Theileriosis of sheep and goats

Previous Authors: 

Current Authors:
J A LAWRENCE - Extraordinary Professor, DPhil, BSc, MRCVS (ret.), DTVM, Department of Paraclinical Veterinary Science, University of Zimbabwe, Harare, Zimbabwe 
C BYARUHANGA - Post-doc Fellow, BVM, MVPM, PhD, Department of Veterinary Tropical Diseases, Para-clinical Building, Faculty of Veterinary Science, University of Pretoria, 100 Old Soutpan Road, Onderstepoort, Pretoria, Gauteng, 0110, South Africa
M OOSTHUIZEN - Parasitology, PhD, Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, Private Bag X04, University of Pretoria, Pretoria, Gauteng, 0110, South Africa
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 is a genus comprising tick-transmitted parasitic protozoa in the family Theileriidae, order Piroplasmida, subclass Piroplasmia, phylum Apicomplexa. It includes parasites previously allocated to the genera Cytauxzoon, Gonderia and Haematoxenus. The taxonomy of the genus at species level on morphological and biological characteristics in the past has been confused; taxonomy is now based on sequences of the 18S rRNA genes. Three Theileria species infecting small ruminants have been shown to be responsible for heavy mortality. Two of them, T. uilenbergi and T. luwenshuni were first described in China, whereas T. lestoquardi causes malignant theileriosis which is a major problem in Sudan and the Middle East. Other small ruminant Theileria species are considered as non-pathogenic; their importance lies in the confusion they may cause in diagnosis. The name T. recondita was given in the past to a non-pathogenic parasite in Europe transmitted by Haemaphysalis spp. but is regarded as invalid ⁸;the parasite was probably T. luwenshuni.

Table 1: Theileria species affecting sheep and goats

The life cycle of the Theileria species of sheep and goats is similar to that of the bovine Theileria species (See East Coast fever). The parasite is transmitted to its vertebrate host by the bite of an infected tick. Sporozoites injected into the host invade mononuclear leucocytes, either lymphocytes or macrophages, depending on the species, where they develop into schizonts. Some species have the ability to transform the host cell so that it can proliferate and disperse throughout the body, others do not. The schizonts release merozoites into the circulation, where they can be seen as piroplasms in erythrocytes. Ticks ingest the organisms when they feed. The organism undergoes multiplication and a sexual cycle in the tick gut and moves to the salivary gland where it is ready to start the next cycle. The organisms are transmitted from stage to stage of the tick, not transovarially from one tick generation to the next.

Malignant Ovine Theileriosis

Aetiology and life cycle

Theileria lestoquardi (T. hirci) is the best known causal organism of malignant theileriosis of small ruminants and has a life cycle and morphology typical of the genus Theileria. Theileria lestoquardi schizonts infect macrophages and B lymphocytes ¹⁴. The parasite infects both sheep and goats. Antigenically it is closely related to T. annulata of cattle31, and it is occasionally detected by PCR in this species ⁵¹.The parasiteis transmitted from stage to stage through the ticks Hyalomma anatolicum anatolicum, H. impeltatum, H. excavatum, H. scupense (H. detritum), Rhipicephalus sanguineus and R. turanicus, and through vertical transmission ¹⁴.

Schizont-infected cells can be grown readily in suspension culture, in a manner similar to those of T. parva and T. annulata (see East Coast fever and Theileria annulata theileriosis) ²⁶ Cultured schizonts are of value in immunization and as antigen for indirect immunofluorescence testing.³¹

Epidemiology

Malignant theileriosis of sheep and goats caused by T. lestoquardi occurs in the Mediterranean basin, the Middle East, Sudan, western Asia and the Indian subcontinent ¹⁴, and infection has been identified in the absence of clinical disease in Tanzania ⁴⁹ and southern Tunisia ⁴⁶. The disease is widespread in some important sheep breeding areas and causes heavy losses in both indigenous and exotic breeds and their crosses, especially in Iran, Sudan and the Sultanate of...