Research Note: Detection of Salmonella Serogroup D-Specific Antibodies in the Yolks of Eggs Laid by Hens Infected with Salmonella enteritidis RICHARD K. GAST and C. W. BEARD United States Department of Agriculture, Agricultural Research Service, Southeast Poultry Research Laboratory, 934 College Station Road, Athens, Georgia 30605 (Received for publication October 18, 1990)
1991 Poultry Science 70:1273-1276 INTRODUCTION
The implication of eggs as a major source of Salmonella enteritidis infections in humans has become an important international public health issue (Centers for Disease Control, 1988,1990; St. Louis et al., 1988; Coyle et al., 1988; Cowden et al, 1989). Salmonella enteritidis has been isolated from the contents of clean, intact eggs produced by both naturally and experimentally infected laying hens (Humphrey et al, 1989a,b; Timoney et al, 1989; Gast and Beard, 1990a). Eggborne transmission of S. enteritidis, which threatens both the safety of human consumers and the economic soundness of the egg industry, can be prevented only if accurate and dependable methods are available for identifying infected flocks. Testing for specific serum antibodies effectively detects hens experimentally infected with S. enteritidis (Gast and Beard, 1990b), but application of serological methods to naturally infected flocks has produced inconsistent results (Cooper et al, 1989; Chart et al, 1990). The presence of serum antibodies establishes that a flock has been exposed to S. enteritidis but does not necessarily indicate current infection. Serological testing is thus used primarily as a screening tool to select flocks or houses for further bacteriological investigation in the present USDA testing program (Anonymous, 1991).
Egg yolk offers a source of antibodies that can be detected by the same methods applied to serum but which can be collected with less labor, less risk to the biological security of the flock, and less stress on the birds. Egg yolk extracts have been found to provide a suitable alternative to serum for the detection of specific antibodies to several avian pathogens (Schmittle and Millen, 1948; Hela etal, 1984; Mohammed et al, 1986; Brown et al, 1989; Silim and Venne, 1989). The objective of the present study was to determine whether experimental S. enteritidis infections of hens could be efficiently detected by testing egg yolk for the presence of specific antibodies in an agglutination assay. This assay was also applied to eggs from commercial flocks naturally infected with S. enteritidis. MATERIALS AND METHODS
Experimentally Infected Hens Laying Hens. Two experiments were conducted, using Single-Comb White Leghorn hens from the authors' laboratory's specific-pathogen-free flock. All birds were housed in singlebird laying cages in a disease-containment isolation building and provided ad libitum access to water and pelleted antibiotic-free layer-breeder ration. Forty-seven hens (55 wk old at the time of inoculation) were used in
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ABSTRACT Eggs laid by hens experimentally infected with Salmonella enteritidis were assayed for the presence of Serogroup D-specific yolk antibodies. Yolk: antibodies were detected with S. enteritidis and Salmonella pullorum antigens in the microantiglobulin test as early as 9 days after inoculation of hens with S. enteritidis. Yolk antibody titers reached peak levels at 3 to 5 wk postinoculation and remained at detectable levels for at least 7 wk postinoculation in eggs from both orally inoculated and horizontally contact-exposed hens. Eggs laid by hens from commercial flocks implicated in epidemiological investigations of human S. enteritidis outbreaks were also tested. Serogroup D-specific yolk antibodies were detected in 5 to 22% of eggs from hens in houses identified as infected by bacteriological culturing of internal organs of hens for S. enteritidis. (Key words: Salmonella enteritidis, microantiglobulin test, eggs, yolk antibodies, Serogroup D)
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Naturally Infected Commercial Flocks Eggs from three commercial laying flocks, all implicated as sources of eggs responsible for human S. enteritidis outbreaks and provided by
'University of Pennsylvania, Kennet Square, PA 19348. Hyattsville, MD 20782.
2
Kay Wheeler and Daryl Johnson of the USDAAPHIS SE Task Force,2 were tested for yolk antibodies by the method described above. Agglutination reactions at a 1:20 dilution of yolk extract were accepted as positive test results. Thirty-six eggs (representing two houses) from Flock A, 25 eggs (representing two houses) from Flock B, and 99 eggs (representing five houses) from Flock C were sampled. RESULTS
Experimentally Infected Hens Because no significant differences in mean microantiglobulin titers of yolks were found between the two antigens in either experiment (Figures 1 and 2), all comparisons and conclusions made in this section are equally applicable to both antigens. No positive agglutination reactions were observed with any of the 13 yolk samples from uninfected control hens. The mean yolk antibody titers of eggs laid by inoculated hens on Days 3 to 8 postinoculation (PI) in Experiment 1 (Figure 1) were not significantly different from those of eggs laid by control hens. The mean titers (expressed as dilutions of the extracted yolk samples) of eggs from inoculated hens were significantly higher (P
1991 Poultry Science 70:1273-1276 INTRODUCTION
The implication of eggs as a major source of Salmonella enteritidis infections in humans has become an important international public health issue (Centers for Disease Control, 1988,1990; St. Louis et al., 1988; Coyle et al., 1988; Cowden et al, 1989). Salmonella enteritidis has been isolated from the contents of clean, intact eggs produced by both naturally and experimentally infected laying hens (Humphrey et al, 1989a,b; Timoney et al, 1989; Gast and Beard, 1990a). Eggborne transmission of S. enteritidis, which threatens both the safety of human consumers and the economic soundness of the egg industry, can be prevented only if accurate and dependable methods are available for identifying infected flocks. Testing for specific serum antibodies effectively detects hens experimentally infected with S. enteritidis (Gast and Beard, 1990b), but application of serological methods to naturally infected flocks has produced inconsistent results (Cooper et al, 1989; Chart et al, 1990). The presence of serum antibodies establishes that a flock has been exposed to S. enteritidis but does not necessarily indicate current infection. Serological testing is thus used primarily as a screening tool to select flocks or houses for further bacteriological investigation in the present USDA testing program (Anonymous, 1991).
Egg yolk offers a source of antibodies that can be detected by the same methods applied to serum but which can be collected with less labor, less risk to the biological security of the flock, and less stress on the birds. Egg yolk extracts have been found to provide a suitable alternative to serum for the detection of specific antibodies to several avian pathogens (Schmittle and Millen, 1948; Hela etal, 1984; Mohammed et al, 1986; Brown et al, 1989; Silim and Venne, 1989). The objective of the present study was to determine whether experimental S. enteritidis infections of hens could be efficiently detected by testing egg yolk for the presence of specific antibodies in an agglutination assay. This assay was also applied to eggs from commercial flocks naturally infected with S. enteritidis. MATERIALS AND METHODS
Experimentally Infected Hens Laying Hens. Two experiments were conducted, using Single-Comb White Leghorn hens from the authors' laboratory's specific-pathogen-free flock. All birds were housed in singlebird laying cages in a disease-containment isolation building and provided ad libitum access to water and pelleted antibiotic-free layer-breeder ration. Forty-seven hens (55 wk old at the time of inoculation) were used in
1273
Downloaded from http://ps.oxfordjournals.org/ at NERL on May 11, 2015
ABSTRACT Eggs laid by hens experimentally infected with Salmonella enteritidis were assayed for the presence of Serogroup D-specific yolk antibodies. Yolk: antibodies were detected with S. enteritidis and Salmonella pullorum antigens in the microantiglobulin test as early as 9 days after inoculation of hens with S. enteritidis. Yolk antibody titers reached peak levels at 3 to 5 wk postinoculation and remained at detectable levels for at least 7 wk postinoculation in eggs from both orally inoculated and horizontally contact-exposed hens. Eggs laid by hens from commercial flocks implicated in epidemiological investigations of human S. enteritidis outbreaks were also tested. Serogroup D-specific yolk antibodies were detected in 5 to 22% of eggs from hens in houses identified as infected by bacteriological culturing of internal organs of hens for S. enteritidis. (Key words: Salmonella enteritidis, microantiglobulin test, eggs, yolk antibodies, Serogroup D)
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GAST AND BEARD
Naturally Infected Commercial Flocks Eggs from three commercial laying flocks, all implicated as sources of eggs responsible for human S. enteritidis outbreaks and provided by
'University of Pennsylvania, Kennet Square, PA 19348. Hyattsville, MD 20782.
2
Kay Wheeler and Daryl Johnson of the USDAAPHIS SE Task Force,2 were tested for yolk antibodies by the method described above. Agglutination reactions at a 1:20 dilution of yolk extract were accepted as positive test results. Thirty-six eggs (representing two houses) from Flock A, 25 eggs (representing two houses) from Flock B, and 99 eggs (representing five houses) from Flock C were sampled. RESULTS
Experimentally Infected Hens Because no significant differences in mean microantiglobulin titers of yolks were found between the two antigens in either experiment (Figures 1 and 2), all comparisons and conclusions made in this section are equally applicable to both antigens. No positive agglutination reactions were observed with any of the 13 yolk samples from uninfected control hens. The mean yolk antibody titers of eggs laid by inoculated hens on Days 3 to 8 postinoculation (PI) in Experiment 1 (Figure 1) were not significantly different from those of eggs laid by control hens. The mean titers (expressed as dilutions of the extracted yolk samples) of eggs from inoculated hens were significantly higher (P
