Ovine babesiosis

Previous authors: I YERUHAM AND A HADANI

Current authors:
D T DE WAAL - Associate Professor, BVSc, PhD, DipDatMet, HDipUTL, DipEVPC, MRCVS, School of Veterinary Medicine, University College Dublin, Room 034, UCD Veterinary Sciences Centre, Belfield, Dublin, D04 W6F6, Ireland

GENERAL INTRODUCTION: BABESIOSES

Introduction

Ovine babesiosis is a tick-borne disease caused by the intraerythrocytic protozoan parasites Babesia ovis¹⁰, Babesia motasi⁷⁰ and several uncharacterized Babesia spp.^{12, 33, 36, 59} The acute form of the disease is characterized by fever, progressive anaemia, icterus and bilirubinaemia. Haemoglobinuria is rare but may occur in the late stage of the disease. Pregnant animals may abort. Susceptible flocks may suffer a high rate of mortality. Recovered animals that are latently infected are usually immune for a certain period. There is no cross-immunity between the parasites.

Ovine babesiosis caused by B. ovis is of considerable economic importance in the southern part of the Palaearctic zone (Mediterranean basin, Balkans, Kazakhstan, Kirghizstan, Turkmenistan, Iraq and Iran) overlapping the distribution area of the vector tick, Rhipicephalus bursa.¹⁶ The presence of B. ovis in sheep in Madagascar⁹⁷ is questionable.

Rhipicephalus bursa is widely distributed between the 31 and 45° parallels north, including the Mediterranean basin,^{45, 47, 55, 58, 87, 110} the Balkans,^{24, 69, 75} southern former USSR,⁸¹ Iraq⁸⁵ and Iran.⁴⁸ It has not been found in the southern hemisphere.

The geographical distribution of B. motasi is very wide and includes western and northern Europe, the Mediterranean basin, Asia (Turkey, India, China) and Africa (Somalia, Nigeria, Madagascar and southern Africa) and is transmitted mainly by Haemaphysalis spp. ticks. Babesia motasi was first reported in southern Africa in 1902,⁴⁹ and subsequently ovine babesiosis was diagnosed in other countries in the African continent (Somalia, Nigeria, Madagascar).^{20, 26, 56, 63, 95, 97} Since the report of Hutcheon and Robertson in 1902,⁴⁹ there are no other records on the occurrence of ovine babesiosis in southern Africa. Babesia motasi causes a severe disease and mortality in sheep in Asia,^{13, 50} Africa²⁰ and China.¹² It is regarded as a benign parasite in western Europe,^{21, 57, 82, 98} but occasionally causes anaemia in lambs.¹⁷

Babesiosis caused by B. ovis and B. motasi affects all breeds of sheep and, more rarely, goats, with B. ovis being the more pathogenic. It appears that some isolates of B. motasi are infective for sheep and goats while others only infect sheep.⁶

More recently several large Babesia strains (Babesia sp. BQ1 (Lintan), Babesia

sp. BQ1 (Ningxian), Babesia sp. Tianzhu, Babesia sp. Hebei, Babesia sp. Madang, Babesia sp. Liaoning and Babesia sp. Xinjiang) have been isolated in China with variable degree of virulence to sheep and goats.³⁴ One isolate in particular Babesia sp. Xinjiang is morphologically very different from B. motasi and B. ovis.³³ Based on the 18S rRNA gene sequences these Babesia isolates can be broadly divided into two groups – Babesia motasi-like; and a second group, consisting of Babesia sp. Xinjiang,  that falls between B. ovis and B. orientalis.³³

Three other Babesia species have been reported to occur in sheep—B. sergenti,¹⁰² B. foliata⁸⁴ and B. crassa⁴² (in Algeria, India and Iran respectively). These have a restricted local importance and their validity is still debated.

Aetiology and life cycle

Babesia ovis is a small parasite of 1,0 to 2,5 μm in length. The majority of parasites are oval or pyriform, tending to occur at the periphery of the erythrocytes.^{40, 93, 106} The angle between the paired pyriform parasites is obtuse. Cross-forms are uncommon¹¹² (Figure 1). Babesia crassa, described in Iran,⁴² has a similar morphology.

Babesia motasi is a relatively large parasite measuring 2,2 to 3,8 μm in length by about 2,0 μm in width. It commonly appears as a pair of pear-shaped organisms, separated by an acute angle, occupying two-thirds or more of the affected erythrocyte.^{5, 98} Ring, oval, elongated and budding forms are less common. Babesia motasi often possesses a double nucleus, which is invariably single in B. ovis. The intraerythrocytic stages include single tropozoites, dividing forms and paired merozoites. The fine structure of the tropozoites and merozoites are apparently similar to those described for other species of Babesia.²⁸

Babesia sp. Xinjiang is also large approx. 2.42 (±0.35) µm in length and 1.06 (±0.22) µm wide and typically occurs in most cases as single or paired pirifom bodies at an acute angle within erythrocytes. A variety of other morphological forms has also been found, including ring, trifoliate, rod- shaped and oval.³³

Rhipicephalus bursa, a two-host tick,¹⁰⁷ is the only known vector of B. ovis.¹⁰⁶ Haemaphysalis punctata, a three-host tick, is the only proven vector of B. motasi in Europe⁵ while H. bispinosa and H. intermedia are the only proven ones in India.^{3, 50} In China the large Babesia isolates are transmitted by a number of three-host ticks including H. longicornis, Hyalomma anatolicum anatolicum  and H. qinghaiensis^{33, 34, 36}

Markov (cited by Erashov²³ and Warnecke¹⁰¹) reported on the transmission of B. motasi by Haemaphysalis otophila, a three-host tick. The possibility that R. bursa may be a vector of B. motasi is extremely doubtful.⁹⁸ Opasina⁷⁴ suggested that Amblyomma variegatum, a...