Molecular basis for vaccine development against anaplasmosis and babesiosis

Immunization of livestock against the erythroparasitic pathogens Anaplasma marginale, Babesia bigemina, and Babesia bovis with safe and effective killed vaccines is not yet feasible on a practical basis. However, the immune protection afforded by recovery from natural infection and premunition indicates that microbial epitopes capable of inducing immunity exist and that the bovine immune system can be primed appropriately. Induction of protection by immunization with killed parasite fractions, enriched for polypeptides with surface exposed epitopes, supports a focus on surface epitopes, including apical complex organellar epitopes in Babesia, for vaccine development. Cloning, sequencing, and expression of genes encoding these key surface polypeptides has allowed examination of polypeptide function and detailed analysis of epitope conservation in light of genetic polymorphism. In this paper, the characterization of these polypeptides at the epitope level and their roles in inducing protective immunity are reviewed. Definition of these epitopes, in combination with improved understanding of immune mechanisms, provides the basis for development of effective recombinant vaccines against anaplasmosis and babesiosis.

Induction of opsonizing antibodies after injection of recombinant Plasmodium falciparum vaccine candidate antigens in preimmune Saimiri sciureus monkeys

We have previously shown that Plasmodium falciparum recombinant antigens PfEB200, R23, and Pfi72 inhibit opsonization of infected erythrocytes by hyperimmune Saimiri sera, indicating that they contain target epitopes involved in the phagocytosis of infected erythrocytes. We have investigated in this study the immune response of Saimiri monkeys with previous experience of malaria infections (preimmune monkeys) after injection of these recombinant antigens, administered alone or simultaneously. The humoral response to the recombinant antigens was monitored by radioimmunoassay, and the response to P. falciparum blood stages was assayed by immunofluorescence. The relative proportion of protective versus nonprotective immunoglobulin subtypes was investigated by using 3A2/G6 and 3E4/H8 monoclonal antibodies, and the capacity of the antisera to promote in vitro phagocytosis of infected erythrocytes was evaluated. The antigens evoked in most cases a secondary-type antibody response, resulting in important increases in antigen-specific antibody titers and concomitantly in anti-P. falciparum titers. The ratio of 3A2/G6 to 3E4/H8 immunoglobulin subtypes varied with the immunogen used. Opsonizing antibodies were boosted in several animals, the most promising combination being the mixture of PfEB200 and R23 that induced long-lasting production in five of five animals. The detectable opsonizing activity appearing after immunization of the animals was antigen specific, as it was lost after adsorption of the recombinant antigens. The challenge of the animals with blood stage parasites confirmed previous findings showing a correlation between the presence of detectable opsonizing antibodies in serum and protection.

Protection of calves with a vaccine against Cryptosporidium parvum

Cryptosporidium parvum causes enteric infection and diarrhea in calves, other species of economically important livestock, and humans. There are no effective treatments currently licensed for this parasite, and preventive measures are difficult. In addition to direct economic losses to the cattle industry, infected calves may contaminate water supplies with oocysts and contribute to human cryptosporidiosis. We have developed a vaccine offering partial protection against C. parvum infection in calves. Nine calves received an oral preparation of lyophilized C. parvum oocysts shortly after birth, and 10 calves served as nonvaccinated controls. All calves received colostrum. At 1 wk of age, all calves were administered 10(4) viable C. parvum oocysts orally. Clinical disease and oocyst shedding were monitored daily. Mean duration of diarrhea was 4 days for control calves and 1.7 days for vaccinated calves. Mean duration of oocyst shedding was 5.3 days for control calves and 2 days for vaccinated calves. These differences were statistically significant and suggest that this vaccine has the potential to reduce diarrhea and oocyst shedding caused by C. parvum.

Identification of Schistosoma mansoni antigens recognized by spleen cells of C57B1/6 mice immunized with ultraviolet-irradiated cercariae

Spleen cells of C57B1/6 mice immunized twice with Schistosoma mansoni cercariae attenuated by ultraviolet irradiation proliferated and produced interleukin-(II)-2 and/or II-4 in response to both soluble schistosomular and adult worm antigens of 72-68, 60-62, 50, 45, 29.5 and 28 kDa. All of these bands, except the 45 kDa, were also recognized by serum antibodies in Western blotting.

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.

Scientific hypothesis: a new strategy for the design of anti-protozoal drugs--DNA polymerase as a drug target

Chemotherapy against protozoan diseases has been practised since ancient times. Although still widely used, antiparasitic drugs are seldom completely effective. Complete elimination of an extensive protozoan infection usually requires a combination of drugs which target parasitic mechanisms very similar to the human host, as manifested by a multitude of side effects of varying severity. Furthermore, there is an escalating problem of widespread resistance to commonly used chemotherapeutic agents. Here we present an alternative strategy for the design of antiprotozoal agents using the principal enzyme involved in DNA replication as a possible drug target--DNA polymerase.

[Coccidiosis control: current status concerning preventive and therapeutic possibilities]

A summary is presented of the present situation in coccidiosis control in poultry, regarding the use of anticoccidial drugs and of therapeutics. Also attention is payed to the development of vaccines, which usage is limited up till now. Monitoring the incidence of coccidiosis will be necessary in future to enable efficient use of anticoccidial drugs and therapeutics.

Developmentally regulated localization and phosphorylation of SmIrV1, a Schistosoma mansoni antigen with similarity to calnexin

Potential molecular targets of a protective humoral immune response against schistosomiasis have previously been identified based on their enhanced immunogenicity in mice vaccinated with irradiated cercaria as compared to chronically infected mice. One of these antigens, IrV1, has been molecularly cloned and its sequence shown to be similar to the molecular chaperone calnexin. In this investigation, we partially characterized IrV1 from different developmental stages of the schistosome. Immunoprecipitation studies with antibodies raised against a portion of recombinant IrV1 demonstrated its presence in cercaria, schistosomula, and adult worms with an apparent molecular mass on SDS-polyacrylamide gel electrophoresis of 90 kDa. There was an approximate 6-fold increase in protein expression level during the cercaria to schistosomula transformation. Consistent with a potential role as a molecular chaperone, IrV1 was associated with several metabolically labeled proteins in co-immunoprecipitation studies with the adult worm tegumental fraction. Similar to calnexin, IrV1 was metabolically labeled with 32P in adult worms on serine and threonine residues and was one of the major phosphoproteins of this stage. This phosphorylation was developmentally regulated and coincided with the transformation of cercaria into schistosomula. The localization was also stage-specific as IrV1 was transported from internal regions of cercaria to the outer tegumental layer of schistosomula. The presence of IrV1 on the surface of schistosomula, an unprecedented localization for this family of endoplasmic reticulum proteins, supports additional studies of the immunoprophylactic potential of this molecule.

Immunogenicity of the Plasmodium falciparum glutamate-rich protein expressed by vaccinia virus

The glurp gene of Plasmodium falciparum F32 has been inserted into a vaccinia virus, and the recombinant virus was designated VVG4. Expression of glurp in VVG4-infected Vero cells was analyzed by immunoprecipitation and revealed a primary GLURP product of approximately 220,000 Da; GLURP was detected both intracellularly and in culture supernatants. To study the immunogenicity of vaccinia virus-expressed GLURP, mice were immunized with VVG4 and serum samples were analyzed for antibody reactivity with three polypeptides, covering almost the entire GLURP molecule; these three polypeptides were produced in recombinant form in Escherichia coli. The immune response was primarily directed against a carboxy-terminal repeat region. The mouse anti-GLURP serum recognized authentic GLURP by immunoprecipitation analysis from P. falciparum grown in vitro. These results demonstrate that vaccinia virus-expressed glurp product can induce a humoral immune response against GLURP derived from blood-stage parasites.

Evaluation of the safety and efficacy of vaccination of nursing pigs with living tachyzoites of two strains of Toxoplasma gondii

The safety of vaccination and persistence and distribution of Toxoplasma gondii stages within tissues following vaccination were examined in 3-day-old nursing pigs vaccinated with living tachyzoites by intravenous and subcutaneous routes of either the TS-4 mutant strain or its parent RH strain of T. gondii. The efficacy of vaccination of nursing pigs with the TS-4 mutant was also examined in pigs challenged orally with oocysts following vaccination. Pigs were vaccinated with 3 x 10(5) living tachyzoites when 3 days old and boosted with 3 x 10(5) living tachyzoites when 17 days old. Group 1 had 2 pigs vaccinated intravenously (i.v.) with Hanks' balanced salt solution (HBSS) and served as a vaccination control. Group 2 and 5 pigs vaccinated i.v. with tachyzoites of the TS-4 mutant; 3 pigs were used to examine the safety, persistence, and distribution of the TS-4 mutant and 2 were used for oocyst challenge. Group 3 had 5 pigs vaccinated i.v. with tachyzoites of the RH strain and all were used to examine the safety, persistence, and distribution of the RH strain within their tissues. Group 4 had 3 pigs vaccinated subcutaneously (s.c.) with tachyzoites of the TS-4 mutant; 1 was used to determine the persistence and distribution of the TS-4 mutant within its tissues and the other 2 pigs were used for GT-1 oocyst challenge studies. Group 5 had 3 pigs vaccinated s.c. with tachyzoites of the RH strain and all were used to examine the safety, persistence, and distribution of the RH strain within their tissues. None of the control pigs or pigs vaccinated with the TS-4 mutant developed clinical signs of disease or died prior to oocyst challenge. The TS-4 mutant was not reisolated from the tissues of vaccinated pigs nor were microscopic lesions present in the tissues of pigs that had been killed and examined at necropsy. Severe disease with clinical signs consisting of dyspnea, inactivity, diarrhea, and ocular lesions was observed in the group 3 pigs vaccinated i.v. with the RH strain. One pig died 7 days after initial vaccination. Microscopic lesions were observed in numerous tissues of all group 3 pigs. Swelling, erythema, and ulcers were observed at the site of inoculation in the group 5 pigs that were vaccinated s.c. with the RH strain. Minimal to no microscopic lesions were observed in these group 5 pigs. The RH strain was reisolated from pigs in both groups vaccinated with this strain.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunity to erythrocytic stages of malarial parasites

In those individuals who live in endemic areas, immunity to malaria is slow to develop and stage-specific. The nature and antigenic specificity of this response, which may involve components of both cell-mediated and humoral immunity, is not well understood. Rodent models provide useful systems to explore the spectrum of host responses that may contribute to resolution of erythrocytic-stage infection or possibly to pathogenesis. Moreover, these models allow identification of plasmodial molecules that can induce different types of host responses. Two different mouse model systems, Plasmodium yoelii yoelii and P. chabaudi adami are presented. These have been selected because resolution of infection by P. yoelii yoelii has been shown to require B cell-dependent mechanisms, while control of acute P. chabaudi adami infection can be achieved by T cell-dependent mechanisms. A monoclonal antibody that provides passive protection to P. yoelii challenge infection has been shown to recognize the cysteine-rich, carboxyl-terminal region of the merozoite surface protein-1. This region, obtained in an appropriate configuration from recombinant Escherichia coli, can induce significant protective immune responses in naive mice. In contrast, cell-mediated immune mechanisms make a major contribution to resolution of asexual-stage P. chabaudi adami infection. An empirical approach using continuous flow electrophoresis has identified several low molecular weight plasmodial proteins that can induce partial protective responses in susceptible hosts. These observations are briefly discussed with respect to human malaria.

Salmonella typhi vaccine strain CVD 908 expressing the circumsporozoite protein of Plasmodium falciparum: strain construction and safety and immunogenicity in humans

rcsp, encoding amino acids 21-398 of Plasmodium falciparum circumsporozoite protein (CSP), under control of tacP was integrated into the chromosomal delta aroC locus of attenuated delta aroC, delta aroD Salmonella typhi CVD 908. By immunoblot and ELISA, rCSP expression was greater from a multicopy plasmid than from the single chromosomal gene. CVD 908 omega (delta aroC1019::tacP-rcsp) was well tolerated by 10 volunteers who were fed two doses of 5 x 10(7) organisms 8 days apart. Seven subjects excreted the vaccine strain for 1-3 days. All subjects developed serologic responses to O and H antigens of the live vector, whereas 3 vaccinees responded to the foreign antigen: 1 developed an 80-fold rise in serum anti-sporozoite antibody, another had a 4-fold rise in antibody to a recombinant portion of CSP (residues 309-345), while a third vaccinee developed CSP-specific CD8+ cytotoxic T lymphocyte activity. This is the first report of attenuated S. typhi eliciting a human serologic or a cytotoxic T lymphocyte response to a foreign protein. Improved foreign gene expression should enhance immunogenicity.

Protein expression in yeast as an approach to production of recombinant malaria antigens

The selection of a system suitable for expression of recombinant malaria antigens for vaccine development is, in the final analysis, empirical. However, experience gained with both malaria antigens and other recombinant proteins has provided helpful guidelines. Recombinant DNA technology has been successfully applied to the development of vaccines against a number of human diseases. For example, recombinant DNA-derived hepatitis B virus surface antigen has been produced from both prokaryotic and eukaryotic systems. Yeast has been demonstrated to be an excellent host for the expression of recombinant proteins with uses in diagnostics, therapeutics, and vaccine production. Both intracellular and secretory systems have been developed and optimized for the production of high levels of recombinant proteins. Recombinant DNA technology, and in particular yeast expression systems, have been successfully used to produce malaria antigens, several of which have been protective in various animal models. In contrast, attempts to produce sufficient quantities of antigens for a malaria vaccine from in vitro cultures of the malaria parasite have been unsuccessful. Recombinant proteins can be produced and purified from yeast in large quantities and at low cost, each being requirements for a vaccine to be used in a global vaccination program against malaria.

Copolymer adjuvants in malaria vaccine development

Protection against virulent challenge with murine Plasmodium yoelii malaria was induced by immunization with whole killed blood-stage parasites in copolymer P1004 and detoxified lipopolysaccharide as adjuvant. Similar immunization with Freund's complete adjuvant and other water-in-oil emulsions failed to protect. Protection was associated with the production of antibody of the IgG2a isotype against epitopes measured by immunofluorescence. Several formulations that did not protect elicited high antibody titers measured by enzyme-linked immunosorbent assays or titers of other isotypes measured by indirect immunofluorescent assay. The results provide additional evidence that the adjuvants influenced both the isotype and specificity of antibody. The implications of these findings for vaccine development are discussed.

Vaccination of dogs against Babesia canis infection using antigens from culture supernatants with emphasis on clinical babesiosis

Groups of five dogs were vaccinated with Babesia canis antigens from in vitro culture in combination with saponin as adjuvant. Protection against challenge infection was evident as diminished clinical disease, decrease in parasitaemia, and a less marked fall in haematocrit values. Recovery from infection occurred at the time a memory immune response became effective (from Days 5 to 6 after challenge infection onwards). The effect was dose dependent, the highest antigen dose being most effective. A lysate of normal erythrocytes did not have protective activity, indicating that a parasite component was responsible for protection. Unlike the malaria situation, disease was not associated with elevated levels of tumour necrosis factor in the plasma, nor with hypoglycaemia. Disease appeared to be the result of the activity of a parasite product, which could have triggered reactions which led to sequestration of erythrocytes from the peripheral venous blood. As a result, the packed cell volume decreased, and organs such as lymph nodes and spleen became congested. As soon as immunity had developed there was a rapid increase in the peripheral erythrocyte number, and congestion of the spleen diminished, indicative of restored capillary blood flow. The results further suggest that vaccination with a soluble parasite product blocks the trigger of this pathological process.

The potential for recruiting immune responses toward type 1 or type 2 T cell help

The design of vaccine strategies in general, and those for malaria in particular, need to take into account the balance of T helper subsets (TH) they induce. The TH1 cells, which secrete interferon-gamma and interleukin-2 (IL-2), are associated with cell-mediated immunity (CMI), rather than humoral responses, and afford protection against intracellular infections, including those caused by parasites. In contrast, the TH2 cells secrete IL-4, IL-5, and IL-10, elicit high titer antibody responses, provide poor CMI, and are often correlated with susceptibility to infection. Depending on the type of TH cell bias required, it is possible to manipulate the immune response to a protein/peptide by 1) using different adjuvants, 2) conjugating the protein to various carriers, 3) immunizing in the presence of cytokines, or 4) using alternative routes of administration. To apply these approaches to malarial vaccines, it is necessary to identify which stage(s) of the parasite to target and what type of TH cell bias is protective against that particular stage. We favor using carriers such as Brucella abortus, which focus the antigen on a specific particle and which can trigger a TH1 cell response.

Liposomes, muramyl dipeptide derivatives, and nontoxic lipid A derivatives as adjuvants for human malaria vaccines

Studies indicate that adjuvant formulations based on liposomes, nontoxic lipid A, and muramyl peptide derivatives are safe and effective for vaccine use. Future research on the immunobiology of these adjuvants as well as the mechanisms by which adjuvants can alter the quality of immune responses may play an important role in determining their efficacy in malaria vaccines.

The promise of asexual malaria vaccine development

This review presents several reasons to support the belief that an asexual vaccine for human malaria caused by Plasmodium falciparum will be developed. It lists many of the current vaccine candidates that we think are ready for human trials and it summarizes a strategy for the development of an asexual vaccine.

Clinical trials of Plasmodium falciparum erythrocytic stage vaccines

Efficacy trials for malaria blood-stage vaccines are currently underway in several field sites. Numerous issues surround the design and execution of such trials, and there are many opportunities for failure that have little to do with the vaccines per se. This review highlights some of the key issues to be considered by investigators designing such trials, including those that are unique to trials for erythrocytic stage vaccines.

Malaria vaccine study site in Irian Jaya, Indonesia: Plasmodium falciparum incidence measurements and epidemiologic considerations in sample size estimation

Malaria epidemiologic and entomologic studies were performed during both the high transmission and low transmission seasons to characterize the Plasmodium falciparum malaria transmission at a proposed malaria vaccine trial site in Irian Jaya, Indonesia. The study population consisted of two subsets: native Irianese men with lifelong exposure to malaria and transmigrants who arrived from a nonmalarious area 2.5 years before the start of the study. All subjects received a radical cure for malaria and were then monitored weekly by blood film. Both P. falciparum malaria attack rates and incidence densities were calculated; transmigrants had a significantly higher rate (P = 0.003) than the Irianese during the low transmission season study (20-weeks long) but not during the high transmission season study (12-weeks long). Lack of exposure-induced immunity left the transmigrants at a minimum 17-25% greater relative risk of becoming parasitemic compared with the Irianese during the low transmission season study. During the high transmission season study, 50% of the transmigrants were P. falciparum positive by week 6 and 50% of the Irianese by week 9. During the low transmission season, 50% of the transmigrants were positive by week 10 and 43% of the Irianese were positive by week 17. Entomologic studies showed that Anopheles koliensis was the predominant vector (> 98% of anopheline catch). Entomologic inoculation rates for P. falciparum were 0.018 and 0.39 infective bites/person/night for the low and high transmission seasons, respectively. New P. vivax cases represented between 16% and 42% of all initial malaria cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Safety, immunogenicity and limited efficacy study of a recombinant Plasmodium falciparum circumsporozoite vaccine in Thai soldiers

Thai soldiers were vaccinated with a recombinant protein derived from the central repeat region of the circumsporozoite (CS) protein of Plasmodium falciparum conjugated to Toxin A (detoxified) of Pseudomonas aeruginosa (R32Tox-A) to evaluate its safety, immunogenicity and efficacy. In a randomized, double-blind manner, 199 volunteers received either R32Tox-A or a control vaccine at 0, 8 and 16 weeks. Immunization was performed in a malaria non-transmission area, after completion of which volunteers were deployed to an endemic border area and monitored closely to allow early detection and treatment of infection. The vaccine was found to be safe and to elicit antibody responses in all vaccinees. Peak CS antibody (IgG) concentrations in malaria-experienced vaccinees exceeded those in malaria-naive vaccinees (mean 40.6 versus 16.1 micrograms ml-1; p = 0.005) as well as those induced by previous CS protein-derived vaccines and observed in association with natural infections. A log-rank comparison of time to falciparum malaria revealed no differences between vaccinated and non-vaccinated subjects. Secondary analyses revealed that CS antibody levels were lower in vaccinee malaria cases than in non-cases, 3 and 5 months after the third dose of vaccine (p = 0.06 and p = 0.014, respectively). Because antibody levels had fallen substantially before peak malaria transmission occurred, the question of whether high levels of CS antibody are protective remains to be resolved.

A trial of the synthetic malaria vaccine SPf66 in Tanzania: rationale and design

The development of a safe, affordable and effective malaria vaccine to form part of control schemes in malaria endemic countries is a priority for researchers and public health officials. SPf66 is the first malaria vaccine to have shown partial protection against natural challenge in a phase III trial carried out in a hypoendemic area of Colombia. This paper describes the rationale and design of the first field trial of SPf66 outside South America, and the first to be conducted in an area of high perennial transmission.

Polymerase chain reaction-based diagnostic assay to detect cattle chronically infected with Babesia bovis

From a B. bovis gene sequence coding for a 60 kDa merozoite surface protein previously published, two sets of primers were designed for the Polymerase Chain Reaction (PCR) assay. Primer set BoF/BoR was used to prime Taq Polymerase DNA amplification of a 350 bp fragment of the target B. bovis DNA. Primer set BoFN/BoRN was used to prepare a PCR-synthesized, Digoxigenin-dUTP-labeled probe (291 bp) which would hybridize to a sequence within the PCR-amplified parasite target DNA. PCR amplification of target DNA obtained from in vitro-cultured B. bovis and nucleic acid hybridization of amplified product with the nonradioactive DNA probe showed that a 350 bp fragment could be detected when as little as 10 pg of genomic parasite DNA was utilized in the assay. A fragment of similar size was amplified from genomic DNA from four other B. bovis isolates but not from B. bigemina, Anaplasma marginale, or bovine leukocyte DNA. The PCR product was detected in blood samples containing approximately 3 B. bovis-infected erythrocytes (20 microliters of packed cells with a parasitemia of 0.000001%). By using the PCR/DNA probe assay, 16 out of 20 animals experimentally inoculated with B. bovis were detected positive, whereas no PCR product was observed in bovine blood samples collected from 20 B. bigemina-infected, and 20 uninfected cattle tested. The PCR-DNA probe assay was shown to be sensitive in detecting some cattle with B. bovis-chronic infection. The specificity and high analytical sensitivity of the test provides a valuable tool to apply in conducting epidemiological studies.

Cross-protection between species of the Schistosoma haematobium group induced by vaccination with irradiated parasites

Mice vaccinated with irradiated cercariae of Schistosoma haematobium, S. bovis and S. margrebowiei showed good levels of resistance (38-62%) against an homologous challenge, and varying degrees of resistance (19-46%), against challenges with closely related species. No protection against S. mansoni was induced by vaccination with any of these species. This restricted cross-protection reflects the close phylogenetic relationship between species of the S. haematobium group and indicates that immunologically important epitopes are conserved within this species complex.

Evaluation of the infectivity of a vaccinal and a pathogenic Babesia bovis strain from Argentina to Boophilus microplus

Three trials were performed to test the infectivity of a vaccinal and a pathogenic strain of Babesia bovis for the tick Boophilus microplus. The progeny of engorged female ticks fed on calves inoculated with the pathogenic strain were able to transmit the infection to splenectomised calves (measured by the presence of the parasite in their blood and seroconversion), whereas the progeny of engorged females fed on calves inoculated with the vaccinal strain did not transmit the organisms. These results appear to show that this strain of Babesia bovis would not be transmitted by ticks under natural conditions. This is a considerable advantage since the organisms used in the vaccine would not be able to establish foci in the field although they would be able to confer a long-lasting immunity.

Leishmaniasis: report of 33 cases and a review of the literature

Leishmaniasis are parasitic diseases in extension. They appear in new foci, because of important displacements of populations, and they affect immunocompromised patients (under chemotherapy, transplanted, or HIV infected). Study of 33 cases of leishmaniasis, 22 visceral and 11 cutaneous, at the Hôpital du Kremlin-Bicêtre, France, showed predominant contamination in Maghreb and in the south of France. In the case of Kala-Azar, fever (18 cases) and hepatosplenomegaly (19 cases) are frequent, and the serodiagnosis and the search of parasites by myelogram are always positive. In HIV-infected individuals, clinical signs are similar, but the serodiagnosis is less reliable. Evolution is bad in transplanted patients who must remain under immunosuppressive drugs. In the case of cutaneous leishmaniasis, diagnosis is based on local sample, while the serodiagnosis remains negative. Treatment is sometimes long, necessitating repeated treatments.

Immunization of cats with tissue cysts, bradyzoites, and tachyzoites of the T-263 strain of Toxoplasma gondii

Previous studies have demonstrated that oral administration to cats of tissue cysts of the oocyst-negative mutant strain of Toxoplasma gondii, T-263, induces immunity to oocyst shedding following challenge. Experiments were designed to compare the levels of protection induced by T. gondii T-263 when tissue cysts, bradyzoites released from tissue cysts, and tachyzoites were administered to cats. In 1 experiment, groups of cats received 2 oral doses of intact tissue cysts or released bradyzoites of T. gondii T-263 and were challenged 47 days later with the oocyst-producing strain of T. gondii T-265. All cats seroconverted following immunization and none of them shed oocysts following challenge. In a second experiment, groups of cats received tachyzoites of T. gondii T-263 as a single oral dose and either 1 or 2 intraduodenal doses; they were challenged 60 days after the last vaccination. All cats seroconverted following immunization. Following challenge, all cats shed oocysts except for 2 of 7 cats that received 2 intraduodenal doses of tachyzoites. Thus, orally administered bradyzoites of T. gondii T-263, either contained in intact tissue cysts or liberated from cysts, induced immunity to oocyst shedding. In contrast, tachyzoites did not completely protect against oocyst shedding, even when delivered directly to the duodenum and despite the development of high antibody titers.

Production of vaccines against gastrointestinal nematodes of livestock

Three international collaborations involving Australian research scientists are currently developing vaccines against Haemonchus, Trichostrongylus and Ostertagia parasites using recombinant DNA technology. The variety of protective antigens identified can be classified as 'conventional' (stimulate naturally acquired immunity) or 'novel'/'convert'/'concealed' (protective once immunity is induced by vaccination). To date, the most gratifying progress has resulted in 60-90% protection against Haemonchus and other blood-sucking parasites (e.g. ticks) using novel antigens, where high titres of serum antibody ingested by feeding worms leads to their demise. A great deal of research effort is unravelling the complexity of naturally acquired immunity so that conventional antigens, which may be the principal means of removing 'mucosal-browsing' parasites, may be formulated and delivered to achieve optimal efficacy. This work reveals that to remove early stages of parasites before they take up residence, deliberate induction of hypersensitivity responses akin to asthma, may be a desirable goal for vaccines and that the two models have much in common.

Protective oral immunization of chickens against Eimeria tenella with sporozoite surface antigens

Antigens were extracted from the surface of Eimeria tenella sporozoites with a solution containing Triton X 100 (1%), sodium dodecyl sulphate (0.5%) Na deoxycholate (1%) and EDTA (1 mM). After removal of the detergents, these surface antigen preparations conferred an immunity that protected chickens against a subsequent infection (10(4) sporulated oocysts). The best results were obtained after two 250 micrograms injections of Al(OH)3 adsorbed antigens (oocyst output per g caecal material on Day 7 post infection: 2.39 x 10(7) +/- 0.32 x 10(7) oocysts for controls and 7.37 +/- 10(6) +/- 3.19 x 10(6) oocysts for vaccinated birds) and after four gastric intubations of liposome entrapped antigens (oocysts output on Day 7 postinfection: 2.75 x 10(6) +/- 2.02 x 10(6) g-1 caecal material). These results represented respectively 70 and 88% protection indexes. Studies on the systemic and local antibody response after one or several infections of chickens with the parasite indicated at least 20 different molecules in the detergent antigens which are classified after immunoblotting according to their properties.

Vaccination against cestode parasites

Cestodes are tapeworm parasites. Infection in the intermediate host with larval (metacestode) parasites causes medically and economically important diseases known as hydatidosis and cysticercosis. Immunization against experimental infection with metacestode parasites has been highly successful, in marked contrast with the relative ineffectiveness of vaccines against infection with most parasitic organisms. High levels of immunity against a challenge infection with taeniid cestode eggs can be stimulated by immunization with extracts of the parasites, particularly with extracts of the oncosphere life-cycle stage. This led to the production of a recombinant antigen vaccine against infection in sheep with the parasite Taenia ovis, the first highly effective, non-living vaccine against a parasitic infection in animals or humans. This paper reviews immunity to the adult and metacestode life-cycle stages of cestode parasites, development and application of the T. ovis vaccine, and prospects for vaccines against other cestode infections.

Experimental challenge of sheep 18 months after vaccination with a live (S48) Toxoplasma gondii vaccine

Sheep can be immunised against toxoplasma abortion with a live vaccine composed of S48 tachyzoites. In this study the immunity induced in sheep was examined 18 months after vaccination. Seventy-three sheep were divided into three groups. Group 1 was vaccinated and 18 months later, when they were 90 days pregnant, the 27 sheep were challenged orally with Toxoplasma gondii oocysts. These ewes produced significantly more live lambs, after a significantly longer gestation period, than the 34 unvaccinated, challenged ewes in group 2. In addition there was less placental damage and fewer lambs were born with antibody to the parasite in the vaccinated group than in group 2. The 12 ewes in group 3 served as unvaccinated, unchallenged control animals. The degree of protection induced by the S48 tachyzoites was as good 18 months after vaccination as previously observed six months after vaccination.

Cryopreservation of Babesia divergens from jirds as a live vaccine for cattle

A Babesia divergens live vaccine can be produced in jirds (Meriones unguiculatus). The major drawback of this live vaccine is the short shelf-life. We evaluated different methods for the cryopreservation of this vaccine. Blood from jirds infected with B. divergens was frozen to -196 degrees C using cooling rates of 1, 10, 30, 100, 196 and 250 degrees C min-1, and a two-step cooling rate. The cryoprotectants dimethyl sulfoxide (DMSO), polyvinylpyrrolidone 40 and glycerol were used at different concentrations. Aliquots were stored in liquid nitrogen for 10-20 days and rapidly thawed in a water bath at 40 degrees C. Infectivity of blood before and after cryopreservation was tested in jirds by i.p. inoculation. The prepatent periods recorded were used to calculate the infectivity of the inocula. The highest infectivity of 46% was recorded from blood cryopreserved with 3 M DMSO and cooled at 10 degrees C min-1. Infectivity of the frozen vaccine was tested in 4 heifers by inoculation of 2.5 x 10(7) parasites. All animals showed Ab titres 12 days after inoculation.

Leishmaniasis

Leishmanial infections include three major clinical syndromes: visceral, cutaneous, and mucosal leishmaniasis. Visceral leishmaniasis, usually due to Leishmania donovani, has received increasing attention in the United States because of the growing number of cases seen in AIDS patients and the occurrence of viscerotropic L. tropica disease among Persian Gulf war participants. Cutaneous leishmaniasis is a relatively benign disease caused by L. Mexicana and L. (Viannia) species in the New World, and L. major, L. tropica, and L. aethiopica in the Old World. Many of these cutaneous lesions are self-healing, and treatment recommendations vary and continue to undergo evolution and study. Mucosal disease, caused by Leishmania (V.) braziliensis, is difficult to diagnose as well as to treat. A practical approach to the clinical presentation, diagnostic measures, and some treatment options of these syndromes is presented in relation to specific case studies.

[Soluble antigens of intra-erythrocyte stages of Plasmodium falciparum: diagnostic and vaccinal value]

A wide variety of soluble antigens also called exoantigens or excretory-secretory antigens are released by asexual blood stages of P. falciparum at the time of schizont burst and subsequent merozoite invasion. At present, most of these soluble malarial proteins have been purified and have had their genes cloned; the primary sequence displays considerable polymorphism upon tandem repeats. However, its function and the relevance of the polymorphism for the induction of host immune response, predominantly IgM and T-cell-independent type, has yet to be determined. The potential of P. falciparum exoantigens as immunodiagnostic tools has been the focus of numerous studies. Enzyme immunoassays or radioimmunoassays for the detection of malarial antigens in blood of suspected peoples have been reported. Their development requires increasing both specificity and sensitivity. Recently, some P. falciparum exoantigens have been proposed as candidates for inclusion in an anti-disease vaccine which induces a clinical but not parasiticidal immunity against malaria. There is much interest in this concept and their efficiency remains to be confirmed, as new strategies are needed in the prevention of malaria.

Expression of a recombinant Entamoeba histolytica antigen in a Salmonella typhimurium vaccine strain

The expression of a major surface antigen of the intestinal protozoal parasite Entamoeba histolytica in an attenuated Salmonella typhimurium vaccine strain is described. A polymerase chain reaction fragment derived from cDNA encoding the serine-rich Entamoeba histolytica protein, SREHP, was introduced into S. typhimurium chi 3987 (delta cya delta crp delta asd) using a plasmid expression vector (pYA292) containing the aspartate semialdehyde (asd) gene. S. typhimurium expressing recombinant SREHP as a SREHP/maltose binding protein fusion protein was administered orally to mice and gerbils (an important animal model for E. histolytica infection) and was recovered from splenic tissue in both species. Our study demonstrates the feasibility of expressing recombinant amoebic proteins in attenuated S. typhimurium strains, and shows that vaccine strains of S. typhimurium can successfully infect the gerbil, a widely used model for amoebic liver abscess and intestinal amoebiasis.

Treatment of theileriosis in crossbred cattle in the Punjab

One hundred and nine cases of bovine tropical theileriosis (Theileria annulata infection) in Punjab State, India, were treated with oxytetracycline (23 cases) or buparvaquone (86 cases). Ages of affected cattle ranged from 6 days to 3 years. Oxytetracycline cured only 7 animals (30.4%), all of them calves below 15 days old, while buparvaquone cured all but one (98.8%), a severely affected 10 day old calf. Cured cattle remained theileriosis-free for 12 to 18 months following recovery. Theileriosis in Punjab is predominantly a disease of young calves that cannot be protected by available cell-culture vaccines. It is suggested that the most economical way to control theileriosis in India would be to immunise calves by infection with sporozoite stabilate and simultaneous treatment with tetracycline, and to reserve buparvaquone for the treatment of clinical cases, in cattle of all ages.

Towards a synthetic malaria vaccine: cyclization of a peptide eliminates the production of parasite-unreactive antibody

In a previous study, human beings were vaccinated with a P. falciparum malaria vaccine candidate consisting of tetanus toxoid coupled to linear (Asn-Ala-Asn-Pro)3 ((NANP)3). The vaccine initiated protection in some people, but some individuals mainly produced anti-peptide antibodies that did not react with the pathogen. A likely contributor to the formation of epitopes that give rise to pathogen-unreactive antibodies is the free terminal proline which is not a terminal residue in the native protein. To avoid the elicitation of antibodies against terminal epitopes, (NANP)3 was cyclized. In contrast to monoclonal antibodies to the linear peptide where 35% were unreactive with the parasite, all monoclonal antibodies to the cyclized peptide were found to react with the parasite.

Efficacy of an Australian Babesia bovis vaccine strain in Malawi

Three calves vaccinated with the Australian Ka strain of Babesia bovis were fully protected against experimental infection with an isolate from a farm on which four of 210 vaccinated cattle had died from B bovis infection. A degree of cross protection against the isolate was demonstrated in one calf which had been infected previously with Babesia bigemina.

Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia

Preclinical and clinical studies have established the safety and immunogenicity of the chemically synthesised SPf66 malaria vaccine. The present study is a phase III randomised, double-blind, placebo-controlled, efficacy trial completed in La Tola, Colombia. 1548 volunteers over one year of age received three doses of either the vaccine (n = 738) or placebo (n = 810). Active and passive case detection methods were used to document clinical episodes of malaria among the study population. The follow-up period began one month after the third dose and lasted for one year. 168 and 297 episodes of Plasmodium falciparum malaria were documented in the SPf66 group and the placebo group, respectively; this corresponds to a crude protective efficacy of 38.8%. Incidence rates for first or only P falciparum malarial episodes were 22.3% per annum among the vaccinee group and 33.5% among the placebo group (RR = 1.5; 95% Cl 1.23, 1.84). Therefore, the protective efficacy of SPf66 against first or only episodes was 33.6% (95% Cl 18.8, 45.7), being highest in children aged 1-4 years (77%) and adults older than 45 years (67%). The estimated protective efficacy against second episodes was 50.5% (95% Cl 12.9-71.9). Our study shows that the chemically synthesised SPf66 malaria vaccine is safe, immunogenic, and protective against P falciparum malaria in semi-immune populations subject to natural challenge.