Line differences in resistance to Salmonella enteritidis PT4 infection

A within-flock model of Salmonella Heidelberg transmission in broiler chickens

As part of the development of a quantitative microbial risk assessment (QMRA) model of third-generation cephalosporins (3GC)-resistant Salmonella Heidelberg, a compartmental (SEIR) model for S. Heidelberg transmission within a typical Canadian commercial broiler chicken flock was developed. The model was constructed to estimate the within-flock prevalence and the bacterial concentration in the barn environment at pre-harvest, and to assess the effect of selected control measures. The baseline scenario predicted an average within-flock prevalence of 23.5 % (95 % tolerance interval: 15.7-31.4) and an average bacterial concentration of 3.579 (0-4.294) log CFU/g of feces in the barn environment at pre-harvest (on the day the flock is sent to slaughter). Because vertical introduction of S. Heidelberg into the barn was already uncommon in the baseline scenario, vaccination of broiler parent flocks appeared to have a negligible effect, while vaccination of broiler chicken flocks substantially reduced the bacterial concentration at pre-harvest. Cleaning and disinfection between batches markedly reduced the within-flock prevalence at pre-harvest, but the effect on bacterial concentration was limited outside of the beginning of the production period. Extending downtime between batches by 7 days had little effect on within-flock prevalence or bacterial concentration of S. Heidelberg when compared to the baseline scenario. This study provides a basis to describe S. Heidelberg dynamics within a broiler chicken flock and to predict the within-flock prevalence and bacterial concentration at pre-harvest, and includes a description of the limitations and data gaps. The results of these analyses and associated uncertainties are critical information for populating QMRA models of the downstream impacts on public health from on-farm and other food-chain practices. Specifically, the study findings will be integrated into a broader farm-to-fork QMRA model to support the risk-based control of S. Heidelberg resistant to 3GC in broiler chicken in Canada.

A genome-wide association study explores the genetic determinism of host resistance to Salmonella pullorum infection in chickens

Background

Salmonella infection is a serious concern in poultry farming because of its impact on both economic loss and human health. Chicks aged 20 days or less are extremely vulnerable to Salmonella pullorum (SP), which causes high mortality. Furthermore, an outbreak of SP infection can result in a considerable number of carriers that become potential transmitters, thus, threatening fellow chickens and offspring. In this study, we conducted a genome-wide association study (GWAS) to detect potential genomic loci and candidate genes associated with two disease-related traits: death and carrier state.

Methods

In total, 818 birds were phenotyped for death and carrier state traits through a SP challenge experiment, and genotyped by using a 600 K high-density single nucleotide polymorphism (SNP) array. A GWAS using a single-marker linear mixed model was performed with the GEMMA software. RNA-sequencing on spleen samples was carried out for further identification of candidate genes.

Results

We detected a region that was located between 33.48 and 34.03 Mb on chicken chromosome 4 and was significantly associated with death, with the most significant SNP (rs314483802) accounting for 11.73% of the phenotypic variation. Two candidate genes, FBXW7 and LRBA, were identified as the most promising genes involved in resistance to SP. The expression levels of FBXW7 and LRBA were significantly downregulated after SP infection, which suggests that they may have a role in controlling SP infections. Two other significant loci and related genes (TRAF3 and gga-mir-489) were associated with carrier state, which indicates a different polygenic determinism compared with that of death. In addition, genomic inbreeding coefficients showed no correlation with resistance to SP within each breed in our study.

Conclusions

The results of this GWAS with a carefully organized Salmonella challenge experiment represent an important milestone in understanding the genetics of infectious disease resistance, offer a theoretical basis for breeding SP-resistant chicken lines using marker-assisted selection, and provide new information for salmonellosis research in humans and other animals.

Evaluation of genetic resistance to Salmonella Pullorum in three chicken lines

Resistance to diseases varies considerably among populations of the same species and can be ascribed to both genetic and environmental factors. Salmonella Pullorum (SP) is responsible for significant losses in the poultry industry, especially in developing countries. To better understand SP resistance in chicken populations with different genetic backgrounds, we orally challenged 3 chicken lines with SP-a highly selected commercial breed (Rhode Island Red, RIR), a local Chinese chicken (Beijing You, BY), and a synthetic layer line (dwarf, DW)-at 4 d of age. Two traits related to SP resistance, survival, and bacterial carriage in the spleen were evaluated after infection. Survival rates were recorded up to 40 d of age when all chickens still alive were killed to verify the presence of SP in the spleen to determine carrier state. Mortalities for RIR, BY, and DW chicks were 25.1%, 8.3%, and 22.7%, respectively, and the corresponding carrier-states in the spleens were 17.9%, 0.6%, and 15.8%. Survival and carrier-state heritabilities were estimated using an animal threshold model. Survival heritability was 0.197, 0.091, and 0.167 in RIR, BY, and DW populations, respectively, and the heritabilities of carrier state for DW and RIR were 0.32 and 0.16, respectively. This is the first time that the heritability of the SP carrier state has been evaluated in chickens. Our study provides experimental evidence that chickens with various genetic background exhibited significantly different SP-resistant activities and heritabilities. These results may be useful for selecting lines with better disease resistance.

Susceptibility to Salmonella carrier-state: a possible Th2 response in susceptible chicks

Infection of chicken with Salmonella may lead to a carrier-state characterized by the persistence of bacteria in the ceca for a long period of time and result in their excretion in feces. This excretion is the source of contamination of their congeners and food. During infection, enterocytes are the primary target cells for Salmonella, the producers of soluble factors which launch immune response and cells which are reciprocally responsive to surrounding immune cells. This study used microarrays to compare the gene expression profile during carrier-state of enterocytes purified from infected and control chicks which are either resistant or susceptible to Salmonella Enteritidis carrier-state. In total, we identified 271 genes significantly differentially expressed with an absolute fold change greater than 1.5. A global analysis determined interaction networks between differentially regulated genes. Using an a priori approach, our analyses focused on differentially expressed genes which were transcriptionally linked to cytokines playing a major role in the fate of the immune response. The expression of genes transcriptionally linked to type I interferon and TGF-β was down-regulated in infected chicks from both lines. Gene expression linked to the Th1 axis suggests the latter is inhibited in both lines. Finally, the expression of genes linked to IL-4, IL-5 and IL-13 indicates that susceptibility to carrier-state could be associated with a Th2 bias. Overall, these results highlight that the response to Salmonella during the acute phase and carrier-state is different and that enterocytes play a central role in this response.

Toward integrative genomics study of genetic resistance to Salmonella and Campylobacter intestinal colonization in fowl

Salmonella enterica serotypes Enteritidis and Typhimurium and Campylobacter jejuni are responsible for most cases of food poisoning in Europe. These bacteria do not cause severe disease symptoms in chicken, but they are easily propagated by symptomless chicken carriers which cannot be easily isolated. This animal tolerance is detrimental to food safety. In this particular case, increasing animal's resistance is not sufficient, since some animals considered as resistant are able to carry bacteria during several weeks without displaying disease symptoms. We review studies aimed at evaluating the resistance of chicken to Salmonella and Campylobacter intestinal colonization, either a few days or several weeks after infection. While studies of the genetic control of Campylobacter colonization are only beginning, mostly due to technical difficulties in infection protocols, genetic studies of Salmonella colonization have been conducted for now more than 20 years. They have initially reported an estimation of the genetic parameters associated with resistance to Salmonella colonization and are now aimed at identifying the genomic regions controlling variation of this trait in experimental lines and commercial populations. With the advent of high-throughput genomics, we are closer than ever to identify the true genes controlling resistance to Enterobacteria colonization in chicken. The comparison of genes involved in early resistance to intestinal colonization with genes controlling resistance to bacteria persistence several weeks after infection (i.e., carrier-state) should soon highlight the differences between the molecular mechanisms underlying those two distinct phenotypes. It will also be highly interesting to compare the genes or genomic regions controlling Campylobacter and Salmonella, in order to evaluate the feasibility of a selection conducted on both bacteria simultaneously.

Genomic selection for carrier-state resistance in chicken commercial lines

Background

Salmonella propagation by apparently healthy chicken and subsequent food security concerns could be decreased by the selection and use of chicken lines more resistant to carrier-state. In the present study we applied the first steps of the genomic selection methodology to assess the interest of including genetic markers for the genetic evaluation of hen lines infected with Salmonella Enteritidis.

Methods

We studied commercial laying hen lines divergently selected for resistance to Salmonella carrier-state at two different ages. A total of 600 animals were typed with 1536 SNP markers and artificially infected with S. Enteritidis. Phenotypes were collected four weeks (young animals) or five weeks (adults) later. Two types of variance component analyses, including or not including SNP data, were performed and compared. All variance components were estimated by Bayesian methods and Gibbs sampling.

Results

The comparison of both genetic analyses shows that SNP are efficient in capturing genetic variation, although none of them captures a large affect on the traits studied. Average accuracies do not change between analyses, showing that using SNP data does not really increase information.

Conclusions

These preliminary results show that genomic selection for Salmonella carrier-state resistance in laying hens is promising, although a denser SNP coverage of the genome on a higher number of animals is needed to assess its feasibility and efficiency compared to classical pedigree evaluation.

Use of a reduced set of single nucleotide polymorphisms for genetic evaluation of resistance to Salmonella carrier state in laying hens

Salmonella propagation by apparently healthy chickens could be decreased by the selection and use of chicken lines that are more resistant to carrier state. Using a reduced set of markers, this study investigates, for the first time to the authors' knowledge, the feasibility of a genomic selection approach for resistance to carrier state in hen lines. In this study, commercial laying hen lines were divergently selected for resistance to Salmonella carrier state at 2 different ages: young chicks and adults at the peak of lay. A total of 600 birds were typed with 831 informative SNP markers and artificially infected with Salmonella Enteritidis. Phenotypes were collected 28 d (389 young animals) or 38 d (208 adults) after infection. Two types of variance component analyses, including SNP data or not, were performed and compared. The set of SNP used was efficient in capturing a large part of the genetic variation. Average accuracies from mixed model equations did not change between analyses, showing that using SNP data does not increase information in this data set. These results confirm that genomic selection for Salmonella carrier state resistance in laying hens is promising. Nevertheless, a denser SNP coverage of the genome on a greater number of animals is still needed to assess its feasibility and efficiency.

New QTL for resistance to Salmonella carrier-state identified on fowl microchromosomes

Chicken's ability to carry Salmonella without displaying disease symptoms leads to an invisible propagation of Salmonella in poultry stocks. Using chicken lines more resistant to carrier state could improve both animal health and food safety. Previous studies identified several QTL for resistance to carrier state. To improve genome coverage and QTL detection power we produced a new set of 480 informative SNP markers and genotyped a larger number of animals. Ten additional microchromosomes could be covered when compared with previous studies. These new data led to the identification of 18 QTL significant at the chromosome-wide level. The only QTL significant at the genome-wide level were identified on microchromosomes 14 and 22 and have never been identified previously. Using a higher number of animals improved the power and the precision of QTL detection. Some of the QTL newly identified are located close to candidate genes or microsatellite markers previously identified for their involvement in the genetic control of resistance to Salmonella, which confirms their interest for selection purposes.

Genetic control of resistance to salmonellosis and to Salmonella carrier-state in fowl: a review

Salmonellosis is a frequent disease in poultry stocks, caused by several serotypes of the bacterial species Salmonella enterica and sometimes transmitted to humans through the consumption of contaminated meat or eggs. Symptom-free carriers of the bacteria contribute greatly to the propagation of the disease in poultry stocks. So far, several candidate genes and quantitative trait loci (QTL) for resistance to carrier state or to acute disease have been identified using artificial infection of S. enterica serovar Enteritidis or S. enterica serovar Typhimurium strains in diverse genetic backgrounds, with several different infection procedures and phenotypic assessment protocols. This diversity in experimental conditions has led to a complex sum of results, but allows a more complete description of the disease. Comparisons among studies show that genes controlling resistance to Salmonella differ according to the chicken line studied, the trait assessed and the chicken's age. The loci identified are located on 25 of the 38 chicken autosomal chromosomes. Some of these loci are clustered in several genomic regions, indicating the possibility of a common genetic control for different models. In particular, the genomic regions carrying the candidate genes TLR4 and SLC11A1, the Major Histocompatibility Complex (MHC) and the QTL SAL1 are interesting for more in-depth studies. This article reviews the main Salmonella infection models and chicken lines studied under a historical perspective and then the candidate genes and QTL identified so far.

Resistance to Salmonella carrier state: selection may be efficient but response depends on animal's age

Increasing resistance to acute salmonellosis (defined as bacteraemia in animals showing symptoms) is not sufficient for food safety, because of the risk of carrier state (when animals excrete bacteria without showing any symptoms). Increased resistance to Salmonella carrier state is therefore needed. Two experiments of divergent selection on resistance at a younger and a later age lead to significant differences between lines and allowed estimating genetic parameters on 4262 animals. Heritability of resistance was estimated at 0.16 in chicks, while it varied from 0.14 to 0.23 with analysed organ in adult hens. Genetic correlations between contamination of the different organs ranged from 0.46 to 0.67, while correlations between resistance at both ages were estimated at -0.50, showing that increasing genetic resistance of hens will reduce resistance in chicks. Highest estimated absolute values of genetic correlations between resistance and production traits were, for chicken contamination level, with number of eggs laid between 41 and 60 (0.37) and, for adult contamination, with number of eggs laid between 18 and 24 (0.37) or 25 and 40 (-0.33) weeks of age.

QTL for resistance to Salmonella carrier state confirmed in both experimental and commercial chicken lines

The ability of chickens to carry Salmonella without displaying disease symptoms is responsible for Salmonella propagation in poultry stocks and for subsequent human contamination through the consumption of contaminated eggs or meat. The selection of animals more resistant to carrier state might be a way to decrease the propagation of Salmonella in poultry stocks and its transmission to humans. Five QTL controlling variation for resistance to carrier state in a chicken F(2) progeny derived from the White Leghorn inbred lines N and 6(1) had been previously identified using a selective genotyping approach. Here, a second analysis on the whole progeny was performed, which led to the confirmation of two QTL on chromosomes 2 and 16. To assess the utility of these genomic regions for selection in commercial lines, we tested them together with other QTL identified in an [Nx6(1)] x N backcross progeny and with the candidate genes SLC11A1 and TLR4. We used a commercial line divergently selected for either low or high carrier-state resistance both in young chicks and in adult hens. In divergent chick lines, one QTL on chromosome 1 and one in the SLC11A1 region were significantly associated with carrier-state resistance variations; in divergent adult lines, one QTL located in the major histocompatibility complex on chromosome 16 and one in the SLC11A1 region were involved in these variations. Genetic studies conducted on experimental lines can therefore be of potential interest for marker-assisted selection in commercial lines.

Mechanisms of egg contamination by Salmonella Enteritidis

Salmonella Enteritidis (SE) has been the major cause of the food-borne salmonellosis pandemic in humans over the last 20 years, during which contaminated hen's eggs were the most important vehicle of the infection. Eggs can be contaminated on the outer shell surface and internally. Internal contamination can be the result of penetration through the eggshell or by direct contamination of egg contents before oviposition, originating from infection of the reproductive organs. Once inside the egg, the bacteria need to cope with antimicrobial factors in the albumen and vitelline membrane before migration to the yolk can occur. It would seem that serotype Enteritidis has intrinsic characteristics that allow an epidemiological association with hen eggs that are still undefined. There are indications that SE survives the attacks with the help of antimicrobial molecules during the formation of the egg in the hen's oviduct and inside the egg. This appears to require a unique combination of genes encoding for improved cell wall protection and repairing cellular and molecular damage, among others.

Infection and excretion of Salmonella Enteritidis in two different chicken lines with concurrent Ascaridia galli infection

Studies on the impact of interaction of Salmonella enterica serovar Enteritidis and the parasitic nematode Ascaridia galli with the avian host were undertaken with particular emphasis on infection and excretion of these pathogens in two different layer lines. A total of 148 salmonella-free 1-day-old chickens (73 Hellevad and 75 Lohmann Brown) were randomly divided into five groups for each line. Group 1 served as an uninoculated control group. Groups 2 and 3 were infected with A. galli and S. Enteritidis, respectively. Group 4 was first infected with S. Enteritidis and subsequently with A. galli, and vice versa for group 5. The number of chickens excreting S. Enteritidis was significantly higher (P < 0.001) in the groups infected with both S. Enteritidis and A. galli compared with those only infected with S. Enteritidis over time. Furthermore, excretion of S. Enteritidis over time was significantly higher (P < 0.001) in the group first infected with S. Enteritidis and subsequently with A. galli compared with the group infected in the reverse order. No significant differences were observed between the two lines concerning excretion of S. Enteritidis over time in any group (P = 0.61 (group 3), P = 0.73 (group 4), P = 0.31 (group 5)). A. galli established itself significantly better (P = 0.02) in the group first infected with A. galli and subsequently with S. Enteritidis compared with the group infected in the reverse order. Furthermore, the A. galli infection rate was significantly higher (P = 0.02) in Hellevad chickens compared with Lohmann Brown chickens at the end of the experiment.

A model of Salmonella infection within industrial house hens

Salmonella is one of the major sources of toxi-infection in humans. Incidences of human salmonellosis have greatly increased over the past 20 years and this can largely be attributed to epidemics of Salmonella enteritidis phage type 4 within poultry. The main concern with this bacterium is the existence of silent carriers, i.e. animals harbouring S. enteritidis without expressing any visible symptoms. In this article, we formulate a model for S. enteritidis transmission in hen houses, considering both the hens and the environmental bacterium contamination. By considering the hen's individual development of the disease, we build a model for the production of eggs contaminated by S. enteritidis. The objectives are to analyse the dynamic of the disease, and to provide understanding of measures to avoid the endemicity of S. enteritidis in industrial hen houses.

Salmonella carrier-state in hens: study of host resistance by a gene expression approach

Salmonellosis is one of the main causes of food-borne poisoning due to the consumption of contaminated poultry products. In the flocks, Salmonella is able to persist in the digestive tract of birds for weeks without triggering any symptom. In order to identify molecules and genes involved in the mechanism of host resistance to intestinal carrier-state, two different inbred lines of laying hens were orally inoculated with Salmonella Enteritidis. Bacterial colonization and host gene expression were measured in the caecum and its sentinel lymphoid tissue, respectively. Significantly increased expression of chemokine, anti-infectious cytokine, bacterial receptor, antimicrobial mediator and particularly, defensin genes was observed in the line carrying a lower level of bacteria in the caecum. These innate immunity molecules were either constitutively or inductively highly expressed in resistant adult birds and thus present candidate genes to play an important role in the host defence against Salmonella colonization.

Association of twelve candidate gene polymorphisms and response to challenge with Salmonella enteritidis in poultry

Breeding for disease resistance to Salmonella enteritidis (SE) could be an effective approach to control Salmonella in poultry. The candidate gene approach is a useful method to investigate genes that are involved in genetic resistance. In this study, 12 candidate genes that are involved in the pathogenesis of Salmonella infection were investigated using five different genetic groups of meat-type chicken. The genes were natural resistance associated macrophage protein 1 (SLC11A1, previously known as NRAMP1), inhibitor of apoptosis protein 1 (IAP1), prosaposin (PSAP), Caspase-1 (CASP1), inducible nitric oxide production (iNOS), interferon-gamma (IFNG), interleukin-2 (IL2), immunoglobulin light chain (IGL), ZOV3, and transforming growth factors B2, B3 and B4 (TGFB2, B3 and B4). In total, 117 birds of all groups were challenged with SE at the age of 3 weeks. In all birds at 7-day post-infection SE load in caecum content, spleen and liver were quantified. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assays were used to genotype all animals for each gene. Overall we found the most significant associations with caecum content, nine of 12 genes showed a significant association (SLC11A1, IAP1, PSAP, CASP1, iNOS, IL2, IGL, TGFB2 and TGFB4). For liver, five genes (SLC11A1, CASP1, IL2, IGL, and TGFB4) and for spleen, only one gene (TGFB3) showed a significant association with SE load. By showing associations of 12 PCR-RFLP assays with SE load after a pathogen challenge, this study confirmed the polygenic nature of disease resistance to SE.

Effect of two candidate genes on the Salmonella carrier state in fowl

Selection for increased resistance to Salmonella carrier-state (defined as the persistency of the bacteria 4 wk after inoculation) could reduce the risk for the consumer of food toxi-infections. The effects of two genomic regions on chromosomes 7 and 17 harboring two genes, NRAMP1 (SLC11A1) and TLR4, known to be involved in the level of chicken infection 3 d after inoculation by Salmonella were thus tested on a total of 331 hens orally inoculated at the peak of lay with 10(9) bacteria. The animals and their parents were genotyped for a total of 10 microsatellite markers mapped on chromosomes 7 and 17. Using maximum likelihood analysis and interval mapping, it was found that the SLC11A1 region was significantly involved in the control of the probability of spleen contamination 4 wk after inoculation. Single nucleotide polymorphisms (SNP) within the SLC11A1 and TLR4 gene were tested on those animals as well as on a second batch of 279 hens whose resistance was assessed in the same conditions. As the former was significantly associated with the risk of spleen contamination and the number of contaminated organs, SLC11A1 appears to be involved in the control of resistance to Salmonella carrier state. The involvement of the TLR4 gene was also highly suspected as a significant association between SNP within the gene, and the number of contaminated organs was detected.

Resistance of broiler outbred lines to infection with Salmonella enteritidis

Salmonella infections originating from poultry are one of the major causes of food-borne disease. For the control of salmonella in poultry a multifactorial approach is more likely to be effective, and the genetic resistance of poultry breeds to salmonella infections may be a valuable contribution. Experimental Salmonella enteritidis infections were examined in three different broiler outbred lines: the FC line, which had been selected for feed conversion efficiency; the R line, which had been selected for growth rate; and the C line, a commercially available line. The FC line had the highest mortality rate after intramuscular inoculation with 5 x 10(6) colony forming units (CFU) of S. enteritidis at 2 weeks of age (40% versus 21 and 20% in the other lines). However, at slaughter age, the number of birds carrying salmonella in caecal contents, and the concentration of salmonella in the caecal contents, was lowest in the FC line. The FC and R lines were compared by inoculation with doses ranging from 10(2) to 10(7) CFU S. enteritidis. At sublethal doses (10(5) CFU or less), the FC line carried significantly less salmonella in caecal contents and the rate of systemic infection was lower. The start of shedding was also delayed compared with the R line. At doses of 10(6) CFU S. enteritidis or higher, there were no differences in salmonella carriage between the lines, and the FC line showed higher mortality. In conclusion, resistance to mortality and resistance to the carriage of S. enteritidis do not necessarily coincide within lines, as the FC line showed high mortality but low carriage, both in survivors of high infection doses and in all birds at lower infection doses.

Effect of low and high doses of Salmonella enteritidis PT4 on experimentally infected chicks

Chicks (1-d-old, three groups, each containing 50 chicks) were inoculated with 2 x 10(2) and 2 x 10(8) CFU of Salmonella enteritidis; the third group were kept as uninoculated control. Five birds from each group were euthanized at intervals from 6 h to 4 weeks post-inoculation (pi). In the low-dose group S. enteritidis was isolated from 60% cecal samples at 18 h pi, and from 20% of livers at 3 d pi. Individual variation in the frequency of S. enteritidis recovery was observed in this group. The clearance of salmonella from the organs was faster in the low-dose group, and salmonella was not isolated from the liver and cecum at 21 and at 27 d pi, respectively. However, in the high-dose group, S. enteritidis was isolated from all ceca and 80% of liver 6 h pi, and salmonella was detected in the cecum and liver throughout the experiment. Serous typhlitis and unabsorbed yolk sac were the most prevalent lesions in both groups. Granulomatous nodules in the cecum were found occasionally in some cases in both inoculated groups, which can play a role as reservoirs in carrier chicks.

Candidate genes for resistance to Salmonella enteritidis colonization in chickens as detected in a novel genetic cross

Salmonellosis is a zoonotic disease that is problematic for both animal production and food safety. A novel genetic cross, named the Iowa Salmonella response resource population (ISRRP), was established to elucidate the genetic control of resistance to Salmonella enteritidis (SE) colonization in young chicks, to characterize unique resistance alleles, and to estimate gene interaction effects. Outbred broiler sires were mated with dams of diverse, highly inbred, light-bodied lines to produce an F(1) generation that was informative for all heterozygous alleles of the sires. Mating F(1) sires back to dams of the corresponding inbred line produced a backcross generation. To mimic the natural route of exposure and thus afford the opportunity to investigate mucosal immunity, pathogenic SE were inoculated into the esophagus of day-old chicks. After 1 week, the SE colonizing the cecal lumen and the spleen were enumerated. Candidate genes were selected for analysis based upon one of the two criteria. Functional candidates were genes with reported activity related to the tested traits. Positional candidates were genes mapped near microsatellites that were linked, in other phases of this project, with antibody levels to SE vaccine. Broiler sire alleles of the MHC class I, NRAMP1, PSAP, and IAP1 genes showed association with SE colonization in the F(1) generation of this novel disease resistance resource population.

Salmonella enterica serovar enteritidis burden in broiler breeder chicks genetically associated with vaccine antibody response

The relationship between antibody response to Salmonella enteritidis vaccine and internal organ burden of S. enteritidis is not fully understood. The genetic relationship, therefore, between postchallenge S. enteritidis burden and antibody response to S. enteritidis vaccine was determined in broiler breeder chicks. Sibling chicks from a broiler breeder male line were either inoculated with a pathogenic S. enteritidis or vaccinated with a commercial S. enteritidis vaccine. Spleen, liver, cecal wall, and cecal content samples from S. enteritidis-challenged chicks (n = 120) were cultured for enumeration of bacteria. Unchallenged chicks (n = 314) were vaccinated at 11 days of age, and serum samples were taken at 10 days postvaccination. Antibody response to vaccination and number of S. enteritidis in cecal content cultures were negatively correlated (-0.772), demonstrating that genetic potential for greater antibody response to S. enteritidis vaccine is associated with lesser S. enteritidis bacterial burden in cecal content of broiler breeder chicks. The findings suggest that genetic selection for vaccine antibody responsiveness can lower bacterial burden in the gut lumenal content and, thus, potentially reduce contamination of poultry products at processing.

Genetic line differences in survival and pathogen load in young layer chicks after Salmonella enterica serovar enteritidis exposure

Early infection may result in long-term colonization of layers with Salmonella enterica sv. enteritidis (S. enteritidis, SE), resulting in shedding into table or hatching eggs. To evaluate genetic factors underlying early response to SE, genetic line differences in mortality and pathogen load at two sites (cecal lumen and spleen) were investigated. At day of hatch, chicks of four genetic lines were intra-esophageally inoculated with one of three doses of SE phage type 13a. There was a significant effect (P < 0.001) of genetic line on chick 6-d survival. The effect of genetic line was significant (P < 0.05) on survivors' SE burden in cecal content but not on SE burden per gram of spleen. The SE pathogen load of the spleen and the cecal content were not significantly correlated, indicating that independent host mechanisms are partly responsible for these two traits. Genetic line differences in chick survival and SE colonization of cecal content were demonstrated in young layer chicks.

Characterization of the innate and adaptive immunity to Salmonella enteritidis PT1 infection in four broiler lines

Four broiler lines were inoculated orally with Salmonella enteritidis phage type 1 at the age of 7 days (experiment A: lines 1 and 2) and at the age of 1 day (experiment B: lines 3 and 4). At various days post-infection chickens were sacrificed and the number of Salmonella in the caeca, liver, and spleen were determined. Furthermore, phagocytic activity, cellular immune responses, and humoral responses were determined using, respectively, single-cell suspensions of spleen or intestine and serum. In both experiments, similar trends were seen. Increased numbers of S. enteritidis were found in the caeca of lines 1 and 3, whereas at the same time a decreased colonization was found in the spleen and in the liver, as compared to lines 2 and 4. In the latter two lines, the phagocytic activity of the phagocytes was higher and the humoral responses were lower. Observations from this study suggest that lower activity of phagocytes and higher humoral activity prevent systemic S. enteritidis infection.

Impact of genetics on disease resistance

The genetics of a bird or flock has a profound impact on its ability to resist disease, because genetics define the maximum achievable performance level. Careful attention should be paid to genetics as an important component of a comprehensive disease management program including high-level biosecurity, sanitation, and appropriate vaccination programs. Some specific genes (e.g., the MHC) are known to play a role in disease resistance, but resistance is generally a polygenic phenomenon. Future research directions will expand knowledge of the impact of genetics on disease resistance by identifying non-MHC genetic control of resistance and by further elucidating mechanisms regulating expression of genes related to immune response.

Estimated heritability of the resistance to cecal carrier state of Salmonella enteritidis in chickens

Previously, we have shown differences in susceptibility to the cecal carrier state in chicks orally infected with Salmonella enteritidis (SE) at 1 wk of age for four outbred lines: L2, B13, PA12, and Y11. The egg-type line L2 was one of the most susceptible lines and presented a large variability in cecal SE colonization. The heritability (h2) of the resistance to SE colonization in ceca was estimated in L2 chickens to determine whether genetic factors might be involved in its control. In three independent trials, a total of 819 L2 chicks produced from 88 sires and 232 dams were challenged orally with SE at 1 wk of age. Each week after inoculation, the frequency of cecal colonization was estimated. When this value had fallen to 50%, all the remaining animals were killed. The extent of cecal colonization by SE was estimated directly by counting the viable organisms in organs and determining the numbers of positive ceca. Enrichment culture was used in Trials 2 and 3. The effects of trial, of room within trial, and of cage within room on the frequency of SE contaminated ceca were often significant. No significant effect of sex was observed. Estimation of h2 using the frequency of SE positive ceca was low, 0.06 +/- 0.07, when results of direct culture were considered. In contrast, when considering the frequency obtained after enrichment, the h2 was estimated at 0.20 +/- 0.12. This result suggests a genetic basis for the expression of the resistance to colonization. An experiment of selection for resistance to SE carrier state in the chicken ceca should definitively confirm the genetic origin of the resistance.

Resistance to salmonellosis in the chicken is linked to NRAMP1 and TNC

Natural resistance to infection with Salmonella typhimurium in mice is controlled by two major loci, Bcg and Lps, located on mouse chromosomes 1 and 4, respectively. Both Bcg and Lps exert pleiotropic effects and contribute to cytostatic/cytocidal activities of the macrophage. Bcg encodes for a membrane phosphoglycoprotein designated Nrampl (natural resistance-associated macrophage protein 1), which belongs to an ancient family of membrane proteins, Lps has not been cloned yet, but its location on mouse chromosome 4 has been refined for positional cloning. As in mice, chicken inbred lines differ in their susceptibility to infection with Salmonella typhimurium. We have tested the candidacy of the chicken homologs of Nrampl and Tnc (a locus closely linked to Lps), in the differential resistance of chicken inbred lines to infection with S. typhimurium. We have first analyzed six inbred chicken lines of Salmonella-resistant or Salmonella-susceptible phenotypes for the presence of nucleotide sequence variations within the coding portion of NRAMP1. We have identified 11 sequence variations within NRAMP1 in the chicken inbred lines tested: 10 of these represented either silent mutations or conservative changes. However, one G-->A substitution at nucleotide 696 resulted in the nonconservative replacement of Arg223 to Gln223 within the predicted TM5-6 region. This allelic variant was specific to the susceptible line C and not observed in any of the resistant strains. To investigate the effect of NRAMP1 and TNC on resistance to infection with S. typhimurium, 425 (W1 x C)F1 x C chicken progeny were examined during a period of 15 days postinfection. Together, NRAMP1 and TNC explain 33% of the early differential resistance to infection with S. typhimurium of parental lines C and W1. Our data established that resistance to infection with S. typhimurium in chickens is inherited as a complex trait and that comparative mapping has proven to be useful to identify Salmonella-resistance genes in the chicken.