Ovine and caprine anaplasmosis

Ovine and caprine anaplasmosis

W H STOLTSZ

Introduction

Anaplasmosis of sheep and goats is an arthropod-borne disease caused by the intra-erythrocytic rickettsial organism, Anaplasma ovis. It is usually a subclinical or mild condition, but moderate to severe clinical disease is generally characterized by fever and a variable degree of anaemia and icterus that may occasionally lead to death. Recovered animals remain carriers of the organism, resulting in premunity, which persists for life.

Anaplasmosis in sheep was first reported in 1912 in Zimbabwe. 4 Although a number of reports of Anaplasma infections in sheep and goats in Africa soon followed,51, 52, 62 the report by Di Domizio in 19198 is considered to be the first authentic description of anaplasmosis in non-bovine hosts. In 1924 Sergent et al.49 reported an Anaplasma infection in sheep that was not transmissible to cattle, and Lestoquard22 differentiated the causative organism from the Anaplasma spp. of cattle, describing and naming it A. ovis.

Ovine and caprine anaplasmosis occurs in many parts of Africa, North and South America, Asia (including the Middle and Far East), the southern and central parts of eastern and western Europe, and the former USSR.2, 13, 19, 28, 32, 35, 43, 53, 54, 56, 66, 69 However, the disease may be more widely distributed than present data indicate.

The condition in southern Africa is of relatively little economic importance. Because of its often insidious nature, the deleterious effects caused by A. ovis infection, such as weight loss and unthriftiness, are probably erroneously ascribed to other causes. Although mortality due to anaplasmosis in sheep and goats is seldom observed in southern Africa, it would appear that under certain circumstances, overt disease, abortion and mortality may occur.1, 3, 35

Aetiology and life cycle

Anaplasma ovis is the main cause of ovine and caprine anaplasmosis. It is more pathogenic for goats than for sheep56, 63, 69 and only very rarely is it infective to cattle.20, 48

Anaplasma mesaeterum, the name proposed for a hitherto unrecognized anaplasm of sheep and goats,63 is closely related to A. ovis and may also cause clinical disease in sheep and goats, but, unlike A. ovis, it appears to be less pathogenic for goats than for sheep and is not infective for cattle.63 Anaplasma marginale, a species pathogenic for cattle, may also, under certain circumstances, cause latent infection in sheep and goats.9, 16, 24, 29, 34, 36

In Giemsa-stained blood smears A. ovis is morphologically indistinguishable from A. marginale,56, 57 and appears as irregularly shaped, almost spherical, intra-erythrocytic granules staining a deep purple colour. The organisms (inclusion bodies) vary in diameter from approximately 0,4 to 0,8 μm,22, 35, 57 with an average of 0,5 μm.35 Within erythrocytes, 60 to 70 per cent of the inclusion bodies are located marginally and 30 to 40 per cent submarginally or centrally.22, 34, 56, 57 In comparison, up to 90 per cent of the inclusion bodies of A. marginale are situated marginally.34 Differences between A. ovis and A. marginale with regard to the intra-erythrocytic location of organisms have been found to be statistically significant.54

The ultrastructure of the marginal bodies of A. ovis in sheep erythrocytes is very similar to that of A. marginale in bovine erythrocytes.15, 30, 44 Development of A. ovis in sheep and goat erythrocytes, and in the tick vector, is presumably similar to the cycle of development proposed for A. marginale. 16, 17, 45, 57 The inclusion bodies of A. mesaeterum are morphologically indistinguishable by both light and electron microscopy from A. ovis, but a distinctive feature is that fewer than 30 per cent of the inclusion bodies are situated marginally in erythrocytes.

Apart from the morphological similarities, complete serological cross-reactivity has been demonstrated between A. ovis and A. marginale.57 They are nevertheless immunologically distinct as infection with one does not provide immunity against the other.56 Anaplasma ovis and A. mesaeterum, on the other hand, appear to be immunologically more closely related, but cross-immunity is nevertheless incomplete, a situation somewhat reminiscent of the relationship between A. centrale and A. marginale in cattle.63

Epidemiology

Anaplasmosis in sheep and goats has been reported from most tropical and subtropical parts of the world.2, 19, 28, 35, 43, 53, 54, 56, 66, 69 With the exception of Namibia, A. ovis has been reported from all the countries in southern Africa and is believed to be prevalent in most sheep- and goat-farming areas. In these areas, small numbers of parasites may frequently be detected during routine microscopic examination of blood smears from sheep and goats. Considering that microscopic examination of blood smears and serological techniques fail to distinguish between A. ovis and A. marginale,20, 56 the fact that sheep and goats exposed to A. marginale may yield serologically positive results without being immune to A. ovis may be of epidemiologic significance. Similarly, calves infected with A. ovis have been noted to react positively to A. marginale antigens in the complement fixation and rapid card agglutination tests, and yet were fully susceptible to A. marginale infection.20 Thus far A. mesaeterum has only been recorded on the Dutch island, Ameland, but circumstantial evidence seems to...

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