Pharynx: lymphoid hyperplasia in Horses (Equis) | Vetlexicon
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Pharynx: lymphoid hyperplasia

ISSN 2398-2977


Synonym(s): Pharyngeal lymphoid hyperplasia

Introduction

  • Proliferation of pharyngeal lymphoid tissue in young horses, previously thought to be a cause of poor performance.
  • Cause: is a normal feature of the nasopharynx in younger horses but increased size has been proposed to be a response to respiratory infection, but in studies no association found.
  • Signs: follicular tissue in the pharyngeal recess.
  • Diagnosis: endoscopy.
  • Treatment: none required.
  • Prognosis: the lymphoid tissue will reduce in size from about 5 years of age.

Presenting signs

  • Once thought to be a cause of poor performance in racehorses, but present work suggests it may be purely an incidental finding.

Acute presentation

  • Commonly indentified as an incidental finding in horses investigated for poor performance or as part of routine airway endoscopy.

Geographic incidence

  • Reported worldwide.

Age predisposition

  • Young horses - under 5 years of age.

Cost considerations

  • Minimal as rarely of clinical significance.

Pathogenesis

Etiology

  • The equine pharyngeal tonsil consists of mucosal-associated tissue, which is made up of lymphoid follicles containing B-cells and surrounded by interfollicular T-cells.
  • The nasopharynx is exposed to multiple allergens, viruses, bacteria and irritant particles, which results in response of the local lymphoid tissue to stimulate mucus-producing cells and production of local immunoglobulins.
  • Young horses are exposed to many new antigenic stimuli as a result of travel, mixing with other young horses, competition and stabling.

Predisposing factors

General
  • No association found with poor stable management, although stabling is thought to cause persistence of pharyngeal lymphoid hyperplasia in comparison with horses at pasture.
  • No association found with lower respiratory tract disease, presence of a cough or poor performance.

Specific

  • Age is the only significant risk factor for pharyngeal lymphoid hyperplasia, with lower age associated with highest prevalence (2 year old racehorses in training and 3-4 year old Sports horses).

Pathophysiology

  • Lesions are unlikely to alter gas exchange unless very severe.
  • Presence of pharyngeal lymphoid hyperplasia significantly associated with pharyngeal instability in Sports horses, as identified during overground endoscopy.
  • Pharyngeal inflammation may result in inflammation of the pharyngeal branch of the vagus nerve, leading to conditions such as dorsal displacement of the soft palate, nasopharyngeal collapse and aryepiglottic fold collapse, however this has not been proven.

Timecourse

  • Not known, but self-resolution without treatment seen with increasing age of horse.

Epidemiology

  • Young racehorses and Sports horses.

Diagnosis

Presenting problems

  • Hyperplasia of pharyngeal tissue identified on upper airway endoscopy.

Client history

  • No specific history, but usually identified in young racehorses and Sports horses.

Clinical signs

  • Other underlying signs of poor performance or respiratory tract disease may prompt the client to seek veterinary attention.

Diagnostic investigation

Endoscopy

Confirmation of diagnosis

Discriminatory diagnostic features

  • Presence of follicular hyperplasia.

Definitive diagnostic features

  • Endoscopic grading systems allow scoring of the pharyngeal lymphoid hyperplasia from Grade 0-4:
    • Grade 0 = no evidence of follicular hypoplasia.
    • Grade 1 = a few small, white lymphoid follicles scattered over the dorsal pharyngeal wall. Appear inactive and shrunken.
    • Grade 2 = mainly small, white follicles with occasional larger pink follicles over the dorsal pharyngeal wall and extending laterally to the level of the guttural pouch ostia.
    • Grade 3 = pink and white follicles covering the entire dorsal and lateral pharyngeal walls often also involving the dorsal surface of the soft palate.
    • Grade 4 = larger pink, edematous follicles covering all visible mucosa of the pharynx, and sometimes including polyps.

Differential diagnosis

  • Pharyngeal neoplasia.

Treatment

Initial symptomatic treatment

  • None necessary in the majority of horses.
  • Rest and NSAIDs sometimes advised in severe cases.
  • Additional therapies have been described (see below).

Standard treatment

  • Systemic and inhaled corticosteroids are advised by some authors.
  • Topical anti-inflammatories include administration of a mixture of glycerine, dimethylsulfoxide, nitrofurazone and prednisolone as a topical spray.
  • Immunomodulatory drugs such as oral administration of human interferon-a have also been reported.

Prevention

Control

  • No preventative measures possible as factor of age.

Outcomes

Prognosis

  • Excellent: few clinical signs reported and no apparent effect on performance unless very severe.
  • Higher pharyngeal lymphoid hyperplasia scores are associated with reduced likelihood of racing, although there is no association with race placing and no evidence that pharyngitis influences performance.
  • Regression of pharyngeal hyperplasia after about 5 years of age.

Expected response to treatment

  • Reduced size/inflammation of lymphoid follicles.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • van Erck E (2011) Dynamic respiratory videoendoscopy in ridden sport horses: effect of head flexion, riding and airway inflammation in 129 cases. Equine Vet J 43 (s40), 18-24 PubMed.
  • O'Neill H & Giorio M E (2010) Obstructive lesions of the equine upper respiratory tract Part 2: Surgical options and expected outcomes: Part 2. UK Vet 15 (5), 4-6 VetMedResource.
  • Saulez M N & Gummow B (2009) Prevalence of pharyngeal, laryngeal and tracheal disorders in Thoroughbred racehorses, and effect on performance. Vet Rec 165 (15), 431-435 PubMed.
  • Widmer et al (2009) Association of increased tracheal mucus accumulation with poor willingness to perform in show-jumpers and dressage horsesVet J 182 (3), 430-435 PubMed.
  • Holcombe S J et al (2006) Effect of tracheal mucus and tracheal cytology on racing performance in thoroughbred racehorsesEquine Vet J 38 (4) 300-304 PubMed.
  • Robinson N E et al (2006) Airway inflammation in Michigan pleasure horses: prevalence and risk factorsEquine Vet J 38 (4) 293-299 PubMed.
  • Holcombe S J et al (2001) Stabling is associated with airway inflammation in young Arabian horsesEquine Vet J 33 (3) 244-249 PubMed.
  • Clarke A F & Mandelin T M (1987) The relationship of air hygiene in stables to lower airway disease and pharyngeal lymphoid hyperplasia in two groups of Thoroughbred horses. Equine Vet J 19 (6), 524-530 PubMed (a good account of the relationship between respiratory disease and stable management).

Other sources of information

  • Holcombe S J & Ducharme N G (2007) Disorders of the nasopharynx and soft palate. In: Equine Respiratory Medicine and Surgery. Eds: McGorum B C, Dixon P M, Robinson N E & Schumacher J. Saunders Elsevier, UK. pp 442-444.