Ovine babesiosis

Ovine babesiosis

I YERUHAM AND A HADANI

Introduction

Ovine babesiosis is a tick-borne disease caused by the intraerythrocytic protozoan parasites Babesia ovis9 and Babesia motasi.62 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 two 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.14 The presence of B. ovis in sheep in Madagascar86 is questionable.

Rhipicephalus bursa is widely distributed between the 31 and 45° parallels north, including the Mediterranean basin,38, 40, 48, 50, 79, 92 the Balkans,22, 61, 69 southern former USSR,66 Iraq74 and Iran.41 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 probably by Haemaphysalis spp. ticks. Babesia motasi was first reported in southern Africa in 1902,42 and subsequently ovine babesiosis was diagnosed in other countries in the African continent (Somalia, Nigeria, Madagascar). 18, 24, 47, 55, 76, 86 Since the report of Hutcheon and Robertson in 1902,42 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 Asia11, 51 and Africa.18 It is regarded as a benign parasite in western Europe,19, 49, 67, 87 but occasionally causes anaemia in lambs.15

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 Three other Babesia species have been reported to occur in sheep—B. sergenti,90 B. foliata73 and B. crassa35 (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.32, 84, 97 The angle between the paired pyriform parasites is obtuse. Cross-forms are uncommon93 (Figure 28.1). Babesia crassa, described in Iran,35 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.7, 87 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.26

Rhipicephalus bursa, a two-host tick,100 is the only known vector of B. ovis.97 Haemaphysalis punctata, a threehost tick, is the only proven vector of B. motasi in Europe7 while H. bispinosa and H. intermedia are the only proven ones in India.3, 51

Markov (cited by Erashov21 and Warnecke89) 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.87 Opasina68 suggested that Amblyomma variegatum, a three-host tick, is a vector of B. motasi in goats in Nigeria.

Babesia ovis and B. motasi develop only in erythrocytes and multiply by binary fission into merozoites (merogony).

Figure 28.1  Cross-form of Babesia ovis in a blood smear

Figure28.2  Kinetes of Babesia ovis in a smear of the haemolymph of a tick

The life cycle of B. ovis in the vector R. bursa has been thoroughly studied1, 12, 25, 27, 59, 60 and consists of gametogony (sexual cycle) in the gut of engorged female ticks, which results in the formation of ookinetes (zygotes). A primary schizogony (= sporogony) produces sporokinetes. A number of schizogonic cycles occur in the gut and various tissues. Schizonts have been observed in the haemolymph, eggs and salivary glands. During the final schizogony in the salivary glands, the infective forms (merozoites) are produced. Young kinetes of B. ovis in haemolymph and eggs of R. bursa are round in shape, but subsequently become elongated and cigar-shaped (Figures 28.1 and 28.2).

Transmission of B. ovis by R. bursa99 and of B. motasi by H. punctata is transovarial as well as transstadial.7, 12, 97 All instars of H. punctata are capable of transmitting B. motasi parasites.7

Adult R. bursa ticks become infected with B. ovis during a blood meal. Transmission to a susceptible host occurs by all instars of the succeeding generation.97 Rhipicephalus bursa can be infected with B. ovis both...

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