Listeria organisms are commonly found in the soil which has been contaminated with manure of apparently normal carrier animals. When making silage it is very difficult to complete the job without some soil contamination. If the silage is not properly packed tight to exclude oxygen, or the pH of the silage is not low enough the Listeria will multiply in these small pockets. Sheep that are cutting teeth such as newborn lambs, yearlings and mature ewes will have the greatest incidence via the bacteria getting in the tooth socket and following the facial nerve to the brain. This will take 2-4 weeks after eating the affected silage.
Listeria is also a human pathogen.
Reduce pocket gopher mounds in the field. Do not cut silage too close to the ground. Use a silage additive to lower the pH. Pack tightly and seal air tight. Test the pH, if greater than 4.5 do not feed to the sheep. Do not feed moldy or suspect silage to the sheep.