Frequently asked questions Nobilis SG 9R
Is Salmonella Enteritidis a problem?
Salmonella in general is recognized world-wide
as a major cause of human gastroenteritis.
Salmonella Enteritidis is among the most commonly isolated serotypes from humans. Eggs and raw egg products are considered to be the major sources for human Salmonella Enteritidis infections.
Therefore, in many countries programs have been implemented with the aim of eradicating S. Enteritidis and also S. Typhimurium from food products.
For more information on the incidence of S. Enteritidis see Epidemiology
Why should we
vaccinate against Salmonella enteritidis? We prefer ‘screen
and slaughter’
The approach of ‘screen and slaughter’ was successful
for the eradication of S. Gallinarum and S.
Pullorum in chickens. The reason for this success is that these
pathogens are host-specific for poultry. For Salmonella
Enteritidis the situation is entirely different. S. Enteritidis
belongs to the paratyphoid group of salmonellas and has a much
broader host-specificity (chicken, man, rats, mice, pigs, dogs,
cats). In consequence, there are many more sources of contamination,
which makes complete eradication (if possible) far more difficult.
Eradication or control of Salmonella Enteritidis will require a large number of measures focussing on all stages of the production process (good hygiene, screening, slaughter of contaminated flocks, use of antibiotics etc), each with its own part to play.
Experience in the UK has been that ‘screen and slaughter’ reduced the number of incidents drastically, but after that the proportion of infected flocks remained stable. Following vaccination, this number was again drastically reduced. Vaccination, therefore, makes a valuable contribution to existing measures for the control of Salmonella Enteritidis and contamination of food products.
Can the vaccine
strain be distinguished from S. Gallinarum and S.
Pullorum field isolates?
The 9R strain can be easily distinguished from S.
Gallinarum and S. Pullorum field strains by agglutination
with 0.1% acriflavin or 3.5% NaCl. More complicated methods are
based on biochemistry: the 9R strain forms acid from rhamnose
and dextrin and differs in its fatty-acid profile (Ryll and Hinz,
1995).
Does the serological
response to the vaccine strain interfere with the serum plate
agglutination (SPA) test for Pullorum Disease in poultry?
Although subcutaneous vaccination with the S. Gallinarum
9R strain can result in some positive reactions in the SPA, however
the reaction induced by 9R vaccination can be distinguished from
true Pullorum Disease by titration of these positive antisera
(Snoeyenbos, 1991). This method, used by the Dutch Animal Health
Service, consists of the following steps: first, undiluted antisera
are tested in the SPA. Next, a series of dilutions of the positive
samples are evaluated. The cut-off value used for a positive antiserum
is that a 1/16 dilution of the antiserum is still positive in
the SPA. During the current field trial in the Netherlands, blood
samples were taken at different time points (at week 24, 32 and
40) from five flocks and tested by SPA. So far, no specific SPA
reactions have been found (personal communication, Dr. T. De Vries,
Drs. A. Feberwee, Dutch Animal Health Service, Deventer, The Netherlands).
The issue of interference by the vaccine strain with S. Pullorum and S. Gallinarum serology, is not relevant for layers, since these birds are not tested routinely for S. Pullorum or S. Gallinarum.
Comment.
Infection with Group D Salmonella other than S.
Pullorum or Gallinarum such as S. Enteritidis, or vaccination
with inactivated S. Enteritidis vaccines may also give
rise to positive results in the SPA test. Therefore, rapid plate
agglutination with S. Pullorum antigen should only be used as
a flock screening test. Only negative results are conclusive.
When positive results are obtained in this test, the results should
be confirmed by bacteriological methods.
Can we still
use the LPS – (Group D) ELISA for Salmonella Enteritidis
serology?
After vaccination with the 9R vaccine a serological response
is induced against the LPS, which is detected in the LPS (D) ELISA.
Salmonella Gallinarum, Salmonella Pullorum and
Salmonella Enteritidis all contain Group D LPS and, therefore,
no distinction can be made between the serological response against
these salmonellas and the vaccine strain using the LPS ELISA.
Can we distinguish
the serological response to the vaccine strain from S. Enteritidis
field infections?
The 9R vaccine strain (and S. Gallinarum and S.
Pullorum field strains) are not flagellated, whereas S.
Enteritidis contains g,m flagella. Therefore, ELISAs based on
the detection of antibodies to the g,m flagella (e.g. GM-DAS blocking
ELISA, Idexx, France) can be used to distinguish a S.
Enteritidis field infection from the immune response elicited
by the 9R vaccine strain.
Can the vaccine
strain be distinguished from S. Enteritidis strains?
The 9R strain can be distinguished from the S. Enteritidis
strains by investigating motility. S. Enteritidis is
motile, whereas the 9R vaccine strain is not.
How long after vaccination with SG 9R can the vaccine strain be reisolated?
In laboratory safety studies the vaccine strain cannot be isolated 6 weeks and longer post vaccination. However in a field situation it has been seen that the vaccine strain may be isolated for a prolonged period of time (from post mortem material) depending on the health status of the birds.