In F1, 139 farrowings were followed, comprising 41 young sows (parities 1–2), 65 middle-aged sows (parities 3 to 6), and 33 old sows (parity ≥ 7). In this farm, VT of PRRSV occurred in 32 cases (23.0%; CI95% = 16.5–31.1%), of which 14 (43%) happened in young sows (p < 0.05). Of note, young sows only accounted for 30% sampling of this farm. In F2, 111 farrowings were followed, including 51 young, 49 middle-aged, and 11 old sows. VT was detected in 21 farrowings (18.9%; CI95% = 12.4–27.1%) with 15 (71%) happening in young sows (p < 0.001). The aggregated data from both farms examined in the generalized linear model (GLM) model (Table 1) showed that the age of sows had a significant influence on the occurrence of VT (p = 0.012). Overall, PRRSV VT occurred more likely, 1.7 times (CI95% = 1.22–2.31), in young (parities 1–2) than in older sows.
Table 1
Generalized linear model results of the association of vertical transmission with regard to age.
FORMULA > model <- glmer(VT ~ AGE+ (1 | FARM), data = data, family = binomial, control = glmerControl(optimizer = "bobyqa")) |
AIC 271.8 | BIC 285.9 | logLik -131.9 | Deviance 263.8 | df.resid 245 | |
Scaled residuals | | | | | |
Min | 1Q | Median | 3Q | Max | |
-0.7190 | -0.6454 | -0.4666 | -0.4188 | 2.3876 | |
Random effects | | | | | |
Groups | Name | Variance | Standard deviation | | |
Farm | (intercept) | 0.02988 | 0.1728 | | |
Fixed effects | | | | | |
| Estimate | Standard Error | z value | Pr(>|z|) | Interpretation |
Intercept AgeYoung AgeOld | -1.6344 0.8648 0.3806 | 0.2847 0.3432 0.4453 | -5.741 2.520 0.855 | 9.44e-09 0.0117 0.3927 | p < 0.001 p < 0.05 non-significant |
Correlation of Fixed Effects | (Intr) | AgeYoung | | | |
AgeYoung AgeOld | -0.681 -0.473 | 0.388 | | | |
Regarding the levels of antibodies measured by ELISA, no differences were observed between farms with regard to the same age group of sows (young, middle-aged, old) (data not shown). The global average S/P ratio was 1.4 ± 0.8 for sows that had VT (1.3 and 1.5 for F1 and F2, respectively), and 1.2 ± 0.6 for sows delivering healthy piglets (1.2 and 1.3 for F1 and F2, respectively). Interestingly, younger sows, despite the VT status, had significantly higher S/P values than older ones (1.4 ± 0.6 vs 1.1 ± 0.6, p < 0.05) (Fig. 1A).
Next, we examined whether the titre of NAb against the resident farm strain was correlated with the delivery of infected or healthy piglets on F1 (the same analysis was not performed for F2 because the resident farm strain could not be adapted to MARC-145). Twenty VT-sows (all sows for which enough serum was available for VNT) and 69 non-VT-sows were analysed. The average neutralization titre was 2.3 ± 2.0 log2 for sows delivering healthy piglets versus 3.1 ± 2.3 log2 for sows delivering infected piglets (non-significant, Supplementary Fig. 1).
Then the relationship between the level of PRRSV-specific NAb (vaccine strain Porcilis® PRRS) and the occurrence of VT was analysed. The average titre of sows with occurrence of VT did not differ from sows delivering PRRSV1-negative piglets (2.1 ± 0.5 vs 2.2 ± 0.4 log2 in F1 and 5.1 ± 2.5 and 4.0 ± 2.4 log2 in F2 for VT and non-VT sows, respectively). When taken both farms together, differences were also not observed between VT- and non-VT- sows. Younger sows showed a trend of higher neutralization titres compared to middle-aged or older sows (5.1 ± 2.2 vs 4.0 ± 2.5 vs 3.7 ± 2.0 log2, respectively, p = 0.07, Fig. 1B).
Given differences observed in the antibody level based on age, further analysis of antibody levels and the depth of neutralizing capacities of sera from sows of different ages was performed. Since no more serum of the abovementioned sows was available, these analyses were performed with sera from another group of 51 sows on F1 and F2 collected for routine monitoring purposes. The serum samples were analysed in parallel by ELISA and the neutralization assay against 5 PRRSV-1 strains (Table 2). Consistently, young sows displayed a higher S/P value (data not shown). With regards to the neutralization against vaccine strain, the proportion of sows with neutralization titre ≥ 1:4 was 79.2% (19/24) and 85.2% (24/27) in young (parities 1–2)and older sows (parity ≥ 3), respectively (non-significant). The average titres were 4.30 ± 1.44 and 4.40 ± 1.40 log2, respectively (non-significant). Furthermore, the cross-neutralizing capability was evaluated against four heterologous PRRSV1 stains (3267, F8, Cn13, and Cw2). For 3267, 66.7% and 70.3% of young and adult sows respectively harboured NAb. For three contemporary strains (F8, Cn13 and Cw2), the proportion of positive sows was respectively 4.2%, 16.7%, and 16.7% in young sows, and 14.8%, 40.7%, and 29.6% in older sows, with a difference trend seen for only Cn13 comparing two groups of sows (p = 0.07). Mean titres ranged from 2.0 log2 for the F8 strain to 4.7 log2 for the Cn13 strain, with no significant differences between strains or between young and older sows. Only one serum from an adult sow was able to neutralize all five strains at titres higher than 1:4. Seven sera, all from adult sows (> 2 parities), neutralized 4 strains (all but F8), 9 sera (6 young and 3 adult sows) neutralized 3 strains (Porcilis®, 3267 and one of the three contemporary strains), and 14 sera neutralized 2 strains (Porcilis and 3267, except in one case). The remaining 5 sera (2 young sows and 3 sows of parity ≥ 3) did not neutralize Porcilis® or any of the other strains. The correlation of NAb titers in sows against each of the five stains is shown in Fig. 2. Titres of neutralization against the vaccine strain were not significantly correlated to the titres against any other strain. In contrast, titres against strain Cm2 were significantly correlated (p < 0.05) with all strains but not Porcilis® although a significant trend was shown (p = 0.061).
Table 2
Nucleotide sequence identity matrix for the PRRSV-1 strains used for neutralization in the present study.
| Resident F1 | Cresa3267 | MLV-DV | Cw2 | Cn13 | F8 |
Resident F1 | ID | 0.8450 | 0.8433 | 0.8355 | 0.7871 | 0.7896 |
Cresa3267 | | ID | 0.9487 | 0.8443 | 0.8327 | 0.8403 |
MLV-DV | | | ID | 0.8424 | 0.8468 | 0.8565 |
Cw2 | | | | ID | 0.7899 | 0.7934 |
Cn13 | | | | | ID | 0.7829 |
F8 | | | | | | ID |