by Darian Peters

Created on: July 23, 2009   Last Updated: July 24, 2009

Listeriosis is a bacterial disease of animals that is known as circling disease when it occurs in sheep. Other animals that can be infected include everything from cattle to game birds and poultry. It can lead to a range of symptoms from circling behaviour and drooling to severe neurological impairments and ultimately death. There are treatments available for listeriosis in the form of antibiotics and prevention is helped by good feed management.

The underlying cause of listeriosis is the bacterium Listeria monocytogenes. This is an infectious, but not contagious, microorganism. This means that it does not spread by direct contact. Transmission instead occurs after the bacteria has proliferated in substances such as soil, the faeces of animals, and rotting vegetation, and is then ingested by the sheep. Low quality silage bales are known to have been implicated in some cases of listeriosis outbreaks because the silage has become mouldy. In particular, abrasions in the mouth lining of the sheep can provide a route of entry for the bacteria, which then pass to the brain.

The symptoms of the disease appear around five weeks after infection. Symptoms can be very severe in some cases. This can include the serious brain disease meningo-encephalitis, late-term abortions, septicaemia, and anterior uveitis, which is also known as silage eye, as well as a variety of neurological signs.

Meningo-encephalitis is of particular seriousness in causing small abscesses on the brain. Side-effects of this can include fever, depression, and anorexia. But this also includes the strange characteristic symptoms such as the circling behaviour that gives the disease its name in sheep, accompanied by head tilting and drooling, for example. But how serious the neurological problems are depends on the precise location of the abscesses in the brain. In the very worst cases these can result in frenzy or comatose can occur, with death happening a couple of weeks later.

If diagnosis of listeriosis is achieved early enough then it may be possible to provide antibiotics in high enough doses to treat the disease effectively. But a better way to beat the disease is to provide good management of the feed stuff and the general environment of the sheep in the first place. This could involve providing silage bales of a good quality, for example. It could also involve keeping as many areas as possible clear of faeces and rotting plant matter and keeping storage units for the feed clean.

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