Anticoccidial vaccines for broiler chickens: pathways to success

Acquisition of Immunity to Eimeria maxima in Newly Hatched Chickens Given 100 Oocysts

The acquisition of immunity to Eimeria maxima by chicks infected 18 hr after hatch with a single dose of 100 oocysts was investigated. In the first experiment, birds were moved each day to clean cages in order to prevent the possibility of secondary infection resulting from ingestion of oocysts passed in their feces. Immunity was measured at 4 wk of age by calculation of oocyst production following challenge with 500 oocysts or weight gain following challenge with 100,000 oocysts. Large numbers of oocysts were produced by infected birds following challenge, although numbers were significantly less than those from birds that had been reared in the absence of infection (susceptible controls). The weight gain of infected birds following challenge was significantly greater than that of susceptible controls but less than that of unchallenged controls. Thus, only partial protection had been acquired, whether parasite replication or body weight gain was used to assess the extent of immunity development. In a second experiment, acquisition of immunity at 4 wk by chicks infected 18 hr after hatch with 100 oocysts of E. maxima and reared in floor pens in contact with their droppings was investigated. Infected birds produced no oocysts following challenge, and weight gains were not significantly different from the unchallenged controls, which indicates that full immunity had developed by 4 wk. It is concluded that if oocysts of Eimeria species are used to vaccinate day-old chicks, reinfection by oocysts present in the litter is necessary for the establishment of protective immunity.

Guidelines for evaluating the efficacy and safety of live anticoccidial vaccines, and obtaining approval for their use in chickens and turkeys

These guidelines are intended to aid those engaged in poultry research in the design, implementation and interpretation of laboratory, floor-pen and field studies for the assessment of the efficacy and safety of live anticoccidial vaccines for immunization of chickens and turkeys against Eimeria species. In addition to efficacy and safety requirements, manufacture, quality control and licensing considerations are discussed. The guidelines do not address subunit vaccines comprising non-viable material, but many of the principles described will be relevant to such vaccines if they are developed in the future. Guidelines are available in some countries for avian vaccines of bacterial or viral origin but specific standards for anticoccidial vaccines in poultry have not, as far as we know, been produced. Information is provided on general requirements of registration authorities (based upon regulations applicable in the European Union and the USA) for obtaining marketing authorizations for vaccines. These guidelines may assist poultry specialists in providing specific information for administrators involved in the decision-making process leading to registration of new vaccines, and are intended to facilitate the worldwide adoption of consistent, standard procedures.

Cross protection studies with Eimeria maxima strains

The purpose of this study was to determine whether differences in fecundity of Eimeria maxima isolates were related to their abilities to elicit cross-protective immunity. Immunizations were initiated by low-dose gavages of sporulated oocysts to day-old broiler chicks under conditions that allowed parasite recycling, and chickens were challenged with homologous and heterologous strains. Immunization efficacies were measured using a protective index calculated from weight gain, gross lesion score, plasma carotenoid, and NO2- + NO3- data. A 4x4 cross- immunization study of four E. maxima strains (designated A-D) showed that strain A, which displayed the lower fecundity, provided no cross-protection against the other three strains. Following several maintenance passages, the fecundity of strain A was increased to that of strain C, and infection with strain A oocysts was able to provide cross-immune protection against challenge with strain C. This study indicates that parasite fecundity is important in providing good immune stimulation, and should be carefully monitored when characterization of the unique immune potentials of Eimeria strains is undertaken.

Intercurrent coccidiosis and necrotic enteritis of chickens: rational, integrated disease management by maintenance of gut integrity

Coccidiosis and necrotic enteritis (NE) are globally common, sometimes intercurrent, diseases of poultry. The risk of NE, due to the Gram-positive bacterium Clostridium perfringens, has increased in recent years because of the voluntary or legally required withdrawal of the use of certain in-feed antibiotic growth promoters with anticlostridial activity. In-feed ionophorous anticoccidial drugs incidentally also possess anticlostridial activity. Such ionophores, although not banned, are usually precluded when live anticoccidial vaccines are used, potentially increasing yet further the risk of NE. This review provides information for the design of rational, integrated management strategies for the prevention and control of coccidiosis and NE in chickens by maintaining gut integrity. Because of differences in local availability of feed ingredients and national legislations regarding antibiotic growth promoters and anticoccidial vaccine licensing, no universal strategy is applicable. The diseases and their interactions are described under the headings of forms of disease, diagnosis, sources of infection, pathophysiological effects, predisposing factors, and control methods. Elements of gut integrity, which influences host predisposition and clinical responses to disease, include physical development, immune competence, gut enzyme activity, mucin production, gut flora and epithelial damage. Experimental studies of coccidiosis and NE are compared, and where possible reconciled, with field observations. Gaps in knowledge and necessary further experiments are identified. Insights are provided regarding interactions between coccidiosis, NE, and the use of live anticoccidial vaccines. Recent changes in NE prevalence in commercial flocks, and their possible causes, are discussed. The necessarily wide range of topics reviewed emphasizes the enormous complexity of this disease combination, and indicates the importance of a multidisciplinary approach in order to reduce its harmful impact on the world's poultry industry.

Integrating genetics and genomics to identify new leads for the control ofEimeriaspp

Eimerian parasites display a biologically interesting range of phenotypic variation. In addition to a wide spectrum of drug-resistance phenotypes that are expressed similarly by many other parasites, theEimeriaspp. present some unique phenotypes. For example, unique lines ofEimeriaspp. include those selected for growth in the chorioallantoic membrane of the embryonating hens egg or for faster growth (precocious development) in the mature host. The many laboratory-derived egg-adapted or precocious lines also share a phenotype of a marked attenuation of virulence, the basis of which is different as a consequence of thein ovoorin vivoselection procedures used. Of current interest is the fact that some wild-type populations ofEimeria maximaare characterized by an ability to induce protective immunity that is strain-specific. The molecular basis of phenotypes that defineEimeriaspp. is now increasingly amenable to investigation, both through technical improvements in genetic linkage studies and the availability of a comprehensive genome sequence for the caecal parasiteE. tenella. The most exciting phenotype in the context of vaccination and the development of new vaccines is the trait of strain-specific immunity associated withE. maxima. Recent work in this laboratory has shown that infection of two inbred lines of White Leghorn chickens with the W strain ofE. maximaleads to complete protection to challenge with the homologous parasite, but to complete escape of the heterologous H strain, i.e. the W strain induces an exquisitely strain-specific protective immune response with respect to the H strain. This dichotomy of survival in the face of immune-mediated killing has been examined further and, notably, mating between a drug-resistant W strain and a drug-sensitive H strain leads to recombination between the genetic loci responsible for the specificity of protective immunity and resistance to the anticoccidial drug robenidine. Such a finding opens the way forward for genetic mapping of the loci responsible for the induction of protective immunity and integration with the genome sequencing efforts.

Responses of chickens vaccinated with a live attenuated multi-valent ionophore-tolerant Eimeria vaccine

Coccidiosis, caused by Eimeria species, is a serious economic disease of chickens (Gallus gallus) and the search for vaccines to control the disease is intensifying especially with the increasing threat of drug resistance. A live attenuated multi-valent ionophore-tolerant Eimeria vaccine has been developed that contains three ionophore-resistant Eimeria species, E. tenella, E. maxima and E. acervulina. The attenuated lines were derived from virulent field strains resistant to monensin ionophore by selection for early development in chicks. The vaccine was administered by gavage and through drinking water to broiler chickens, Chinese Yellow strain, reared in wire cages. Vaccinated medicated birds performed better than vaccinated unmedicated and medicated unvaccinated groups. The final mean weights of vaccinated medicated birds were significantly higher (P<0.05), and a better vaccine protection index, using both vaccinating methods, was achieved. Results indicated that concomitant use of ionophores and vaccines could be a useful adjunct to planned immunization in the control of coccidiosis.

Clostridium perfringens in poultry: an emerging threat for animal and public health

The incidence of Clostridium perfringens-associated necrotic enteritis in poultry has increased in countries that stopped using antibiotic growth promoters. Necrotic enteritis and the subclinical form of C. perfringens infection in poultry are caused by C. perfringens type A, producing the alpha toxin, and to a lesser extent type C, producing both alpha toxin and beta toxin. Some strains of C. perfringens type A produce an enterotoxin at the moment of sporulation and are responsible for foodborne disease in humans. The mechanisms of colonization of the avian small intestinal tract and the factors involved in toxin production are largely unknown. It is generally accepted, however, that predisposing factors are required for these bacteria to colonize and cause disease in poultry. The best known predisposing factor is mucosal damage, caused by coccidiosis. Diets with high levels of indigestible, water-soluble non-starch polysaccharides, known to increase the viscosity of the intestinal contents, also predispose to necrotic enteritis. Standardized models are being developed for the reproduction of colonization of poultry by C. perfringens and the C. perfringens-associated necrotic enteritis. One such model is a combined infection with Eimeria species and C. perfringens. Few tools and strategies are available for prevention and control of C. perfringens in poultry. Vaccination against the pathogen and the use of probiotic and prebiotic products has been suggested, but are not available for practical use in the field at the present time. The most cost-effective control will probably be achieved by balancing the composition of the feed.

Recent advances in immunomodulation and vaccination strategies against coccidiosis

Coccidiosis is a ubiquitous intestinal protozoan infection of poultry seriously impairing the growth and feed utilization of infected animals. Conventional disease control strategies rely heavily on chemoprophylaxis, which is a tremendous cost to the industry. Existing vaccines consist of live virulent or attenuated Eimeria strains with limited scope of protection against an ever-evolving and widespread pathogen. The continual emergence of drug-resistant strains of Eimeria, coupled with the increasing regulations and bans on the use of anticoccidial drugs in commercial poultry production, urges the need for novel approaches and alternative control strategies. Because of the complexity of the host immunity and the parasite life cycle, a comprehensive understanding of host-parasite interactions and protective immune mechanisms becomes necessary for successful prevention and control practices. Recent progress in functional genomics technology would facilitate the identification and characterization of host genes involved in immune responses as well as parasite genes and proteins that elicit protective host responses. This study reviews recent coccidiosis research and provides information on host immunity, immunomodulation, and the latest advances in live and recombinant vaccine development against coccidiosis. Such information will help magnify our understanding of host-parasite biology and mucosal immunology, and we hope it will lead to comprehensive designs of nutritional interventions and vaccination strategies for coccidiosis.

The Biology of Avian Eimeria with an Emphasis on their Control by Vaccination

Studies on the biology of the avian species of Eimeria are currently benefiting from the availability of a comprehensive sequence for the nuclear genome of Eimeria tenella. Allied to some recent advances in transgenic technologies and genetic approaches to identify protective antigens, some elements are now being assembled that should be helpful for the development of a new generation of vaccines. In the meantime, control of avian coccidiosis by vaccination represents a major success in the fight against infections caused by parasitic protozoa. Live vaccines that comprise defined populations of oocysts are used routinely and this form of vaccination is based upon the long-established fact that chickens infected with coccidial parasites rapidly develop protective immunity against challenge infections with the same species. Populations of wild-type Eimeria parasites were the basis of the first live vaccines introduced around 50 years ago and the more recent introduction of safer, live-attenuated, vaccines has had a significant impact on coccidiosis control in many areas of the world. In Europe the introduction of vaccination has coincided with declining drug efficacy (on account of drug resistance) and increasing concerns by consumers about the inclusion of in-feed medication and prospects for drug residues in meat. The use of attenuated vaccines throughout the world has also stimulated a greater interest in the vaccines that comprise wild-type parasites and, during the past 3 years worldwide, around 3x10(9) doses of each type of vaccine have been used. The need for only small numbers of live parasites to induce effective protective immunity and the recognition that Eimeria spp. are generally very potent immunogens has stimulated efforts to develop other types of vaccines. None has succeeded except for the licensing, within several countries in 2002, of a vaccine (CoxAbic vaccine; Abic, Israel) that protects via the maternal transfer of immunoglobulin to the young chick. Building on the success of viral vaccines that are delivered via the embryonating egg, an in ovo coccidiosis vaccine (Inovocox, Embrex Inc.) is currently in development. Following successful field trials in 2001, the product will be ready for Food and Drug Administration approval in 2005 and a manufacturing plant will begin production for sale in late 2005. Limited progress has been achieved towards the development of subunit or recombinant vaccines. No products are available and studies to identify potential antigens remain compromised by an absence of effective in vitro assays that correlate with the induction of protective immunity in the host. To date, only a relatively small portfolio of molecules has been evaluated for an ability to induce protection in vivo. Although Eimeria are effective immunogens, it is probable that to date none of the antigens that induce potent protective immune responses during the course of natural infection has been isolated.

Characterisation of the antigenic and immunogenic properties of bacterially expressed, sexual stage antigens of the coccidian parasite, Eimeria maxima

Coccidiosis in poultry is caused by the intestinal parasite Eimeria; it causes significant financial losses to the commercial poultry industry worldwide. CoxAbic is the first commercially available subunit vaccine against coccidiosis. The vaccine consists of affinity purified sexual stage (gametocyte) antigens (APGA) isolated from Eimeria maxima. Production of this vaccine is time-consuming and laborious and, therefore, a recombinant subunit vaccine substitute for CoxAbic is desirable. The genes encoding the two immunodominant components of CoxAbic, gam56 and gam82, were cloned into the bacterial expression vector, pTRCHisB, and the proteins expressed and purified. Both recombinant proteins were recognised by protective chicken antibodies that were raised to APGA, by immunoblotting. In a competitive ELISA, a combination of the recombinant proteins inhibited the binding of anti-APGA antibodies to APGA by 76%, which was comparable to the inhibition of 98% observed when APGA was used as the competing protein in the assay. In two breeds of chicken (Australorp and Cobb500), the recombinant proteins alone, or in combination, elicited a dose-dependent, antibody response that recognised APGA by ELISA, and gametocytes by immunoblotting. Together, the results suggested that the development of a recombinant subunit vaccine that maintains the antigenic and immunogenic properties of the native protein vaccine, CoxAbic, is feasible.

Veterinary parasitic vaccines: pitfalls and future directions

Most available antiparasitic drugs are safe, cheap and highly effective against a broad spectrum of parasites. However, the alarming increase in the number of parasite species that are resistant to these drugs, the issue of residues in the food chain and the lack of new drugs stimulate development of alternative control methods in which vaccines would have a central role. Parasite vaccines are still rare, but there are encouraging signs that their number will increase in the next decade. The modern paradigm is that an understanding of parasite genes will lead to the identification of useful antigens, which can then be produced in recombinant systems developed as a result of the huge investment in biotechnology. However, we should also continue to devote efforts to basic research on the host-parasite interface.

Eimeria maxima TRAP family protein EmTFP250: subcellular localisation and induction of immune responses by immunisation with a recombinant C-terminal derivative

EmTFP250 is a high molecular mass, asexual stage antigen from Eimeria maxima strongly associated with maternally derived immunity to this protozoan parasite in hatchling chickens. Cloning and sequence analysis has predicted the antigen to be a novel member of the thrombospondin-related anonymous protein (TRAP) family of apicomplexan parasites. Members of the TRAP family are microneme proteins and are associated with host cell invasion and apicomplexan gliding motility. In order to assess the immunogenicity of EmTFP250, a C-terminal derivative encoding a low complex, hydrophilic region and putative transmembrane domain/cytosolic tail was expressed in a bacterial host system. The recombinant protein was used to immunise mice and chickens and found to induce strong IgG responses in both animal models as determined by specific ELISAs. Using Western blotting, protective maternal IgG antibodies previously shown to recognise native EmTFP250 recognised the recombinant protein and, in addition, antibodies raised against the recombinant protein were shown to recognise native EmTFP250. Localisation studies employing immuno-light microscopy and immuno-electron microscopy showed that antibodies to the recombinant protein specifically labeled micronemes within merozoites of E. maxima. Furthermore, antibodies to the recombinant EmTFP250 derivative showed similar labeling of micronemes within merozoites of Eimeria tenella. This study is further suggestive of a functional importance for EmTFP250 and underscores its potential as a candidate for a recombinant vaccine targeting coccidiosis in chickens.

Immunization of Broiler Chicks by in Ovo Injection of Infective Stages of Eimeria

Immunization of chickens by in ovo injection of infective stages of 5 species of Eimeria was investigated. Fertile Hubbard x Petersen broiler chicken eggs were injected through the air cell on d 18 of incubation with oocysts of E. acervulina, E. maxima, E. mitis, E. praecox, or E. brunetti. Injected doses of all species ranged from 1 x 10(2) to 1 x 10(6) sporulated oocysts per egg. Chicks receiving oocysts in ovo shed oocysts posthatch. After 2 wk in wire-floored cages, birds were given a challenge infection with the homologous Eimeria species. Chicks immunized by in ovo injection of oocysts had significantly reduced lesion scores, improved weight gain, or reduced oocyst output compared with their nonimmunized counterparts. In additional studies, eggs were injected with 1 x 10(5) sporozoites of E. tenella, E. maxima, or E. acervulina per egg. Sporozoites of E. acervulina were not infective for chick embryos when administered in phosphate-buffered saline, but if sporozoites were suspended in tissue culture medium when injected in ovo, hatched chicks shed oocysts with peak output occurring 3 to 4 d posthatch. Sporozoites of E. maxima and E. tenella were infective for 18-d-old embryos regardless of the vehicle. The results demonstrate that immunization of broiler chickens against several species of coccidia by in ovo injection of oocysts is feasible. The infectivity of sporozoites for 18-d-old chick embryos varied depending on the species of Eimeria and the vehicle in which the sporozoites were suspended prior to injection.

Effect of Ibuprofen on Coccidiosis in Broiler Chickens

Ibuprofen (IBU)-a nonsteroidal anti-inflammatory drug-inhibits the biosynthesis of prostaglandins with pro-inflammatory and immunosuppressive properties and is therefore proposed as a candidate molecule for the treatment of coccidiosis in broiler chickens. In all experiments, IBU was administered via drinking water. In a first experiment, chickens were infected at 10 or 21 days of age with oocysts of Eimeria acervulina (5 X 10(4)), Eimeria maxima (3 X 10(4)), and Eimeria tenella (7.5 X 10(3)) and medicated with IBU at a dose of 15 mg/kg body weight (BW). In a second experiment, chickens were infected at 6 days of age with 10(4) oocysts of E. acervulina and medicated with IBU at a dose of 100 mg/kg BW. In the third experiment, an inoculum consisting of 5 x 10(4) or 10(5) E. acervulina oocysts was administered at 6 days of age to chickens medicated with IBU at a dose of 100 mg/kg BW. In a fourth experiment, the effect of IBU on sporulation and infectivity of E. acervulina oocysts was studied. Coccidial lesion scores (CLSs), oocyst shedding, and weight gain were used as evaluation parameters in all experiments except the fourth, where weight gain was not taken into account. In addition, the sporulation percentage was determined in the last experiment. No influence of IBU on the indicated parameters was observed after providing the drug at a dose of 15 mg/kg BW, whereas CLSs and oocyst shedding were reduced when IBU was provided at a dose of 100 mg/kg BW. However, IBU did not significantly show any effect on the degree of sporulation and infectivity of E. acervulina oocysts at a dose of 100 mg/kg BW.

Immunization of broiler chicks by in ovo injection of Eimeria tenella sporozoites, sporocysts, or oocysts

Immunization of chickens by in ovo injection of Eimeria tenella parasite stages was investigated. Fertile Hubbard x Petersen broiler chicken eggs were injected through the air cell on d 18 of incubation with sporozoites, sporocysts, or oocysts of E. tenella. Injected doses were in the range of 1 x 10(2) to 1 x 10(6) sporozoites, 2 x 10(5) to 2 x 10(7) sporocysts, or 1 x 10(2) to 5 x 10(6) oocysts per egg. Hatch rates were generally unaffected. Hatched chicks shed oocysts, with oocysts per gram of feces reaching a maximum at 3 d posthatch for chicks injected with sporozoites and at 7 d posthatch for chicks receiving oocysts or sporocysts in ovo. After 2 wk in wire-floored cages or 3 wk on litter, birds were challenged with 2.5 x 10(4) sporulated oocysts of E. tenella. Chicks immunized by in ovo injection of parasite stages had significantly reduced lesion scores compared to their nonimmunized counterparts. The results demonstrate the feasibility of immunizing broiler chickens against E. tenella infection by in ovo injection of sporozoites, sporocysts, or oocysts.

A new method for the experimental production of necrotic enteritis and its use for studies on the relationships between necrotic enteritis, coccidiosis and anticoccidial vaccination of chickens

A new method for the experimental production of necrotic enteritis in chickens is described. The main features are the use of a diet high in wheat and fish meal content; oral administration of a non-lethal inoculum of the coccidium Eimeria maxima followed 6 days later by the bacterium Clostridium perfringens type A per cloaca, so that the bacterial inoculum is deposited at the time and place when and where the intestinal coccidial lesions are maximal; grading of coccidial and clostridial lesions in individual birds sampled during the 14 days following the coccidial infection. The new method was used to examine the relationship between clostridial and coccidial infections. Frank coccidiosis, caused by virulent E. maxima, exacerbated the lesions of necrotic enteritis and other clinical effects due to a subsequent challenge with virulent C. perfringens type A. Immunization with a live, pentavalent, attenuated anticoccidial vaccine (Paracox-5) protected against a severe challenge with heterologous E. maxima. Furthermore, vaccination with Paracox-5, by virtue of its protection against clinical coccidiosis due to the E. maxima challenge, indirectly protected birds against a subsequent challenge with virulent C. perfringens. The results are reconciled with previous field observations on concomitant coccidiosis and necrotic enteritis in chicken flocks.

Fifty years of anticoccidial vaccines for poultry (1952-2002)

Although earlier investigators experimented with anticoccidial vaccines, the world's first commercially successful product was developed by Prof S. A. Edgar of Auburn University, Auburn, AL. This product contained live, nonattenuated Eimeria tenella oocysts and was first marketed by Dorn and Mitchell, Inc., in 1952. Under the trade names of DM Cecal Coccidiosis Vaccine, Coxine, NObiCOX, and CocciVac, it went through several formulations containing various Eimeria species that parasitize chickens, and a further product containing turkey Eimeria species was also developed. After many product and company changes, one turkey and two chicken formulations of CocciVac are still marketed worldwide by Schering-Plough Animal Health, Inc. Chicken and turkey formulations of Immucox, a similar type of vaccine, were developed by Dr. E.-H. Lee and first marketed in 1985 in Canada by Vetech Laboratories, Inc. In 1974, Dr. T. K. Jeffers of Hess and Clark, Inc., Ashland, OH, published his discovery of precocious lines of coccidia, which facilitated the development of the first attenuated anticoccidial vaccine. For commercial reasons, Jeffers was unable to do this himself, but this first attenuated vaccine was designed by Dr. M. W. Shirley and colleagues at the Houghton Poultry Research Station (HPRS) in the United Kingdom. The vaccine was commercially developed under license in the United Kingdom by Glaxo Animal Health Ltd. and then Pitman-Moore, Inc., and launched in The Netherlands during 1989 under the trade name Paracox. After further changes in company ownership, two formulations for chickens are now marketed worldwide by Schering-Plough Animal Health, Inc. Attenuation of coccidia by embryo adaptation was reported in 1972 in the United Kingdom by Dr. P. L. Long, who originally worked at the HPRS and later became a professor at the University of Georgia, Athens, GA. An embryo-adapted line of E. tenella was included with precocious lines of other species in a series of three attenuated vaccines for chickens under the trade name Livacox, developed by Dr. P. Bedrník and launched in the Czech Republic in 1992 by Biopharm. The formulations of all other commercially available live anticoccidial vaccines for poultry are currently based upon the scientific principles established for the CocciVac, Paracox or Livacox vaccines.