In late 1987 the Centers for Disease Control (CDC) reported a sharp increase in Salmonella enteritidis (SE) infections in consumers of shell eggs in the Northeast States. At the time, little was known about the epidemiology of SE in either laying flocks or through the distribution chain for eggs. Except for rare sporadic cases, SE had not previously caused apparent disease in laying flocks or emerged as a public health problem.

In April 1989 Maine began the first cooperative Industry-State-University SE program of surveillance and suppression of SE. This program continues today.

The goal of this program has been to work proactively to reduce the risk of SE infection in laying hens and eggs by identifying, prioritizing and eliminating sources of SE. The program evolved over time, consisting originally of an SE prevalence study of grandparent and parent farms, hatcheries, commercial pullet and production flocks, feed mills, rendering plants and rodents.

Three phases of study

During this time techniques for environmental and direct bird testing were validated and procedures for remodeling and effective cleaning and disinfection between flocks were developed. The use of inactivated emulsion SE vaccine was initiated during this phase. During the second phase, these steps were supplemented by rodent surveillance to routinely evaluate control and serology to verify effective vaccination. In the third phase of development of the Maine program, wet cleaning and disinfection was replaced by dry cleaning of contaminated buildings and in selected flocks, egg testing for SE was conducted.

Five documented outcomes of the program:

1. Infected parent flocks  

Two Maine parent flocks and two NPIP-certified hatcheries in other states were found to be the source of SE prior to 1995. These Maine parent flocks, their fertile eggs and potentially infected chicks were eliminated within two weeks of confirming SE infection.

Following the initial incidents, no SE has been detected in parent flocks or imported pullet chicks in the Maine program for 15 years. At no time have parent flocks been vaccinated against SE.

Over the life of the Maine program, the level of Salmonella spp. of any serotype in fecal samples from day-old parent chicks declined from 19% to 6%, while the overall detection of Salmonella spp. in parent flocks increased from 29% to 79%.  

2. Environmental contamination  

During the first phase of the Maine program, all SE contaminated houses were depopulated, wet cleaned, disinfected and tested before re-stocking with replacements. This method worked well for SE-contaminated multiplier breeder, pullet and free-standing layer houses. In the last 15 years of the Maine program, using standard NPIP monitoring procedures, SE was not detected in these types of houses. 

In contrast, post-cleaning and disinfection (C&D) of larger, automated, multiple in-line house layer complexes failed to completely eliminate SE. The failure of wet C&D was considered to be due to inadequate drying of surfaces after each stage, allowing bacterial overgrowth to occur.

In 2000 it was decided to require only dry-cleaning of SE-contaminated houses after depopulation. The adequacy of cleaning was visually verified by a state inspector. In addition, increased criteria for effective decontamination were applied, mandating the absence of SE at the end of the laying cycle of at least two successive flocks in all houses on a given complex.

No cleaning method can guarantee total elimination of SE from all surfaces but proper environmental management and routine sanitary operating practices can prevent bacterial “blooms” of SE. In properly maintained poultry houses, environmental SE may eventually be out-competed by less harmful indigenous bacteria.  

3. Rodent control  

Rodent control was critical to eliminate SE from contaminated houses, regardless of the cleaning method used. Rodents act as reservoirs and multipliers of SE, and they can quickly re-contaminate recently sanitized houses. Decontamination of clusters of multiple SE contaminated houses was only achieved following effective rodent control within and between all houses. This goal often required repairing and sealing houses. Rodent control is challenging to impossible when buildings have cracks in foundation walls and concrete slabs or have other entry points.

When the effectiveness of rodent control was continuously monitored, it allowed higher awareness of the potential for recontamination. Due to the high reproductive rate of rodents, control efforts must be continuous in all buildings on the premises to achieve effective suppression of SE.

4. Vaccination  

Although vaccination without adequate sanitation and rodent control is unlikely to achieve adequate risk reduction, vaccination of replacement flocks was considered to be critical to SE control from the beginning of the SE program. At the inception of the program, inactivated SE emulsion vaccines were used in replacement flocks to be housed in buildings with a history of SE. Serological testing at 4 to 8 weeks post vaccination was used to confirm that proper vaccination techniques had been followed. 

5. Monitoring  

Environmental monitoring of hatcheries, breeder, pullet and layer houses was implemented routinely throughout the 20-year period following accepted standard procedures. Layer houses were tested at the end of each layer cycle to determine the SE status of the house and the need for decontamination.

In a pilot study, 20 houses with a history of SE were tested when flocks were 45 weeks old and at the end of production. More houses tested positive at the end of production then at 45 weeks.  However none of the 38,000 eggs assayed from these houses yielded SE.