Issue |
Vet. Res.
Volume 31, Number 1, January-February 2000
|
|
---|---|---|
Page(s) | 44 - 45 | |
DOI | https://doi.org/10.1051/vetres:2000016 | |
How to cite this article | Vet. Res. (2000) 44-45 |
Passive immunity due to PRRS in vaccinated and unvaccinated sow herds
C. Dewey and A. RajicPopulation Medicine, University of Guelph, Guelph, Ontario, Canada N1G 2W1
Abstract -
If a sow has a high level of antibodies to the Porcine Reproductive and Respiratory
Syndrome (PRRS) virus we assume that her piglets will get a high level of passive
immunity from her colostrum. Although this measurable immunity is not directly
related to the protection of pigs against PRRS, it may indicate a surrogate measure of
immunity for the piglet. Recent work suggests that repeatedly vaccinated sows lose their
measurable antibody titre to PRRS. However, in herds in which sows are not vaccinated,
there may be various sub-populations of sows, some with high titres to PRRS and some with
no titres to PRRS. The objectives of the current study were to determine (i) whether there
is a relationship between the sow's PRRS titre and that of her piglets; (ii) if the
transmission of passive immunity from a PRRS positive sow to her piglets is 100%; (iii)
if sow vaccination increases the proportion of piglets with passive immunity. Sixteen
PRRS positive herds were included in the study. Ten farms vaccinated the sows against
PRRS at weaning, the other six farms did not vaccinate sows. The producers included in
this study were those who were concurrently participating in other research at the
University of Guelph and were not a random sample of herds. No herds were experiencing
a clinical outbreak of PRRS at the time of sampling. Blood samples were taken from three
to seven lactating sows per farm and from three to fourteen piglets per sow. These
included all pigs nursing the sow that were born to that sow. On the average, eight
piglets were sampled per litter at 2 to 10 days of age. The PRRS IDEXX ELISA test
was used to determine each pig's level of immunity. The S/P ratios were categorized
by group and both the protected and active infection groups were considered positive:
the PRRS negative group had PRRS Elisa S/P ratios
; the PRRS protected group had
PRRS Elisa S/P ratios between 0.41 and 2.50; the PRRS active infection group had PRRS
Elisa S/P ratios
> 2.50. Pig information was analysed at the pig level, whereas sow
information was analysed at the sow level. Pig and sow titres were transformed into
titre groups and then the data was analysed with a Chi-square using the Statistix
software program. Differences in S/P ratios were determined with the Kruscal Wallace
ANOVA for non-parametric data. Of the 491 pigs, 74% had a positive PRRS S/P ratio at
2-10 days of life. Most pigs (88%) nursing PRRS positive sows had a positive titre at
3 to 10 days of life. Pigs nursing positive sows were 6.2 times more likely to have a
positive titre than pigs nursing negative sows
. In both vaccinated and
unvaccinated herds, the correlation between piglet and sow S/P ratios was 0.55. Passive
immunity due to PRRS appears to have a short half-life. Pigs 2-8 days old were 2.8 times
more likely to be PRRS positive than pigs 9-10 days old (P = 0.002). In vaccinated herds,
66% of the sows were positive, whereas in unvaccinated herds, 48% were positive. Sows
from vaccinated herds were 2.9 times more likely to be positive than sows from unvaccinated
herds (P = 0.04). In vaccinated herds, 81% of the pigs were positive, whereas 63% were
positive in unvaccinated herds
. Piglets from vaccinated herds were 2.3
times more likely to be positive than piglets from
unvaccinated herds
.
In vaccinated herds, 91% of pigs nursing positive sows were positive, whereas in
unvaccinated herds, 86% of pigs nursing positive sows were positive. In these
data, 26% of the positive pigs came from negative sows. These represented 41% of
the pigs from unvaccinated herds and 19% of pigs from vaccinated herds. In unvaccinated
herds, 40% of the pigs nursing sero-negative sows were positive whereas in vaccinated herds,
51% of the pigs nursing sero-negative sows were positive
. We concluded that the
antibody was concentrated in the colostrum. In unvaccinated herds, the negative sows may
represent those with a rising or a declining antibody titre. Also, sows vaccinated
repeatedly with the same commercial product have a lower titre than sows vaccinated
only once. In our data set, parity one sows tended to have the highest average S/P
titre (1.12 SD = 0.99), followed by parity 3+ sows (0.74 SD = 0.57). The lowest S/P
titre was observed in parity 2 sows (0.48 SD = 0.34). These values were not statistically
different
(p = 0.12).
Corresponding author: C. Dewey Tel.: (1) (519) 824 4120 ext. 4070; fax: (1) (519) 763 3117;
e-mail: cdewey@uoguelph.ca
© INRA, EDP Sciences 2000