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Current methods for the control of the cattle tick Boophilus microplus and the agent of bovine babesiosis, Babesia bovis are unsatisfactory. Effective immunological control of both parasites would have great advantages. However, naturally acquired immunity to the tick is generally unable to prevent serious production losses. A vaccine against the tick, based on a novel form of immunization, is being developed. A protective antigen has been isolated from the tick, characterized and produced as an effective, recombinant protein. A vaccine incorporating this antigen is currently undergoing field trials. In the Australian situation, improved tick control will probably increase endemic instability with respect to B. bovis. Fortunately, a trivalent, recombinant B. bovis vaccine has also been developed. This too is now undergoing pre-registration field trials.

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Current methods for the control of the cattle tick Boophils microplus and the agent of bovine babesiosis, Babesia bovis are unsatisfactory. Effective immunological control of both parasites would have great advantages. However, naturally acquired immunity to the tick is generally unable to prevent serious production losses. A vaccine against the tick, based on a novel form of immunization, is being developed. A protective antigen has been isolated from the tick, characterized and produced as an effective, recombinant protein. A vaccine incorporating this antigen is currently undergoing field trials. In the Australian situation, improved tick control will probably increase endemic instability with respect to B. bovis. Fortunately, a trivalent, recombinant B. bovis vaccine has also been developed. This too is now undergoing pre-registration field trials

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The plasma of cattle recovering from severe babesia bovis infection contained cold precipitable protein which consisted of immune complexes formed in antibody excess. The major immunoglobulin in the complexes was IgM although IgG1 and IgG2 were also present at lower concentrations. In addition, fibrinogen, alpha 2 macroglobulin and C3 component of complement were detected. As the complexes were produced after parasiteamia fell below the detectable level and the inflammatory response to B. bovis was waning, the complexes did not appear to have much pathological significance.

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Soluble antigen which protected susceptible cattle against challenge with Babesia bovis was extracted from B. bovis-infected erythrocytes by sonic disintegration and separation of the soluble from the insoluble matter by ultracentrifugation. The material was then fractionated by the precipitation of fibrinogen-like proteins. The precipitate contained the babesial antigens that were located on the stroma of the infected erythrocytes. Antigen originally located on the parasite remained in solution. Both fractions conferred protection on splenectomized calves against challenge with B. bovis. However, the fraction containing the parasite antigens appeared to have more potential for development as a killed vaccine because it was not heavily contaminated with antigenic material from bovine erythrocytes.

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Preliminary studies have demonstrated changes to the lipid metabolism of cattle acutely infected with Babesia bovis. Total lipid, total cholesterol, and phospholipids decreased significantly during infection. Associated with this was a decrease in concentration of serum lipoproteins and a loss of their electrophoretic heterogeneity. These changes are discussed in relation to the pathophysiological changes during infection.

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The proteins in plasma and serum from cattle infected with Babesia bovis that react with protamine sulphate and ethanol have been characterized. Their sizes and chain structures suggest they are intermediates in the conversion of fibrinogen to cross-linked fibrin. In addition, comparison with reference proteins and in vitro systems strongly indicate the proteins are, in the main, the products of thrombin activation in vivo and not those of fibrinolysis.

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Fibrinogen and fibrinogen-like proteins (FLP) were isolated from plasma and serum of cattle acutely infected with Babesia bovis. The sizes and chain structures of these proteins were examined and clotting assays performed. The results indicated that the blood was in a hypercoagulable state due mainly to enhanced production of hydrogen bonded fibrin and offset partly by slight inhibition of chain cross-linking. The latter appeared due to a Factor XIII inhibitor. Reduction of A alpha chains of plasma FLP was not evident, nor could lower molecular weight remnants be regularly detected strongly suggesting that fibrin(ogen) lysis rarely occurred. Similarly the size and chain structure of the majority of noncoagulable FLP of serum was consistent with their being the product of coagulation and not fibrinolysis. Only in heavily infected splenectomized cattle were products from lysed cross-linked fibrin detected and these constituted only about 3% of total serum FLP.

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A cryofibrinogen complex, found in the plasma of cattle acutely infected with Babesia bovis (argentina), was characterized. The fibrinogen-like proteins of the complex were isolated and the structure of their polypeptide chains analysed. In general, the chain structure was similar to that of soluble non-crosslinked fibrin (fibrinogen) although chains indicating some degree of fibrin crosslinking were often detected. Only rarely did the chain structure suggest that fibrinolysis occurred. It was concluded that the complex was produced by activation of the coagulation system but that fibrinolysis did not occur to any marked degree. The complex was implicated in assistance to the sludging of erythrocytes in the internal organs which is a feature of the pathogenesis of the disease.

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1. Kinin levels began rising on day 3 after infection of cattle with Babesia bovis (= B. argentina) and attained a maximum value of 98% above preinfection levels by day 7. 2. Kininogen levels began falling on day 3 and reached minimum levels of 83% below preinfection levels on day 8. 3. Changes in both kinin and kininogen levels on day 3 coincided with the detection of low levels of parasites, and with a fall in packed cell volume. 4. Plasma kininase levels rose significantly 6 to 9 days after infection. Preparations of lysed and sonicated uninfected and infected red cells contained kininase activity, the respective red cell preparations being 23.9 and 11.4 times more active per mg protein than uninfected red cell preparations. The effect of pH, and the inhibitors disodium edetate, 1,10 phenanthroline and aprotinin on normal and infected plasma and on the various red cell preparations suggested that the rise in plasma kininase levels during infection was probably at least partly due to parasite products. 5. These results are discussed in relation to previous data showing that both kallikrein activation and the onset of hypotension also occur on or about day 3.

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The results of this paper indicate that cattle infected with B. bovis (argentina) have a markedly altered and activated coagulation system. A degree of thrombin activation occurs due partly to release of thromboplastin-like substances from lysed erythrocytes but due primarily to activation of kallikrein by babesial proteases. This produces a hyperfibrinogenaemia, particularly in intact cattle, with soluble fibrin complexes constituting up to one-third of the total fibrinogen concentration. High molecular weight non-coagulable fibrinogen-like proteins are detected terminally but more so in splenectomized cattle. Plasminogen concentration decreases in splenectomized but not intact cattle while low molecular weight fibrinogen degradation products are not easily detected. It is suggested that a hypercoagulable intermediate state with little or no fibrin deposition occurs rather than terminal disseminated intravascular coagulation.

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Three groups of four splenectomised calves, three months old, were infected with Babesia argentina parasites. One group was inoculated intravenously with 2000 kallikrein inhibitor units (kiu)/kg body weight of the broad-spectrum proteinase inhibitor, Trasylol, three times a day for 5 days commencing on the 3rd day after infection (early treatment). The same dose of Trasylol was given to a second group for 3 days commencing 8 days after infection (late treatment). The third group was untreated. Parasite multiplication rates were similar in the three groups. In the early treated group levels of activated kallikrein in plasma were significantly lower than those in the other two groups. The early treatment group also showed significantly higher levels of plasma kallikrein inhibitor. No significant differences in total plasma kallikrein levels were seen among the three groups. Packed cell volumes in the treated groups remained significantly higher than those in the controls. The implications of these findings are discussed.

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