Salmonella enterica serovar enteritidis (SE) emerged in the United States during the late 1980s and is considered as the main serotype associated with contamination of table eggs.

The probability of egg-borne infection in humans can be prevented by effective implementation of a comprehensive Egg Quality Assurance Program (EQAP). Surveys conducted by the U.S. Department of Agriculture (USDA)-National Animal Health Monitoring System (NAHMS) have demonstrated the presence of SE in the environment of laying hens with a special concentration in high-rise units in multi-aged, in-line operations.

EQAPs involve a coordinated approach to suppress SE infection. The chain of multiplication from elite stock to commercial pullets is regularly monitored to ensure freedom from vertically transmitted SE infection. Egg operations functioning in accordance with an EQAP are required to implement biosecurity procedures to limit the possibility of introduction of SE infection, requiring an active rodent control program.

Vaccination represents an important component of a preventive program, with the selection of commercial vaccines and frequency of administration based on the history of the farm and the risk of exposure.

Currently, monitoring programs for SE vary in intensity. They range from comprehensive sequential assays performed during the life of the flock, as required in the state of Pennsylvania and by a producer of nationally branded eggs, to a single assay performed at the time of flock depletion, as designated by the United Egg Producers (UEP).

The most recent Food and Drug Administration (FDA) initiative to suppress salmonellosis in consumers proposes a comprehensive protocol for sampling flocks, essentially in compliance with the Pennsylvania EQAP, which prescribes procedures to establish the SE status in the event of detecting environmental contamination.

Epidemiology of SE infection

The organism can be vertically transmitted through the egg to successive generations. For this reason, the National Poultry Improvement Plan (NPIP) has introduced a certification program involving breeding flocks and hatcheries. As with all salmonella serotypes, SE is readily transmitted through direct contact with infected chickens by the oral-fecal route.

In exposed flocks, the intestinal tract becomes colonized with SE organisms, which are excreted in an inconsistent pattern when hens are subjected to environmental stress. Indirect infection can occur by transfer of contaminated equipment, footwear or egg packing material. In the context of multi-age, in-line operations, mice exposed to SE become infected and serve as reservoirs of infection capable of disseminating the infection among houses and to successive flocks.

Intestinal colonization with SE, as detected by environmental swabs, does not directly represent a hazard to consumers since contamination of the eggshell during oviposition should be effectively removed by current washing and disinfection practices. The possibility exists that internal contamination of the egg will occur by passive penetration of SE through the shell.

A potential public health problem arises when hens are subjected to stress or are immunosuppressed, resulting in extension of infection from the intestinal tract to the liver, spleen, ovary and oviduct. If the reproductive tract is colonized, a proportion of eggs which are laid may contain SE organisms, albeit at a low concentration, perhaps in the hundreds per egg.

Prolonged storage, especially when commercial table eggs are subjected to temperatures above 50 F, will result in proliferation of SE to levels sufficient to infect consumers.

Circumstances which contribute to systemic dissemination of SE from the intestinal tract include stress associated with the onset of egg production and maintaining high production extending from twenty to thirty-five weeks.

Molting represents the second period of concern. The transition from initiation of molting by starvation to restricting the energy content in the feed has reduced the risk of systemic infection. Despite this improvement, reduction in feed intake, change in the consistency of diets, alteration of the intestinal flora and physiological changes may predispose flocks to extension of infection from the intestinal tract to the reproductive system.

Under commercial conditions, it is impossible to absolutely prevent the introduction of infection by the indirect route. With acceptable biosecurity and management, the risks of introducing infection in a flock are low, as long as adequate operating procedures are implemented.

Cost of infection

Financial consequences of infection can be considerable, especially if contamination results in human infection and can be traced-back by public health authorities to the plant and farm of origin. For this reason, vaccination is as an essential protective measure.

Vaccination against SE

Live attenuated Salmonella typhimurium (ST) USDA-approved vaccines are commercially available. Live ST vaccines have been developed by deletion of specific genes [aroA, cya, crp], resulting in a defective organism devoid of pathogenic properties. Live attenuated vaccine strains have a reduced capacity for replication, although they have the ability to colonize the intestinal tract, competing with wild salmonella strains for attachment receptors on the enterocytes (cells lining the intestinal tract).

One commercial ST vaccine does not interfere with routine SE assays since the defective metabolism of the selected strain results in identification, if present, as Hafnia alvei. Attenuated, gene-deleted ST vaccine strains can be administered either in drinking water or by coarse spray.

Introducing the vaccine into the intestinal tract of chicks within the first 48 hours after hatch usually results in colonization of enterocytes, followed by stimulation of protective cell-mediated immunity.

This is especially important in pullets during the first 21 days after placement, when the chicks do not have a fully developed intestinal flora able to compete with wild-type salmonella strains present in the feed or the environment. Generally, chicks which receive a live gene-deleted mutant ST vaccine at the hatchery or within a day of placement develop protection within hours against salmonella organisms.

Administration of antibiotics concurrently with Marek's vaccine at one day of age in the hatchery may adversely affect vaccine colonization and protection. Accordingly, the administration of the ST vaccine can be delayed until the second day of placement, although it is stressed that spray vaccination at the hatchery is the most effective method of obtaining early colonization and protection of the intestinal tract.

Vaccination at one day of age has not been associated with any adverse effect on livability in chicks of normal quality when handling and transportation procedures prevent chilling.

Successive administration of the attenuated, gene-deleted ST vaccine results in circulating immunity, which will provide some reproductive tract protection in the event of extension of infection from the cecum and terminal ileum of the intestine into the vascular system.

Inactivated oil emulsion vaccines, which were the first line of defense when SE emerged in the late 1980's, should be part of a coordinated protection program. Inactivated vaccines containing various phage types of SE stimulate the production of immunoglobulin G [IgG] antibodies, which are transferred to the yolk and will inactivate SE organisms which may be incorporated into eggs produced by infected hens.

Since inactivated SE vaccines can be combined with other antigens, including infectious bronchitis and Newcastle disease, protection against all three infections can be enhanced, justifying the cost of handling and administration of an inactivated emulsion to hens.

Vaccination programs

Attenuated gene-deleted ST vaccines can be administered in either drinking water or by coarse spray at the hatchery or during the first 36 hours after placement. Vaccination should be repeated during the 21- to 30-day period following the development of a mature intestinal flora and then again at 12 to 14 weeks-of-age before transfer. The three-dose program is recommended for all flocks where there is no previous history of SE on the laying farm to which flocks are to be transferred.

Additional protection

An alternative program, which provides a higher degree of protection, would involve the first two attenuated gene-deleted ST vaccines followed by the administration of an inactivated SE vaccine at 14 to 16 weeks-of-age, preferably at least two weeks before transfer. The live and inactivated program is considered essential for farms or complexes with a history of SE.

Extra-label administration of attenuated ST vaccines has been applied on farms with a history of SE infection. Vaccines are administered on the specific recommendation of a veterinarian, usually two weeks before molting.

Financial projections have demonstrated highly positive benefit to cost ratios attributable to flock vaccination.

Depending on future FDA requirements, which at the least will involve diversion of eggs from infected flocks to breaking and pasteurization, the cost of vaccination is offset ten-fold even if an infection occurs at the mid-cycle of only one of eight consecutive flocks.

Calculation of the attractive financial benefits of vaccination, as reported at a regional meeting, did not take into account the degradation of brand image or the substantial legal costs associated with a confirmed infection resulting in lawsuits following a confirmed traced back case.