Early vaccination for protection against Salmonella infections of poultry

Hatchlings are highly susceptible to Salmonella with infection by a few organisms resulting in rapid colonization of the intestinal tract (7). Infection of young birds results in heavier shedding of salmonellae and a longer duration than occurs in older birds.

Resistance to Salmonella infection increases with age with older birds requiring a higher level of salmonellae to become infected (6). It is for these reasons that early vaccination of poultry, preferably at day of hatch, should be used as a critical
intervention to reduce infection by salmonellae when chicks are most susceptible.

Passive immunity to progeny chicks has been exploited as an intervention to break the chain of infection vertically against salmonellae by vaccinating breeders with a combination of both live and killed Salmonella vaccines (3,5,7).

Live Salmonella vaccines are strains of pathogenic bacteria that are weakened and, thus, no longer pathogenic. Vaccination with live vaccines is designed to stimulate maximal immunity and provide a first-line of defense that directly increases the level of resistance to Salmonella infection in the bird. The mechanism of resistance can be two fold, one provided by the physical presence of the live vaccine organisms causing an exclusionary effect in the gut preventing colonization by other salmonellae, and the other provided by early activation of cell-mediated immune responses that serve to block pathogens at the gut’s “portal of entry.” Pathogen pattern recognition receptors play an important role with toll-like receptors in stimulating early immune responses (8,11). van Immerseel et al (13) showed early vaccination of chicks with a live Salmonella vaccine stimulated innate immunity as early as 12 hours after hatch. As a result of the combination of a competitive exclusion effect and early onset of immunity, a reduction of Salmonella in intestinal colonization leads to reduced fecal shedding into the environment (2,9,10,12). 

Vaccination with a live Salmonella vaccine at one-day of age is the best opportunity for earliest colonization by the vaccine when done at the hatchery or at placement. Live vaccine organisms initially colonize the gut lining and prevent colonization by other Salmonella sp. Live Salmonella vaccines can be applied en masse using spray application either in a hatchery spray cabinet or with a backpack portable sprayer. Use of a food-grade dye in the vaccine water helps to visualize the spray pattern for full coverage of birds to ensure uniform administration of the vaccine. Water stabilizer products are commercially available that neutralize iron and chlorine, and contain dye that aids in spray or drinking water application of live vaccines. Bright lights provide an effective stimulus to encourage active preening and uptake of the vaccine.

Vaccination of breeders and good hatchery management is a critical component of a Salmonella prevention and control program. However, vaccination practices and resulting inherent immunity must work efficiently with current hatchery and medication practices to be practical and economical. Compatibility with other products used during the hatching of poultry to support the health of birds must be considered. Water stabilizers are often used in vaccine mixing water and in drinking water. Vaccines, such as those used to protect against Newcastle disease virus, infectious bronchitis virus, Marek’s disease virus and coccidiosis, are administered either in ovo or soon after hatch. Antibiotics, such as gentamicin, are provided either in ovo or soon after hatch, for the prevention of early mortality associated with colibacillosis and paratyphoid bacteria. Concurrent use of live Salmonella vaccines and these types of products should be investigated for their interference effects or compatibility with live Salmonella vaccines prior to administration to birds in the hatchery or at placement in the house.

Vaccination with live Salmonella vaccines has been shown to be useful in reducing infections in poultry, however, there are limitations with their use. Live Salmonella vaccines have the capability of protecting against serogroups B and D, but cross protection against serogroups C and E is moderate (1,4). Combining a vaccination program with killed vaccines to target specific serotypes is needed for optimal protection against other serogroups, with live vaccines inducing mucosal and cellular immunity and killed vaccines providing long-lived systemic or humoral immunity. Including a live Salmonella vaccine in a primary breeder or layer prevention program should not interfere with bacteriological or serological monitoring programs. Live vaccines should be easily recognized and differentiated from wild-type salmonellae either biochemically or serologically. Live bacterial vaccines are susceptible to antimicrobial agents commonly used in poultry production and this can reduce or even eliminate their efficacy if antimicrobials are unintentionally left activated in vaccine water before spray or drinking water administration. Vaccination is not 100% effective; immune compromised birds may become carriers and can persistently shed the vaccine or wild type organisms. Additionally, a heavy burden of salmonellae in the environment can overcome the protective effects of vaccination.

A range of interventions should be implemented in addition to vaccination to control salmonellae in young poultry including biosecurity, pest and rodent control, judicious use of antibiotics and competitive exclusion with probiotics. These interventions, if  practiced and monitored vigilantly, will effectively reduce the incidence and prevalence of salmonellae infections of poultry. Poultry are an important reservoir for Salmonella and shed and spread are the modus operandi for horizontal transmission of infection to flockmates through contact and environmental contamination. Use of a live Salmonella vaccine to prevent colonization of the gut early by wild-type salmonellae and to reduce shedding of the organisms will effectively control spread and the prevalence of salmonellae in poultry.

REFERENCES

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