Pasteurella multocida , the causal agent, is a small, gram-negative, nonmotile rod that may exhibit pleomorphism after repeated subculture. In freshly isolated cultures or in tissues, the bacteria have a bipolar appearance when stained with Wright’s stain. Although P multocida may infect a wide variety of animals, strains isolated from nonavian hosts generally do not produce fowl cholera. Strains that cause fowl cholera represent a number of immunotypes, which complicates widespread prevention by using bacterins. The organism is susceptible to ordinary disinfectants, sunlight, drying, and heat. Turkeys are more susceptible than chickens, older chickens are more susceptible than young ones, and some breeds of chickens are more susceptible than others. Chronically infected birds are considered to be a major source of infection. Dissemination of P multocida within a flock is primarily by excretions from mouth, nose, and conjunctiva of diseased birds that contaminate their environment.
These vary greatly depending on the course of disease. In acute fowl cholera, dead birds are usually the first indication of disease. Fever, depression, anorexia, mucoid discharge from the mouth, ruffled feathers, diarrhea, and increased respiratory rate are usually seen. Pneumonia is particularly common in turkeys. In chronic fowl cholera, signs and lesions are generally related to localized infections. Sternal bursae, wattles, joints, tendon sheaths, and footpads are often swollen because of accumulated fibrinosuppurative exudate. There may be exudative conjunctivitis and pharyngitis. Torticollis may result when the meninges, middle ear, or cranial bones are infected.