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1.
Eighteen chickens were immunized subcutaneously with purified type 1 fimbriae from Salmonella enterica serotype Enteritidis at 18 and 21 weeks of age. Evidence of IgG and IgA responses was found in the eggs and in the sera of the immunized hens. Three weeks later, immunized and non-immunized chickens (n=18) were challenged intravenously with 2x10(7) live Salmonella enterica serotype Enteritidis. There was no significant difference in the numbers of eggs laid by immunized and non-immunized birds. The percentage of Salmonella contaminated eggs was significantly higher in the non-immunized group than in the immunized group due to a higher percentage of contamination of the externally disinfected egg shells. There were no statistical differences in the percentages of contaminated yolks and egg whites between control and immunized birds. No differences in the number of colonizing bacteria could be found in the spleen nor in the liver between the immunized and the control groups throughout the experiment. Salmonella was cleared from the ovary of the immunized birds in the second week p.i., in contrast to the control birds where Salmonella was isolated till the third week after infection. Oviducts were significantly more infected in the control group than in the immunized group. Salmonella was cleared from the oviducts at 3 weeks p.i. in the immunized hens but not in the control hens. In conclusion, we demonstrated that the immunization of laying hens with type 1 fimbriae reduced the number of contaminated eggs and reduced the colonization of the reproductive organs.  相似文献   

2.
This study evaluates the immune responses of single avian influenza virus (AIV) HA DNA vaccine immunization using attenuated Salmonella enterica sv. Typhimurium as an oral vaccine carrier and intramuscular (IM) DNA injection. One-day-old specific-pathogen-free (SPF) chicks immunized once by oral gavage with 10(9) Salmonella colony-forming units containing plasmid expression vector encoding the HA gene of A/Ck/Malaysia/5858/04 (H5N1) (pcDNA3.1.H5) did not show any clinical manifestations. Serum hemagglutination inhibition (HI) titer samples collected from the IM immunized chickens were low compared to those immunized with S. typhimurium.pcDNA3.1.H5. The highest average antibody titers were detected on day 35 post immunization for both IM and S. typhimurium.pcDNA3.1.H5 immunized groups, at 4.0±2.8 and 51.2±7.5, respectively. S. typhimurium.pcDNA3.1.H5 also elicited both CD4(+) and CD8(+) T cells from peripheral blood mononuclear cells (PBMCs) of immunized chickens as early as day 14 after immunization, at 20.5±2.0 and 22.9±1.9%, respectively. Meanwhile, the CD4(+) and CD8(+) T cells in chickens vaccinated intramuscularly were low at 5.9±0.9 and 8.5±1.3%, respectively. Immunization of chickens with S. typhimurium.pcDNA3.1.H5 enhanced IL-1β, IL-12β, IL-15 and IL-18 expressions in spleen although no significant differences were recorded in chickens vaccinated via IM and orally with S. typhimurium and S. typhimurium.pcDNA3.1. Hence, single oral administrations of the attenuated S. typhimurium containing pcDNA3.1.H5 showed antibody, T cell and Th1-like cytokine responses against AIV in chickens. Whether the T cell response induced by vaccination is virus-specific and whether vaccination protects against AIV infection requires further study.  相似文献   

3.
A novel Salmonella Enteritidis (SE) vaccine candidate strain, JOL919 was constructed by deleting the lon and cpxR genes from a wild-type SE using an allelic exchange method. The study was carried out to evaluate the strain as a vaccine candidate against salmonellosis. The strain showed the enhanced macrophage invasion, early bacterial clearance and higher immune responses as compared to the other mutants, JOL917 (Δlon) and JOL918 (ΔcpxR), and the wild type. In further analysis, the chickens immunized with JOL919 showed a significant increase in plasma IgG and intestinal secretory IgA levels, which was an indication of robust humoral and mucosal immune responses induced by the candidate. The lymphocyte proliferation response and CD45(+)CD3(+) T cells, associated with an activation of T helper and cytotoxic cells, were also significantly increased in the immunized group, which indicated that the candidate also induced cellular immune responses. The immune cell influx into caecal tissues analyzed by immunohistochemistry showed that CD8(+) T cells were predominated in the immunized group, suggesting that the candidate can clear the invaded pathogen in the intestines by a more direct way involving cytotoxic activity. By the examination of the protection efficacy measured by observations of gross lesions in the organs and bacterial recovery, the candidate can provide an efficient protection upon virulent challenge.  相似文献   

4.
Using a deletion mutant in the regulator of SPI-2, ssrA, we investigated the role of SPI-2 in invasion, intestinal colonization and reproductive tract infection of chickens by Salmonella Enteritidis. The ssrA mutant was fully invasive in phagocytic and non-phagocytic cells but failed to persist within chicken macrophages. The ability of Salmonella Enteritidis to cause disease in orally infected 1-day-old chicks was not altered when ssrA was deleted. Furthermore, caecal colonization was not affected, while spleen and liver showed reduced colonization. Following intra-peritoneal and intravenous infection of 1-day-old chicks, internal organ colonization was strongly reduced. After intravenous inoculation in adult laying hens bacterial numbers of the ssrA mutant were significantly lower in oviducts and ovaries as compared to the wild type strain. The chickens showed less reproductive tract lesions and the recovery of egg production were faster compared to the wild type strain infected chickens. These findings indicate that the SPI-2 regulator ssrA promotes reproductive tract colonization, but is not essential for intestinal colonization of chickens with the host non-specific serotype Enteritidis.  相似文献   

5.
This study investigated protection against Eimeria acervulina (E. acervulina) following vaccination of chickens with an Eimeria recombinant profilin in conjunction with different adjuvants, or by changing the route of administration of the adjuvants. Day-old broilers were immunized twice with profilin emulsified in Montanide IMS 1313 N VG PR adjuvant (oral, nasal, or ocular routes), Montanide ISA 71 VG adjuvant (subcutaneous route), or Freund's adjuvant (subcutaneous route) and orally challenged with virulent E. acervulina parasites. Birds orally immunized with profilin plus IMS 1313 N VG PR, or subcutaneously immunized with profilin plus ISA 71 VG, had increased body weight gains compared with animals nasally or ocularly immunized with profilin plus IMS 1313 N VG PR, or subcutaneously immunized with profilin plus Freund's adjuvant. All adjuvant formulations, except for IMS 1313 N VG PR given by the nasal or ocular routes, decreased fecal parasite excretion and/or reduced intestinal lesions, compared with non-vaccinated and infected controls. Compared with animals vaccinated with profilin plus Freund's adjuvant, chickens immunized with profilin plus IMS 1313 N VG PR or ISA 71 VG showed higher post-infection intestinal levels of profilin-reactive IgY and secretary IgA antibodies. Finally, immunization with profilin in combination with ISA 71 VG was consistently better than profilin plus IMS 1313 N VG PR or Freund's adjuvant for increasing the percentages of CD4(+), CD8(+), BU1(+), TCR1(+), and TCR2(+) intestinal lymphocytes. These results indicate that experimental immunization of chickens with the recombinant profilin subunit vaccine in conjunction with IMS 1313 or ISA 71 VG adjuvants increases protective mucosal immunity against E. acervulina infection.  相似文献   

6.
The adjuvant effect of chicken interferon-gamma (ChIFN-gamma) was examined for protecting chickens against intestinal colonization of Salmonella Enteritidis (SE) following oral exposure. Ten 7-week-old chickens per group were immunized with inactivated SE twice with or without co-administration of ChIFN-gamma intramuscularly, and all chickens were challenged with SE. Sera collected from immunized groups with or without ChIFN-gamma, and from unimmunized group were measured for SE antibody by agglutination test. The levels of antibodies were raised by 1 week post-immunization and did not show any difference between groups with and without ChIFN-gamma. No antibodies were detected in unimmunized group before challenge. Fecal samples from each group were cultured at 1, 4, 7, and 13 days post-challenge to determine the incidence of intestinal colonization and the numbers of SE shed into the environment. Co-administration of ChIFN-gamma, significantly reduced the incidence of intestinal colonization (P<0.05). At 13 days post-challenge, the bacterial counts of SE in organs were also reduced in ChIFN-gamma administered group. These data suggest co-administration of ChIFN-gamma with SE antigen enhances protection against SE challenge without acceleration of antibody production.  相似文献   

7.
Egg contamination by Salmonella Enteritidis has remained a significant public health problem for nearly two decades, and Salmonella Heidelberg has also been recently implicated in egg-transmitted human illness. Colonization of the intestinal tract is a necessary precursor to the invasion of reproductive organs and subsequent deposition inside eggs laid by infected hens, but the relationship between the persistence of Salmonella in the intestinal tract and the likelihood of egg contamination has been uncertain. In this study, groups of laying hens were inoculated with large oral doses of strains of Salmonella Enteritidis and Salmonella Heidelberg, including variants of the original parent strains that had been reisolated from eggs laid by infected hens in a prior study. The shedding of Salmonella in voided feces was monitored for 6 wk postinoculation, and all eggs laid by infected hens between 5 and 22 days postinoculation were cultured for Salmonella in their contents. The mean duration of fecal shedding was significantly longer for the previously passaged Salmonella strains (26.7 days) than for the original parent strains (17.5 days), and the passaged strains caused a significantly higher frequency of egg contamination (6.4%) than did the parent strains (3.3%). However, the duration of fecal shedding and the frequency of egg contamination were not correlated for any of the Salmonella Enteritidis or Salmonella Heidelberg strains.  相似文献   

8.
Poultry products are an important source of Salmonella enterica. An effective way to reduce food poisoning due to Salmonella would be to breed chickens more resistant to Salmonella. Unfortunately host responses to Salmonella are complex with many factors involved. To learn more about responses to Salmonella in young chickens, a cDNA microarray analysis was performed to compare gene expression profiles between two chicken lines under control and Salmonella infected conditions. Newly hatched chickens were orally infected with S. enterica serovar Enteritidis. Since the intestine is the first barrier the bacteria encounter after oral inoculation, intestinal gene expression was investigated at different timepoints. Differences in gene expression between the two chicken lines were found in control as well as Salmonella infected conditions. In response to the Salmonella infection a fast growing chicken broiler line induced genes that affect T-cell activation, whereas in a slow growing broiler line genes involved in macrophage activation seemed to be more affected at day 1 post-infection. At days 7 and 9 most gene expression differences between the two chicken lines were identified under control conditions, indicating a difference in the intestinal development between the two chicken lines which might be linked to the difference in Salmonella susceptibility. The findings in this study have lead to the identification of novel genes and possible cellular pathways, which are host dependent.  相似文献   

9.
In the last 2 decades, the prevalence of Salmonella enterica serotype Enteritidis (Salmonella Enteritidis) has dramatically increased worldwide, becoming the leading cause of food-borne illnesses and an important public health issue. Many studies have suggested the role of the SEF14 fimbrial protein in the adhesion of Salmonella Enteritidis to the host. In the present study, the sefA gene, which encodes the main subunit of the SEF14 fimbrial protein, was cloned into a temperature-sensitive expression vector and transformed into a nonpathogenic, avirulent strain of Escherichia coli. The recombinant strain was used as a vaccine to elicit specific immune response against the SefA protein of Salmonella Enteritidis in 1-day-old chickens. The recombinant strain was reisolated from the intestines of treated birds for up to 21 days posttreatment, demonstrating its ability to colonize the intestinal tracts of 1-day-old chickens. In addition, immunoglobulin A (IgA) against the SefA protein was detected in intestinal secretions from treated birds at 7 days posttreatment and in bile samples from 14 to 21 days posttreatment by enzyme-linked immunosorbent assay. Nontreated birds did not show any evidence of intestinal colonization by the recombinant strain or anti-SefA IgA response in their bile or intestinal secretions. Preliminary evaluation of the recombinant strain showed a potential use of this strain to elicit protection against Salmonella Enteritidis infection in chickens. Further experiments are needed to study the ability of the recombinant strain to protect birds against Salmonella Enteritidis colonization.  相似文献   

10.
Distribution of maternally transmitted Salmonella antibodies and their protective effects were studied in the progeny of broiler breeder birds which had been vaccinated with live S. Typhimurium and inactivated S. Enteritidis vaccines. Vaccination resulted in a significant increase of the antibody concentration in yolk of hatching eggs and in serum and jejunum of the progeny of immunized breeder birds. Higher antibody titres for isotypes IgG and IgA were still seen on day 21 of age. Antibody production of isotypes IgA and IgM by the chickens themselves was found between 14 and 21 days of age. Two challenge models (10(2) cfu/bird on day 1 of age and a seeder bird model, respectively) were used to evaluate the efficacy of maternal antibodies against challenge with S. Enteritidis. Using both models numbers of challenge organisms were lower in the caeca of the progeny of immunized parent birds between day 7 and day 21 of age (maximum about 1.5 log10 units) compared with control chicks. The results indicate the efficacy of maternally transferred antibodies but it remains the question of their practical relevance. The effects of acquired maternal antibodies on an active immunization of the progeny of immunized breeder birds with live Salmonella vaccines are discussed.  相似文献   

11.
Fowl typhoid is under control in poultry farms of developed countries, but it still endemically subsists in commercial laying hen farms of some countries. It has been demonstrated that Salmonella live vaccines can elicit cross-immunity against members of the same Kauffmann-White scheme serogroup. In this work, we explored the protection conferred by TAD Salmonella vac E, a live Salmonella enterica serovar Enteritidis vaccine, against fowl typhoid. Three groups of laying hens were vaccinated with different vaccination schedules starting on the first day of life, and afterwards were infected with 2 x 10(5) CFU of a virulent Salmonella Gallinarum strain, either at wk 28 or wk 52. Mortality, fecal shedding, and organ invasion of Salmonella Gallinarum were assessed. In this work we demonstrated that this Salmonella Enteritidis vaccine is able to cross-immunize against Salmonella Gallinarum. At wk 28, hens vaccinated with three oral doses or with two oral doses combined with one subcutaneous dose were protected by the vaccine. At wk 52, when hens were infected 36 wk after the final immunization, the vaccine was not able to confer protection. Thus, revaccination every 3 mo would be highly recommended. In countries where Salmonella Gallinarum subsists together with Salmonella Enteritidis, control programs should include vaccination of laying hens using safe attenuated Salmonella strains.  相似文献   

12.
FliC, the flagellin antigen of Salmonella Enteritidis, was tested as a vaccine candidate for protective effect against a homologous challenge in chickens. After immunization with recombinant FliC (rFliC) or administration of phosphate-buffered saline (PBS) at 56 days old, the chickens were challenged with 10(9) colony-forming units of Salmonella Enteritidis at 76 days old. The vaccinated birds showed significantly decreased bacterial counts in the liver and cecal contents compared to those administered PBS at 7 days postchallenge, but the protection was partial. The replication experiment also showed a similar result. In both experiments, vaccination induced an increased level of serum anti-rFliC IgG, which was also reactive to the native flagella. The intestinal IgA level was slightly higher in the vaccinated birds than in the control. However, neither the proliferative response nor interferon-gamma secretion of splenic cells upon stimulation with rFliC was induced. Therefore, the effect of rFliC as a vaccine is limited, and further improvement is needed.  相似文献   

13.
为了检测能够表达传染性法氏囊病毒VP2蛋白的重组沙门菌在雏鸡体内主要组织和器官的感染情况,分别用亲本菌株和重组沙门菌免疫雏鸡,免疫后24~216 h每隔24 h对雏鸡的盲肠、血液、肝脏、脾脏、法氏囊中的细菌数量进行分析。结果显示:雏鸡接种重组减毒沙门菌后,其在血液中的数量不断升高并在96 h达到峰值,在脾脏中的数量不断升高并在120 h左右达到峰值,在肝脏、法氏囊中的数量不断升高并在168 h达到峰值,在盲肠中,细菌数量在不断升高并在72 h达到峰值;达到峰值后细菌数量逐渐下降。亲本菌株和重组菌株在相应组织器官中的数量变化趋势一致,但是亲本菌株的数量始终高于重组菌株。结果表明:重组菌株能在雏鸡体内生长定植,能为诱导雏鸡机体产生保护性免疫应答奠定重要基础,有望作为预防传染性法氏囊病毒的口服疫苗。  相似文献   

14.
To evaluate the efficacy of a novel attenuated Salmonella Enteritidis (△lon△cpxR) vaccine candidate (JOL919), chickens were immunized through oral and intramuscular routes to reduce egg contamination against S. Enteritidis challenge. Birds were orally immunized with JOL919 on the first day of life and were subsequently boosted in the 6th and 16th weeks through oral (group B) or intramuscular (group C) route, while control birds were unimmunized (group A). The chickens of all groups were challenged intravenously with the virulent S. Enteritidis strain in the 24th week. The immunized groups B and C showed significantly higher plasma IgG and intestinal secretory IgA levels as compared to those of the control group. The lymphocyte proliferation response and CD45+CD3+ T‐cell number in the peripheral blood of the groups B and C were significantly increased. In addition, the egg contamination rates were significantly lower in the group B (0%, 10.7% and 0%) and the group C (3.6%, 14.3% and 3.6%) as compared to the group A (28.6%, 42.8% and 28.6%) in the 1st, 2nd and 3rd weeks post‐challenge. All animals in the groups B and C showed lower organ lesion scores in the liver and spleen and lower bacterial counts in the liver, spleen and ovary at the 3rd week post‐challenge. These results indicate that this vaccine candidate can be an efficient tool for prevention of Salmonella infections by inducing protective humoral and cellular immune responses. In addition, this vaccine did not prevent egg contamination, but did appear to reduce incidence. Booster immunizations, especially via oral administration route, showed an efficient protection against internal egg contamination with S. Enteritidis.  相似文献   

15.
Rodents play a major role in the transmission and maintenance of Salmonella contamination cycles in poultry facilities. However, very limited field data are available regarding the transmission routes, infection cycle, and shedding patterns of Salmonella by naturally infected wild rodents from commercial layer farms. In this study, a total of 128 resident wild roof rats (Rattus ratus) were captured from a Salmonella-contaminated layer facility. All roof rats were divided into 51 laboratory cages, and weekly monitoring of Salmonella fecal shedding patterns was conducted for 53 wk. Seven roof rats from cages that were observed to frequently shed Salmonella were isolated in individual cages, and daily Salmonella monitoring was performed for 35 days. At the end of monitoring, each roof rat was euthanatized, and isolation of Salmonella from different organs was performed. Results of weekly monitoring of Salmonella showed that 21 of 51 cages (41.2%) were positive for Salmonella Infantis, while two cages (3.92%) were positive for Salmonella Enteritidis. Moreover, 11 cages were positive for Salmonella for at least two sampling weeks. Isolation of Salmonella from fecal droppings was mainly observed during the first 12 wk of captivity. The longest interval between two Salmonella-positive fecal dropping was 24 wk. In the daily Salmonella monitoring, only Salmonella Infantis was isolated from fecal droppings, in which the highest number of Salmonella Infantis organisms per fecal dropping was at 1 x 10(8) colony-forming units (cfu), while the lowest measured quantity was 1 x 10(3) cfu. It was noted that the frequency of Salmonella shedding in fecal droppings appeared to have a linear correlation (r = 0.85) with the number of Salmonella organisms (cfu) per fecal pellet (P < 0.05). Moreover, pulsed-field gel electrophoresis analysis of Salmonella Infantis isolates revealed a single identical pulsed-field pattern. Salmonella Enteritidis isolates from fecal droppings and internal organs also generated a single identical pulsed-field pattern. Interestingly, Salmonella Infantis was not isolated from any of the organs examined, while Salmonella Enteritidis was isolated from the spleen and liver of one roof rat. These results may indicate that wild roof rats could persistently carry Salmonella and contaminate commercial poultry facilities through intermittent fecal shedding. Moreover, Salmonella Enteritidis in wild roof rats appears to be more of a systemic infection, in which isolation is most likely to occur in internal organs, whereas Salmonella Infantis is more likely an enteric type of infection, in which isolation is most likely to occur in the intestinal contents. It is very plausible that layer chickens could become infected with Salmonella through ingestion of Salmonella-positive fecal droppings or feeds contaminated with these fecal droppings from infected resident roof rats. This is likely one of the major reasons why layer houses can be persistently infected by Salmonella even if the facilities are thoroughly cleaned and disinfected and if replacement stocks are obtained from Salmonella-free breeders and rearing units. It is therefore a noteworthy suggestion that rodent control programs inside poultry premises comprise an essential and effective tool in the management and control of Salmonella contamination in layer flocks.  相似文献   

16.
The superior conservation of antigenic determinants on the surface of genetically inactivated bacterial ghosts makes them attractive immunogenic inactivated vaccine candidates. The efficacy of Salmonella Enteritidis (SE) ghost vaccination was evaluated in chickens by characterizing the nature of the adaptive immune response. Chickens from the immunized group demonstrated significant increases in SE-specific plasma IgG, intestinal secretory IgA, and lymphocyte proliferative response. The populations of CD4, CD8, and TCR γδ T-cells in immunized chickens were significantly greater than in the controls. Increased levels of IFN-γ, IL-2, IL-6 and IL-10 were observed in peripheral blood mononuclear cells stimulated with SE specific antigen. After virulent SE challenge, the immune system of immunized chickens was rapidly stimulated, as indicated by significantly increased population of CD4 and CD8 T-cells. Furthermore, the immunized group exhibited decreased challenge strain recovery of the internal organs compared to the non-immunized group. Together, these data indicate that the immunization induced humoral and cell-mediated immunity might be responsible for significant reduction of the virulent challenge strain load in the internal organs of immunized chickens.  相似文献   

17.
The effects of sugar cane extracts (SCE) on immune responses in chickens were studied. Two- or 10-month-old chickens orally administered SCE (500 mg/kg/day), for 3 consecutive days before immunized with sheep red blood cells, Brucella abortus and Salmonella Enteritidis organisms, showed significantly increased and prolonged antibody responses to these antigens, compared to control chickens without SCE. Furthermore, chickens orally administered SCE also revealed enhanced delayed type hypersensitivity responses to human gamma globulin. These results indicated that SCE has immunostimulating and adjuvant effects in chickens.  相似文献   

18.
根据已发表的马链球菌兽疫亚种MGCS10565酮基转移酶(transketolase)的基因序列,设计并合成引物。以ATCC35246株基因组DNA为模板,通过PCR技术,扩增出目的基因并定向克隆至表达载体pET-28a(+)中,然后将重组质粒转化入大肠杆菌BL21(DE3)中,分析并纯化表达产物。选用ICR小鼠作为实验动物模型,以纯化的重组融合蛋白通过皮下注射途径免疫小鼠,并用间接ELISA法监测小鼠血清中的抗体效价。结果表明重组蛋白免疫小鼠后能产生有效的免疫应答,血清中抗体水平有明显的升高。加强免疫2周后,以5LD50的ATCC35246强毒株攻击免疫组及对照组,结果免疫组小鼠的保护率可达37.5%。表明原核表达产物免疫ICR小鼠,可使其对同源菌株攻击产生一定的保护作用,在亚单位疫苗研制中具有潜在的应用价值。  相似文献   

19.
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20.
This study was conducted to evaluate the influence of a vancomycin pretreatment on the ability of marker (nalidixic-acid resistant) Salmonella Enteritidis (SE(M)), field Salmonella Enteritidis (SE(E)), and marker Salmonella Typhimurium (ST(M)) strains to colonize within the intestinal and reproductive tracts and translocate to other organs of leghorn laying hens. In each of three trials, caged laying hens (76, 26, and 33 wk ofage) were divided into six groups designated to receive SE(M), SE(F), or ST(M), and half were pretreated with vancomycin (n = 11-12 hens). Vancomycin-treated hens received 10 mg vancomycin in saline/kilogram body weight orally for 5 days to inhibit Gram-positive bacteria within the intestines. On Day 6, all hens were concurrently challenged by oral, intravaginal, and intracolonal routes with Salmonella and placed into separate floor chambers by Salmonella strain. Two weeks postinoculation, all hens were euthanatized and the ceca, spleen, liver/gall bladder (LGB), upper (URT), and lower (LRT) reproductive tracts, and ovarian follicles were aseptically collected, and analyzed for Salmonella. Results did not differ for the three hen's ages and were therefore combined. The vancomycin pretreatment also had no significant effect on the colonization ability of SE(M), SE(F) or ST(M), and therefore results were combined within Salmonella strain. The marker strain of Salmonella Enteritidis was recovered from 21% of ceca, 4% of LGB, 9% of LRT, and 17% of the fecal samples. The field strain of Salmonella Enteritidis was recovered from 88% of ceca, 96% of spleen, 92% of LGB, 30% of LRT, 4% of URT, 13% of follicle, and 42% of the fecal samples. The marker strain of Salmonella Typhimurium was recovered from 100% of ceca, 74% of spleen, 91% of LGB, 30% of LRT, 9% of URT, 9% of follicle, and 100% of the fecal samples. Among ceca, spleen, LGB, and fecal samples, SE(F) and ST(M) colonization was significantly greater than SE(M) colonization. Overall prevalence of Salmonella in the reproductive tracts of challenged hens was relatively low, ranging from 4% to 30%.  相似文献   

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