Advances and prospects for subunit vaccines against protozoa of veterinary importance.

Protozoa are responsible for considerable morbidity and mortality in domestic and companion animals. Preventing infection may involve deliberate exposure to virulent or attenuated parasites so that immunity to natural infection is established early in life. This is the basis for vaccines against theilerosis and avian coccidiosis. Vaccination may not be effective or practical with diseases, such as cryptosporidiosis, that primarily afflict the immune-compromised or individuals with an incompletely developed immune system. Strategies for combating these diseases often rely on passive immunotherapy using serum or colostrums containing antibodies to parasite surface proteins. Subunit vaccines offer an attractive alternative to virulent or attenuated parasites for several reasons. These include the use of bacteria or lower eukaryotes to produce recombinant proteins in batch culture, the relative stability of recombinant proteins compared to live parasites, and the flexibility to incorporate only those antigens that elicit "protective" immune responses. Although subunit vaccines offer many theoretical advantages, our lack of understanding of immune mechanisms to primary and secondary infection and the capacity of many protozoa to evade host immunity remain obstacles to developing effective vaccines. This review examines the progress made on developing recombinant proteins of Eimeria, Giardia, Cryptosporidium, Toxoplasma, Neospora, Trypanosoma, Babesia, and Theileria and attempts to use these antigens for vaccinating animals against the associated diseases.     

Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis

Recent studies have shown that fish are able to mount protective immune responses against various parasites. One of the best characterized parasite-host system in this context is the ciliate Ichthyophthirius multifiliis (Ich) parasitizing a range of freshwater fishes. Both specific and non-specific host defence mechanisms are responsible for the protection of fish against challenge infections with this ciliate. The specific humoral components comprise at least specific antibodies. The non-specific humoral elements included are the alternative complement pathway and probably lectins. Cellular factors involved in the specific response are B-cells and putative T-cells. The non-specific effector cells recognized are various leukocytes. In addition, goblet-cells and mast cells (EGC-cells) may have a function. The NCC-cell (suggested analogue to NK-cells in mammals) seems to play a role in the non-specific response. This well documented protective response in freshwater fishes against Ich has urged the development of anti-parasitic vaccines. Indeed, such products based on formalin killed parasites have been developed and found to offer the vaccinated host a satisfactory protection. However, the collection of parasites for vaccine production is extremely laborious. It involves keeping infected fish due to the fact that in vitro propagation of the parasite is still insufficiently developed. Gaining knowledge of amino acid sequences and its encoding DNA-sequences for the protective antigens (i-antigens) in the parasite was a major breakthrough. That achievement made it possible to produce a recombinant protein in E. coli and preliminary results indicated a certain protection of fish vaccinated with this product. Recent work has shown that the free-living and easily cultivated ciliate Tetrahymena can be transformed and express the i-antigen. This path seems to be promising for future development of vaccines against Ich. A novel approach in fish is the development of DNA-vaccines. Successful DNA-vaccination trials have been conducted in fish against viral infections and the technology also makes it possible to develop a DNA-vaccine against Ich. Other approaches to immuno-protection against Ich have been the use of heterologous vaccines. Thus, both bath and injection vaccination using live or killed (un-transformed) Tetrahymena has been reported to offer treated fish a certain level of protection. Such protection could be explained by non-specific reactions and the efficacy and duration of this vaccination type should be further evaluated.     

Immunization with recombinant surface antigens p26 with Freund's adjuvants against Babesia rodhaini infection

The surface proteins of Babesia rodhaini have previously been shown to induce a high degree of protective immunity. In the present study, one of those proteins, B. rodhaini antigen p26 was expressed in Escherichia coli and in insect cells infected with a recombinant baculovirus. These proteins were recognized by immune serum from a drug-cured BALB/c mouse. While BALB/c mice immunized with both recombinant antigens and Freund's adjuvants showed 40-100% survival rate against challenge infection with B. rodhaini, saponin failed to induce protection, although significant levels of B. rodhaini-specific antibodies were produced in both immunized mice (1:1,000-2,000 by indirect immunofluorescent antibody test). The immunization of IFN-gamma-deficient mice with the recombinant proteins was not protective against B. rodhaini infection, indicating that IFN-gamma is one of the important factors for the survival against lethal B. rodhaini infection.     

Evaluation of serum and secretory antibody responses to an immunodominant recombinant merozoite surface antigen, p150, using a sensitive enzyme-linked immunosorbent assay

An equation obtained from linear-regression analysis of positive/negative ratios and log of enzyme-linked immunosorbent assay (ELISA) titers of coccidia-immune serum samples was used to accurately predict the antibody titers of chickens immunized with a recombinant merozoite surface protein (p150). Chickens immunized twice intramuscularly with the recombinant p150 antigen emulsified in complete Freund's adjuvant developed a dose-dependent anti-p150 antibody response 14 days after primary immunization. Serum IgG and IgM and secretory biliary IgA antibodies were detected 2 months after primary immunization. Oral challenge with live Eimeria parasites significantly enhanced both the serum and secretory anti-p150 antibody titers. These results indicate that vaccination of chickens with the p150 recombinant merozoite antigen can induce a parasite-specific host immune response.     

Fatal pentastomiasis in captive African dwarf crocodile hatchlings (Osteolaemus tetraspis).

Six eggs from a captive African dwarf crocodile (Osteolaemus tetraspis) nest were artificially incubated. Two abnormal hatchlings with incomplete absorption of their yolk sacs died within 48 hr of hatching. No parasites were detected grossly or histologically in either hatchling. The remaining four apparently healthy hatchlings died without clinical signs at 3 wk of age, 1 wk after being fed live wild-caught fish and commercially raised goldfish. Necropsy revealed multiple white small wormlike organisms within the lungs, liver, and gastrointestinal tract of all four hatchlings. Histopathologic cross sections suggested that pentastomiasis was the cause of death, and whole parasites were identified as Sebekia mississippiensis. This infection was probably contracted by ingestion of live fish intermediate hosts infected with nymphal parasites. Avoiding the use of infected live fish or untreated fish products can prevent mortality of hatchling crocodilians caused by S. mississippiensis within zoological collections.     

Reduced cerebral infection of Neospora caninum in BALB/c mice vaccinated with recombinant Brucella abortus RB51 strains expressing N. caninum SRS2 and GRA7 proteins

Neospora caninum, an obligate intracellular protozoan parasite, is the causative agent of bovine neosporosis, an important disease affecting the reproductive performance of cattle worldwide. Currently there is no effective vaccine available to prevent N. caninum infection in cattle. In this study, we examined the feasibility of developing a live, recombinant N. caninum vaccine using Brucella abortus vaccine strain RB51 as the expression and delivery vector. We generated two recombinant RB51 strains each expressing SRS2 (RB51/SRS2) or GRA7 (RB51/GRA7) antigens of N. caninum. BALB/c mice immunized by single intraperitoneal inoculation of the recombinant RB51 strains developed IgG antibodies specific to the respective N. caninum antigen. In vitro stimulation of splenocytes from the vaccinated mice with specific antigen resulted in the production of interferon-gamma, but not IL-5 or IL-10, suggesting the development of a Th1 type immune response. Upon challenge with N. caninum tachyzoites, mice vaccinated with strain RB51/SRS2, but not RB51/GRA7, showed significant resistance to cerebral infection when compared to the RB51 vaccinated mice, as determined by the tissue parasite load using a real-time quantitative TaqMan assay. Interestingly, mice vaccinated with either strain RB51 or RB51/GRA7 also contained significantly lower parasite burden in their brains compared to those inoculated with saline. Mice vaccinated with strain RB51/SRS2 or RB51/GRA7 were protected to the same extent as the strain RB51 vaccinated mice against challenge with B. abortus virulent strain 2308. These results suggest that a recombinant RB51 strain expressing an appropriate protective antigen(s), such as SRS2 of N. caninum, can confer protection against both neosporosis and brucellosis.     

The adjuvant effect of a single dose of interleukin-12 on murine immune responses to live or killed Brucella abortus strain RB51.

This study was designed to determine if a single 0.5 microg administration of recombinant murine interleukin-12 (IL-12) would influence immune responses of mice vaccinated with live or killed Brucella abortus strain RB51 (SRB51). Mice were vaccinated intraperitoneally with 5 x 10(8) cfu of live or gamma-irradiated SRB51 bacteria alone, or in combination with 0.5 microg of IL-12. Control mice received saline or 0.5 microg of IL-12. Serologic responses and spleen weights after vaccination were greater in mice vaccinated with live SRB51 when compared to mice receiving killed SRB51 or control treatments. Administration of a single dose of IL-12 as a vaccine adjuvant did not influence immune responses, clearance of live SRB51, or resistance against B. abortus strain 2308 (S2308) challenge. The results of this study suggest that a single administration of 0.5 microg of IL-12 at the time of vaccination does not have significant adjuvant effects on vaccine-induced immune responses against live or killed Brucella.     

Immune responses of cattle following vaccination with living and non-living Babesia bovis antigens

Twenty-four yearling Hereford (Bos taurus) cattle were vaccinated against Babesia bovis using either live parasites or non-living antigens obtained from the supernatant of in vitro cultures. A single dose of live parasites was given subcutaneously, while the non-living supernatant antigen (NLSA) was combined with saponin and 2 doses given, 2 weeks apart. Following vaccination with live parasites, serum antibodies remained at high levels for 6 months, but the lymphocyte transformation response was low and lasted only 10-18 days. In contrast, NLSA vaccination was followed, after 21-28 days, by a peak of serum antibodies which then slowly declined. The lymphocyte transformation response in these animals was much higher and persisted for 6 months. Following heterologous challenge all unvaccinated cattle had severe reactions and required treatment to prevent death. Cattle vaccinated with live parasites had mild reactions with only 1 of the 12 requiring treatment. Cattle vaccinated with NLSA were only partially protected and 6 of the 12 required treatment.     

Role of immunoglobulin and peritoneal phagocytes in the protection of rats against salmonella infection

Intradermal vaccination of rats with heat-killed Salmonella dublin protected against an intraperitoneal challenge of live S dublin. Serum from vaccinated animals administered intraperitoneally protected normal rats against intraperitoneal challenge with S dublin and four other serotypes. Protection was attributed to opsonic antibodies which promoted phagocytosis by mononuclear phagocytes. The opsonin was identified as IgG2a which prevented lysis of macrophages and enabled them to contain the pathogen at the site of infection. In vitro, mononuclear phagocytes killed S dublin for up to two hours after phagocytosis in the presence of immune rat serum.     

Antiparasitic and immunomodulatory effect of innovative treatments against Myxobolus sp. infection in Diplodus puntazzo

The potential antiparasitic and immunomodulatory effect of three treatments against myxosporean parasites on the innate immune system of sharpsnout sea bream (Diplodus puntazzo) was investigated. Fish naturally infected with Myxobolus sp. (Bivalvulida/Platysporina), a histozoic parasite mainly affecting the renal interstitial tissue, were treated by oral administration of a combination of salinomycin with amprolium, Origanum essential oil or fumagillin in a small-scale field trial. Various leucocyte functions influenced by myxosporean infection were examined in order to determine treatment effects on leucocyte immunocompetence of treated fish. One month post treatment all drugs caused a significant decrease in prevalence and intensity of infection in comparison to untreated, infected fish. The effect was most prominent in salinomycin with amprolium treated fish, which 1-month post treatment contained either no cysts at all or a few spores free in melanomacrophage centres revealing almost total elimination of the parasite and the antiparasitic action of the treatment. There was no histopathological evidence of drug toxicity. Antiparasitic action was accompanied by a significant enhancement of phagocytic activity demonstrated by ingestion of large numbers of latex beads and the secretion of high levels of reactive nitrogen intermediates by phagocytes in vitro. Complete restoration of the diminished mitogenic responses and serum lysozyme secretion was also detected in salinomycin with amprolium-treated fish compared to untreated, infected fish. These data suggest that salilomycin with amprolium may be a promising treatment for myxosporean infections in intensively cultured warm-water fish, exhibiting action partially via the enhancement of host, innate immune functions and leading to parasite elimination.     

Priming for local and systemic antibody memory responses to bovine respiratory syncytial virus: effect of amount of virus, virus replication, route of administration and maternal antibodies

We studied the conditions under which calves can be primed for mucosal and serum antibody memory responses against bovine respiratory syncytial virus (BRSV), and the relationship between such responses and protection against the virus. Calves were primed via the respiratory tract with a low or high amount of live virus, with killed virus, or intramuscularly with live virus. Calves were challenged via the respiratory tract. Priming with live virus via the respiratory tract induced primary antibody responses in serum and on the mucosae, which were identical after the low and the high amount of virus. These responses were suppressed by maternal antibodies. Intramuscular priming of seronegative calves induced serum IgG1 and sometimes serum IgM and IgG2 responses, but no responses were detected on the mucosae. Sera of calves primed by the intramuscular or the respiratory route recognized the same viral proteins. No responses were observed after priming with killed virus, or after intramuscular priming of calves with maternal antibodies. After challenge, mucosal and serum antibody memory responses developed in calves that had been primed via the respiratory tract with live virus, whether they had maternal antibodies or not. One colostrum-fed calf showed a mucosal memory response, although serum responses were still suppressed by maternal antibodies. None of the calves thus primed shed virus after challenge. Intramuscular priming also primed for mucosal and serum memory responses after challenge, which however started perhaps slightly later and were not associated with protection against virus shedding. Priming with killed virus, or with live virus intramuscularly in the presence of maternal antibodies proved least effective in inducing memory and protection against virus shedding. Thus, protection against virus shedding was afforded by priming with live virus via the respiratory tract, both in calves with an without maternal antibodies. Protection was associated with a strong and rapid mucosal antibody memory response, but the reverse was not necessarily true. Protection against virus excretion had no relationship to titers of serum neutralizing or serum IgG1 or nasal IgA antibodies at the time of challenge.     

A preliminary study to evaluate the immune responses induced by immunization of dogs with inactivated Ehrlichia canis organisms

Ehrlichia canis is an intracellular pathogen that causes canine monocytic ehrlichiosis. Although the role of antibody responses cannot be discounted, control of this intracellular pathogen is expected to be by cell mediated immune responses. The immune responses in dogs immunized with inactivated E. canis organisms in combination with Quil A were evaluated. Immunization provoked strong humoral and cellular immune responses, which were demonstrable by Western blotting and lymphocyte proliferation assays. By Western blotting antibodies to several immunodominant E. canis proteins were detected in serum from immunized dogs and antibody titres increased after each immunization. The complement of immunogenic proteins recognized by the antisera were similar to those recognized in serum from infected dogs. Upon challenge with live E. canis, rapid anamnestic humoral responses were detected in the serum of immunized dogs and primary antibody responses were detected in the serum from control dogs. Following immunization, a lymphocyte proliferative response (cellular immunity) was detected in peripheral blood mononuclear cells (PBMNs) of immunized dogs upon stimulation with E. canis antigens. These responses were absent from non-immunized control dogs until after infection with live E. canis, when antigen specific-lymphocyte proliferation responses were also detected in the PBMNs of the control dogs. It can be thus concluded that immunization against canine monocytic ehrlichiosis may be feasible. However, the immunization regimen needs to be optimized and a detailed investigation needs to be done to determine if this regimen can prevent development of acute and chronic disease.     

Blood changes in carp (Cyprinus carpio) induced by ulcerative Aeromonas salmonicida infections

Only recently Aeromonas salmonicida has been recognized as a significant bacterial pathogen in ulcerative disease of cyprinid fish. Our attempts to formulate a vaccine based on bacterial surface antigens were unsuccessful in conferring reliable protection against lethal challenge. This lead us to study pathological changes in the humoral defense system during ulcerative A. salmonicida infection in carp. High numbers of opportunist pathogens such as A. hydrophila and Pseudomonas sp. were frequently recovered from the internal organs of moribund fish, in addition to A. salmonicida. These findings together with leucopenia in moribund fish suggest that pathogenesis is characterized by a state of immune suppression. In addition, fish which had sustained a sublethal infection were not protected against a subsequent lethal challenge. However, fish previously injected with a concentrated and inactivated culture supernatant showed protection. Differential blood cell counts did not differ between experimental and control groups during sublethal infection in contrast to serum proteins. Furthermore infected non-immune carp showed a progressive decrease of immunoglobulin and total serum protein levels before the day of peak mortality whereas protected carp maintained the immunoglobulin concentration despite a decrease in protein. Our observations suggest the involvement of multiple pathogenic events, affecting different parts of the humoral defense system during ulcerative A. salmonicida infection. The immunosuppressive effects can be minimized by prior vaccination with culture supernatant.     

接种泰氏锥虫及其抗原使家兔和小鼠产生抗伊氏锥虫的交叉免疫力

用培养的泰氏锥虫制备的死、活两种虫体抗原,接种于家兔和小鼠,一定时间后用伊氏锥虫强毒株攻击,观察其血清抗体的变化及交叉免疫保护能力。试验表明,泰氏锥虫虫体抗原可刺激机体产生较高的体液抗体;免疫动物有一定的抵御伊氏锥虫攻击的能力,与对照组相比,免疫动物血液中虫体出现的时间较迟,最初的数量少,临床症状轻。死虫抗原组的交叉免疫保护能力较活虫抗原组强。     

Leukocyte migration inhibition after experimental trypanosome infection in cattle

Blood serum from cattle experimentally infected with trypanosomes was tested for its activity influencing granulocyte migration. Pooled porcine granulocytes were used in the migration assay. The inhibitory migration activity observed in serum samples of trypanosome infected animals implies the presence of mediators of cellular immunity. Values of migration indices express reciprocal events of inhibitory and stimulating events in infected animals. This study allowed to follow at least some aspects of the very complex cellular immune system and its functioning. The observed differences in migration values in serum of each animal speak for an individual immunological capacity to defy trypanosome infections. The variability in occurrence of the first inhibitory activity after infection supports the view of individual responsiveness. Distinct differences in migration values were observed in Dahomey cattle after primary and secondary challenge with trypanosomes. Serum from reinfected animals exhibited a marked decrease in inhibitory activity as compared to samples obtained after first infection. These observations suggest a modulation of the individual immune response after multiple challenge with trypanosomes of the same strain. This study demonstrates the involvement of cell mediated immune responses to trypanosome infections.     

Parasite kinetics and immune responses in efferent prefemoral lymph draining skin reactions induced by tsetse-transmitted Trypanosoma congolense

Localised skin reactions (chancres) occurred on the flanks of cattle at the sites of deposition by tsetse flies of metacyclic forms of Trypanosoma congolense. Marked enlargement of the draining prefemoral lymph nodes accompanied the development of the skin reactions. Lymph from these nodes was collected through polyethylene cannulae inserted into the efferent lymphatics, and examined for trypanosomes, cells and antibody content. Within 6-9 days after infected tsetse fly bite, trypanosomes were detected in the efferent lymph; this preceded their appearance in the blood by 3-6 days, indicating that the lymphatic system acted as a major route for the passage of trypanosomes from the skin into the bloodstream. Responses induced in the draining lymph node as a result of trypanosome migration included a 2-3-fold increase in the volume of lymph and up to a 10-fold increase in lymphocyte output, including blast lymphocytes and plasma cells. Neutralising antibodies to metacyclic trypanosomes were detected in lymph and serum by Day 14 after infection, although in 2 out of 4 animals investigated, they were not demonstrated in serum until Day 18. Trypanosomes were also found in small numbers in efferent lymph of the prefemoral lymph node on the flank contralateral to the infected tsetse bites after development of parasitaemia. Increases in lymph flow and cellular output occurred about the same time in the ipsilateral and the contralateral efferent lymphatics, but were significantly less in the latter. Homologous challenge of immunised calves with tsetse-transmitted parasites revealed that trypanosomes were eliminated at the level of the skin or within the draining lymph node, as no parasites were detected in efferent lymph.     

An update on antileishmanial vaccine candidates and prospects for a canine Leishmania vaccine

Dogs are the domestic reservoir for Leishmania infantum, the parasite causing zoonotic visceral leishmaniasis (VL) in both the Old and New Worlds. Since the available methods for canine leishmaniasis treatment and control have limited efficacy, the development of a canine Leishmania vaccine is highly desirable. Mechanisms of antileishmanial immune responses in murine, human, and canine infections are briefly presented. Vaccine candidates, including live or killed parasites, Leishmania purified fractions, defined recombinant parasite antigens, live recombinant bacteria expressing Leishmania antigens and antigen-encoding DNA plasmids, are reviewed. Finally, some practical requirements for the evaluation of vaccine candidates in dogs are indicated.     

Trypanosomes in the lymph nodes of cattle and sheep infected with Trypanosoma congolense

The prefemoral lymph nodes of two calves and a sheep infected with a stock of Trypanosoma congolense transmitted by Glossina morsitans were examined histologically for the presence of trypanosomes. Ten days after infection trypanosomes were found in the subcapsular sinuses of the nodes of a calf and the sheep but parasites were absent from the blood at this time. Trypanosomes were also detected in the prefemeral lymph node of the other calf on examination 30 days after infection, when parasites were also present in the blood. These observations provide further evidence that extravascular foci of trypanosomes develop in infections with T congolense and indicate that it should not be regarded as a strict plasma parasite.     

Microhabitat use, not temperature, regulates intensity of Gyrodactylus cichlidarum long-term infection on farmed tilapia--are parasites evading competition or immunity?

Gyrodactylids (Monogenea) are ectoparasites of fish, some of which negatively affect commercially valuable fishes. Temperature strongly regulates population dynamics of these viviparous flatworms in farmed and wild fish populations, with most gyrodactylid species showing positive temperature-abundance associations. In agreement with epidemiological theory, numerous laboratory studies demonstrate that these parasites cannot persist in confined fish populations without periodic introduction of susceptible hosts. Extinction of gyrodactylid populations is due to host immunity, which develops in several fish species. In this one-year study, we followed populations of the recognized pathogen Gyrodactylus cichlidarum infecting four genetic groups of confined tilapia (wild type Nile tilapia Oreochromis niloticus niloticus, red O. n. niloticus, Mozambique tilapia O. mossambicus and a red synthetic population called Pargo-UNAM) kept under farming conditions and subject to natural environmental fluctuations. Based on the antecedents given, we postulated the following three hypotheses: (1) parasite abundance will be regulated by water temperature; (2) parasites will induce host mortality, particularly during periods of rapid infrapopulation growth; and (3) gyrodactylid populations will eventually become extinct on confined fish hosts. We disproved the three hypotheses: (1) parasite numbers fluctuated independently of temperature but were associated to changes in microhabitat use; (2) although gyrodactylid populations exhibited considerable growth, no evidence was found of negative effects on the hosts; and (3) infections persisted for one year on confined fish. Microhabitat use changed over time, with most worms apparently migrating anteriorly from the caudal fin and ending on the pectoral fins. Gyrodactylid populations followed similar trajectories in all fish, aggregating and dispersing repeatedly. Several instances were found where increased parasite dispersion coincided with increased intensity of infection; as well as the opposite, where increased aggregation coincided with parasite population declines. Three alternative explanations could account for these observations: that parasites (1) experience differential mortality on different anatomical regions of the fish; (2) migrate to avoid intraspecific competition; and (3) migrate to escape localized immune responses induced by infection. Our data do not allow us to demonstrate which of these alternatives is correct, so we discuss the merits of each. We provide circumstantial evidence in support of the third explanation, because as shown in other fish host-gyrodactylid interactions where immune responses have been characterized, in this study worms progressively moved away from fins with high mucus cell density to those with low density - what would be anticipated if immune defenses occur and reach the fish surface through mucus.     

Developing vaccines to control protozoan parasites in ruminants: dead or alive?

Protozoan parasites are among some of the most successful organisms worldwide, being able to live and multiply within a very wide range of hosts. The diseases caused by these parasites cause significant production losses in the livestock sector involving reproductive failure, impaired weight gain, contaminated meat, reduced milk yields and in severe cases, loss of the animal. In addition, some protozoan parasites affecting livestock such as Toxoplasma gondii and Cryptosporidium parvum may also be transmitted to humans where they can cause serious disease. Data derived from experimental models of infection in ruminant species enables the study of the interactions between parasite and host. How the parasite initiates infection, becomes established and multiplies within the host and the critical pathways that may lead to a disease outcome are all important to enable the rational design of appropriate intervention strategies. Once the parasites invade the hosts they induce both innate and adaptive immune responses and the induction and function of these immune responses are critical in determining the outcome of the infection. Vaccines offer green solutions to control disease as they are sustainable, reducing reliance on pharmacological drugs and pesticides. The use of vaccines has multiple benefits such as improving animal health and welfare by controlling animal infections and infestations; improving public health by controlling zoonoses and food borne pathogens in animals; solving problems associated with resistance to acaricides, antibiotics and anthelmintics; keeping animals and the environment free of chemical residues and maintaining biodiversity. All of these attributes should lead to improved sustainability of animal production and economic benefit. Using different protozoan parasitic diseases as examples this paper will discuss various approaches used to develop vaccines to protect against disease in livestock and discuss the relative merits of using live versus killed vaccine preparations. A range of different vaccination targets and strategies will be discussed to help protect against: acute disease, congenital infection and abortion, persistence of zoonotic pathogens in tissues of food animals and passive transfer of immunity to neonates.