Fusobacterium necrophorum in necrotic laryngitis of feedlot cattle.

Anaerobic culture is essential for diagnosis; β-lactams and tetracyclines remain first-line treatments for necrotic laryngitis.

This case series describes the clinical, pathologic, and microbiologic findings of necrotic laryngitis (calf diphtheria) in four 20-month-old feedlot steers. Affected animals presented with classic signs of upper airway obstruction including inspiratory stridor, increased respiratory effort, and flared nares. Endoscopic examination revealed laryngeal chondritis, mucopurulent diphtheritic membranes, and necrotic inflammation of the vocal folds. Crucially, aerobic cultures were negative while anaerobic cultures confirmed the causative role of Fusobacterium necrophorum, with isolates identified as either subsp. necrophorum or subsp. funduliforme. Hematologic findings included leukocytosis and elevated plasma fibrinogen, and serum biochemistry revealed marked hyperglycemia and mild hepatic enzyme elevations, attributed to stress-related metabolic changes. Antimicrobial susceptibility testing demonstrated consistent susceptibility to β-lactams and tetracyclines, supporting their use as first-line therapeutic agents. F. necrophorum isolates were also susceptible to florfenicol, tiamulin, and trimethoprim/sulfamethoxazole. Resistance was noted to aminoglycosides and fluoroquinolones. Macrolide susceptibility was variable: four strains were susceptible to tylosin, tilmicosin, and tulathromycin, while one strain was resistant. These findings reinforce the necessity of anaerobic culture techniques for accurate diagnosis and highlight that β-lactams and tetracyclines remain appropriate empirical treatment choices for necrotic laryngitis in feedlot cattle.

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