Listeriosis is caused by the bacterium Listeria monocytogenes. Although many species of birds, including chickens, turkeys, pigeons, ducks, geese, canaries, and cockatiels, are susceptible to infection, clinical disease in birds is rare. Generally, young birds are more susceptible to infection and more likely to develop clinical disease than older birds. In chickens, the disease occurs sporadically as either septicemia or encephalitis.
Etiology, Epidemiology, and Pathogenesis
L monocytogenes is a gram-positive, nonsporeforming, facultative intracellular, rod-shaped bacterium. It is widely distributed and commonly found in the environment. In temperate zones, the primary habitats of the organism are soil and decaying vegetation. The organism is common in poorly preserved stored corn silage. L monocytogenes has been isolated from the intestinal tract of healthy animals, including different species of mammals, birds, and fish. Transmission occurs via ingestion, inhalation, or wound contamination. In ruminants, the encephalitic form of listeriosis develops after entry of the organism through minor injuries in the conjunctiva or oral and nasal mucosa with subsequent migration along peripheral nerves to the brain. It is unknown if this same route of infection occurs in birds. Contamination of poultry farms with fecal material from nearby food animal farms (eg, cattle or swine farms) is an important source of infection, especially after a rain or flooding. Wounds from beak trimming and vaccine injection are possible sites of entry for the organism.
In the septicemic form of listeriosis, clinical signs are not specific and include depression, lethargy, and sudden death. In the encephalitic form, lateral recumbency, ataxia, torticollis, leg paddling, opsisthotonos, paresis, and paralysis have been seen.
Birds affected with the septicemic form of listeriosis often have extensive degeneration and necrosis of the myocardium, with splenomegaly, necrotic foci in the liver, and pericarditis. Other lesions reported in broilers include ascites and petechial hemorrhages in the myocardium, liver, kidneys, and spleen. In the encephalitic form, no gross lesions are seen in the brain, but histopathologic lesions are remarkable and include disseminated microabscesses, extensive fibrinous thrombosis, foci of hemorrhages, necrosis (malacia) of the parenchyma, perivascular cuffs of lymphocytes and macrophages, and gliosis. Gram stain of tissues reveals typical gram-positive bacteria within the lesions. Lesions are found in the medulla oblongata, where they are generally most severe, and in the optic lobes and cerebellum.
In the septicemic form, gross lesions should arouse suspicion, and histopathologic lesions should allow preliminary diagnosis of bacterial septicemia. Diagnosis is confirmed by immunohistochemistry to demonstrate L monocytogenes in the tissues or by isolation of the organism, usually from the liver and/or spleen in the septicemic form and brain in the encephalitic form. Direct culture of affected tissues, especially the brain, may not always be successful because of the low concentration of organisms in affected tissues. Recovery of L monocytogenes increases significantly if a portion of the specimen is refrigerated for 4–8 wk and subcultured weekly. Alternatively, tissue may be macerated or blended with a general nutrient broth (eg, trypticase soy broth, brain heart infusion) at a ratio of 1:10. The broth media is incubated at 35°C (95°F) for 5–7 days and examined daily for growth.
Differential diagnoses for septicemic listeriosis include other bacterial septicemias such as colibacillosis, pasteurellosis, and erysipelas. For encephalitic listeriosis, differential diagnoses include viral encephalitides, eg, Marek disease and exotic Newcastle disease. With the latter, neurologic signs (torticollis, opisthotonos) typically follow high mortality in the flock, and lesions of the disease are present in visceral organs.
Treatment and Prevention
Antibiotics may be used successfully to treat the septicemic form of the disease. In vitro, L monocytogenes is susceptible to penicillin, tetracycline, erythromycin, gentamicin, and trimethoprim-sulfamethoxazole. Treatment of the encephalitic form is usually unsuccessful. Prevention should focus on identifying and eliminating potential sources of infection.
Listeriosis is a serious zoonotic disease. L monocytogenes is recognized as an important food-borne pathogen in people and is of great concern to the public and poultry industry. Outbreaks of listeriosis usually follow exposure to raw or uncooked poultry products but have also occurred after contaminated ready-to-eat poultry meat products were eaten.
Last full review/revision November 2013 by Tahseen Abdul-Aziz, BVMS, MS, PhD, DACPV