1. Introduction   

Necrotic laryngitis is an acute or chronic Fusobacterium necrophorum infection of the laryngeal mucosa and cartilage of young cattle, characterized by fever, cough, inspiratory dyspnea, and stridor. It occurs primarily in feedlot cattle 3–18 months old; however, cases have been documented in calves as young as 5 weeks and in cattle as old as 24 months. Cases occur worldwide and year-round but appear to be more prevalent in fall and winter (Campbell, 2023).

2. Aetiology

2.1 Bacterial characteristics

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 (i.e., necrobacillosis), including necrotic laryngitis (Campbell, 2023).

2.2 Classification

A summary of the group specificities of different molecular markers (CSIs) is presented in Figure 2. Of these molecular markers, 7 CSIs in proteins involved in important functions are uniquely found in all sequenced Fusobacteria species, providing molecular markers for this phylum; Six and three additional CSIs present in the protein homologs of other important proteins are specific for members of the families Leptotrichiaceae and Fusobacteriaceae, respectively.
These molecular markers provide independent evidence that the members of these families are distinct from each other, and they provide novel means for identifying and distinguishing them from each other and all other bacteria (Gupta and Sethi, 2014).

3. Epidemiology

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 (Campbell, 2023).

4. Pathogenesis

The infection more often is referred to as ‘calf diphtheria’, which is somewhat of a misnomer, because the disease occurs in cattle up to 3 years of age. The disease is characterized by necrosis of the mucus membrane of the larynx, particularly in the lateral arytenoids cartilage, and adjacent structures. The infection can be acute or chronic and is non-contagious (Mackey, 1968). The lesions appear as erosions progressing to ulcers and abscesses. In severe cases, cattle could die from aspiration pneumonia. Clinically, an initial fever is followed by dyspnea that causes a roaring noise on inspiration (‘hard breathers’) and, in severe cases, painful swallowing and cough. Necropsy lesions include necrosis of the larynx and vocal cords and a mucus membrane covered by inflammatory exudate. Occasionally, bronchopneumonia may be evident.

F. necrophorum is also a normal inhabitant of the respiratory tract of cattle. In a study involving bacteriological investigation of laryngeal swabs from animals with clinical laryngitis, F. necrophorum and A. pyogenes were isolated most frequently (Panciera et al., 1989). 

5. Clinical signs and pathology

5.1 Clinical signs

These cattle tend to be dull with inappetance or anorexia. Often there is pyrexia (40.5°C, 105°F) and there may be stertor. Usually, respirations are dyspnoeic to a varying degree. There may be a cough that is moist and painful. Palpation of the larynx is resented and can elicit the cough. The mouth may be foul smelling. Many of these animals do not respond well to treatment and the diphtheritic area may become detached resulting in sudden asphyxiation or lung infection (Lopez and Martinson, 2017).     

The clinical signs of necrotic laryngitis are fever, anorexia, depression, halitosis, moist painful cough, dysphagia, and inspiratory dyspnea and ventilatory failure because of fatigue of the respiratory muscles (diaphragmatic and intercostal). Calves affected with calf diphtheria usually have abscesses in the cheek region, have mild salivation, and may refuse solid feed (Figure 3).

5.2 Pathology

The gross lesions, regardless of location in the mouth or larynx (most common in the mucosa overlying the laryngeal cartilages), consist of well-demarcated, dry, yellow-gray, thick-crusted, and fibrinonecrotic exudate (Figure 4) that in the early stages is bounded by a zone of active hyperemia. Deep ulceration develops, and if the lesion does not result in death, healing is by granulation tissue formation. Microscopically, the necrotic foci are first surrounded by congested borders, then by a band of leukocytes, and finally the ulcers heal by granulation tissue and collagen (fibrosis). The lesions can extend deep into the submucosal tissue. Numerous bacteria are evident at the advancing edge (Lopez and Martinson, 2017).Additionally, the larynx will be found to be uniformly swollen and may appear to have cartilaginous deformities in chronic cases (Figure 5) (Peek and Divers, 2018).

Post-mortem examination (Figure 6) reveals a caseous infection (A), typically located bilaterally between the vocal processes (B) and the medial angles of the arytenoid cartilages (C), where it restricts air passage. In other cases (Figure 7) the caseous infection may be in the deeper tissues, such as on the left arytenoid area (A) of this 4-month-old Limousin cross calf, where the only superficial change is a soft tissue swelling, the caseous material being deeper within. Note the shape of the normal cartilage (B) on the right (Blowey and Weaver, 2011).

6. Diagnosis

6.1 Clinical evaluation

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 (Campbell, 2023).

6.2 Visual inspection of larynx

The diagnosis 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 (Campbell, 2023).

6.3 Differential diagnoses

The main differential diagnoses are foreign bodies in the mouth, papular stomatitis, mouth and jaw injuries and mucosal disease. All are quite easy to eliminate by thorough oral examination (Andrews et al., 2008).

7. Treatment

Long-term therapy is required because infection of cartilaginous structures usually exists. Acute cases should be treated with penicillin (22,000 U/kg IM, twice daily). A tracheostomy is essential for treatment of calves that have severe dyspnea. This will provide a patent airway and rest the infected larynx from further exertional irritation while the infection is controlled. The prognosis for acute cases is fair. Chronic cases have a poor prognosis because laryngeal deformity and cartilaginous necrosis or abscesses within the laryngeal cartilage already have developed. Treatment is similar to that described for acute cases but should be extended to 14 to 30 days in patients valuable enough to warrant treatment, or the necrotic cartilage should be surgically removed or debrided. A tracheostomy may be necessary for the reasons listed above, and some clinicians recommend concurrent treatment with sodium iodide in the hope of penetrating the deep-seated infection of cartilage. A. pyogenes frequently contributes to or replaces F. necrophorum as the causative organism in chronic infections because these two organisms are synergistic. For valuable cattle with the chronic form, referral to an expert surgeon familiar with the tracheolaryngostomy technique described by Gasthuys should be considered (Peek and Divers, 2018).

8. Control and prevention

If more than a single case occurs, hygiene should be improved. The calves should be fed with their own buckets and quality feed should be used. The milk and water buckets should be cleaned and disinfected after each feed and the feed bucket disinfected at least twice a week. Occasionally, it is necessary to give oral antibiotics as a prophylactic measure. Suitable agents include chlortetracycline and oxytetracycline (Andrews et al., 2008).

9. References

Andrews, A.H., Blowey, R.W., Boyd, H., Eddy, R.G., 2008. Bovine medicine: diseases and husbandry of cattle. John Wiley & Sons.

Blowey, R., Weaver, A.D., 2011. Color atlas of diseases and disorders of cattle e-book. Elsevier Health Sciences.

Campbell, J., 2023. Necrotic Laryngitis in Cattle. MSD Manual. Veterinary Manual.

Dowell, V.R., 1972. Public health image library. CDC. Centers for Disease Control and Prevention.

Gupta, R.S., Sethi, M., 2014. Phylogeny and molecular signatures for the phylum Fusobacteria and its distinct subclades. Anaerobe 28, 182-198.

Lopez, A., Martinson, S.A., 2017. Respiratory system, mediastinum, and pleurae. Pathologic basis of veterinary disease, 471.

Mackey, D., 1968. Calf diphtheria. Journal of the American Veterinary Medical Association 152, 822-823.

Panciera, R., Perino, L., Baldwin, C., Morton, R., Swanson, J., 1989. Observations of calf diphtheria in the commercial feedlot. Agri-practice 10, 12-17.

Peek, S.F., Divers, T.J., 2018. Rebhun’s diseases of dairy cattle-E-book. Elsevier Health Sciences.