Theileria mutans infection

Previous authors: J A LAWRENCE

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

Introduction

Theileria mutans was first described as a benign parasite of the ox by Theiler,³² but its identity was inextricably confused with that of other benign species of Theileria for many years. It was assumed to be the only benign bovine Theileria in Africa until 1977, when it was demonstrated that Theileria taurotragi was also capable of causing a mild clinical reaction in cattle.⁴¹ Elsewhere in the world the benign Theileria of cattle is considered to be T. buffeli/orientalis.³⁴ Theileria mutans is transmitted by Amblyomma ticks and is now known to be confined to eastern, western and southern Africa and to the Caribbean Islands where it was introduced in cattle from Africa. The parasite also infects the African buffalo (Syncerus caffer), in which it was first described under the name Theileria barnetti.² Its only practical significance in southern Africa is the confusion that it causes in the differential diagnosis of Theileria parva (see below). In eastern Africa, pathogenic strains of the parasite occur, which may cause severe clinical illness and death.

Aetiology and life cycle

In African buffalo, extensive heterogeneity in the 18S rRNA gene of T. mutans is present in populations in South Africa ^{5, 6, 19}.  On the other hand, in cattle, micro-heterogeneity of up to 2 nucleotides was observed between T. mutans sequences in Uganda and South Africa ^{3, 19}.  Out of the five recognised genotypes of T. mutans, two (T. mutans, T. mutans MSD) infect cattle and buffalo, while the other three (T. mutans-like 1, T. mutans-like 2 and T. mutans-like 3) have so far been found only in African ^{4, 5, 19, 20}  In addition to their significantly higher prevalence, T. mutans-like 1 to 3 genotypes also occur at higher parasitaemias than T. mutans and T. mutans MSD ^{18, 20} suggested that they should perhaps be assigned to a separate species on the basis of host specificity.

Theileria mutans has a typical theilerial life cycle (see East Coast fever: Figure 29.2) but unlike T. parva, T. mutans undergoes limited lymphocytic merogony; the main mode of replication occurs in the erythrocytes, and this may cause high piroplasm parasitaemia ³⁰.

Theileria mutans is transmitted by several species of Amblyomma: A. astrion,³³ A. cohaerens,⁴⁰ A. gemma,²⁸ A. hebraeum,^{9, 16} A. lepidum ³⁷ and  A. variegatum.³⁵ Amblyomma hebraeum is the principal vector south of the central watershed of Zimbabwe, and A. variegatum is the principal vector to the north (see Vectors: Ticks). Infection occurs transstadially from larva to nymph, or nymph to adult. Recovered animals remain carriers indefinitely. The piroplasm is readily transmitted by blood inoculation.³²

The piroplasms of T. mutans are larger than those of T. parva and dividing forms may be seen during the acute phase of infection. The schizonts are also larger and the chromatin granules are more numerous and less uniform in size and shape than those of T. parva.^{2, 40} It has not yet been possible to culture T. mutans-infected lymphocytes in vitro.

Epidemiology

Theileria mutans is widespread throughout the range of its tick vectors in sub-Saharan Africa and on some Caribbean islands. The pathogen was confirmed using reverse line blot (RLB) hybridisation assay in cattle in Uganda ^{1, 3, 17, 23 25, 27}, Kenya ²⁴, Nigeria ¹⁷, and South Africa ¹⁹, and in African buffalo in Uganda ²⁶, Botswana ¹² and South Africa ^{7, 19}. The prevalence of T. mutans varies, depending on the geographical area and the different genotypes being detected; it can reach 100% in adult cattle, with most calves becoming infected by five to six months of age ^{1, 21}.  High prevalence of T. mutans in Uganda has been attributed to high tick infestation of cattle with A. lepidum and A. variegatum ³, and to the long carrier state at high levels after infection ¹ Amblyomma cohaerens, which is commonly found on African buffalo and cattle and develops high infection rates with T. mutans, is also likely to promote high levels of T. mutans transmission where buffalo and cattle come into contact.²

Fatal infections due to T. mutans have been described ¹⁴ often in situations where animals have been exposed to T. mutans infection after immunization against T. parva with a reduction in tick control.^{29, 31}.  It is probable that stress caused by poor nutrition, intercurrent disease or other factors predisposes cattle to anaemia when infected with T. mutans. On the other hand, a protective effect of benign T. mutans and T. velifera co-infections against T. parva