It is often difficult to determine how Salmonella issues in poultry begin and what measures should be implemented to prevent them. Chickens may become colonized through both vertical (from parent stock) and horizontal (environmental) means. This article describes how Salmonella colonizes poultry and explains effective measures for preventing or controlling it.
Breeder chicken origin
Many scientists have implicated breeder chickens as vehicles for vertical transmission of Salmonella to the fertile egg. For example, Cox and other researchers, in 1991, evaluated egg fragments, paper pads from chick boxes, and chick fluff from six commercial broiler breeder hatcheries for the presence and level of salmonellae. Overall, 42 of 380 samples (11.1%) from the six hatcheries were contaminated with salmonellae. Salmonella was detected in 22 of 145 (15.2%), five of 100 (4.6%), and 15 of 125 (12%) samples of egg fragments, chick fluff, and paper pads, respectively.
The incidence and extent of salmonellae-positive samples in the breeder hatcheries were found to be much less than previously found in broiler hatcheries, meaning that overall the industry is reducing Salmonella in breeder chicken populations. The researchers concluded, however, that the cycle of salmonellae contamination will not likely be broken until contamination at these critical points is eliminated.
Efforts to break the cycle are complicated by several factors. Breeders infected with salmonellae may not always be easily detectable on the farm because egg production rates for infected chickens may be unaffected and Salmonella may not be detected in fecal samples. Furthermore, in 2002, Bailey and fellow researchers concluded that there was poor correlation between the Salmonella serotypes found on breeder farms and those found in the hatchery.
Interventions for breeders
Over the years, a number of methods have been used to eliminate Salmonella from breeder chicken populations, including slaughtering positive flocks, medications and competitive exclusion, and vaccination.
Slaughtering positive flocks. In Denmark, Sweden and the Netherlands breeder flocks are tested and, if found to be positive for Salmonella, slaughtered.
In the Danish program, birds from Salmonella-positive flocks are slaughtered on separate slaughter lines or late in the day to avoid cross-contamination of Salmonella-negative birds. Farmers there get a better price for birds from Salmonella-free flocks, and slaughterhouses can use the label “Salmonella-free” for birds that meet criteria determined by the authorities. The effect of this program may be seen in Figure 1.
How effective are these programs? By comparison, there are 42.8 cases of human salmonellosis per 100,000 people in Sweden versus only 14.9 per 100,000 in the U.S. The question is, in a country where extraordinarily expensive measures are used to eliminate Salmonella from flocks prior to processing, what are they getting for their money?
Medications and competitive exclusion. Scientists have worked for decades to control Salmonella in chickens by using bacterial cultures to colonize baby chicks and thus, prevent Salmonella from colonizing the chicks. Another approach, sometimes used in conjunction with competitive exclusion (CE), is to use antibiotics to prevent colonization of the chickens with Salmonella.
In one study involving 13 trials, an antimicrobial treatment was administered in breeder flocks followed by CE. The researchers concluded that enrofloxacin significantly reduced the prevalence of S. Enteritidis in tissues from birds, and reduced the level and prevalence of Salmonella in the bird’s environment.
Any program of antibiotic treatment and competitive exclusion must be part of a coordinated program, which will achieve a decrease in the prevalence of S. Enteritidis over time by contemporary use of disease security measures. This may not be acceptable in all cases in the U.S. as many companies are decreasing the use of prophylactic antibiotics.
Vaccination. Most efforts to control Salmonella in breeder flocks in the U.S. have been concentrated on vaccination programs.
Inoue and fellow researchers in 2008 showed that vaccinating breeders helped to increase the resistance of the progeny against early Salmonella Enteritidis (SE) infections. However, the bacteria were not completely eliminated, suggesting that additional procedures are needed to effectively control SE infections.
In some cases, vaccination is very effective; however, in others it not. It is important to identify the serotypes of Salmonella that are most frequently isolated and targeting those in the vaccine.
Hatchery origins and intervention
Many opportunities exist for Salmonella to be transferred from contaminated eggs to uninfected baby chicks during the hatching process. Salmonella may be found in the nest boxes where breeders lay eggs, in the cold storage egg room at the breeder farm, on the truck that transports baby chicks to the grow-out houses, or in the hatchery environment.
The most common approaches to eliminate Salmonella from hatcheries and to control cross-contamination from contaminated eggs to uncontaminated eggs during hatching involve disinfection of the surfaces in the incubator and hatching cabinets and disinfecting the hatching eggs. In disinfecting hatching eggs, however, it is difficult to get the sanitizer to coat the egg effectively and it is not advisable to wet the egg during this process.
Research by the author was conducted to determine if spraying of electrolyzed oxidative (EO) water was effective for disinfecting hatching eggs contaminated with Salmonella. Results for Salmonella typhimurium prevalence in the lower intestines of broiler chickens from hatching eggs treated electrostatically with tap water or EO water during hatch are in Table 1. For electrostatically-treated eggs using EO water, Salmonella was able to colonize only one chicken out of forty tested over two repetitions under actual grow-out conditions.
Even if “Salmonella-free” birds are delivered to the grow-out house, it is still possible for them to become colonized during grow-out. The USDA, Agricultural Research Service (ARS), conducted an epidemiological study to determine the relative importance of all known sources of Salmonella from the hatchery through grow-out and processing in high- and low- production flocks from four integrated operations located in four states across four seasons (Bailey et al., 2001-Table 4).
Data in Table 4 show that Salmonella may be transferred to birds during grow-out from rodents, wild birds, and insect vectors, such as beetles and flies. This data makes it clear that controlling Salmonella during grow-out requires a comprehensive approach.
Grow-out interventions: Competitive exclusion
Researchers have conducted studies using cecal cultures from healthy, Salmonella-free birds in an attempt to develop colonization of the bird’s intestines with good bacteria that will preclude later colonization of the bird by Salmonella.
Hofacre (2000) reported that establishment of adult intestinal flora in day-old turkeys using competitive exclusion has been shown highly effective in reducing Salmonella colonization.
The efficacy of two commercial products, a chicken-origin lyophilized culture (Aviguard, Bayer Animal Health. Merriam, KS) and a probiotic culture (Avian Pac Soluble Plus, Loveland Industries, Greeley, CO) containing only Lactobacillus acidophilus, was compared with that of fresh and 24-hr-old turkey cecal material using the challenge Mead model. The Aviguard was protective, but fresh turkey cecal material (undefined cecal culture from healthy turkeys) was significantly more protective in three of the four trials.
CE with starches, probiotics
Another approach in competitively excluding Salmonella has been to use starches such as isomaltooligosaccharide (IMO) to enrich cecal bifidobacterial populations and thus, reduce colonization levels of Salmonella in the ceca of broiler chickens.
A study by Thitaram in 2005 found IMO-supplemented diets resulted in significantly higher cecal bifidobacteria compared with the control diet. Chickens fed diets with 1% IMO had a significant 2-log reduction in the level of inoculated S. Typhimurium present in the ceca compared with the control group.
No differences in feed consumption, feed conversion, or feed efficiency compared with the control group were observed. However, the study showed a significant reduction in weight for birds fed 1% IMO diet compared with those fed the control diet.
Higgins in 2007 evaluated the ability of a commercially available lactic acid bacteria-based probiotic culture (LAB) to reduce Salmonella Enteritidis or Salmonella Typhimurium in day-of-hatch broiler chicks. In these experiments, LAB significantly reduced the incidence of Salmonella Enteritidis (60% to 70% reduction) or Salmonella Typhimurium (89% to 95% reduction) recovered from the cecal tonsils of day-old broiler chicks 24 hours following treatment as compared with controls. In these studies, LAB treatment significantly reduced recovery of Salmonella in day-of-hatch broilers.
Many companies have been investigating the use of vaccination of broiler chickens at the hatchery or during grow-out to reduce Salmonella colonization of the live birds.
Research by Babu showed that cell-mediated immunity (CMI) was enhanced by live Salmonella vaccine (LV). The study evaluated the impact of live and killed Salmonella vaccines on Salmonella enteritidis (SE) clearance. Chickens were first immunized at two weeks of age followed by a booster dose at four weeks, challenged with live SE vaccine two weeks later (six-week-old) and tested. The results suggested that the live Salmonella vaccine protected against SE infection, probably by enhancing cell mediated immunity.
Acid in waterers during feed withdrawal
Salmonella in the crops of chickens that have consumed litter may be spread from carcass to carcass during the crop removal process at processing. Thus, efforts should be made to control Salmonella in the crop prior to the crop removal process.
Some companies have been successful at controlling Salmonella in the crop by acidifying the bird’s drinking water during the feed withdrawal process. Acetic, citric or lactic acids and Poultry Water Treatment (PWT) have all been used to acidify the crop to the extent that Salmonella are unable to survive.
Research by Byrd found that lactic acid was most effective and that 0.44% lactic acid in the waterers of broilers during the feed withdrawal period reduced Salmonella contaminated crops by 80%. This effect carried over to the pre-chill carcasses on which the prevalence of Salmonella was reduced by 52.4 %.
Cost must be evaluated
Preventing colonization of chickens during breeding, hatching, and grow-out is challenging. This article details a number of methods that may be used to decrease or eliminate Salmonella from broiler chickens. However, each company must balance the cost of implementation of these techniques as there may be more effective methods that can be used at a lower cost to the company during processing, to allow the company to maintain Salmonella at low levels.