The effect of dietary protein level on performance characteristics of coccidiosis vaccinated and nonvaccinated broilers following mixed-species Eimeria challenge.

A series of experiments were conducted to investigate the effect of starter diet protein levels on the performance of broilers vaccinated with a commercially available live oocyst coccidiosis vaccine before subsequent challenge with a mixed-species Eimeria challenge. Data indicated that an increasing protein concentration in the starter diet improved broiler performance during coccidiosis vaccination. Prechallenge performance data indicated that vaccination could decrease BW and increase feed conversion ratio. The time period most important for the observed effects appeared to be between 13 and 17 d of age. This reduction in performance parameters of vaccinated broilers compared with nonvaccinated broilers was eliminated by the conclusion of the experiments (27 d) in the diet groups with higher protein. Vaccination was effective at generating protective immunity against Eimeria challenge, as evidenced by increased (P < 0.05) BW gain, improved feed conversion, reduced postchallenge mortality, and reduced lesion development in vaccinated broilers compared with nonvaccinated broilers. These observations support numerous other reports that confirm live oocyst vaccination can be used effectively as a preventive against avian coccidiosis in commercially reared broilers. More important, these findings suggest that reduced protein concentration of starter diets can lead to significant losses in broiler performance when using a vaccination program to prevent coccidiosis.

Immunostimulating complexes incorporating Eimeria tenella antigens and plant saponins as effective delivery system for coccidia vaccine immunization.

Immunostimulating complexes (ISCOMs) are unique, multimolecular structures formed by encapsulating antigens, lipids, and triterpene saponins of plant origin, and are an effective delivery system for various kinds of antigens. The uses of ISCOMs formulated with saponins from plants collected in Kazakhstan, with antigens from the poultry coccidian parasite Eimeria tenella, were evaluated for their potential use in developing a vaccine for control of avian coccidiosis. Saponins isolated from the plants Aesculus hippocastanum and Glycyrrhiza glabra were partially purified by HPLC. The saponin fractions obtained from HPLC were evaluated for toxicity in chickens and chicken embryos. The HPLC saponin fractions with the least toxicity, compared to a commercial saponin Quil A, were used to assemble ISCOMs. When chicks were immunized with ISCOMs prepared with saponins from Kazakhstan plants and E. tenella antigens, and then challenged with E. tenella oocysts, significant protection was conveyed compared to immunization with antigen alone. The results of this study indicate that ISCOMs formulated with saponins isolated from plants indigenous to Kazakhstan are an effective antigen delivery system which may be successfully used, with low toxicity, for preparation of highly immunogenic coccidia vaccine.

Intestinal mucosal mast cell immune response and pathogenesis of two Eimeria acervulina isolates in broiler chickens.

Four experiments were conducted comparing intestinal immune responses to 2 isolates of Eimeria acervulina (EA), EA1 and EA2. In experiments 1 and 2, broiler chicks of 2 commercial breeds were divided into control (nonchallenged), EA1-, or EA2-challenged groups. On d 6 postchallenge (PC), changes in BW were determined, intestinal lesions were scored, and duodenal tissue was evaluated for morphometric alterations and mucosal mast cell numbers. EA1 produced classical duodenal lesions and reduced villus height to crypt depth ratios compared with controls; however, no differences were found in mast cell counts. EA2 produced different results, and observed data were suggestive of an anaphylactic-like intestinal secretory response compared with EA1 or controls. In experiment 3, tissues were analyzed from d 2 through 6 PC. Villus atrophy and crypt hyperplasia were increased on d 5 PC in both challenged groups. Mast cell counts were significantly greater on d 3 and 4 PC in EA1-challenged birds. In experiment 4, EA2 oocysts were cleaned with 5.25% sodium hypochlorite to evaluate the possibility of a bacterial contaminant contributing to the pathogenesis of intestinal alterations. No evidence of a bacterial contaminant contributing to the pathology was observed. These data are indicative of differential host response and immunovariability between different isolates of the same Eimeria species in 2 breeds of commercial broiler chickens.

Immunogenicity of ad libitum drinking water administration of bovine serum albumin in Leghorn chickens.

Oral administration of protein antigen in solution routinely leads to development of oral tolerance in most mammals but has been reported to be fully immunogenic in chickens. Previous studies, including several performed by our laboratory, have demonstrated that oral administration of discrete amounts of BSA for 6 consecutive days is fully immunogenic. This study was performed to determine immunoresponsiveness to protein antigen administered ad libitum at low levels in drinking water compared with i.p. and oral gavage routes of administration. Seven days following the last oral immunization, serum was assayed for IgG, bile for IgA, and tissue culture supernatant from 3 distinct lower intestinal regions for IgG and IgA in immunized and nonimmunized single-comb White Leghorn chickens. Systemic responses in the serum of experimental birds revealed a greater (P < 0.001) IgG response when BSA was administered via i.p. injection or by drinking water compared with gavage administration or nonimmunized controls. Responses measured in bile revealed that BSA administration in the drinking water resulted in a greater (P < 0.001) secretory IgA response compared with i.p. or gavage administration, and negative control groups. Intestinal antigen specific IgG, but not IgA, was elevated (P < 0.05) in all intestinal areas tested in birds immunized against BSA by drinking water and i.p. routes of administration, compared with other experimental groups. Taken together, the present experiments demonstrate that ad libitum drinking water administration of a protein antigen is as effective as i.p. administration or gavage routes of antigen exposure and potentially describe a novel approach to immunization of commercial poultry with purified protein antigens.

Development of a protective index to rank effectiveness of multiple treatments within an experiment: application to a cross-protection study of several strains of Eimeria maxima and a live vaccine.

Vaccination of chickens with live oocysts has become a more widely used method for controlling avian coccidiosis as resistance to anticoccidial medication increases. However, some coccidia strains are not useful in multispecies vaccines because antigenic variation has made them generally less protective. In order to experimentally test a number of strains for the best cross protection, we have devised an evaluation method using four independently measured variables: weight gain, lesion score, plasma carotenoids, and plasma NO2- + NO3-. These values, when measured at 6 days postchallenge, tend to be significantly correlated. A protective index (PX) is calculated for each chicken using the following algorithm: PX = (Ngain + Ncarotenoids) - (Nsqrls + N[NO2- + NO3-]), where the prefix N indicates values for a variable normalized against a mean of that variable from a control group. Nsqrls values are normalized values of the square roots of lesion scores. The PX can then be treated as a dependent variable. In this study, mean PX values of unchallenged groups cluster around 0. Mean PX values of protected chickens are statistically close to those from unchallenged groups, whereas unprotected chickens have highly negative mean PX values.

Participation of the intestinal epithelium and mast cells in local mucosal immune responses in commercial poultry.

The intestinal mucosa of commercial poultry is continually subjected to invasion or colonization by a wide array of potentially hostile enteric pathogens. Although, recent investigations have focused on lymphocyte involvement in immune responses in the intestine, lymphocyte-mediated immunity alone will not explain the barrier nature of mucosal membranes associated with rejection of many enteric pathogens upon secondary homologous challenge. Our laboratories have focused on nontraditional elements of mucosal immunity in poultry to better understand host-pathogen interactions in the intestine. Following classical and novel immunization procedures, we have identified an antigen-specific mechanism of immediate responsiveness of the mucosal epithelium characterized by epithelial chloride secretion. This mechanism, characteristic of intestinal anaphylaxis, is mediated by local immune elements. Similar mechanisms in mammals contribute to the barrier nature of mucosal membranes during pathogen challenge. To identify cells participating in these and similar responses, additional studies have described a role for mast cells in acute phase responses in the intestines of chickens experimentally challenged with Eimeria. To a more practical end, other experiments in our laboratories have characterized drinking water administration of BSA for elicitation of local and systemic antibody responses. These experiments have shown ad libitum drinking water administration of BSA to be as effective as i.p. administration of BSA; they present a novel approach to immunization of commercial poultry with protein vaccines. These investigations support continued research on host-pathogen interactions within the intestine of commercial poultry to better understand and control enteric pathogens through vaccination or immunomodulation.

Sustainable coccidiosis control in poultry production: the role of live vaccines.

The development of new methods of administering coccidiosis vaccines has facilitated their use in the hatchery and thereby improved prospects for the economic vaccination of broilers. The acquisition of protective immunity to Eimeria species is boosted by further exposure to infection after vaccination. Factors that affect the reproductive efficiency of non-attenuated and attenuated vaccines are considered and the key role that oocyst production plays in establishing and maintaining uniform immunity in a flock of chickens is discussed. In addition to immunisation, a possible advantage to the application of certain vaccines is that their use could repopulate poultry houses with drug-sensitive organisms. Theoretical rotation programmes in which the use of drugs is alternated with that of vaccines are described. Variability of the cross-protective immune response between strains of the same species should be considered during vaccine development and subsequent use. The significance of less common species of Eimeria, not included in all vaccines, also needs to be assessed. An important consideration is the occurrence of pathogens other than Eimeria (such as the bacterium Clostridium) in flocks given coccidiosis vaccines and the methods by which they might be controlled. More research is required into the relationship between bacterial and viral infections of poultry and coccidiosis vaccination. Vaccines need to be developed that are simple to apply and cost effective for use in areas of the world where small-scale poultry production is commonplace. In the near future it is likely that more live vaccines based upon oocysts derived from attenuated strains of Eimeria will be developed but in the longer term vaccines will be based on the selective presentation to the host of specific molecules that can induce protective immunity. This achievement will require significant investment from the private and public sectors, and, if successful, will facilitate the sustainable control of coccidiosis in poultry production.

Use of monoclonal antibodies developed against chicken coccidia (Eimeria) to study invasion and development of Eimeria reichenowi in Florida sandhill cranes (Grus canadensis).

Eimeria gruis and Eimeria reichenowi are common coccidial parasites of a number of species of cranes. Until recently, little was known about either the site for invasion or the dynamics of early development of the crane coccidia because of the difficulty of identifying sporozoites and early developmental stages of these parasites by conventional staining methods. In the present study, monoclonal antibodies (MAbs) elicited against Eimeria spp. of chickens and turkeys were found to cross-react with sporozoites and developmental stages of E. reichenowi in the tissues of Florida sandhill cranes (Grus canadensis). With these Mabs, E. reichenowi sporozoites were found in specimens taken at 6 hr postinoculation (PI) from just proximal to Meckel's diverticulum in the jejunum to the ileocecal juncture. Fewer were found in the ceca and rectum and none in the duodenal loop. At 24 hr PI, there were markedly fewer sporozoites and their location had shifted to the duodenum. No stages were seen in intestinal cells at 5 days PI (DPI), but trophozoites had developed in the liver and spleen. At 10 DPI, sexual stages were detected in the intestine from the duodenal loop through Meckel's diverticulum but not in other organs. By 14 DPI, numerous developmental stages were detected in the intestine (ceca and jejunum), liver, and lungs but not in the heart, kidney, or brain. The number, location, and maturity of the stages in the ceca differed markedly from those in the jejunum.

Partial characterization of a non-proteinaceous, low molecular weight antigen of Eimeria tenella.

A low molecular weight (LMW) antigen of Eimeria tenella, initially identified using a murine monoclonal antibody (mAb C(3)4F(1)) raised against E. tenella sporozoites, was partially characterized using enzymatic degradation. solvent extraction, and immunization into various inbred lines of mice. The LMW antigen could be isolated using Folch extraction (methanol/chloroform/ water) and the epitope recognized by mAb C(3)4F(1) was resistant to degradation by alpha-amylase, pronase, and proteinase K, but was sensitive to sodium m-periodate treatment or digestion using mixed glycosidases (from Turbo cornutus). These observations suggest that the antigenic epitope recognized by mAb C(3)4F(1) is carbohydrate-dependent and, based on our ability to isolate the LMW antigen by Folch extraction, the epitope probably resides on a polar glycolipid. The inability of sporozoite-immunized nude mice to elicit a serum antibody response to this molecule indicates that it acts as a T-dependent antigen. Furthermore, sporozoite-immunized male CBA/N mice (with an X-linked immunodeficiency) also failed to elicit a serum antibody response to this molecule, which is consistent with a carbohydrate antigenic epitope. We propose that this antigenic molecule be designated ET-GL1 to reflect its origin and probable structure (E. tenella glycolipid 1).

Effects of nutritionally balanced and stabilized flaxmeal-based diets on Eimeria tenella infections in chickens.

Twenty Sex Sal cockerels were randomly assigned to each of eight groups; each of four nutritionally balanced diets were fed to two groups from 1 d through 4 wk of age. These diets contained 0, 2, 5, or 10% stabilized flaxseed meal that provided a calculated 0, 0.45, 1.11, or 2.22% n-3 fatty acids, primarily linolenic acid. At 3 wk of age, one group of chickens from each diet treatment was infected with Eimeria tenella and was housed in separate but similar conditions to uninfected control chickens. At 6 d postinfection, chickens were weighed, bled, killed, and scored for lesions. No level of dietary flaxmeal tested provided protection against weight gain depression, increased feed conversion ratios, or lesions. We concluded that these diets did not protect against E. tenella infection because levels of linolenic acid were not high enough, and the oxidative potentials were well suppressed by vitamin E and other stabilizers present.

Efficacy evaluation of lasalocid plus roxarsone combination medication with different geographic field strains of Eimeria acervulina.

Performance of broiler chickens medicated with lasalocid alone (at 125 ppm) or in combination with roxarsone (at 50 ppm) was evaluated in battery and floorpen trials after challenge with geographically different field strains of coccidia containing predominately the upper intestinal species Eimeria acervulina. No significant difference in bird performance measured at 6 days postinfection (PI) was observed between lasalocid plus roxarsone-medicated (L+RM) or lasalocid-medicated (LM) birds challenged in separate battery trials with mixed-species inocula from Alabama or Georgia containing 92% or 88% E. acervulina, respectively. In contrast, L+RM birds challenged in another battery trial with a Louisiana mixed-species inoculum containing 92% E. acervulina showed significant reduction in average weight gain at 6 days PI compared with LM-challenged birds. A floorpen trial done with the same Louisiana inoculum showed significant reduction in average bird weight gain at 27 and 35 days of age (6 and 14 days PI) for L+RM-challenged birds compared with both unmedicated-nonchallenged (UMNC) control and LM-challenged birds. The LM+R groups were significantly lower in average bird weight at 27 days of age than the unmedicated-challenged controls. Feed conversions (FCs) for L+RM birds were significantly higher than those for the UMNC control birds during time of challenge (21-27 days of age) and for the 1-to-27-day-of-age time period. No significant difference in FC was seen between the UMNC and LM groups. Results of this study showed that performance of broiler birds medicated with lasalocid plus roxarsone could vary for geographically different mixed-species challenge inocula that contained predominately E. acervulina.

Influence of betaine and salinomycin on intestinal absorption of methionine and glucose and on the ultrastructure of intestinal cells and parasite developmental stages in chicks infected with Eimeria acervulina.

The effect of betaine and salinomycin on absorption of methionine and glucose in tissue from the duodenal loops of Eimeria acervulina-infected chicks was determined. Differences in the ultrastructure of the intestinal cells and parasite developmental stages were also examined. With a drug-resistant isolate of E. acervulina, methionine absorption was significantly higher in chicks fed a basal diet supplemented with 0.15% betaine as compared with absorption in chicks fed the unsupplemented basal diet. Addition of 66 ppm salinomycin to the diet containing betaine did not further enhance absorption. Conversely, with a drug-sensitive isolate, methionine absorption was significantly higher in chicks fed a diet supplemented with both betaine and salinomycin than in chicks fed the unsupplemented basal diet. Tissue from chicks fed any of the supplemented diets was usually significantly heavier than that from chicks fed the unsupplemented diet, even when weight gains of the birds were similar. Glucose absorption was similar in all diet groups. Epithelial cells in coccidia-infected and uninfected chicks fed diets supplemented with betaine or betaine plus salinomycin were less electron dense than cells from chicks fed diets that were not supplemented with betaine. Merozoites of E. acervulina in chicks fed diets supplemented with salinomycin had extensive membrane disruption and vacuolization, but the damage was prevented when betaine was added to the diet. Numerous merozoites and intact schizonts were seen in the intestinal lumen of chicks fed the diet containing betaine plus salinomycin.

Evaluation of the effect of peptidyl membrane-interactive molecules on avian coccidia.

This study examined the lytic effect of seven different synthetic peptidyl membrane-interactive molecules (Peptidyl-MIMs) on sporozoites of five different species of Eimeria infecting chickens and merozoites of two different species that infect chickens. All Peptidyl-MIMs (pMIMs) demonstrated antiparasitic effects at concentrations of 1-50 microM during incubation periods varying from 1 to 20 min. In addition, electron microscopy showed that ultrastructural degeneration of the pellicle of sporozoite stages of the parasites occurred within 5-10 min of exposure to 5-microM concentrations of three different pMIMs. Pore-like openings were seen in the pellicle of the sporozoites at the ultrastructural level, which indicated that the pMIMs had the same mechanism of action on the parasites as that reported from studies done on bacteria. A reduction in lesion scores was seen in chickens treated orally with 10-, 50-, or 75-microM concentrations of two different proteolytic stabilized (methylated) pMIMs after challenge with three different species of avian coccidia in battery-cage trials. Collectively these data indicate that pMIMs may be useful in the control of coccidiosis in poultry.

Effects of dietary supplementation with n-3 fatty acid ethyl esters on coccidiosis in chickens.

The ethyl esters of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids were added to a broiler starter diet singly or in combination [as bulk purified ethyl ester concentrate from menhaden oil (n3FAC)] in quantities similar to those found in a diet supplemented with 5% menhaden oil (MO). Diets were fed to chickens from 1 d of age through 3 wk of age. At 2 wk of age, the chickens were infected with Eimeria tenella, Eimeria acervulina, or Eimeria maxima. At 6 d postinfection (PI), the effects of the diets were assessed on weight gains, plasma carotenoids, gross lesion scores, and histological parasite scores in gut cross sections, or oocyst output. Significant ameliorating effects of diet on lesion scores and parasite scores were only seen in E. tenella infections and were only produced by the n3FAC and MO supplements. These two supplements, which contained higher molar concentrations of double bonds than the other supplements, also significantly reduced plasma carotenoids in uninfected chickens, indicating that they promoted a state of oxidative stress. These results are consistent with previous reports on the interaction of coccidiosis with dietary n-3 fatty acids and strengthen the hypotheses that dietary-induced oxidative stress is an effective deterrent against cecal coccidiosis in chickens.

Use of live oocyst vaccines in the control of avian coccidiosis: experimental studies and field trials.

Areas addressed in this study on the use of live oocyst vaccines to control coccidiosis include: the influence of immunocompetency of the strains and sex of the birds used; methods of delivery of vaccine; immunological variation between different strains of the same coccidial species; and the effects of combining vaccine with anticoccidial medication. The results show that vaccination with live oocysts elicited significant protection against coccidiosis, both with experimentally induced and naturally acquired coccidial infection, resulting in average bird weight gains and feed efficiency similar to that obtained with conventional anticoccidial medication.

Dietary modulation of avian coccidiosis.

During the past several years, our laboratory has been investigating the anticoccidial activities of various natural products that have potential use as dietary supplements for coccidiosis control. Sources of fats containing high concentrations of n-3 fatty acids such as menhaden oil and flaxseed oil and flaxseed, when added to starter rations and fed to chicks from one day of age, effectively reduce lesions caused by the caecal parasite Eimeria tenella, but not lesions caused by Eimeria maxima. Our results are consistent with reports of effects of diets high in n-3 fatty acids on other protozoan parasites which suggest that the state of oxidative stress induced by these diets in the cells of both host and parasites is responsible for their parasitic actions. Artemisinin, a naturally occurring (Artemisia annua) endoperoxide and effective antimalarial significantly lowers lesions from E. tenella when given at low levels as a feed additive. The mechanism of its action is also considered to involve induction of oxidative stress. Diets supplemented with 8 p.p.m. gamma-tocopherol (abundant in flaxseeds) or with 1% of the spice tumeric, reduce mid-small intestinal lesion scores and improve weight gains during E. maxima infections. These compounds may exert their anticoccidial activity because they are effective antioxidants. Betaine, a choline analogue found in high concentrations in sugar beets, improves nutrient utilisation by animals under stress. When provided as a dietary supplement at a level of 0.15% it has enhanced the anticoccidial activity of the ionophore, salinomycin. Betaine may act as an osmoprotectant whereby it improves the integrity and function of the infected intestinal mucosa. In in vivo studies, betaine plus salinomycin significantly inhibit invasion of both E. tenella and E. acervulina. However, subsequent development of E. acervulina is inhibited more effectively with this combination treatment than development of E. tenella.

Use of monoclonal antibodies against chicken coccidia to study invasion and early development of Eimeria gruis in the Florida sandhill crane (Grus canadensis).

Eimeria gruis and E. reichenowi are common coccidial parasites of a number of crane species. In the present study, monoclonal antibodies (McAbs), elicited against Eimeria spp. of chickens and turkeys, cross-reacted with sporozoites and developmental stages of E. gruis in the tissues of Florida sandhill cranes. These McAbs were used to define the area of the intestine that was invaded by sporozoites of E. gruis and to demonstrate the feasibility of using McAbs to study the early development of E. gruis in the intestines and visceral organs of cranes. At 6 hr postinoculation (PI), E. gruis sporozoites were found primarily from just proximal to Meckle's diverticulum in the jejunum to the ileocecal juncture. Fewer sporozoites were found in the ceca and rectum, and none were found in the duodenum. Most of the sporozoites were in the middle third of the villi and within the lamina propria. At 14 days PI, developmental stages were detected in the ceca, jejunum, liver, and lungs but not in the heart, kidney, or brain. In the ceca and jejunum, the number, location, and maturity of the stages differed markedly.

Analysis of infraspecific variation among five strains of Eimeria maxima from North America.

Two laboratory strains from the eastern shore of Maryland 15 years ago and from an Ontario broiler house 23 years ago and three recent field strains of Eimeria maxima (isolated in Maryland, North Carolina and Florida) were examined for phenotypic and genotypic variation using protein profiles, random amplified polymorphic DNA-PCR analysis and DNA sequences obtained from the internal transcribed spacer regions of the rRNA genes. Staining profiles obtained by one-dimensional SDS-PAGE of sporozoite proteins were identical in all five strains. Using random amplified polymorphic DNA-PCR analysis with high %G-C content decamers as primers, we were able to confirm that the five strains are all E. maxima, but were unable to discern any relationships among them because of the limited number of shared polymorphisms identified. In contrast, cloning and sequencing of the internal transcribed spacer-1, 5.8S rDNA and internal transcribed spacer-2 regions of the rRNA genes provided sufficient sequence information to infer phylogenetic relationships among the strains. Almost all of the infraspecific variation was located in the internal transcribed spacer regions. Only two base changes were identified within the 5.8S rRNA gene. Evolutionary relationships among the strains inferred using parsimony analysis of the aligned internal transcribed spacer sequences were well supported, but the hypothesised relationships did not correlate well with the demonstrated immunological cross-reactivities of these strains.

Effects of combined treatment with recombinant bovine somatotropin and immunization with live oocysts on performance of broiler chicks raised in coccidia-seeded floor pens.

In our laboratory, preliminary studies have indicated that recombinant bovine somatotropin (rbST) can stimulate protective immunity against coccidia infection. A floor pen trial on coccidia-seeded litter was run to further test its activity as an adjuvant during immunization of chicks with a live oocyst vaccine. Five hundred day-old male broiler strain chicks were randomly assigned to five experimental Treatments: 1, medicated controls; 2, unimmunized, not treated with rbST; 3, unimmunized, rbST-treated; 4, immunized, not treated with rbST; 5, immunized, rbST-treated. Each treatment consisted of four pens of 25 chicks each. At the end of the growout period (7 wk), the chicks in Treatment 1 (medicated controls) had the highest mean BW, but mean BW of chickens in Treatment 3 (rbST treatment only) were not significantly less. On the other hand, the mean weights of chicks in Treatments 4 (immunized only) and 5 (immunized plus rbST) were significantly reduced, and not different from those of the untreated chickens (Treatment 2). However, when challenged at 3 wk, the chicks in Treatment 5 had a mean combined total lesion score that was significantly lower than that from Treatment 3, indicating that they had developed a higher degree of specific immunity, but of the expense of weight gain. The results suggest that rbST has a potential for use as an adjuvant with live oocyst vaccination, but that the ratio between rbST dose and numbers of oocysts in the live vaccine needs to be carefully controlled.

Assessment of recombinant bovine somatotropin as an immunomodulator during avian coccidiosis: immunization with living oocysts.

Coccidiosis, a disease of great economic importance to the poultry industry, is generally controlled prophylactically by additions of anticoccidial drugs to the feed. However, increasing development of drug-resistant coccidia species has stimulated searches for alternative control methods, one of which is vaccination. As part of this effort, recombinant bovine growth hormone (rbST) was tested as a possible immune stimulator in combination with live oocyst vaccination. At a dose of 0.045 mg per chick, given by s.c. injection at 1 d of age, rbST did not improve immunity developed by immunization with 500 or 2,500 oocysts of Eimeria maxima as judged by weight gain and lesion scores. At a single dose of 0.09 mg per chick given at 1 d of age in combination with IMMUCOX, rbST provided some protection against challenge infection with Eimeria tenella but not Eimeria acervulina as judged by reduction in lesion scores. Treatment with 0.09 mg rbST per chick alone at 1 and 3 d of age was protective against challenge with E. tenella but not E. acervulina or E. maxima as judged by reduction in lesion scores. These results strongly indicate that rbST can act as an immune modulator in chickens infected with coccidia, and provide a basis for further investigations of its use as a vaccine adjuvant.