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Small ruminant babesiosis

Babesiosis is a disease complex caused by unicellular Babesia parasites, which are transmitted by ticks and invade and proliferate in the red blood cells of vertebrate hosts. They cause a febrile disease of domestic and wild animals characterised by extensive erythrocytic lysis leading to anaemia, icterus and haemoglobinuria, which can be fatal. In contrast to Theileria, Babesia parasites do not have a pre-erythrocytic stage in the vertebrate host. Babesia parasites are among the most widely distributed blood parasites and have a considerable economic, veterinary and medical impact worldwide. A number of species are gaining interest as emerging zoonosis in humans. For example, approximately 60 human cases mostly due to infection with B. divergens have been recorded in Europe [1].

Sheep grazing

A number of Babesia species capable of infecting small ruminants have been described, many of which are highly pathogenic for small ruminants:

•  B. ovis: This parasite is transmitted by Rhipicephalus ticks (R. bursa) and is highly pathogenic especially in sheep with mortality in susceptible hosts ranging from 30% and 50% in field infections [2].

•  B. motasi: This parasite appears to be of moderate virulence and the acute form of the disease is manifested by anorexia, fever, fast and audible heart beats, pallor of the mucous membranes, icterus and haemoglobinuria. Other features include abdominal pains, diarrhoea, prostration and death. The chronic form of the disease is manifested mainly by emaciation but coughing and oedema are also features. It is transmitted by Haemaphysalis ticks. B. motasi is recognised as pathogenic for small ruminants.

•  B. species (China): A number of unclassified Babesia parasites infecting small ruminants have recently been reported in China, which have temporarily been called Babesia spp (China). These parasites appear to be most closley related to B. motasi. However, in contrast to B. motasi, they are transmitted by the tick species Haemaphysalis longicornis, are highly pathogenic and infect goats as well as sheep.

Diagnosis

Clinical observation can support a provisional diagnosis of the disease. The demonstration of Babesia in thin or thick smears made from peripheral blood and stained with Giemsa is confirmatory. Indirect haemagglutination, complement fixation tests and ELISA have been established for a number of Babesia species [3]. PCR-based molecular diagnostic tools have also been established for the detection and differentiation of parasites DNA in blood samples of affected animals [4].

Babesia

Vaccination

Live attenuated vaccines for B. bovis and B. bigemina have been applied in many countries such as Australia, Argentina, South Africa, Brazil and Israel. It is postulated that attenuation through passage in culture results in enrichment of less virulent parasite populations or in down-regulation of virulence [5]. To date, no vaccine has been developed against small ruminant babesiosis. However, due to great similarities between the bovine pathogen B. bovis and B. ovis, the techniques developed for cultivation and attenuation of bovine babesiosis are likely to be applicable for establishing culture systems for Babesia of small ruminant and to achieve their attenuation.

Immune response

Both cellular and humoral immune responses are activated during Babesia infection. Thus, the resolution of acute B. bovis infection in naive animals is largely achieved by activation of the innate immune response, involving activation of macrophages via IFN-gamma , and resulting in killing of the organisms by phagocytosis and production of toxic macrophage metabolites, including nitric oxide (NO) [6]. On the other hand, the protection against clinical disease in persistently infected or vaccinated cattle relies upon the activation of memory and effector CD4+ T cells that secrete IFN-gamma [6]. Activated macrophages also secrete other products that include, TNF-alpha , IL-18 and IL-12, the latter activates natural killer (NK) cells to produce enhanced levels of IFN-gamma and activated effector function [6]. During Babesia infection, antibodies can reach high titres shortly after the infection and can neutralise sporozoites or merozoites at the extracellular stage [7]. The mechanisms of the immune response against small ruminant Babesia have not been adequately investigated. Due to the similarities between bovine and ovine Babesia species, particularly between B. bovis and B. ovis, it is possible that similar immune reactions are induced in sheep after an infection with B. ovis.

References

1. Meliani P, Khatibi S, Randazzo S, Gorenflot A, Marchou B. [Human babesiosis]. Med Mal Infect. 2006 36(10):499-504.

2. Hashemi-Fesharki R. Tick-borne diseases of sheep and goats and their related vectors in Iran. Parassitologia. 1997 39(2):115-7.

3. MA Bakheit, U Seitzer, PA Mbati, JS Ahmed. Serological Diagnostic Tools for the Major Tick-Borne Protozoan Diseases of Livestock. Parassitologia. 2007 49(Suppl. 1): 53-62

4. Criado-Fornelio A. A review of nucleic-acid-based diagnostic tests for Babesia and Theileria, with emphasis on bovine piroplasms. Parassitologia. 2007 49 Suppl 1:39-44.

5. Bock R, Jackson L, de Vos A, Jorgensen W. Babesiosis of cattle. Parasitology. 2004 129 Suppl:S247-69.

6. Brown WC, Norimine J, Goff WL, Suarez CE, McElwain TF. Prospects for recombinant vaccines against Babesia bovis and related parasites. Parasite Immunol. 2006 28(7):315-27.

7. Hines SA, McElwain TF, Buening GM, Palmer GH. Molecular characterization of Babesia bovis merozoite surface proteins bearing epitopes immunodominant in protected cattle. Mol Biochem Parasitol. 1989 37(1):1-9.