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Listeriosis

 Listeriosis, also referred to as Circling Disease or Silage Sickness, is a sporadic bacterial infection caused by Listeria monocytogenes. Listeriosis is a worldwide disease, and affects a wide variety of mammalian and avian species, including man. Encephalitis is the most frequently recognized form of listeriosis of animals. The infection most commonly occurs in adult ruminants that are being fed contaminated silage.

 Listeria monocytogenes is a small, motile, gram-positive nonspore-forming cocco-bacillus. This ubiquitous saprophyte lives in a plant-soil environment and can be found in soil, vegetables, sewage, genital secretions and nasal mucous of apparently healthy animals. The organism is very resistant to drying and can survive up to two years in dry soil and feces. It is also capable of growing well under a wide range of temperatures, 4 - 44 C.

 There are four common manifestations of listeriosis: encephalitis in adult ruminants, septicemia in monogastrics and neonatal ruminants, abortion and perinatal deaths in all species, and mastitis in ruminants. It is uncommon for all forms of listeric infections to occur at one time within a flock/herd and for the majority, the infections are subclinical. However, when animals become stressed, immunocompromised or pregnant, clinical listeriosis often develops.

 The route of infection seems to vary in accordance with the different clinical syndromes: encephalitis by small wounds in the buccal mucosa, while septicemia and abortions come from ingestion and inhalation.

 Encephalitis is most prevelant in late winter and early spring when animals are confined and silage feeding is greatest. Silage fed from trenches or pits is most often the source of infection. The sides and lower, damper layers of silage from these pits seem to be the most contaminated because these sites are often exposed to air. This results in an aerobic decomposition of silage and an increase in pH (>5) which enhances the multiplication of L. monocytogenes.

L. monocytogenes commonly enters abrasions in the oral mucosa or at sites where teeth have fallen out and ascends via the trigeminal nerve to the brainstem. Here a unilateral, localized lesion is produced within the medulla oblongata and pons.

The number of animals affected clinically in an outbreak of listeriosis is usually low but mortality is extremely high. Sheep and goats are most susceptible to Listeria infections and are overcome by an acute disease with death occurring 4-48 hours after the onset of clinical signs. In cattle, infections are sporadic, less acute and most survive for 4-14 days. Spontaneous recovery may occur, but permanent CNS injury is frequent in these animals.

The clinical signs of affected animals include depression, fever, disorientation, and an indifference to their surroundings. They often separate themselves and crowd into corners and head press. They stumble and circle continuously. An associated head tilt is also commonly seen in these animals. Facial paralysis characterized by a drooping ear, dilated nostril and lowered eyelid (ptosis) on the same side as the lesion often develops. Intermittent twitching and paralysis of facial, throat and tongue muscles are usually present resulting in tongue protrusion, excessive salivation and dysphasia. A progressive paralysis develops throughout the course of the disease and in the terminal stages, the animal often falls and is unable to rise. Exhaustion followed by coma and death rapidly occurs once the animal has become recumbent.

Histological examination of brain tissue, preferably the brainstem, is necessary to demonstrate the monocytic perivascular cuffing and microabscesses that are characteristic of the disease.

Septicemic or visceral listeriosis is commonly observed among monogastric animals, including pigs, dogs, cats, rabbits and chinchillas, as well as neonatal ruminants. The clinical signs include an acute onset of depression, fever, anorexia, coughing and respiratory distress, diarrhea, prostration and death. The principle lesion is focal hepatic necrosis. For the septicemic form, the finding of multiple necrotic foci in any organ, especially the liver, is often highly suggestive of Listeriosis.

All pregnant domestic animals are susceptible to Listeria and an infection at this time often results in placentitis, fetal deaths, abortions, stillbirths, and neonatal deaths. Abortions in cattle are sporadic and occur within the last third of pregnancy. Abortion storms are more common in sheep and most abortions occur after the 12th week of pregnancy. When the placental tissues are retained, a secondary metritis often develops. This metritis is long lasting but has little or no effect on the animal's reproductive future. These animals commonly shed the organism in milk and vaginal secretions for a period of two months following an abortion episode.

Several suggestive gross lesions can be demonstrated in aborted fetuses. These include small yellow foci of necrosis in the liver, shallow abomasal erosions and a yellow-orange meconium. The fetus is often edematous and autolyzed masking these lesions. Dams and ewes should also be examined for placentitis and endometritis.

Mastitis is a rare manifestation of listeriosis. It affects only a single quarter and is unresponsive to antibiotics. Although uncommon, it does occur, and should be considered when dealing with chronic cases of mastitis in cattle.

The definitive diagnosis can only be made by the isolation and identification of L. monocytogenes. Brain tissue, aborted placenta and fetus are the preferred specimens for culture. The organism is often difficult to grow and is commonly missed if not specifically requested. In the past, cold enrichment procedure incubated at 4 C was used for the isolation of L. monocytogenes. Recently, a more selective media established by USDA scientists, were adopted by the ADDL bacteriology lab. This method will decrease the long incubation time (1-2 months) to the cold enrichment protocol of 1-2 weeks. In addition, a PCR for L. monocytogenes has been developed by the ADDL bacteriology lab to detect L. monocytogenes. Other tests like immunohistochemical testing, using the perioxidase-antiperoxidase (PAP) method, has been used for the detection of L. monocytogenes antigen within brain tissue of infected animals. Both PCR and immuno-histochemistry methods are rapid and confirmatory, especially when inappropriate materials are submitted for other diagnostic methods.

Due to its ubiquitous nature, minimizing the opportunity for exposure seems to be the best preventive alternative against listeriosis. When an outbreak occurs, the affected animals should be immediately treated and isolated and those that have died should be destroyed or removed from the premises. Buildings should be thoroughly disinfected and cleaned and all bedding and feed should be burned. Silage feeding should be reduced and if spoiled, should be avoided. Other recommendations for silage fed herds/flocks may include: minimize soil contamination when making silage and filling the trench, perform routine silage testing and use additives to improve the fermentation process. It is extremely hard to totally eliminate L. monocytogenes from a surrounding environ-ment but the incidence of disease within a herd or flock can be significantly reduced if these recommendations are followed.

Listeriosis is also an important zoonotic disease. Aborted fetuses and necropsies of septicemic animals present the greatest hazard for veterinarians and agricultural workers. People have been reported to develop meningitis, septicemia, and a papular exanthema on their hands and arms after handling such tissues. Pregnant women are susceptible and at great risk of aborting if they are not properly protected from infection. The young, the elderly, and the immuno-compromised are most susceptible to infection and need to take precaution as well. The sources of infection include milk that was improperly pasteurized or contaminated after pasteurization, cheese by-products, and raw vegetation. Just as in animals, the only reasonable alternative for prevention is to minimize exposure and ensure the use of good personal and food hygiene to reduce the incidence of listeriosis within the human population.

- by Julie Stone, Class of 1998

- edited by Ching Ching Wu, DVM, PhD

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ADDL-West Lafayette:
406 S. University
West Lafayette, IN 47907
Phone: 765-494-7440
Fax: 765-494-9181

ADDL-SIPAC
11367 E. Purdue Farm Road
Dubois, IN 47527
Phone: (812) 678-3401
Fax: (812) 678-3412

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