Listeriosis is an infectious disease caused by a bacterium, Listeria monocytogenes. This is a zoonotic disease that can be spread from animals to humans and has been implicated as a potential human health risk, associated with consumption of contaminated milk or meat.
In cattle, the most common manifestations of listeriosis are meningoencephalitis, abortion, mastitis and septicaemia. Very little current information on the prevalence of Listeria infections in British cattle is available. Many animals excrete L. monocytogenes in their faeces as a normal part of the intestinal flora. Oral infection via contaminated silage is the most common route of infection, but nasal or venereal infections can also occur. This type of infection is seen most often in 1-2-year old animals as result of cutting of molar teeth. There are a number of predisposing factors that cause disease by the agent. Nutritional deficiency, particularly malnutrition, poor quality silage (high pH), heavy silage feeding, cold and wet periods, stress caused by long transport, potential feed contamination by vermin and the entry of clinically normal carrier animals into a clean herd have been recognised as risk factors for listeriosis.
The bacteria persist up to two years in slurry, manure and straw and appear to persist better in cold than warm conditions.
Abortion caused by L. monocytogenes tends to occur in late gestation (6-8 months), during the winter months, is sporadic by nature and recurs year after year.
Certain serotypes of L. monocytogenes are particularly associated with listerial encephalitis. Many reported cases are associated with fermented silage and are sporadic. Listerial encephalitis symptoms can be confused with those of Chronic mastitis caused by L. monocytogenes has also been reported in dairy cows and is of particular concern for farms that process unpasteurised milk for consumption