Development and evaluation of an experimental vaccination program using a live avirulent Salmonella typhimurium strain to protect immunized chickens against challenge with homologous and heterologous Salmonella serotypes

Combination of vaccination and competitive exclusion to prevent Salmonella colonization in chickens: experimental studies

Vaccination and competitive exclusion (CE) represent accepted prophylactic measures to control Salmonella infections in chickens. To use the advantages of both the CE technique and vaccination with live Salmonella vaccines the combination of these methods was studied. In three experiments, SPF chickens were pre-treated using combined or unique administration of CE and vaccination with a live Salmonella typhimurium strain on days 1 and 2 of life and challenged with the antibiotic resistant, but otherwise isogenic mutant of this Salmonella typhimurium strain on days 3, 15 or 40 of life. The caecal colonization of both the vaccine and the challenge strain and the antibody response after infection were examined to evaluate the protective effects of the different combinations. The exclusion effect of CE cultures against Salmonella infection could be seen in very young chicks and was still considerable on day 40 of life of the birds. The Salmonella wild-type strain used as vaccine alone also resulted in a substantial protective effect against homologous challenge. The combined administration of competitive exclusion and immunization using the Salmonella wild-type strain as vaccine resulted in a considerable additional protective effect above the level of the respective exclusive application of these prophylactic measures. Administration of the Salmonella vaccine strain prior to or simultaneously with the CE culture produced the best protective effect, because such combinations ensure an adequate persistence of the vaccine strain as prerequisite for the expression of colonization inhibition effects and a strong immune response. The full exploitation of this potential using attenuated live Salmonella vaccines will require the presence of high inhibitory and immunogenic properties in the vaccine strain after attenuation of a selected parent strain. The combination of competitive exclusion and vaccination as a new measure in integrated control programmes against Salmonella infection in poultry could result in a considerable increase of protection in both very young and older chickens.

Vaccination for control of Salmonella in poultry

Salmonella spp. are facultative intracellular pathogens causing localised or systemic infections, in addition to a chronic asymptomatic carrier state. They are of worldwide economic and public health significance. In poultry, which represent important sources of cheap protein throughout the world, fowl typhoid and pullorum disease continue to cause economic losses in those parts of the world where the poultry industries are continuing to intensify and where open sided housing is common. A number of serotypes that cause human gastro-enteritis are also increasing. The costs or impracticality of improvements in hygiene and management together with the increasing problems of antibiotic resistance suggest that vaccination in poultry will become more attractive as an adjunct to existing control measures. However, our understandings of the immunology of Salmonella infections in poultry is rudimentary and much poorer than that of equivalent infections in mice and live vaccine development for poultry has therefore been largely empirical. In addition to the killed Salmonella vaccines which have been used over the past few years with variable efficacy, a number of live vaccines have become available and some new vaccines will appear on the market over the next few years. These new vaccines should fulfil the criteria of efficacy, safety and compatibility with existing systems for monitoring infection before they are released on to a mass market. In this review we attempt to summarise the current understanding of Salmonella immunology in poultry together with the progress that has been made in poultry vaccine development.

Nonrecombinant and recombinant avirulent Salmonella vaccines for poultry

We have constructed and evaluated a live avirulent Salmonella typhimurium vaccine strain, attenuated by deletion (delta) mutations in genes for adenylate cyclase (cya) and the cAMP receptor protein (crp). Immunization of chicks can preclude Salmonella colonization and invasion of challenged vaccinated chickens when compared with the non vaccinated control. Immunization induces significant cross-protective immunity against various Salmonella serotypes and protects laying hens from transmission of Salmonella in or on eggs following challenge with S. enteritidis or S. typhimurium. Immunization of chicks destined to be breeders and then with a booster immunization at 16-18 weeks of age leads to maternal transfer of immunity to chicks which then can be immunized either orally or by coarse spray to display an enhanced immunity to prevent infection of visceral organs by and shedding of Salmonella. The attenuated S. typhimurium vaccine can be genetically manipulated to express foreign antigens specified by cloned genes from other pathogens. Immunization with such recombinant vaccines not only induces immunity to Salmonella but to infection by the pathogen that supplied the genes specifying the protective antigens expressed by the recombinant vaccine.

Virulence and vaccine potential of Salmonella typhimurium mutants deficient in the expression of the RpoS (sigma S) regulon

The alternative sigma factor RpoS (sigma S) is required for Salmonella virulence in mice. We report the immunizing capacity of Salmonella typhimurium rpoS and rpoS aroA mutants to protect susceptible BALB/c mice against subsequent oral challenge with virulent S. typhimurium. When administered orally or intraperitoneally, rpoS derivatives of the mouse-virulent S. typhimurium strains, C52 and SL1344, were highly attenuated and were efficient single-dose live vaccines. rpoS aroA mutants were more attenuated than corresponding single aroA or rpoS mutants, as assessed after oral or intraperitoneal administration, but retained significant ability to protect mice against salmonellosis. Salmonella rpoS and rpoS aroA mutants therefore deserve serious consideration for rational vaccine design. Consistent with this, Salmonella typhi Ty2, a 'wild-type' strain used widely for the development of human live-vaccine candidates against typhoid fever, was shown to be defective for rpoS. In addition, our results demonstrate that rpoS not only controls the growth and persistence of S. typhimurium in deep lymphoid organs, but also plays a role during the initial stages of oral infection.

Protection against oral challenge three months after i.v. immunization of BALB/c mice with live Aro Salmonella typhimurium and Salmonella enteritidis vaccines is serotype (species)-dependent and only partially determined by the main LPS O antigen

The role of the main LPS O antigen in the specificity of protection as mediated by systemic mechanisms following immunization with live attenuated Aro Salmonella vaccines was studied in mice. Innately Salmonella-susceptible (Itys) BALB/c mice were immunized intravenously with a single dose of either Salmonella typhimurium SL3261 aroA (LPS O4,5,12) or Salmonella enteritidis Se795aroA (LPS O1,9,12), and challenged orally 2-3 months later with either S. typhimurium C5 or S. enteritidis Thirsk. Nearly isogenic transductants of the two challenge strains expressing either their own LPS or that of the other serotype (S. typhimurium C5 O4 or O9, and S. enteritidis Thirsk O9 or O4) were also used. Both vaccines conferred similar high protection against the virulent strain of the homologous serotype expressing its own LPS. There was no protection against the heterologous serotype expressing its own LPS. However, when vaccinated mice were challenged with either the same serotype as the vaccine but expressing the heterologous LPS, or with the heterologous serotype expressing the LPS of the vaccine, protection was always lower than protection against the fully homologous serotype. Anti-smooth LPS antibodies showed higher titres against the homologous LPS, but with significant crossreactivity with the heterologous LPS. Antibodies to O-rough S. typhimurium and S. enteritidis LPS were present following immunization with either of the two vaccine strains. The LPS alone cannot fully account for the specificity of protection in this model; other (protein) antigens may be responsible. It remains to be seen whether there is a T-cell mediated component to the specificity of protection conferred by live Salmonella vaccines.

Effect of vaccination of hens with an avirulent strain of Salmonella typhimurium on immunity of progeny challenged with wild-Type Salmonella strains

The avirulent Salmonella typhimurium chi3985 was used to vaccinate white leghorn chickens at 16 and 18 weeks of age, and the effect of maternal antibody on Salmonella colonization of progeny of vaccinated hens was assessed with S. typhimurium F98 or chi3985. Progeny of hens that had been vaccinated at 1 and 3 or 2 and 4 weeks of age with chi3985 were used to determine the effect of maternal immunity on vaccine efficacy. Vaccination of hens induced long-lasting Salmonella-specific antibodies which were transferred into eggs and were detected as immunoglobulin G (IgG) in the egg yolk. Maternal antibody was detected in the progeny of vaccinated birds as IgG and IgA in serum and intestinal fluid, respectively. The titer of maternally transmitted IgG or IgA was highest in the first week of life of the progeny and declined with age. Maternal antibodies prevented colonization of the chicks by S. typhimurium chi3985 and reduced colonization by S. typhimurium F98. Overall, chicks from vaccinated hens had significantly higher antibody responses than did the progeny of nonvaccinated hens after oral infection with Salmonella strains. Maternal antibody reduced the efficacy of vaccination of progeny with chi3985 at 1 and 3 weeks of age. But vaccination at 2 and 4 weeks of age induced excellent protection against challenge with S. typhimurium F98 or S. enteritidis 27A PT 8 in birds from vaccinated hens and in specific-pathogen-free chickens. Vaccination of chickens at 2 and 4 weeks of age has been shown to protect the birds against challenge with homologous and heterologous Salmonella serotypes. A combination of vaccination of adult animals and use of the progeny of vaccinated birds will enhance effective control of Salmonella infections in the poultry industry. This will complement the present control of Salmonella-associated food poisoning caused by Salmonella enteritidis in eggs because the avirulent S. typhimurium vaccine strain chi3985 induced excellent protection against S. enteritidis in chickens.