18       Listeria species

 

Most Listeria species are small, Gram-positive, coccobacillary rods up to 2 μm in length. The genus is composed of 10 species, two of which are pathogenic. Listeria monocytogenes, the more important of these two pathogens, has been implicated worldwide in diseases of many animal species and humans. Listeria ivanovii occasionally causes abortions in cattle and sheep. The organism can grow over a wide temperature range, from 4 to 45°C, and can tolerate pH values between 5.5 and 9.5 and up to 10% sodium chloride. Listeria species can replicate in the environment and are widely distributed in herbage, faeces of healthy animals, sewage effluent and bodies of fresh water. Based on cell wall and flagellar antigens, 16 serotypes are recognized with serotypes 4b and 1/2 responsible for most cases of disease in animals and humans.

Infection with L. monocytogenes usually follows ingestion of contaminated feed and may result in septicaemia, encephalitis or abortion. Organisms probably penetrate the M cells in Peyer's patches in the intestine. Spread occurs via lymph and blood to various tissues. In pregnant animals, infection results in transplacental transmission. There is evidence that the organism can invade through breaks in the oral or nasal mucosa. From this site, migration in cranial nerves is thought to be the main route of infection in neural listeriosis. Lesions in the brainstem are composed of microabscesses and perivascular lymphatic cuffs. Listeria monocytogenes has the ability to invade both phagocytic and non-phagocytic cells with the aid of specific proteins known as internalins. Listeriolysin O and phospholipases mediate escape from the phagosome and cell-to-cell spread following intracellular replication of the organism.

Listeriosis in ruminants may present as encephalitis, abortion, septicaemia or endophthalmitis. Usually only one form of disease occurs in a group of affected animals. Outbreaks of lis-

teriosis tend to be seasonal in European countries and to affect silage-fed animals in late pregnancy. Listeria monocytogenes can replicate in poor-quality silage with pH values above 5.5. The incubation period for neural listeriosis ranges from 14 to 40 days. Dullness, circling and tilting of the head are common clinical signs. Unilateral facial paralysis results in drooling of saliva and drooping of the eyelid and ear. Abortion with evidence of systemic illness may occur up to 12 days after infection. Septicaemic listeriosis, with a short incubation period, is most commonly encountered in lambs. Listeria monocytogenes is an important food-borne pathogen of pregnant women and immunocompromised humans.

Characteristic neurological signs or abortion in association with silage feeding may suggest listeriosis. Appropriate specimens for laboratory examination include cerebrospinal fluid and tissue from the medulla and pons of animals with neurological signs and cotyledons, foetal abomasal contents and uterine discharges from cases of abortion. Suitable samples from septicaemic cases include fresh liver, spleen or blood. Listeria species cultured on blood agar have small, haemolytic, translucent colonies which can be definitively identified using biochemical tests. However, many PCR-based methods have been described for detection of Listeria species in clinical specimens and foods. In addition to serotyping, molecular techniques such as multi-locus sequence typing, pulsed-field gel electrophoresis-based methods and amplified fragment length polymorphism are used for characterization of isolates.

Response to antibiotic therapy may be poor in neural listeriosis and prolonged treatment may be required. In the early stages of septicaemic listeriosis, response to therapy is usually satisfactory. Poor-quality silage should not be fed to pregnant ruminants.