DISEASE (MD), is a common virus that causes internal
lesions (tumors), and kills more birds than any other
disease. It is so common that you should assume you have
it in your flock, even if you detect no evidence.
Mareks is a member of the
herpesvirus family of viruses. It is also known as 'Range
Paralysis'. Mareks is spread through airborne feather
dander so microscopic that it can spread from one farm to
another via the wind, even when no human or bird contact
is made between the two farms. The virus enters through
the bird's respiratory tract.
The turkey version is
Herpes Virus Turkey (HVT), and the waterfowl version is
known as Duck Virus Enteritis (or DVE). All three are
from the same family of viruses.
It's not common for MD,
HVT, and DVE to cross over between chickens, turkeys, and
ducks kept together, but it has happened.
The study of Mareks Disease
in poultry is exciting because it has had a profound
effect on cancer research in all species, including
human. And the Mareks vaccine for chickens was the first
time medical science was able to produce an effective
cancer vaccine for any species.
There are a few different
types of Mareks in chickens. The most common are eye,
visceral (tumor producing), and nerve.
The nerve version is known
by some Fanciers as 'down in the leg', and symptoms range
from slight to severe paralysis in the wings, legs, or
neck, and usually results in death from trampling by
other chickens, and/or the inability to get to food and
water. There can be 'transient' paralysis that disappears
after a few days, such as a dropped wing that suddenly
corrects itself. In the eye version, you'll detect an
irregularly shaped pupil, cloudy eye ('gray eye'), or
sensitivity to light. It can result in blindness. The
visceral version should be considered when a bird is just
Mareks is extremely
contagious but does not spread vertically (to the egg).
Youngsters should develop a natural immunity (called 'age
resistance'), by the time they're five months old. This
is one of the reasons it is important to raise your
youngsters separately from your oldsters. The older birds
that have encountered Mareks and have managed to survive
are carriers. New birds coming in from other flocks are
always potential carriers.
Mareks usually hits between
5 and 25 weeks of age, but can appear even later if the
bird had 'latent' MD and is substantially stressed.
However, if the bird is a few years old, I would suspect
a similar disease called Lymphoid Leukosis (which does
pass to the egg). Both diseases will produce internal
lesions (or tumors), detectable upon post mortem
examination, but LL does not produce paralysis.
Since it is so difficult to
control your birds' exposure to Mareks, (showing,
bringing in new birds, airborne spread from other farms),
the best course is prevention. That starts with
completely sanitizing your brooders. Then consider the
Mareks vaccine, which is available in a freeze-dried form
through a few of the mail-order suppliers, and is easy to
administer to day-old chicks. The downside is that you
have to administer the vaccine within one-half hour of
mixing it with the fluid it comes with (diluent), and you
have to plan your hatching to accommodate the
'all-or-none' vaccination within a day or two of hatch.
(After one hour of mixing the Mareks vaccine, the active
virus dies and the vaccine becomes ineffective.)
You'll inject the vaccine
under the skin at the back of the neck (subtecaneously).
Be careful though, you could stick right through to the
other side and vaccinate the floor instead of the chick!
There are certain 'B
factors' contained in the blood of some chickens that
make them resistant to Mareks. If you have access to a
lab for 'B type' blood testing, 'B factor' birds are
desirable for breeding for a 'Mareks-free' flock.
Overall, the easiest way by
keep Mareks out of your flock (but not the most
effective), is to promote 'age resistance' by keeping
your youngsters separate from the adults and away from
the poultry shows until they're over 5 months old.
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