be milder than in East Coast fever justifies the continued use of the terms January disease and Zimbabwe theileriosis. However, there is at present no good scientific reason to retain the subspecies classification, T. parva bovis, as the parasite causing the disease is indistinguishable from cattle-derived T. parva from elsewhere in eastern and southern Africa.17 It is worth noting that T. parva isolates have been produced in eastern Africa, which show either the lower virulence or parasitological features associated with some T. parva strains causing January disease.1, 18 The disease causes a significant number of deaths each year in Zimbabwe, necessitating the implementation of intensive dipping regulations to control its vector. An apparently identical syndrome in the Southern Province of Zambia is regarded as East Coast fever 14.

Aetiology and life cycle

Theileria parva strains causing January disease are indistinguishable from other T. parva strains in morphology and by serological and molecular methods.

The life cycle is identical (see East Coast fever: Figures 29.2 to 29.8) and transmission is principally by adults of Rhipicephalus appendiculatus. Transmission of the parasite by Rhipicephalus zambeziensis has been demonstrated experimentally.11 Several isolates of the parasite have been studied and differences in virulence and in cross-protective immunity have been demonstrated. Some isolates are relatively benign.4 However, apparent variations in virulence may reflect differences in infectivity of the parasite for ticks, which in turn depends on piroplasm parasitaemia in the animals on which they feed, resulting in differences in sporozoite dose, rather than inherent differences in pathogenicity 5, 6;  the severity of disease is parasite dose-dependent. The parasite remains stable in regard to its virulence, even after rapid repeated passage by ticks through cattle.19

Epidemiology

Zimbabwe theileriosis occurs sporadically on the highveld of Zimbabwe (see East Coast fever: Figure 29.1) usually during the period December to May. The seasonal occurrence of clinical disease coincides with the seasonal distribution of adult R. appendiculatus.13 However, occasional outbreaks do occur during the winter months, coinciding with peaks in nymphal activity, and transmission of mild or subclinical infection has been demonstrated during this period 3, 6, 7. Animals that recover act as carriers of infection for at least 18 months 6.  The conclusions from studies on distribution, prevalence and seasonal transmission of infection carried out in the 1990’s in Zimbabwe and Zambia using immunofluorescent antibody techniques may be compromised by the cross-reaction that occurs with Theileria taurotragi 12, which is widespread in the region.

Most cases occur between January and March and the disease usually affects cattle over the age of one year; it is rarely seen in calves. Primary outbreaks usually occur in herds with a moderate to heavy tick burden and are often associated with new additions to the herd. The disease may appear in the resident cattle (suggesting initiation of infection by introduced carriers) or in the introduced cattle (suggesting introduction of susceptible animals into an endemically stable environment). Morbidity in an outbreak is usually less than 10 per cent. The disease is likely to recur each year on an infected property unless good tick control is achieved, but it shows little tendency to spread to neighbouring properties.

Clinical signs

Zimbabwe theileriosis exhibits the same clinical features as East Coast fever except that the course is...

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