NADIS
disease bulletins are written specifically for farmers, to increase awareness
of prevalent conditions and promote disease prevention and control, in order
to benefit animal health and welfare.
Farmers are advised
to discuss their individual farm circumstances with their veterinary surgeon. Listeriosis
There
were several NADIS reports during January
of listeriosis in silage fed ewes.
While some cases were associated with feeding of poor quality silage
bales, others were due to secondary fermentation in feed rings or
bunkers. The
bacterial cause of listeriosis, Listeria
monocytogenes, proliferates in soil, faeces and rotting vegetation and
can replicate at low environmental temperatures. L. monocytogenes can
also tolerate the high temperatures which are sometimes generated during
aerobic fermentation of poor silage.
In anaerobic conditions, L.
monocytogenes cannot survive below pH 5.6 but in poorly consolidated
silage with some oxygen present it can survive at pH levels as low as
3.8. Poor
quality big bale silage presents a particular risk for listeriosis because it
is usually made later in the season than conventional clamp silage, from
grass with lower sugar and higher dry matter contents. The combined effects of a high pH, due to
inefficient lactic acid fermentation, inadequate consolidation and failure to
totally exclude air from such silage, enable the proliferation of L. monocytogenes. These principles also apply to poorer
quality clamp silages. These
conditions also favour the growth of certain moulds, so mouldy silage
generally presents a high risk for listeriosis.
MOULDY SILAGE PRESENTS A HIGH RISK
FOR LISTERIOSIS Entry
of air into baled silage during storage permits the slow growth of aerobic
bacteria and a slow rise in pH allowing L.
monocytogenes, already present from soil contamination, to multiply. In tightly packed, hard centred bales, or
well-consolidated clamp silage, aerobic deterioration tends to be
superficial, being confined to the site of entry of air, but in soft centred
bales, or poorly-consolidated clamp silage, such as whole crop silages, the
aerobic deterioration and subsequent multiplication of L. monocytogenes may occur throughout the bale or clamp.
A ROUGH SILAGE FACE, REMOVAL OF A
LARGE SECTION OF PLASTIC SHEETING AND SLOW PROGRESS THROUGH THE CLAMP ENABLE
SPOILAGE AND PROLIFERATION OF L.
MONOCYTOGENES
A TIDY SILAGE FACE, CUT WITH A
BLOCK CUTTER AND ONLY REMOVING SUFFICIENT SHEETING TO GAIN ACCESS, RESTRICTS
THE PROLIFERATION OF L. MONOCYTOGENES Secondary
fermentation of previously stable silage, associated with failure to
regularly remove stale feed from bunkers or feed rings, also enables
proliferation of L. monocytogenes.
SPOILAGE AND SECONDARY
FERMENTATION OCCURS IN SILAGE WHICH CANNOT BE REACHED IN THE CENTRE OF A RING
FEEDER OR REMAINS ON THE FLOOR WHEN BUNKERS ARE REFILLED, PROVIDING AN
OPPORTUNITY FOR PROLIFERATION OF L.
MONOCYTOGENES The
feeding of listeria-contaminated silage to sheep can result in several
clinical diseases. The most commonly
encountered is meningo-encephalitis, but anterior uveitis (silage eye),
abortion and neonatal septicaemia are sometimes seen. It is unusual for different forms of
listeriosis to occur simultaneously in a flock. Listerial
meningo-encephalitis is caused by formation of micro-abscesses in the
brainstem. The clinical signs vary in
severity depending upon the degree and precise location of the abscesses, and
include fever, anorexia, profound depression and various specific
neurological signs. Affected animals
frequently drool saliva and have unilateral drooping of the eyelid, lip and
ear. Some sheep show deviation of the
head and walk compulsively in circles.
Severely affected animals may become frenzied or comatose and die in
10-14 days. The important
differential diagnoses are scrapie, pregnancy toxaemia and brain
abscessation.
RECUMBENCY, HEAD DEVIATION,
DROOLING OF SALIVA AND UNILATERAL FACIAL PARALYSIS IN A MULE EWE HOGG
HEAD DEVIATION AND CIRCLING IN A
TEXEL-CROSS HOGG The
route of infection is thought to be via abrasions in the mouth and spread to
the brain along nerves, thus the incidence of infection is highest in two
year old animals as permanent cheek teeth are erupting. Most cases occur 4 - 6 weeks after feeding
of L. monocytogenes contaminated
silage. Cases are usually sporadic,
but outbreaks can occur associated with the feeding of particularly poor
quality silage. When
cases are recognised early, treatment with high doses of antibiotics can be
effective. Control is aimed at ensuring
that only good quality silage is fed to sheep. Neil Sargison BA VetMB DSHP FRCVS Copyright © NADIS 2004
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