High specificity of the Salmonella Pullorum/Gallinarum rapid plate agglutination test despite vaccinations against Salmonella Enteritidis and Salmonella Typhimurium

Figures & data

Table 1. Results of RPA test for antibodies to Salmonella Pullorum and Salmonella Gallinarum on 579 SPF sera and 62,419 field sera from 1641 Dutch layer, breeder and grandparent flocks of varying age and unknown vaccination programmes against Salmonella Enteritidis and Salmonella Typhimurium.

Maximum serum dilution that tested positive	SPF sera (n = 579)	Field sera (n = 62,419)		Flocks (n = 1641)
		Number	Diagnostic specificity^B (%) (95% confidence interval)	Number^C	Flock specificity^D (%) (95% confidence interval)
<1:2 ^A	579	62,200		1565
1:2		145	99.65 (99.60–99.69)	42	95.37 (94.24–96.28)
1:4		58	99.88 (99.85–99.91)	25	97.93 (97.12–98.51)
1:8		15	99.97 (99.96–99.98)	8	99.45 (98.96–99.71)
1:16		1	99.998 (99.99–100)	1	99.94 (99.66–99.99)
1:32		–	100 (99.99–100)	–	100 (99.77–100)

^A 1:2 dilution was negative.

^B Specificity using the dilution as a positive cut-off.

^C Number of flocks with only negative results (all individual sera showed titres below the serum dilution that was used as a positive cut-off).

^D Percentage of flocks with only negative results (all individual sera showed titres below the serum dilution that was used as a positive cut-off).

Table 2. Results of RPA test for antibodies to Salmonella Pullorum and Salmonella Gallinarum on 11,504 sera from 206 young parent flocks with a known vaccination programme against Salmonella Enteritidis and Salmonella Typhimurium that all had been sampled according to the Dutch monitoring programme at 19–24 weeks of age.

Maximum serum dilution that tested positive	Individual RPA SG/SP titres of sera originating from 206 flocks of 19–24-week-old broiler breeders with different Salmonella vaccination programmes				Diagnostic specificity^D for sera from
	No SE/ST vaccine	Live SE/ST vaccines^A	Inactivated SE/ST vaccines^B	Live and inactivated SE/ST vaccines^C	Non-vaccinated flocks and flocks vaccinated with only live SE/ST vaccines (95% confidence interval)	Flocks vaccinated with inactivated SE/ST vaccines, with or without live vaccines (95% confidence interval)
Total number of sera	1969	7168	1216	1151	9137	2367
<1:2^E	1969	7167	1163	1122
1:2	–	1	23	12	99.99 (99.94–100)	96.54 (95.72–97.20)
1:4	–	–	17	16	100 (99.96–100)	98.01 (97.37–98.50)
1:8	–	–	13	1	100 (99.96–100)	99.41 (99.01–99.65)
1:16	–	–	–	–	100 (99.96–100)	100 (99.84–100)
1:32	–	–	–	–	100 (99.96–100)	100 (99.84–100)

^A AviPro Salmonella DUO (3× or 4×).

^B Inactivated vaccines Nobilis Salenvac T, Nobilis Salenvac ETC or Gallimune SE + ST.

^C Avipro Salmonella DUO (*3×) and Nobilis Salenvac T (2×).

^D Specificity using the according serum dilution as a positive cut-off.

^E Negative in the 1:2 dilution after heat inactivation.

Table 3. Results of RPA test for antibodies to Salmonella Pullorum and Salmonella Gallinarum at flock level on 11,504 sera from 206 young parent flocks with a known vaccination programme against Salmonella Enteritidis and Salmonella Typhimurium that all had been sampled according to the Dutch monitoring programme at 19–24 weeks of age.

Maximum serum dilution that tested positive	RPA SG/SP titres at flock level of sera originating from 206 flocks of 19-24-week-old broiler breeders with different Salmonella vaccination programmes				Flock specificity^D for sera from
	No SE/ST vaccine	Live SE/ST vaccines^A	Inactivated SE/ST vaccines^B	Live and inactivated SE/ST vaccines^C	Non-vaccinated flocks and flocks vaccinated with only live SE/ST vaccines (95% confidence interval)	Flocks vaccinated with inactivated SE/ST vaccines, with or without live vaccines (95% confidence interval)
Total number of flocks	34	128	24	20
<1:2^E	34	127	5	10
1:2	–	1	6	3	99.4 (96.6–99.9)	34.1 (21.9–48.9)
1:4	–	–	8	6	100 (97.7–100)	54.5 (40.1–68.3)
1:8	–	–	5	1	100 (97.7–100)	86.4 (73.3–93.6)
1:16	–	–	–	–	100 (97.7–100)	100 (92.0–100)
1:32	–	–	–	–	100 (97.7–100)	100 (92.0–100)

^A AviPro Salmonella DUO (3× or 4×).

^B Inactivated vaccines Nobilis Salenvac T, Nobilis Salenvac ETC or Gallimune SE + ST.

^C Avipro Salmonella DUO (*3×) and Nobilis Salenvac T (2×).

^D Specificity using the according serum dilution as a positive cut-off.

^E Negative in the 1:2 dilution after heat inactivation.

Table 4. Results of RPA SP/SG test international PTS Royal GD from 2010 to 2020.

SPF sera (n)	Number of test results	Percentage positive results in the RPA SP/SG test 1:8 serum dilution or higher	Mean RPA SP/SG titre
SPF (1)	22	4.5	0.3
S. Infantis (2)	16	0	0
S. Virchow (1)	8	0	0
Live ST and/or SE vaccine (6)	40	4.8	0.2
Live SG vaccine (1)	10	0	0
SE field isolate (6)	34	2.9	0.8
ST field isolate (2)	12	17	1.0
SG field isolate (3)	20	55	2.7
SP field isolate (2)	12	100	3.9

Figure 1. 2010–2020 International Proficiency Test results of the RPA test for SG and SP for antisera raised against several Salmonella serovars. Sera were tested at dilutions from 1:2 to 1:32.