In diseases caused by the pathogenic bacterium Listeria, the cerebrospinal fluid may be cloudy, there may be some congestion of meningeal vessels and in some bovine cases there is panophthalmitis but in general the macroscopic findings are not marked. Histological examination of brain tissue using a binocular compound microscope is necessary to demonstrate the micro-abscesses, which are characteristic of the disease. Heavy inoculation of media with material that has been macerated and refrigerated for long periods is advisable when attempting to isolate the organisms, which are often present in small numbers, before stuying them under a binocular compound microscope. Visceral lesions occur as multiple foci of necrosis in the liver, spleen, endocardium and myocardium especially in the septicemic form and in aborted fetuses. Gross lesions suggested as being almost pathognomonic for listeriosis in aborted lambs are small yellow foci of necrosis in the liver, small abomasal erosions and yellow-orange meconium. Macroscopically, aborted fetuses are usually edematous, rarely mummified, and autolysed. In animals that abort there are placentitis and endo¬metritis in addition to the lesions in the fetus.
A characteristic of the pathology of the disease is the formation of granulomas as well as foci of necrosis, and these appear at the beginning of the disease, rather than as sequelae to necrosis.
Diagnosis
Listerial meningoencephalitis may be confused with the nervous form of acetonemia in cattle and with early cases of pregnancy toxemia in sheep. In these diseases dullness, isolation, apparent blindness and circling are also, characteristic but there is no facial nerve paralysis and no endophthalmitis. However, acetonemia occurs in cattle soon after parturition and in ewes pregnancy toxemia is observed only during late pregnancy and usually in association with multiple pregnancy and a declining nutritional status. Also circling in these diseases is accompanied by muscle twitching, par¬ticularly of the face, champing of the jaw and blinking of the eyelids and these signs appear only inter¬mittently. As soon as the convulsive episode has passed a normal posture is adopted and the animal can walk in a straight line. Pregnancy toxemia may, be accom¬panied by a rise in body temperature if muscular activity is much increased. Both these metabolic diseases are accompanied by marked ketonuria. Brain abscess, while rare, may be clinically indistinguishable from listeriosis. Cerebrospinal fluid and blood leucocyte examinations may yield little or no basis for differentiation when examined and compared using binocular compound microscopes. The course is usually much longer, the animal often surviv¬ing for some weeks. Rabies should be considered in the differential diagnosis but there are no localizing signs as there are in listeriosis.
Listerial abortion must be differentiated from the other causes of abortion in cattle and sheep and in goats from Br. melitensis infection (another pathogenic bacterium that can be identified using a binocular compound microscope). Acute septicemias of new¬born animals are due to many causes but association with abortion and stillbirths may suggest the possible presence of listeriosis. The necropsy lesions are dis¬tinctive and positive cultural findings confirm the diagnosis with the help of a binocular compound microscope.
Treatment
Listeria monocytogenes is resistant to many drugs but is sensitive to chlortetracycline. The intravenous injection of chlortetracycline (10 mg/kg BW per day for 5 days) is reasonably effective in meningo-encephalitis of cattle but less so in sheep. The recovery rate depends largely on the speed with which treatment is commenced. If severe clinical signs are already evident death usually follows in spite of treatment. Usually the course of events in an outbreak is that the first case dies but subse¬quent cases are picked out sufficiently early for treat¬ment to arrest further development of the disease.
Control
Most attempts by veterinarians to produce a satisfactory killed vaccine have been unsuccessful although field trials with a killed bacterin are reported to reduce the incidence in sheep flocks. Virulent living culture or attenuated bacteria are also under investigation as immunizing agents with the help of a binocular compound microscope. A common recommendation is to reduce the amount of ensilage fed and in animals in the feedlot the constant feeding of low levels of tetracyclines is recommended for the duration of the fattening period. There may be some merit in the recommendation that a change of diet to include heavy feeding of ensilage should be made slowly, particularly if the ensilage is spoiled or if listeriosis has occurred on the premises previously. Read more on this subject