Heterologous strain immunity in bovine babesiosis using a culture-derived soluble Babesia bovis immunogen

A vaccine for human babesiosis: prospects and feasibility

Babesiosis is a tick-borne disease caused by intraerythrocytic Babesia parasites. It is a well-known illness in companion animals and livestock, resulting in substantial economic losses in the cattle industry. Babesiosis is also recognized as an emerging zoonosis of humans in many countries worldwide. There is no vaccine against human babesiosis. Currently, preventive measures are focused on vector avoidance. Although not always effective, treatment includes antimicrobial therapy and exchange transfusion. In this review, we discuss the host's immune response to the parasite, vaccines being used to prevent babesiosis in animals, and lessons from malaria vaccine development efforts to inform the development of a human babesiosis vaccine. An effective human vaccine would be a significant advance towards curtailing this rapidly emerging disease.

Babesiosis Vaccines: Lessons Learned, Challenges Ahead, and Future Glimpses

The incidence and prevalence of babesiosis in animals and humans is increasing, yet prevention, control, or treatment measures remain limited and ineffective. Despite a growing body of new knowledge of the biology, pathogenicity, and virulence of Babesia parasites, there is still no well-defined, adequately effective and easily deployable vaccine. While numerous published studies suggest that the development of such anti-Babesia vaccines should be feasible, many others identify significant challenges that need to be overcome in order to succeed. Here, we review historic and recent attempts in babesiosis vaccine discovery to avoid past pitfalls, learn new lessons, and provide a roadmap to guide the development of next-generation babesiosis vaccines.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

Protective efficacy of Babesia gibsoni culture-derived exoantigens against the challenge infection in dogs

The aim of this study is to determine the efficacy of exoantigens derived from Babesia gibsoni cultures to induce protective immunity against challenge exposure of virulent organisms. An attenuated B. gibsoni Oita strain was maintained in vitro by the microaerophilus stationary phase (MASP) method, and exoantigens-containing supernatant fluids were collected for preparation of the immunization. Two dogs received three subcutaneous immunizations with a 20-day interval of B. gibsoni exoantigens plus 0.5 mg saponin (Quil A). On day 68 after the prime immunization, the immunized dogs and control dogs were challenged intravenously with 2 × 10(8) virulent parasites of a homologous B. gibsoni strain. The results showed that exoantigens could induce a high degree of protection against virulent homologous challenge exposure. Two dogs immunized with exoantigens showed a lower parasitemia, accompanied by a slight decrease in the PCV that returned to normal values. Control dogs developed typical acute clinical signs, including severe anemia and hyperthermia. The immunization elicited humoral immune responses. In dogs immunized with exoantigens, the maximum antibody titer was 2,560 and 5,120 by indirect fluorescent antibody test (IFAT), respectively. Preliminary Western blot analysis of the immunogen revealed five dominant proteins of molecular weights of 18, 37, 43, 50, and 57 kDa. These results suggested that the culture-derived exoantigens were candidates for non-viable vaccine.

Hydrophobic moeties in recombinant proteins are crucial to generate efficient saponin-based vaccine against Apicomplexan Babesia divergens

Throughout Europe, bovine babesiosis is mainly caused by Babesia divergens, an Apicomplexan parasite transmitted by tick bites. The intra-erythrocytic development of B. divergens merozoites leads to haemolytic anaemia, and bovine babesiosis is responsible for economic losses in the agro-business industry. A totally efficient recombinant vaccine based on the merozoite surface protein Bd37 and saponin QuilA was recently described. In the present study we determined that protective immunity elicited by the Bd37 recombinant protein was related to the presence of hydrophobic residues in the protein. Using polymeric fusion of Bd37 as well as cell-free in vitro protein expression, we successfully expressed recombinant proteins containing hydrophobic sequences without the need of GST fusion. We used different hydrophobic sequences and different recombinant Bd37 proteins to demonstrate that antigen hydrophobicity affects the immune system, turning an inefficient protein into a 100% protective vaccine. Some hypotheses about the hydrophobic effect and its potential application to other parasitic protozoa vaccine are also discussed.

Vaccines against babesiosis using soluble parasite antigens

Babesiosis in cattle and dogs causes severe economical and emotional loss. Although effective chemotherapeutic treatment of infected animals is available, the prevention of babesiosis by vaccination would be preferable. Attenuated parasite lines of Babesia bovis have been used successfully to control tropical babesiosis in cattle. However, among other drawbacks associated with live vaccines, such vaccines bear the risk of variable infectivity and morbidity requiring veterinary surveillance. Soluble parasite antigens derived from different Babesia species have proven to induce immune responses that do not necessarily affect the parasite, per se, but reduce the manifestations of clinical disease upon infection. In this review, Theo Schetters and Sonia Montenegro-James present an overview of the results obtained with vaccines based on soluble parasite antigens and their characterization, and discuss the possible immune effector mechanisms of such vaccines.

Erythrocyte invasion by Babesia bovis merozoites is inhibited by polyclonal antisera directed against peptides derived from a homologue of Plasmodium falciparum apical membrane antigen 1

Apical membrane antigen 1 (AMA-1) is a micronemal protein secreted to the surface of merozoites of Plasmodium species and Toxoplasma gondii tachyzoites in order to fulfill an essential but noncharacterized function in host cell invasion. Here we describe cloning and characterization of a Babesia bovis AMA-1 homologue designated BbAMA-1. The overall level of similarity of BbAMA-1 to P. falciparum AMA-1 was low (18%), but characteristic features like a transmembrane domain near the C terminus, a predicted short cytoplasmic C-terminal sequence with conserved sequence properties, and an extracellular domain containing 14 conserved cysteine residues putatively involved in disulfide bridge formation are typical of AMA-1. Rabbit polyclonal antisera were raised against three synthetic peptides derived from the N-terminal region and domains II and III of the putative extracellular domain and were shown to recognize specifically recombinant BbAMA-1 expressed in Escherichia coli. Immunofluorescence microscopy showed that there was labeling of the apical half of merozoites with these antisera. Preincubation of free merozoites with all three antisera reduced the efficiency of invasion of erythrocytes by a maximum of 65%. Antisera raised against the N-terminal peptide detected a 82-kDa protein on Western blots and a 69-kDa protein in the supernatant that was harvested after in vitro invasion, suggesting that proteolytic processing and secretion take place during or shortly after invasion. A combination of two-dimensional Western blotting and metabolic labeling allowing direct identification of spots reacting with the BbAMA-1 peptide antisera together with the very low silver staining intensity of these spots indicated that very low levels of BbAMA-1 are present in Babesia merozoites.

Association between sequence polymorphism in an epitope of Babesia divergens Bd37 exoantigen and protection induced by passive transfer

In Europe, Babesia divergens is the major agent responsible for babesiosis in cattle and can occasionally infect splenectomised humans. Recently, we reported the characterisation of a 37 kDa exoantigen (Bd37) anchored in the merozoite membrane of B. divergens by a glycosylphosphatidyl-inositol. After phospholipase hydrolyse of the glycosylphosphatidyl-inositol anchor, the Bd37 antigen could be isolated in the plasma of the infected host and from the in vitro culture supernatants. Immunisation of mice with a gel-filtration protective fraction of B. divergens exoantigens, produced a monoclonal antibody (MAb), called F4.2F8-INT, directed against Bd37. In the present study, we report data on passive protection using MAb F4.2F8-INT. This MAb was able to completely protect against virulent challenges with B. divergens isolates Rouen 1987 (Rouen87) and Weybridge 8843 (W8843) but had no protective effect against another French isolate from Massif Central (6303E). Physical characterisation of the epitope recognised by F4.2F8-INT allowed us to explain the differences observed between these isolates by western blotting and passive protection. These results suggest that the antigen carrying this epitope could be used as a target in the development of a recombinant vaccine against B. divergens babesiosis.

Culture-derived parasites in vaccination of cattle against tick-borne diseases

The major economically important tick-borne diseases of cattle are theileriosis, babesiosis, anaplasmosis, and cowdriosis. Culture-derived attenuated schizonts of Theileria annulata have proved to be safe for all types of cattle and they protect against tick-borne theileriosis. T. parva was also successfully grown in vitro; however, inoculation of cattle with allogeneic schizont-infected cells resulted in rejection and destruction of the parasites together with the host cells. The number of schizont-infected cells needed for immunization is greater than for T. annulata theileriosis. Culture-propagated Babesia bovis and B. bigemina were used for large scale vaccination in the field. An avirulent population of Babesia spp. was obtained by in vitro cloning; inoculation of cattle did not induce clinical babesiosis, but produced specific antibodies. Culture-derived exoantigens of Babesia spp. proved to be completely safe for cattle, however, they conferred less protection than live parasites. Cell-cultured Cowdria ruminantium was highly infective for susceptible animals but, attenuated in vitro, could offer a potential source for vaccination. Anaplasma marginale, successfully grown in tick cell culture, may be developed for vaccines. Factors that should be considered in the developing of vaccines against tick-borne diseases include: the protective immune response to the pathogenic parasite developmental stages, virulence, immunological strain differences, and antigenic variations in cattle and in culture.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Immunochemical characterization of in vitro culture-derived antigens of Babesia bovis and Babesia bigemina

Cross-reactivity between Babesia bovis and B. bigemina becomes a problem in discrimination of the two infections in endemic areas where the two species usually occur in association. With the aim of identifying candidate proteins for use as specific diagnostic tools, culture-derived components of three geographically different stocks of B. bovis (Lismore, Kwanyanga and Mexico) and one of B. bigemina (Mexico) were analyzed by immunoprecipitation using acrylamide gel electrophoresis. The approach taken was based on the analysis of 35S-methionine-labelled parasite antigens released into culture supernatant. A variety of serum samples were tested, including a panel of calf sera experimentally produced against the different stocks of Babesia, serum samples from cattle naturally infected in the field in Brazil, and a panel of anti-B. bovis monoclonal antibodies, previously characterized by the indirect fluorescent antibody test, ELISA and Western immuno-blotting. Approximately 28 and 23 bands (with molecular weights ranging from 200 to 14 kDa) were detected in total protein profiles of B. bovis and B. bigemina culture supernatants, respectively, whereas no bands were seen in the uninfected red blood cell culture supernatant (negative control). The immunoprecipitation analysis showed antigenic diversity amongst the stocks of B. bovis and resulted in identification of at least five B. bovis specific antigens common to the three stocks (molecular weights of 80, 72, 58, 38 and 24 kDa) and four B. bigemina specific antigens (molecular weights of 240, 112, 50 and 29 kDa).

Babesia bovis: biosynthesis and localisation of 12D3 antigen in bovine erythrocytes

The 12D3 antigen of Babesia bovis was found to be synthesised rapidly in cultured parasites, and localised to both the apical complex of the merozoite and the cytoplasm of the parasitised erythrocyte. Amino-terminal sequencing suggested that the nascent protein had been processed and differences between the predicted and measured molecular weights suggested post-translational modification. The major proportion of 12D3 appeared in the soluble compartment of the parasitised erythrocytes with a molecular weight consistent with no further processing. A significant proportion of the protein required extraction by sodium carbonate, suggesting association with membranous components. The timing of release of soluble 12D3 was coincident with haemoglobin release and this probably reflects a non-specific lysis of the erythrocyte. Synthesis of recombinant BV12D3 was achieved in baculovirus-infected SF9 insect epithelial cells. The product was of the same molecular weight as the native 12D3 and polyclonal antibodies raised against the recombinant protein reacted with both the recombinant and native forms of the antigen.

Exoantigens of an attenuated strain of Babesia bovis used as a vaccine against bovine babesiosis

Bovine babesiosis caused by Babesia bovis remains a significant constraint to beef and milk cattle production throughout the world. Exoantigens released by the parasites in culture supernatants are a potential source of antigen to induce protective immunity. An attenuated strain of B. bovis from Brazil, catalogued as BbUFV1, was maintained in vitro by the MASP method, and exoantigen-containing supernatant fluids were collected daily to form a pool representing a 72-h culture cycle for preparation of the vaccine. Exoantigen concentration was estimated using a two-site EIA. Three groups of susceptible non-splenectomised male Bos taurus cattle, 14 months old, were used. Group A (vaccinated) received two subcutaneous immunizations with a 21-day interval of B. bovis supernatant, content 6500 EIA units of exoantigens plus 1.5 mg saponin, and Group B (adjuvant control) received two injections of adjuvant alone. Four weeks after the second immunization, Groups A, B and C (control) were challenged intravenously with 10(8) virulent parasites of a heterologous B. bovis strain. The results showed that exoantigens present in in vitro cultures can induce a high degree of protection against virulent heterologous challenge exposure. In Group A only one animal showed discrete parasitaemia; all developed a fever and slight decreases in PCV, with a rapid return to normal values. One animal of Group B died; the survivors showed fever, anaemia and parasitaemia. All animals of Group C died between 7 and 13 days after challenge. Vaccination elicited both humoral and cell-mediated immune responses. In Group A, after the challenge, the maximum antibody titer was 12,800. When vaccinated, cattle were tested at the moment of challenge for B. bovis-specific cell-mediated immunity by the monocytemigration inhibition test. A mean inhibition index of 60 +/- 0.33 was observed. Preliminary Western blot analysis of the immunogen revealed at least four proteins of molecular weight ranging between 30 and 160 kDa.

A 37-kilodalton glycoprotein of Babesia divergens is a major component of a protective fraction containing low-molecular-mass culture-derived exoantigens

The supernatants of in vitro cultures of Babesia divergens Rouen 1987 in human erythrocytes, obtained by using a semidefined medium based on human high-density lipoproteins, were fractionated by gel filtration chromatography into four fractions, F1 to F4. The crude supernatant as well as each fraction adjuvanted with Quil-A protected gerbils from mortality due to a homologous infectious challenge. Analysis of the humoral response of the 10 protected gerbils with fraction F4, containing major proteins with molecular masses lower than 50 kDa, showed that a few antigens (from 50 to 17 kDa) could be important candidates for an improved vaccine against B. divergens babesiosis. As an immunodominant response was directed against the 37-kDa antigen (Bd37) in two different B. divergens strains tested, a polyclonal antibody directed against Bd37 was produced in a rabbit. In an immunofluorescence assay, the anti-Bd37 antiserum strongly labelled small internal vesicles of the merozoites and the cell surface was diffusely labelled after fixation, whereas on live merozoites, this labelling was not observed. [3H]glucosamine-radiolabelling experiments demonstrate that Bd37 is a glycoprotein. The Bd37 protein can also be labelled with [14C]palmitate but not with [3H]myristic acid. In Triton X-114 temperature phase partitioning of B. divergens-infected erythrocyte extracts, Bd37 was exclusively found into the detergent phase, indicating that the palmitoylated Bd37 protein was in the membrane fraction. In the in vitro supernatant, the glycoprotein Bd37 was found in a nonpalmitoylated form, indicating excretion and/or release of the glycoprotein from the merozoite.

Development of conventional subunit vaccines for anaplasmosis and babesiosis

Tick-borne hemoparasitic diseases of cattle continue to impact the beef industry throughout a large portion of the world. A substantial amount of research is currently focused on development of improved vaccines. The two main approaches being followed are: (1) use of conventional inactivated or native protein subunit vaccines, and (2) development of recombinant DNA technology for expression of selected immunogens. Recombinant or synthetic peptide based vaccines hold promise owing to the exquisitely defined nature of the product. However, the development is long-term, and will require extensive testing and risk assessment before field trials can be considered. Until then, more conventional subunit immunogens may offer an attractive alternative, and can be defined immunologically better than before. This paper reviews progress in the development of improved vaccines for anaplasmosis and babesiosis with an emphasis on the characterization of culture-derived babesial exoantigens. Both in vitro and in vivo information is presented.

A review on vaccination against protozoa and arthropods of veterinary importance

The recent advances in immunology and biotechnology have stimulated much research on the control of parasitic diseases through vaccination. This is a review of the state of the art regarding important protozoan and arthropod veterinary parasites. A live oocyst vaccine for avian coccidiosis is still in use but much work has been done on the identification, cloning, and assay of protective antigens. The sporozoites of Eimeria tenella have been the preferred subject and at least four recombinant antigens have already been tested with partial success. Premunization against babesiosis is still widely used in Latin America as is a live vaccine with attenuated parasites in Australia. At least three Babesia bovis and three Babesia bigemina antigens that generate partial protection have been produced as recombinant proteins. A vaccine against canine babesiosis is being commercialized in France. Infection-treatment is still used to vaccinate against Theileria parva and a schizont vaccine against Theileria annulata. Recombinant sporozoite antigens have been assayed with partial success against both species but the identification and administration of protective schizont antigens, regarded as the most important, still requires considerable work. The immunological control of African trypanosomoses is still impaired by the antigenic variation that the parasites experience during the infection. Although some possibilities exist, most specialists are pessimistic about the promise of developing a vaccine in the near future. Control of Boophilus ticks with an occult tick intestine recombinant antigen seems to have potential in inhibiting reproduction of the tick but salivary antigens appear to be more effective at inhibiting feeding and pathogen transmission. Vaccination with a Hypoderma protein, recently cloned, has induced 90% protection against subsequent infestations. It is very likely that effective vaccines against veterinary parasites will become available in the near future.

Cellular and humoral immune responses induced in cattle by vaccination with Babesia divergens culture-derived exoantigens correlate with protection

Previous results with the Babesia divergens gerbil vaccination model were extended in studies with cattle. Two calves were vaccinated with culture-derived B. divergens exoantigens, and two others were treated with control supernatant; both preparations were adjuvanted with Quil-A saponin. A parasite-specific humoral response was observed after the first vaccine injection and was boosted by two succeeding vaccine injections. Sera from the two vaccinated calves immunoprecipitated eight major parasitic proteins (with molecular masses ranging between 17 and 110 kDa) whose patterns were close to those observed in gerbil vaccine assays. The cellular immune response, monitored by lymphoproliferation assays, was slightly delayed in comparison with the humoral response; a significant proliferation occurred only after the second vaccine injection. Mononuclear cell proliferation was dose dependent in the presence of (i) lysates of B. divergens-parasitized erythrocytes, (ii) exoantigens of the whole supernatant, or (iii) protective exoantigens of two low-molecular-mass fractions obtained after supernatant gel filtration chromatography. An infectious challenge was administered 3 weeks after the third vaccine injection, with 3.6 x 10(10) B. divergens-parasitized erythrocytes. Erythrocyte count, rectal temperature, and parasitemia of the animals were monitored daily until they returned to initial values. All parameters indicated that the exoantigens induced protection from B. divergens infection for the two vaccinated calves.

Analysis of immune responses of different hosts to Babesia divergens isolates from different geographic areas and capacity of culture-derived exoantigens to induce efficient cross-protection

The immunoprecipitation of [35S]methionine-radiolabelled antigens from different Babesia divergens isolates by using bovine, gerbil, and human immune sera has shown that many B. divergens proteins contain epitopes shared between isolates. The cross-protective capacity of culture-derived soluble immunogens from the B. divergens Rouen 1987 isolate was tested against different B. divergens isolates. Results showed complete protection against the 7107b French isolate and substantial protection against the Weybridge 8843 English isolate (80% protection) and the Munich 87 German isolate (60% protection). In order to explain these vaccination results and to assess both the common and variable antigenicity of B. divergens, the antigenic patterns of the challenge isolates (Rouen 1987, 7107b, Weybridge 8843, and Munich 87) were compared by immunoprecipitation, using gerbil antisera raised against the Rouen 1987 vaccine isolate. Differences in the antigenic patterns and in the cross-protection of gerbils in these heterologous challenges were examined by studying the virulence and the antigenic status of each isolate.

Cell-mediated immune responses to Babesia bovis merozoite antigens in cattle following infection with tick-derived or cultured parasites

Peripheral blood mononuclear cells from cattle experimentally infected with Babesia bovis were examined for parasite-specific cell-mediated immune responses. Unfractionated merozoites and soluble and membrane fractions derived from merozoites were all antigenic for immune cattle, although the membrane fraction was the most stimulatory. Cattle responded to different antigenic fractions in a differential manner, and only that animal immunized with autologous cultured parasites responded to parasitized erythrocyte culture supernatants. Plastic-adherent cells (presumably monocytes/macrophages) were required for a proliferative response to babesial antigens but not to the T-cell mitogen concanavalin A, suggesting that babesial proteins are not simply mitogenic for T cells. Lymphocyte responses directed against a different hemoparasite from Mexico, Babesia bigemina, indicate that this parasite shares cross-reactive T-cell epitopes with B. bovis. These studies define a system whereby T lymphocytes from babesia-immune cattle can be used in proliferation assays to identify babesial merozoite antigens which are immunogenic for T cells. Because identification of helper T-cell epitopes is important for the design of a babesial subunit vaccine which will evoke anamnestic responses, the studies described here provide a basis for such experiments.

Identification of major Babesia divergens polypeptides that induce protection against homologous challenge in gerbils

[35S]methionine-radiolabeled proteins from the Babesia divergens Rouen 1987 isolate were immunoprecipitated with immune sera from three potential hosts: human, ox, and gerbil. The results showed a constant humoral response against major babesial antigens. Similarly, immunoprecipitation of radiolabeled in vitro culture supernatant demonstrated that the exoantigens of 37, 46, 70, and 90 kDA were the immunodominant polypeptides, whatever the host. The effects of vaccination with concentrated supernatant from B. divergens Rouen 1987 in vitro cultures (30 to 40% parasitemia) were examined in gerbils inoculated with the homologous B. divergens isolate. Gerbils having received two or three injections of a whole vaccine dose (1.5 ml of parasitized culture supernatant equivalent [PCSE]) or of a 1:5 diluted vaccine dose (0.3 ml of PCSE) showed 100% survival after intraperitoneal challenge with 10(6) B. divergens-infected gerbil erythrocytes. Moreover, two or three injections of a 1:25 diluted vaccine dose (0.06 ml of PCSE) or 9% NaCl or 1.5 ml of unparasitized culture supernatant equivalent resulted in a mortality rate of 80 to 90% of the infected gerbils. Immunoprecipitation and immunofluorescence assays performed with antisera from vaccinated and control gerbils demonstrated that a single vaccine injection induced a humoral response, which increased slightly after the second or third injection. After challenge, antibody levels increased significantly, although the immunoprecipitation did not display any modification of Babesia antigen patterns.

Application of exoantigens of Babesia and Plasmodium in vaccine development

The University of Illinois malaria vaccine programme uses culture-derived soluble exoantigens of Plasmodium falciparum and the squirrel monkey as an experimental model. Exoantigens are soluble polypeptides naturally released into the blood plasma of animals infected with Babesia or Plasmodium species, or into the supernatant medium of in vitro cultures of these organisms. Immunization with soluble B. bovis and B. bigemina exoantigens prepared from culture supernatant fluids protected cattle against homologous and heterologous challenge. Similarly, vaccination of squirrel monkeys with supernatant fluids from P. falciparum cultures containing exoantigen induced protective immunity against acute clinical malaria. Susceptible monkeys have been vaccinated with an aluminium hydroxide-fortified antigenic fraction partially purified from supernatants of P. falciparum strains Indochina I and Genève/SGE-1; this conferred significant clinical protection against needle challenge with the homologous Indochina I strain, and a moderate degree of immunity to the heterologous strain. Following sequential purification by high performance liquid chromatography, the N-terminal amino acid sequences of P. falciparum 100 kDa, 83 kDa and 70 kDa exoantigens were determined. A 29 amino acid peptide constructed from the N-terminal sequence of the P. falciparum (Genève strain) 83 kDa exoantigen has been synthesized. When coupled to a carrier protein, the peptide was immunogenic in rabbits, mice and squirrel monkeys, inducing antibodies which were trophozoite-specific, reactive to native parasite proteins in a two-site enzyme immunoassay (EIA) and in Western blots, and which inhibited P. falciparum growth in vitro. Using this synthetic peptide, EIAs are being developed for the detection of antibodies to P. falciparum blood-phase parasites in individuals living in malaria-endemic areas of Africa, Asia and South America.

Immunoprophylactic control of bovine babesiosis: role of exoantigens of Babesia

A major breakthrough in babesiosis vaccine research was achieved with the development of methods for continuous propagation in vitro of Babesia parasites. The development of a soluble, exoantigen-containing immunogen, free of erythrocytic antigens, has been proposed as a practical and realistic means of immunoprophylactic control of babesiosis. Such immunogens have been tested for immunogenicity of different strains, minimal protective dose, and protective efficacy of a combined B. bovis-B. bigemina immunogen. During the last 7 years, most of these aspects have been studied in a co-operative research programme between the University of Illinois and the Veterinary Research Institute of Venezuela. The Babesia exoantigen vaccine prevents clinical disease, is effective against different parasite strains, induces protective immunity for at least 13 months, does not immunize against host blood groups, is antigenically stable for at least 3 years at 4 degrees C, and is available in large quantities. Continued improvement of currently available culture-derived immunogens will best guarantee the successful production of food-producing animals in the tropics. Since July 1984, 16 field vaccination trials have been conducted in 7 states of Venezuela in areas where large-scale dairy and beef production is crucial for the livestock industry. These ranches encompassed a cattle production of 14,000, of which 3000 have been vaccinated with a combined B. bovis-B. bigemina immunogen in 2 subcutaneous inoculations with a 4-week interval. In the first 5 trials, a 1-year monitoring period following vaccination and natural tick exposure indicated that the culture-derived vaccine has effectively controlled the clinical manifestation of infection. Studies on antigen characterization are shifting the emphasis from crude antigenic preparations to purified polypeptides, isolated and identified using new biochemical techniques. Current research efforts are directed toward isolation of protective antigens and the production of synthetic vaccines. Until optimal molecular vaccines are developed, a polyvalent inactivated B. bovis-B. bigemina immunogen may offer the best combination of potency, efficacy and safety available for effective immunoprophylaxis against bovine babesiosis.

Immunological control of ticks and tick-borne parasitic diseases of livestock

Parasitic diseases inflict major losses on livestock production throughout the world. Currently, control of the diseases relies largely on prophylactic or therapeutic application of anti-parasitic drugs. In many instances, these measures are only partially effective. Moreover, they must be applied frequently, are therefore costly and time-consuming, and lead to the selection of drug resistance within the parasite populations. Thus, it has been recognized for several decades that effective methods of vaccination against parasitic diseases would have a major impact on livestock production. However, despite considerable efforts over the last 30 years, only a few parasite vaccines are currently in use and all of these involve the administration of live organisms.

A reappraisal of vector control strategies for babesiosis

This review suggests that we are on the threshold of a new epoch in the control of ticks and the diseases transmitted by them. Ecological studies of parasitic and non-parasitic stages of the tick life cycle have shown how strategic dipping can lead to effective control of Boophilus ticks. However, this approach can lead to drastic reduction in tick populations which may favour the development of enzootic instability. On the other hand, mathematical models and field studies indicate that we can eradicate babesiosis without eradicating the tick vector. As no well-established vaccine exists for babesiosis or its vectors, it is important to anticipate the possible effects of low or fluctuating populations of ticks caused by strategic control programmes. Under these conditions, it is probable that babesiosis will disappear before the tick vector does. Prudent use of tick bionomic data from several centres of field research, as well as models to analyse these data, should accelerate the latter process. Epidemiological models should be included in the analysis of babesiosis in areas that run the risk of converting from stable zones to unstable zones due to strategic dipping. These observations and projections underscore the value of integration of traditional and modern techniques in the control of babesiosis and other vector-borne diseases.

Identification of Babesia bovis merozoite surface antigens by using immune bovine sera and monoclonal antibodies

Three Babesia bovis merozoite surface proteins with relative molecular weights of 37,000, 42,000, and 60,000 were identified by indirect immunofluorescence of live merozoites and by immunoprecipitation of 125I-surface-labeled merozoite proteins with immune bovine sera and monoclonal antibodies. These proteins were clearly of parasite origin, as evidenced by immunoprecipitation of metabolically labeled [( 35S]methionine) merozoites from cultures with specific antimerozoite monoclonal antibodies. In addition, two other proteins were identified with these methods. An 85-kilodalton protein was considered to be of parasite origin based on fluorescence reactivity with a monoclonal antibody. However, this protein was not detected after immunoprecipitation of metabolically labeled parasites, and thus, the exact nature of its origin is equivocal. A fifth protein of 145 kilodaltons was detected by immunoprecipitation after metabolic labeling but was not directly apparent on the surfaces of live merozoites. Since merozoite surface proteins may be important in the induction of protective immunity, those identified here are candidates for vaccine studies.

Identification of Babesia bigemina and Babesia bovis merozoite proteins with isolate- and species-common epitopes recognized by antibodies in bovine immune sera

Immunoprecipitation of radiolabeled antigens with bovine antisera indicated that many Babesia bigemina and Babesia bovis merozoite proteins contain isolate-common epitopes, while at least 16 B. bigemina and 8 B. bovis proteins contain species-cross-reactive epitopes. One immunogenic, isolate-common, and species-specific candidate diagnostic protein from each species was identified.

Induction of protective immunity to Babesia divergens in Mongolian gerbils, Meriones unguiculatus, using culture-derived immunogens

Immunogens derived from microaerophilous stationary phase (MASP) cultures of Babesia divergens grown in bovine erythrocytes were used to inoculate the laboratory host of B. divergens, the Mongolian gerbil, Meriones unguiculatus. Animals inoculated subcutaneously twice with preparations of freeze-thawed merozoites in complete Freund's adjuvant were fully protected against homologous challenge, as were gerbils immunised with a non-viable preparation of parasite-enriched lysed infected bovine erythrocytes. Animals which had been infected with small numbers of parasitised erythrocytes from cultures cooled to 4 degrees C, allowed to recover, then challenged, also survived. All three groups had high antibody titres which dropped immediately after challenge and then rose again. Gerbils given culture supernatants containing soluble merozoite protein coat antigens were partially protected only after receiving a third inoculation. Non-immunised animals all died 4 days after challenge.

Detection of culture-derived Babesia bovis exoantigen using a two-site enzyme immunoassay

Soluble exoantigens in the supernatants of Babesia bovis cultures have been shown to be efficient immunogens against bovine babesiosis. We used a two-site enzyme immunoassay to monitor the release of these antigens during in vitro cultivation. Bovine immunoglobulin G was isolated from serum of an adult cow previously immunized with culture-derived B. bovis exoantigens and challenged via needle with virulent parasites. The specific immunoglobulin G was used as a capture antibody and as an enzyme-conjugated recognizing antibody. The optimal protein concentration of capture antibody was 10 micrograms/ml. The 24-h cultures showed the greatest antigen concentration. The test was sensitive for detection of differences in species-specific antigenic activity among B. bovis isolates, for determining loss of antigenicity during storage and formalinization, and for monitoring the kinetics of exoantigen release during in vitro cultivation. Antigens cross-reactive with the other major Babesia species of cattle, Babesia bigemina, were also detected with this assay. The high specificity, sensitivity, and reproducibility of this technique should facilitate detection and quantitation of Babesia antigens during purification and in standardization of candidate immunogens.

Bovine babesiosis: induction of protective immunity with culture-derived Babesia bovis and Babesia bigemina immunogens

The comparative protective efficacy of culture-derived Babesia bovis and B. bigemina immunogens against challenge exposure of susceptible crossbred cattle to heterologous strains was examined and correlated with the antigenic threshold requirements for induction of protective immunity. Strong protection was observed in animals vaccinated with 10 ml-equivalent doses of soluble, B. bovis exoantigen-containing supernatant fluids. Similar protective responses to B. bigemina exoantigens were evident even at 1 ml-equivalent dosages. In addition, the efficacy of a combined B. bovis-B. bigemina immunogen was assessed with a dose-response analysis in highly susceptible, purebred cattle. Vaccinated animals were protected against clinical babesiosis, and significant weight gains were recorded after challenge infection with virulent parasites.

Biology of Babesia

Recent advances in the knowledge of the biology of the protozoan family Babesiidae is reviewed with special emphasis on those species which cause economically important diseases in cattle. The life cycle of Babesia of cattle is illustrated taking into account these recent advances. Areas of controversy in the life cycles still exist particularly over the presence or absence of a sexual cycle within the tick vector and of exoerythrocytic schizonts in the mammalian host. In the mammalian host, the major parasite division cycle occurs in erythrocytes and it is these stages which induce the pathogenic effects. The recent progress in the understanding of the biology of Babesia parasites may allow important developments in the control of babesiosis.