Listeriosis

Also known as:

Circling disease Silage sickness

Description

Listeriosis is a bacterial infection caused by Listeria species. It affects a wide range of animals and is an important zoonosis. L.monocytogenes is found in silage and is the main source of contamination for animals. Soil and the mammalian gastrointestinal tract are natural reservoirs for L.monocytogenes and if ingested by grazing animals the bacteria will further contaminate the pasture. The bacteria is very resistant to drying and can survive at a variety of temperatures and PH.

Signalment

Affects a range of animals including sheep, cattle, goats and birds. Most commonly affects adult ruminants that are being fed contaminated silage over the winter.

• Zoonosis - consumption of contaminated unpasteurised milk by humans can lead to memingitis and meningoencephalitis and if pregnant abortion.

Diagnosis

Lumbosacral Cerebrospinal Fluid (CSF) can be collected under local anaesthesia and if the animal is infected will show an increased protein count and mild pleocytosis with large mononuclear cells. Isolation of L. monocytogenes provides a definitive diagnosis of listeriosis. The preferred samples for culture are brain, aborted foetus or placenta. Immunofluorescence can also be used to identify L.monocytogenes on smears taken from dead or aborted animals. Serology is not used as many healthy animals have high Listeria titres.

History and Clinical Signs

The most recognised form of listeriosis in ruminants affects the nervous system causing meningoencephalitis. Animals present depressed, sometimes with a head tilt, facial paralysis, a drooping eyelid, profuse salivation and circling towards the side of the lesion. In terminally affected animals who are recumbent it is common to see unvoluntary running movements. Death can follow clinical signs acutely and is often within 24-48 hours.

In monogastric animals and young ruminants listeriosis presents differently causing septicaemia which can also be fatal.

If animals are infected whilst pregnant this can cause abortion up to 12 days later with a purulent exudate covering the placenta. Animals are affected in the last trimester of gestation having shown no previous clinical signs. Following abortion the animal will often recover normally unless the foetus is retained and metritis will develop.

Additionally Listeriosis can also manifest as conjunctivitis when infected silage come in contact with the animals eye, pneumonia, myocarditis and endocarditis.

Pathology

In Meningoencephalitis caused by listeriosis, lesions can be found on the meninges, pons, medulla oblongata and the spinal cord. The lesions are often small and are seen in conjunction with congested meninges.

In septicaemic listeriosis, small focal necrotic lesions can be found in all organs but particularly the liver and in very young animals gastroenteritis may also be present.

Aborted foetuses are autolysed with small necrotic yellow foci at multiple organs sites often including the lungs, liver and the spleen.

Treatment

L monocytogenes is susceptible to a range of antibiotics including penicillin, ceftiofur, erythromycin, and trimethoprim/sulfonamide. In order to achieve high enough levels in the brain high doses are required daily for 1-2 weeks. In an outbreak, affected animals should be isolated, treated and silage feeding should be discontinued. All bedding should be destroyed and buildings should be thoroughly cleaned. To avoid further outbreaks, ensure animals are fed good quality silage and minimise soil contamination when making silage. For ocular listeriosis Sub-conjuntival antibiotics and corticosteroids can be given.

Prognosis

Recovery depends on rapid diagnosis of the disease. If treated, animals can recover however if signs of encepalitis are present treatment is often unsuccessful.

References

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial