The following is from a book I'm writing and may be of help.
Note that Listeriosis can be in the soil and contracted by eating
balage or anything from eating it on infected soil.
Listeriosis
Listeriosis (Listeria monocytogenes) is a bacterial infection of
animals and birds. It affects the nervous system. It is most common
in sheep fed poor quality silage with a pH above 5, and was first
identified in New Zealand in 1931.
It can be transmitted from animals and soils to humans as meningitis
and glandular fever.
Symptoms
Initially the animal will be on its own away from others after which
symptoms start to show. One side of the animal's face can droop (hang
down) and drooling (slobbering) can occur. Eating becomes difficult
so weight loss follows. Abortions can occur. It is known as 'circling
disease' because the animal walks round in unidirectional circles and
shows nervous symptoms, developing to convulsions, coma and death
within a few days.
Causes
Silage sometimes gets the blame for listeriosis when it can be from
feeding it on polluted or very short pasture so when animals lick up
the last of finely chopped silage they pick up enough listeria
bacteria to infect them. Sheep graze shorter and lick more to pick up
that last bit, so can suffer listeria more than cattle. When feeding
silage animals are usually short of feed so are stressed which is
usually a contributing factor of listeria toxicity. Also, change to
silage early (before pasture has finished) and gradually over ten
days to avoid digestive upsets and stress. This means feeding very
little on the first day (10% of diet) and increasing by 10% per day
for ten days. Causes include polluted soil, dirty troughs, dung
infected silage, balage, or haylage. Pasture of reasonable length and
hay are less of a problem because listeriosis seems to thrive in
dirty, moist and mouldy conditions. Good silage with a low pH of 4.5
or less is seldom a problem because the bacteria can't survive in it,
although the bad mouldy top and sides can carry listeriosis.
Infections can also occur from unhygienic conditions in barns and
from animal (abbatoirs) and human sewerage where the people are
infected with listeria. Seagulls have spread it from sewerage. Any
form of stress can aggravate an outbreak which usually affects just a
few sheep in a flock.
Prevention
Avoid harvesting pasture where animal dung can get into the silage.
If earthworms and/or dung beetles are not spreading the dung then
there is a case for harrowing (dragging) at the time of closing the
paddock for silage or hay.
Avoid too much wilting because mould is then more likely, which,
combined with a high pH can allow the silage or balage to harbour the
bacteria. Good vacuum silage, where moulds seldom exist and pH is
usually optimum, is safest, provided no soil or dung pollution has
occurred. Avoid having a wide silage face which takes days to feed
across, because moulds will grow on the face. Long narrow stacks are
best. If vacuum silage is not made, avoid feeding the top and edges
which can be mouldy. Round bale silage can have mould on the ends,
but the listeria bug also has to be present.
Pasture where it is fed should not be too short because sheep can
then pick up listeria bacteria.
Treatment
Brain damage occurs, so treatment is seldom satisfactory.
--
Vaughan Jones
