Despite the diligent commitment of industry and government resources to prevent poultry-associated transmission of salmonella to consumers, significant public health concerns remain – with equally significant economic consequences. Salmonella control efforts include both preharvest strategies (directed toward minimizing the introduction, persistence and transmission of flock infections) and postharvest strategies (designed to safeguard the microbial integrity of food products or to decontaminated them before consumption). This article reviews preharvest salmonella control strategies and assesses what they are, and are not, able to achieve, individually and together.
Vaccinating poultry flocks against salmonella has several goals: reduce the susceptibility of individual birds to infection, horizontal transmission of infection within flocks, vertical transmission of infection to the progeny of infected breeding flocks, the salmonella load in poultry house environments (which affects the likelihood of transmission of infection to subsequent flocks), and the frequency of product contamination.
Types of vaccines. Vaccines can be either inactivated (killed) or attenuated (live). An advantage of inactivated vaccines is that they can be rapidly prepared from specific strains responsible for problems in a particular locale or in a particular commercial enterprise or industry. Multivalent vaccines containing mixtures of strains or serotypes can provide an expanded spectrum of protection. Advantages of attenuated vaccines include their ease of administration (often in drinking water) and their ability to induce a stronger and longer-lasting immune response.
Drawbacks. Both killed and live vaccines have provided significant protection against salmonella infection and egg contamination in challenge studies, but neither type of vaccine has consistently been able to prevent infection entirely (especially against high pathogen doses) or to effectively cross-protect against different serotypes.
Poor vaccine performance. This has sometimes been attributed to severe rodent control or sanitation problems in poultry houses, feed or water deprivation, or environmental stressors such as heat. Nevertheless, significantly lower frequencies of egg contamination and human S. Enteritidis infections were reported after the implementation of a British vaccination program for egg-laying hens.
Bottom line on vaccination. Vaccination may be most valuable as a component in a comprehensive program of risk reduction practices, especially in application to highly susceptible flocks or flocks exposed to severe challenges from environmental sources, when epidemiologic evidence has led to heightened concerns about particular salmonella serotypes.
As illustrated by the successful S. Pullorum program of the National Poultry Improvement Plan, flock testing has sometimes played a leading role in reducing the incidence of salmonella infections. However, a USDA trace-back testing program for S. Enteritidis in the early 1990s did not decrease S. Enteritidis isolation from either hens or eggs (Figure 1). Trace-back testing to identify and eradicate infected flocks is unlikely to be effective when salmonella can be continuously reintroduced from diverse environmental sources.
Assay sensitivity and timing. Decision-making based on testing results always involves some uncertainty because of both assay sensitivity issues and variations over time in the detectable parameters of infection (including fecal shedding, antibody production and egg contamination). Antibody detection can be highly sensitive but is inherently historical and does not necessarily indicate active infection or ongoing egg contamination. The usefulness of culturing eggs for salmonella is limited by the fact that contamination is infrequent, transient and sporadic. Environmental testing is easily performed, but provides only an indirect measurement of the probability of product contamination.
Detection and verification. In salmonella control programs with testing and response components, the potential effectiveness of the response depends on the nature of the testing questions that are asked: what, when and how many samples are collected and how are they tested? Testing in quality assurance programs serves both to detect flocks that pose a potential threat to public health and to verify that the investment of resources in risk reduction practices is cost-effective.
Bottom line on flock testing. Basing decision-making and response on serotype-specific testing results is useful, arguably even essential, for responding to severe public health problems that are tightly linked to individual serotypes, but a more serotype-independent approach has the advantage of detecting and responding to emerging problems (perhaps involving new reservoirs of infection or previously uncommon serotypes) before their impact becomes more severe.
Gastrointestinal colonization control
Poultry are highly susceptible to infection by salmonella for several days after hatching, but they rapidly become much more resistant over the next few weeks of life as they acquire a complete and protective set of intestinal bacteria from their environment. This mature intestinal microflora competes with salmonella for attachment sites in the gut and alters intestinal biochemical conditions to inhibit salmonella growth.
Defined vs. undefined cultures. Providing young chicks with intestinal contents from mature birds or undefined cultures derived from this material has reduced both intestinal colonization and invasion to internal tissues by salmonella. Defined mixtures of microorganisms offer potentially greater consistency of performance and assurances of safety, but typically offer less protective efficacy than undefined preparations. Protection tends to increase along with the complexity of defined mixtures.
Timing of administration. The greatest opportunity for protection by “competitive exclusion” treatment is during the first few days of life when susceptibility to infection is highest and salmonella exposure can lead to very persistent infection. Colonization control is most effective when chicks or poults are treated before exposure to pathogens, so the presence of salmonellae in hatcheries or rearing facilities can nullify protection.
Mature birds. Preventive provision of protective intestinal bacteria to mature birds is of uncertain value except when the normal microflora are compromised (by antibiotic administration or during periods of stress such as induced molting by feed restriction, transportation, vaccination, environmental heat, vaccination or peak egg production). Chemical additives in feed or water (including various acids, complex carbohydrates, and chlorate) can be used to reduce bacterial counts and prevent re-contamination as well as to manipulate intestinal biochemistry to either directly inhibit pathogen colonization or support the growth of protective microflora.
Bottom line on competitive exclusion. Like vaccination, colonization control is seldom able to completely exclude salmonella and its effects can be overcome by severe salmonella challenges. Nevertheless, colonization control can make a significant contribution to risk reduction efforts by diminishing susceptibility to salmonella infection in vulnerable young poultry and during periods of stress for flocks when the normal intestinal microflora is disrupted.
Flock management and sanitation
Because a long list of flock management and sanitation risk factors have been reported for salmonella infection in poultry flocks, identifying the most important issues and determining the most effective preventive or corrective practices can be highly complex.
Hatcheries. Here young birds are at their stage of maximum susceptibility to infection, are especially critical salmonella control points. Contaminated eggs can introduce salmonellae into hatcheries and air circulation can distribute contaminated fluff and other hatchery debris.
Production flocks. Even when intensive pathogen control investments are made in breeding flocks, environmental persistence in laying or broiler houses can lead to flock infections being initiated at these later stages of production. Direct contact between birds, airborne circulation of dust, ingestion of contaminated feces or litter, and movement of contaminated equipment or personnel can all spread infection rapidly through flocks.
Feed and litter. Salmonella can survive in litter and feed for more than two years after the removal of an infected flock. Salmonellae isolated from finished carcasses or table eggs have been occasionally – but not often – linked to contaminated feeds. Levels of moisture in poultry houses are highly influential in determining whether (and where) salmonellae persist.
Rodents (mice and rats) and insects (flies, litter, beetles, and cockroaches) are constant threats to perpetuate and amplify environmental contamination and to re-introduce salmonella after cleaning and disinfection of poultry houses.
Stressful situations, such as feed withdrawal associated with induced molting or transportation can significantly increase the susceptibility of poultry to salmonella infection.
Risk reduction practices
Several specific preharvest risk reduction practices are common to most salmonella control programs.
- Eggs and chicks (or poults) should be obtained only from breeding flocks proven to be salmonella-free.
- Hatching eggs should be disinfected and hatched under stringently sanitary conditions. Poultry houses should be thoroughly cleaned and disinfected between flocks.
- Rodent and insect control measures should be incorporated into house design and management and periodically monitored.
- Rigidly enforced biosecurity practices should be implemented to restrict the movement of personnel and equipment onto and within poultry housing premises.
- Feed should be pelleted or contain no animal proteins.
- Water should come from sources that are treated to ensure microbiological purity.
An improvement in the frequency of human illness due to S. Enteritidis was reported in most states which implemented egg quality assurance programs based on these types of risk reduction practices (Figure 2).
Bottom line on flock management/sanitation. The principal advantage of the risk reduction approach is that it proactively addresses the causes of salmonella infection in poultry flocks rather than merely seeking to identify infected flocks or to render flocks less susceptible to infection. However, the major shortcoming of an emphasis on risk reduction is that this approach is a long-term route to achieving salmonella control and may not provide a rapid response for problems with severe short-term public health or economic consequences.
Preharvest control facts
- No single response is likely to be an effective unilateral solution to the complex public health and economic problems associated with salmonella in poultry.
- Comprehensive quality-assurance programs, incorporating both the coordinated and sustained implementation of risk-reduction practices throughout the production continuum and targeted testing to detect pathogens of concern, have yielded promising results in several nations.
- Although most quality-assurance programs emphasize risk reduction, testing provides essential verification of the efficacy and cost-effectiveness of risk reduction practices (and identifies flocks infected with uniquely problematic serotypes).
- Preventive treatments such as colonization control or vaccination can reduce the salmonella susceptibility of poultry in case risk reduction practices fail to prevent pathogen introduction into flocks. Vaccination also enhances the short-term responsiveness of control programs to address problems associated with specific serotypes of elevated significance.