Listeriosis is the collective name for diseases caused by any of the pathogenic species of the genus Listeria. There are six known species of Listeria, but only two are known to be pathogenic in animals, namely Listeria monocytogenes and Listeria ivanovii. Listeria species occur naturally as saprophytes in decaying plant matter. It may also be found in the intestines of healthy animals as part of the normal flora. L. monocytogenes infects a wide variety of animals such as cattle, sheep, dogs, horses, and pigs. L. ivanovii is less common, it has thus far only been implicated in ruminant infections. Transmission is usually via ingestion and sources of infection is usually soil or contaminated feed (e.g. silage). In the case of ruminants, poor quality silage with a high pH is often incriminated. Listeriosis occurs sporadically and is only seen when the animal’s immune system is overwhelmed by unusually large numbers of Listeria. An infection is established in the intestine and then disseminates via the bloodstream to other sites such as the liver, central nervous system and pregnant uterus. Transmission to the foetus occurs via the placenta and amniotic fluid. The result of a Listeria infection depends on the organ system affected. Infection of the intestine manifests as diarrhoea, central nervous system infection results in rhombencephalitis or encephalitis (‘circling disease’), infection of the placenta may lead to abortion or the birth of pre-term young with septicaemia. In monogastric animals the disease tends to manifest as septicaemia. The encephalitis and abortion are thought to be most common manifestations of the disease in South Africa. Animals that are clinically ill may be treated with Penicillin G and gentamicin, tetracycline or trimethoprim/sulphonamide. If silage is fed, it should be discontinued. Feeding of decomposing silage should be avoided.
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