F necrophorum, a gram-negative, non-spore-forming anaerobe, is a normal inhabitant of the gastrointestinal, respiratory, and urogenital tracts of animals. The organism is an opportunistic pathogen that causes several necrotic conditions in animals (ie, necrobacillosis), including necrotic laryngitis.
Etiology of Necrotic Laryngitis in Cattle
Predisposing factors for necrotic laryngitis are not fully understood. F necrophorum, commonly isolated from laryngeal lesions of affected cattle, is unable to penetrate intact mucous membranes. Laryngeal contact ulcers, a common finding in slaughtered cattle, are thought to provide a portal of entry for F necrophorum.
Transmission, Epidemiology, and Pathogenesis of Necrotic Laryngitis in Cattle
Necrotic laryngitis is most common where cattle are closely confined under unsanitary conditions or in feedlots. The prevalence in feedlot calves is estimated to be 1%–2%. Most cases are sporadic and occur year-round; however, disease peaks in fall and winter. Mixed upper respiratory tract infections (due to infectious bovine rhinotracheitis virus and parainfluenza-3 virus; Mycoplasma spp; and bacteria, including Pasteurella and Haemophilus), and the coughing and swallowing associated with these infections, may predispose feedlot cattle to develop laryngeal contact ulcers. These ulcers on the vocal processes and medial angles of arytenoid cartilages are thought to provide a portal of entry for F necrophorum.
F necrophorum causes inflammation, necrosis, and edema in the laryngeal mucosa, resulting in variable narrowing of the rima glottidis and inspiratory dyspnea and stridor. If infection extends into the laryngeal cartilage, laryngeal chondritis develops, which may lead to a chronically deformed larynx. Pharyngeal invasion by the organism causes discomfort characterized by painful swallowing motions. Systemic clinical signs of illness have been attributed to the exotoxin produced by F necrophorum. Cattle breeds such as Belgian Blue may be predisposed to this condition because of a narrow larynx and smaller lung volume.
Clinical Findings of Necrotic Laryngitis in Cattle
Moist, painful cough
Dyspnea with stridor
Purulent nasal discharge
Fetid odor to breath
Painful swallowing motions
Initially in necrotic laryngitis, a moist, painful cough is noticed. Severe inspiratory dyspnea, characterized by open-mouth breathing with the head and neck extended, and loud inspiratory stridor are common findings. Ptyalism; frequent, painful swallowing motions; bilateral, purulent nasal discharge; and a fetid odor to the breath may also be present. Systemic clinical signs of necrotic laryngitis may include fever (41.1°C [106°F ]), anorexia, depression, and hyperemia of the mucous membranes.
Untreated calves may die in 2–7 days from endotoxemia and upper airway obstruction. Longterm sequelae include aspiration pneumonia and permanent distortion of the larynx, resulting in a chronic, harsh cough and inspiratory dyspnea.
Lesions are typically located over the vocal processes and medial angles of arytenoid cartilages. Acute lesions are characterized by edema and hyperemia surrounding a necrotic ulcer in the laryngeal mucosa; lesions may spread along the vocal folds and processes to involve the cricoarytenoideus dorsalis muscle. In chronic cases, lesions consist of necrotic cartilage associated with a draining tract surrounded by granulation tissue.
Diagnosis of Necrotic Laryngitis in Cattle
Visual inspection of larynx
Clinical signs are usually sufficient to establish a diagnosis of necrotic laryngitis. However, because numerous other conditions can cause clinical signs of upper airway obstruction, the larynx should be visually inspected to confirm a diagnosis. This can be accomplished by means of an orally inserted speculum, laryngoscopy, endoscopy, or radiography; however, care must be exercised to avoid further respiratory compromise. A tracheostomy should be performed before laryngoscopic or endoscopic examination in cattle with severe inspiratory dyspnea. Differential diagnoses include pharyngeal trauma, severe viral laryngitis (eg, infectious bovine rhinotracheitis), actinobacillosis, abscesses, trauma, paralysis, or neoplasia.
Treatment and Control of Necrotic Laryngitis in Cattle
NSAIDs or dexamethasone
Oxytetracycline (11 mg/kg, IV or SC, every 12 hours, or 20 mg/kg of long-acting tetracycline, SC, every 72 hours), procaine penicillin (22,000 U/kg, IM, every 12 hours), or florfenicol (40 mg/kg SQ) is commonly used for antimicrobial treatment in cases of necrotic laryngitis. Prolonged antimicrobial treatment of up to 2–3 weeks may be necessary in cases of severe laryngitis.
NSAIDs (aspirin, 100 mg/kg, PO, every 12 hours; flunixin, 1.1–2.2 mg/kg, IV, once daily or every 12 hours; or ketoprofen, 3 mg/kg/day, IM or IV, for up to 3 days) is used to decrease the fever and laryngeal inflammation and edema. A single dose of dexamethasone (0.2–0.5 mg/kg, IV or IM) may be used to decrease laryngeal edema in animals with severe respiratory distress. A tracheostomy is indicated in cattle with severe inspiratory dyspnea. Good nursing care should be provided. Intravenous fluids may be required in dehydrated animals.
The prognosis is fair for early cases treated aggressively; an overall survival rate of approximately 58%–65% has been estimated for necrotic laryngitis cases surviving to slaughter age. Animals diagnosed at less than 6 months old had higher mortality rates. Chronic cases require surgery under general anesthesia to remove necrotic or granulation tissue and to drain laryngeal abscesses. A 60% success rate has been reported for surgical intervention in advanced cases.
There are no specific control measures for necrotic laryngitis; however, the proposed pathogenesis suggests that control measures for common respiratory pathogens may be beneficial.
Bacterial infection of the larynx is due to Fusobacterium necrophorum.
Antimicrobial treatment with broad-spectrum antimicrobials is needed for 2–3 weeks.
NSAIDs or dexamethasone may be used to help decrease laryngeal inflammation.
Chronic cases may require surgical intervention.