Download key points in fine tuning the use of a specific vaccine against

KEY POINTS IN FINE TUNING THE USE OF A SPECIFIC VACCINE AGAINST VERY
VIRULENT INFECTIOUS BURSAL DISEASE
D.r Marcos Bentué, p oultry technical services department, Laboratorios
Hipra, S.A. Spain
Specific vaccines against infectious bursal disease (IBD) offer satisfactory solutions to most
problems in which the very virulent IBD virus (vvIBD) is implicated. They provide a next step
after the mild and intermediate vaccines in the control of IBD and have proved to be
effective in reducing mortality caused by the disease. However, poultry veterinarians and
producers are sometimes hesitant to administer this kind of product because of past
experience when the application of too ‘hot’ vaccines irreversibly damaged the bird’s
immune system. Nowadays, there is a wide range of this type of vaccine available on the
market, although there is a tendency to choose products that are safer and less damaging to
the bursa. This article provides some insights into the technical characteristics of specific IBD
vaccines, with a view to using them more effectively to control vvIBD. The term ‘specific’
refers to the specificity that the vaccine virus shows to neutralise vvIBD viruses without
causing the bursal damage that some other ‘hot’ vaccines cause.
Back to basics
When dealing with IBD problems, the first step is to decide the appropriate place and time to
switch the IBD vaccines. The use of a specific vaccine should only be considered when
abnormally high mortality is experienced that is clearly related to IBD and that cannot be
controlled with intermediate vaccines. Starting with the basics, an IBD control programme
should always be followed, based on chicks hatched with adequate levels of maternal
antibodies, appropriate vaccination time and good cleaning/disinfection programmes. If,
after revision of all these points, IBD problems are still unmanageable, the decision may be
taken to extend the range of vaccines under consideration to include the specific type.
Step 1: Identify the enemy
However, before proceeding in this direction, it is essential to identify the characteristics of
the viruses circulating on the farm. It is not a good idea to introduce specific vaccines against
vvIBD virus if the viral strains present can be controlled with mild or intermediate vaccines
and/or the implementation of better management practices. To this end, various molecular
diagnostic tools are available not only to confirm the presence of the IBD virus but also to
identify specific traits of the strains. For instance, RT/PCR-RFLP (reverse
transcriptase/polymerase chain reaction-restriction fragment length polymorphisms) is able
to:
- detect all IBD strains independently of their
level of pathogenicity and antigenicity
- differentiate most vaccine strains from wild-type
field viruses
- determine virulent strains of the virus and
fit the strains into molecular groups.
Differentiation of vvIBDV (very virulent IBD) from cvIBDV (classic virulent IBD).
Restriction enzyme BspMI “cuts” very virulent IBD (lane 3) while does not have any effect on
the classic virulent IBD (lane 7) . Restriction enzyme SacI does not “cut” vvIBD (lane 2) while
it does in lane 6 (cvIBD). Thus allowing the identification of both IBD viruses.
RT-PCR followed by enzymatic digestion allows
us to discriminate between the very virulent and classic virulent strains of IBD (cvIBD), as
shown in Figure 1. Subsequently, the sequencing of the PCR products allows further
characterisation of the virus. In the example in Figure 1, restriction enzyme BspMI cuts vvIBD
virus (lane 3), while it does not affect the cvIBD virus in lane 7. Restriction enzyme SacI does
not cut vvIBD virus (lane 2), while it does cut the cvIBD strain in lane 6. This allows the two
viruses to be identified and distinguished. Routine monitoring of poultry flocks by RT-PCR is
highly recommended in order to assess the IBD viruses circulating on the farm and to finetune the vaccination programme. For flocks facing IBD, 5 to 10 bursas should be submitted to
a well-equipped laboratory to identify the characteristics of the IBD virus present in the flock.
For effective control of IBD, it is essential to know beforehand which viruses you are facing.
Step 2: Get the timing for vaccination right
Once the involvement of vvIBD virus is confirmed and the decision is made to administer the
vaccine, the next step is to determine the best vaccination time. It is well known that
maternal immunity might protect the birds against the field virus during the first weeks of
life. The disadvantage is that the maternal immunity may also interfere with the vaccine virus
if vaccination is carried out too early.
Maternal immunity should be seen as a kind of temporary ‘wall’ that is very thick at the
hatching time but it declines linearly at a predictable rate afterwards. At a certain point, this
‘wall’ will have become sufficiently thin for the vaccine virus to penetrate and for active
immunity to be developed. However, if IBD vaccines are administered too early, they bump
against the ‘wall’ and thus, they are neutralised and become ineffective in vaccinating the
flock as a whole.
Vaccine neutralisation can be avoided by delaying vaccination until the chicks become
susceptible to the vaccine and are able to respond to the vaccine virus. However, this opens
up the possibility of an ‘immunity gap’, when the field virus can penetrate before some
intermediate vaccine viruses. In order to maintain the bird’s good health, the vaccine virus
must get there first. After bypassing maternal immunity, the vaccine viruses colonise the
bursal tissue and replicate fast, while humoral and cellular immunity are developing. In
addition, they compete with the field virus for the bursal attachment sites.
Generally, specific IBD vaccines can be administered earlier and the susceptibility period to
the field virus is shorter. A further advantage of specific IBD vaccines is that they are more
diffusible. This means that calculation of the proper vaccination time is less critical than with
intermediate vaccines. The latter can allow the vaccine virus to spread to the unvaccinated
chicks. However, it is still recommended to avoid the unwanted neutralisation and to achieve
the maximum potential from the
vaccine.
In order to estimate the optimal vaccination time, blood samples should be collected from 18
to 20 healthy, one-day-old chicks. Estimations based on fewer samples are unreliable and are
a false economy. The serum should be submitted to the laboratory, which will provide the
level of titres, coefficient of variation (CV) and vaccination time.
The prediction of vaccination time is made based on calculation using one of several formulae
available, e.g. Kouwenhoven or Deventer.
Regular monitoring of day-old chicks is recommended to predict the best time of vaccination.
The example in Figure 2 shows a good level of titre and high uniformity and so the chicks will
be protected with a single vaccination at 12 days of age. If the results show low titres and/or
a high co-efficient of variation, it may be necessary to vaccinate the flock twice. A word of
caution is necessary here. The mathematical formulae do not take into account all the
variables on a particular farm. Vaccination time should be adjusted to allow for the special
circumstances of the flock. Allowances might need
to be made, for example, for:
- broiler flocks originating from different sources
and consequently with heterogeneous maternal
titres and
- multi-age farms where chicks are exposed to high
levels of vvIBD virus upon arrival.
Other considerations
It has already been mentioned that specific IBD vaccine viruses are more diffusible than those
in the intermediate type of vaccine. This means that in-house cycling is a common
phenomenon. After replicating in the bursa, the viruses are shed onto the litter and may be
ingested again, reinforcing the immune status of the same bird or passing on toother chicks
that were poorly vaccinated. This cycling may continue for several days after vaccination.
It is said that the specific type of IBD vaccine is more forgiving when the vaccination is not
doneproperly. However, it is important not to rely on this phenomenon and relax over
vaccination practices because this can jeopardise protection levels and allow IBD to break out
again. This risk is higher for pullet flocks confined to rearing cages as they cannot access their
faeces
and
the
IBD
vaccine
virus
cannot
circulate
in
this
way.
Technically speaking, specific IBD vaccines are more invasive to the bursa and even replicate
to some extent in other lymphoid tissues, e.g. caecal tonsils, spleen, Harderian glands. This
means that earlier and higher levels of ELISA titres are seen after the administration of these
vaccines.
It is not recommended that specific IBD vaccines are administered continuously. After
vaccination of several batches of birds with specific vaccines, IBD problems will, in most
cases, be overcome and the field virus may even have been displaced. As a result, the
possibility of stepping down to vaccines of intermediate type should be evaluated. To
summarise, specific IBD vaccines can effectively control field cases of vvIBD. However, some
technical guidelines should be followed, in order to optimise the protection conferred by the
vaccines. In addition, good use should be made of the available diagnostic techniques, which
are invaluable tools in helping to reach the most appropriate decisions.
Elisa testing of 20 serum samples of broiler day
old chicks to assess the maternal antibodies
level
100.00
90.00
Frequency
80.00
70.00
Age at sampling:
Number of Samples:
Arithmetic Mean Titre:
CV %
Geometric Mean Titre:
Predicted vaccination time:
60.00
50.00
40.00
30.00
20.00
10.00
0.00
0
1
2
3
4
5
6
7
8
9
Titre Group
10
11
12
13
14
15
1 day
20
6600
27 %
5950
12 days