The Farmers Mart Jun-Jul 2020 - Issue 69 | Page 18

18 LIVESTOCK JUN/JUL 2020 • farmers-mart.co.uk
LIVING WITH GUMBORO
What is Gumboro?
Gumboro (Infectious Bursal)
disease caused by a virus of the
Birnaviridae family that can be
found in chickens and turkeys.
There are two serotypes of
this virus; Serotype 1 causes
disease in chickens and can
be a ‘classic’ or ‘variant’ strain.
Serotype 2 affects both chickens
and turkeys but does not cause
disease.
It is spread in the faeces of
infected birds from 2 days after
initial infection for around 2
weeks. Litter beetles have been
known to carry the virus from
flock to flock. It targets both the
gut associated lymphoid tissue
(GALT) and the main immune
system organ, the Bursa of
Fabricius, where it causes the
destruction of the antibody producing
cells (B lymphocytes).
The severity of the disease
in a flock depends upon the
strain, age of bird and the level
of maternally derived antibodies
(MDAs).
‘ By its nature,
the virus readily
mutates’
The disease tends to be characterised
by two forms:
Acute; sudden onset and
apparent return to full health
after the symptoms subside.
These include – white,
watery diarrhoea, soiled
vents, decreased feed intake,
increased water consumption,
depressed birds and sudden
death (mortality rate up to 40%
in broilers and up to 70% in
layers). Usually affects broilers
that are between 4-6 weeks old
and layers up to 17 weeks old.
Subclinical; long term immunosuppression,
poor FCR and
poor growth rates. Birds that
have this form of the disease
will usually have been infected
within the first 3 weeks of life.
The most severe immunosuppression
occurs in birds with the
absence of maternal antibodies
in the first week of life.
Why should I worry
about it?
• The virus can still be present
on your site even if there is
absence of clinical disease.
• Subclinical disease causes
long term production losses
as birds fail to thrive, resulting
in lower than expected
growth rates and FCRs.
• It will also cause immunosuppression
and therefore
increased susceptibility
to other diseases such as
coccidiosis. This results in
ongoing chronic disease
situations.
• This immunosuppression
will also cause suboptimal
response to any vaccines.
• Ultimately the health and
productivity of the birds
will be poor, resulting in last
ditch efforts to control the
opportunistic diseases with
antibiotics and thus increasing
your financial inputs with
downgraded birds being the
output.
• By its nature the virus readily
mutates. This means constant
monitoring is useful.
Copyright 2020 D. G. Parsons MRCVS
Copyright 2020 D. G. Parsons MRCVS
How are we
controlling it?
The last major outbreak was
in Netherlands in the late 1980’s
due to a sudden mutation in the
virus resulting in the ‘variant’
strain of serotype 1. This has
then spread over Europe, Asia
and South America. It has since
been controlled mainly by
vaccination.
It is a very resistant virus
and cannot be killed easily by
disinfectants or by the outside
environment (it can remain viable
for around 120 days in the shed
or 50 days in water/feed), so it
is important to maintain strict
biosecurity and rigorous cleaning
and disinfection protocols to
reduce disease pressure on your
flock. One study showed that
the virus is so hardy that it can
be recovered fully viable from
poultry meat after being cooked
to 71 – 74 °C. Exposure to 0.5%
formaldehyde for 6 hours or
0.5% Chloramine for 10 minutes
are some of the only ways to
decrease the viability of this virus.
All in/ all out systems are also
important as allow for routine
cleaning and disinfection to be
carried out and for reduced internal
cycling of disease between
different age groups.
Vaccination timing and correct
application is essential to assuring
that the effects of the disease
are not seen on farms;
• MDAs (Maternally Derived
Antibodies) from yolk are
important in protecting the
bird in early life. However,
they are often an obstacle to
ensuring you have adequate
immunity generated from
vaccination later in life. This is
because when using a vaccine
in birds younger than 14 – 17
days old the MDAs will block
vaccine uptake. Therefore,
once the MDAs reach the
end of their lives, the birds
will not have an adequate
understanding of the virus in
order to mount an immune
response themselves.
• Parent birds achieve high
levels of MDAs (to pass