Outbreaks of listeriosis in sheep have been recorded at 14 separate properties in the state's south-west, leading to widespread ewe deaths and lamb abortions.
This year has been a remarkable year for clinical listeriosis in sheep, according to Agriculture Victoria district veterinary officer Hannah Manning, who said there had been a significant number of cases of infectious listeriosis causing neurological disease and abortion in adult sheep in winter.
Dr Manning said in almost all cases seen, infected sheep had been in containment, being fed a mix of hay, silage and potentially contaminated grain, with most infected late pregnant ewes of mixed ages.
Producers reported clinical signs including staggering, convulsions, head tilt, circling, paddling of the limbs and death in 0.5-10 per cent of the flock, while some also reported abortions.
Dr Manning said diagnosis was confirmed on laboratory examination of fixed brain tissue and culture of fresh tissue from the affected sheep or aborted foetus.
Listeria can survive in the environment for up to five years, she said, as it was "hardy" and could withstand a range of temperatures and salt conditions.
She said the disease commonly occurred under wet and muddy conditions, with signs evident eight to nine days after infection.
"Other predisposing factors include sudden change in the weather, change in feeding regime, introduction of carrier animals, confinement, overcrowding, poor sanitation, stress, advanced pregnancy, feeding silage, access to rotting hay or vegetation and feeding sheep on the ground," she said.
She advised producers who were affected to remove animals from the potential source of listeria, such as silage, containment, rank feed, or other sick animals.
Dr Manning urged producers to clean water troughs, and feed out on fresh, dry areas.
Producers should avoid excessively muddy and wet containment areas, she said, with treatment limited as antibiotics were ineffective in neurological cases.
"In abortion cases we believe there has been reduced levels of abortions post treatment with antibiotics during an outbreak, but this is always hard to measure," she said.
"If you have seen abortions it is a good idea to investigate the cause as there are many different causes and some of these such as Campylobacter are preventable with vaccination."
Prior to the Campylobacter vaccine being widely used in south-west Victoria, Campylobacter was the major cause of abortion in this district.
She said in the last three years listeria has taken the lead as the major pathogen causing abortion.