Listeriosis

H. E. BIESTER AND L. H. SCHWARTE.

LISTERIOSIS, an infectious disease, is caused by a bacterial organism that affects animals and people.

Three British scientists first recovered the organism from laboratory rabbits and guinea pigs in 1926. They named it Bacterium monocytogenes. Later a similar organism was isolated from some rodents in South Africa. Various names have been used for the organism Bacterium monocytogenes, Listerella hepatolytica, L. monocytogenes, Corynebacterium parvulum, and Bacillus monocytogenes. It has been designated as Listeria monocytogenes since 1948. The disease formerly was known as listerellosis.

Various investigators have found the infection in sheep, cattle, swine, foxes, chickens, and man. Increasing numbers of cases of listeriosis have been reported from many States since 1936.

Generalized gross changes are observed in children, chickens, and rodents, but those changes usually are absent in infected cattle, sheep, and swine. Listeriosis causes changes in the cells of the nervous system. The changes are much like those associated with some virus infections. The organism has been recovered primarily from the central nervous system, but some investigators have found Listeria in other organs.

LISTERIOSIS IN SHEEP often reaches epidemic proportions. An outbreak in large flocks, particularly large feeder units, may cause heavy losses. Losses of 200 to 500 in feeder flocks of 3,000 sheep have occurred. Fewer than 2 percent of the animals with definite clinical symptoms recover. Losses on the range are seldom extensive.

The exact mode of transmission under field conditions is not well understood. The infection may be spread by contact and through contaminated feed and water.

The suspension of operations on an infected area for 6 months or more after thorough cleaning and disinfection is advisable in order to prevent recurrence of the disease.

The first indication of listeriosis in lambs may be a loss of appetite and a temperature up to 106 F. The infected animals appear depressed and weak. They are inclined to tremble and lean against objects. Incoordination of various degrees is frequently observed. Lambs may have a tendency to throw their heads back. Others show unilateral involvement, so that the head turns to one side. The disease usually runs a rapid course. Some infected animals lie on their sides and their legs move, as if trying to run. Convulsions may occur. The corneas of the eyes may appear grayish toward the termination of the disease. Breathing becomes faster. Sometimes strands of mucus, from the mucosa of the turbinates, exude from the nose and mouth. Examination of an infected animal usually discloses no definite pathologic changes in the viscera. The liver and kidneys may be swollen and grayish brown. Slight congestion or inflammation of the meninges and occasionally petechial hemorrhages may be observed in the brain tissue.

MICROSCOPIC EXAMINATION of parts of the central nervous system may show focal areas of concentrations of white blood cells in the brain tissue and vast accumulations of monocytic cells around the walls of the blood vessels. Degeneration is not unusual in the areas of cellular infiltration. The organism is seldom isolated from the blood stream, liver, spleen, or kidney of infected sheep in the advanced stage of the disease.

Routine culture methods of brain tissue often fail to secure positive results in infected animals. More satisfactory results are obtained when composite samples taken from various parts of the brain and cord are triturated in a mortar and shaken for 15 minutes or when the composite samples are thoroughly processed in a Waring blender to break up the infected foci. The bacterial organisms are then uniformly distributed in the emulsion of brain tissue and can be transferred and grown on culture media. General characteristics of this organism are: Small rods 0.4 to 0.5 by 0.5 to 2.0 microns; Gram - positive, flagellated, aerobic, catalase positive; acid but no gas from glucose and a few additional carbohydrates; easy growth on ordinary culture media. The organism is not acid-fast. This brief summary differentiates this bacterial species from others that have similar size and shape.

LISTERIA INFECTION IN SWINE has been reported in a small number of droves in the Corn Belt. The disease has been found in suckling pigs and larger animals of 150 pounds or more. Mature stock is seldom affected.

The early symptoms include restlessness, less appetite, periodic thirst, increased temperature reaction in most cases, and more and more nervousness. The larger animals develop definite central nervous disturbances. Some individuals drag their hind legs or show various degrees of incoordination; the forelegs move in a stilted gait. The suckling pigs develop a progressive central nervous involvement, including spasms and paralysis, which often ends in death. Some cases recover despite severe clinical symptoms. The highest mortality occurs in the young pigs.

The disease occurs usually in large droves. It is believed to be spread by contact and by exposure to infected feed, water, buildings, and equipment.

Gross pathologic changes in the visceral organs are usually vague or absent. Congestion or inflammatory reactions in the meninges may be observed. Congestion of the cerebral blood vessels or a few petechial hemorrhages may or may not be present.