Listeriosis, Biology

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Listeriosis

The organisms have an extensive host range which includes mammal, poultry, fish, crustaceans and ticks including man. The organism has been isolated from sheep, goats, pigs, buffaloes and poultry. The antibodies against this organism have also been reported to be present among sheep, goats, cattle, pig, man and poultry.


The causative organism is Listeria monocytogenes. Several serovars of this organism have been identified, the commonest in natural cases being 1/2a, 1/2b and 4 B. Another species, L. ivanovii is associated with abortions in ruminants.The micro-organism is like a small rod and is gram positive. The organism is destroyed within minutes at 580C and is killed by usual disinfectants. The organism survives in the silage which becomes a source of disease to cattle and sheep. Sheep, cattle, buffaloes, goats, horses, pigs, cats, rabbits, some wild animals and man are susceptible to infection.


Transmission: The organisms are excreted in the faeces, urine, aborted foetuses, uterine discharge and milk of infected animals. The organisms are sufficiently resistant to remain viable in animal and human faeces, sewage, soil, silage and dust for several weeks or months. The blood-sucking arthropods may spread infection since organisms have been isolated from cattle ticks and tabanid flies. Under natural conditions certain predisposing factors are related to clinical infection.


Symptoms: The disease is characterized by meningo-encephalitis, abortion or septicaemia. In farm animals the disease occurs towards the end of winter or early spring. The first signs of meningo-encephalitis are stiffness of neck, incoordinated movement of limbs and tendency to move in circles or to lean against a fence or wall (Circling disease). T here may be paralysis of muscles of jaw and pharynx. Incoordination becomes progressively more severe until the animals can no longer stand. The cattle which are not severely affected may survive. Abortions in cattle usually occur after 4-8 months of pregnancy and at a comparatively later stage in sheep. In pigs and horses, clinical signs are not common but may develop as encephalitis and septicaemia. In poultry, the disease usually causes sudden death, occasionally there are signs of torticollis, weakness and in co-ordination of the legs.


Lesions: The lesions in all species are varied depending upon the tissue involved. In rabbits and guinea-pigs, liver necrosis is the pronounced lesion. In sheep, cattle and pigs lesions are confined to central nervous system where a marked meningitis is produced. Visceral lesions occur as multiple foci of necrosis in the liver, spleen, endocardium and myocardium in the septicaemia form and in the aborted foetus.


Diagnosis:
The encephalitic form of the disease may be diagnosed by examining sections of brain. In farm animals symptoms like encephalitis and abortions may be suggestive of listeriosis but final diagnosis depends upon the isolation and identification of bacteria. Serological tests (indirect haemagglutination) are used for the detection of L. monocytogenes antibodies in infected animals.


Treatment: Tetracyclines are very effective in meningo-encephalitis of cattle but  less so in sheep. The recovery rate depends on the speed with which the treatment is commenced.


Control: When outbreaks occur all affected animals should be slaughtered and buried along with litter and bedding. The vaccines, living or killed, have little effect on the pathogenesis of infection under natural conditions. Tetracyclines are very effective for treatment of listeriosis.


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