Listeriosis is caused by the bacteria Listeria monocytogenes, which has a worldwide distribution. Although many species of birds are susceptible to infection, clinical disease in birds is rare and generally occurs as septicemia or a localized encephalitis. Poultry (chickens, turkeys, geese, and ducks) and pet birds (canaries and parrots) have been affected. Young birds are more susceptible to the effects of the disease. Adult birds often have an acute septicemic form, while young birds tend to develop the chronic form of the disease.
Etiology and Epidemiology
L monocytogenes is a gram-positive, non-sporeforming, rod-shaped bacteria that forms long filaments on stained smears. L monocytogenes is commonly found in the environment, and poultry can become readily colonized. Transmission is via ingestion of contaminated nasal secretions, feces, and soil. Infection can also occur via inhalation and wound contamination. Because infected birds often do not have any clinical signs, they can serve as a reservoir to perpetuate listeriosis in a flock. This disease often occurs secondary to other diseases.
L monocytogenes infections are often subclinical. Chickens and turkeys are relatively resistant to natural infection. However, if infection does occur, signs suggestive of septicemia are observed and include depression, lethargy, and sudden death. In the subacute and chronic forms, encephalitis along with torticollis, paresis, and paralysis have been observed.
Affected birds often have multiple areas of necrosis on the myocardium with congestion, increased pericardial fluid, and pericarditis. Enlargement of internal organs along with focal areas of necrosis are common due to septicemia. In the chronic encephalitic form, no gross brain lesions are observed, but gliosis in the cerebellum with microabscesses containing gram-positive bacteria are present in the midbrain and medulla microscopically.
A presumptive diagnosis of listeriosis can be made based on the history, signs, lesions, and microscopic observation of the bacteria in the lesions. A definitive diagnosis is confirmed via isolation of L monocytogenes from infected blood, liver, heart, spleen, or brain. Direct culture of affected tissues may not always be successful due to the low concentration of organisms in the affected tissues. Recovery of L monocytogenes increases significantly if a portion of the specimen is refrigerated for 4–8 wk and subcultured weekly.
Differential diagnoses include numerous acute and chronic bacterial diseases including colibacillosis, pasteurellosis, erysipelas, and velogenic viscerotropic Newcastle disease.
Treatment and Control
L monocytogenes is often resistant to many of the commonly used antibiotics, making treatment difficult. Tetracyclines have been efficacious in both the acute and subacute forms when given at 25 mg/kg, PO, sid for 1 wk. Treatment of the chronic form is usually unsuccessful. Because treatment is not effective, prevention is of utmost importance. Rigid sanitation and disinfection procedures along with culling and isolation of affected birds may reduce the prevalence of listeria in the flock. Prevention should focus on identifying and eliminating the source of infection.
Conjunctivitis due to L monocytogenes has been reported in individuals handling apparently healthy but infected chickens. Human infections have also resulted from the consumption of contaminated poultry or ready-to-eat poultry products.
Last full review/revision March 2012 by Teresa Y. Morishita, DVM, MPVM, MS, PhD, DACPV