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EQUINE VETERINARY EDUCATION


Equine vet. Educ. (2018) 30 (2) 80-84 doi: 10.1111/eve.12707


Clinical Commentary Equine guttural pouch empyema, why does it become chronic?


P. M. Dixon* and O. A. James Division of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Studies, The University of Edinburgh and The Roslin Institute, Midlothian, UK. *Corresponding author email: p.m.dixon@ed.ac.uk


Keywords: horse; guttural pouch; chondroids; strangles infection


Guttural pouch empyema is defined as the presence of purulent exudate or chondroids within one or both pouches (Freeman and Hardy 2012). The article by DeLoache et al. (2018) in this issue highlights the need to broaden our horizons when considering the aetiology of guttural pouch disease to include empyema caused by C. pseudotuberculosis, the causative agent of ‘pigeon fever’, a common disease in certain areas of the USA.


Aetiopathogenesis of guttural pouch empyema


The guttural pouches are lined by the common respiratory mucosa and are thus affected by all generalised viral respiratory infections. Endoscopy of horses with such viral respiratory infections frequently shows mucopurulent exudate draining bilaterally from the guttural pouches. Similar exudate is also commonly seen draining from the sinonasal ostia (‘sinus drainage angles’) in such infections because the sinuses are also lined by the common respiratory mucosa and are similarly affected by these acute infections. Most such infections are self-limiting and no treatment, other than rest, is usually required. A more chronic and purulent guttural pouch infection may occur with upper respiratory infections caused by bacteria, most commonly a primary nasopharyngeal and guttural pouch infection with Streptococcus equi var equi (strangles) and less commonly by other bacteria including Streptococcus zooepidemicus (Judy et al. 1999), that may be primary infections or secondary to viral respiratory infections. As shown by DeLoache et al. (2018), guttural pouch infection with C. pseudotuberculosis also needs to be considered in some geographical areas. With strangles infection, abscessed retropharyngeal lymph


nodes on the floor of the medial guttural pouch compartment can rupture and drain into the guttural pouch lumen (Fintl et al. 2000) and initiate the empyema (Fig 1). It is unclear if empyema of the guttural pouches by other


types of bacteria are caused by direct pyogenic infection of the guttural pouch mucosa, or occur following rupture of abscessed retropharyngeal lymph nodes into the guttural pouch lumen. Most acute cases of guttural pouch empyema resolve spontaneously and resolution is aided by postural drainage from the pouches that occurs when the head is lowered, in conjunction with the normal opening of the ostia during swallowing. However, such postural drainage, that is encouraged by feeding at ground level, does not necessarily completely empty the guttural pouches of exudate by gravity, because areas lateral to the guttural pouch pharyngeal ostium are ventral to its internal ostium when the head is lowered. The presence of normal guttural pouch mucociliary clearance is also required for complete drainage of exudate. Continuing mucosal inflammation with loss of cilia


© 2016 EVJ Ltd


inhibits normal movement of exudate that has increased quantity due to the local mucosal inflammation and is also of a more viscous nature than normal guttural pouch mucus secretions due to the leucocyte breakdown products it contains. Inflammation of the guttural pouch ostia may also impede drainage at this stage. If guttural pouch empyema does not fully drain for the


above reasons, the exudate can gradually become viscous (Fig 2) and may eventually dehydrate further and form firm chondroids, that become spherical or ovoid from kneading type movements on the floor of the pouch during head movements and eventually become solid (Fig 2). The presence of chondroids, that are essentially infected, porous foreign bodies, will cause ongoing mucosal inflammation and thus make the guttural pouch empyema permanent until they are removed. A similar situation occurs with chronic paranasal sinus empyema, if the more dependent sinuses (especially the ventral conchal and rostral maxillary sinuses) develop inspissated pus that, in turn, makes the sinus empyema permanent. It is unclear if equine pus is more prone to become inspissated than exudate of other species, or whether this predisposition to inspissation is anatomical, due to species-related poor drainage of their guttural pouches and sinuses.


Fig 1: Drainage of pus from a ruptured lymph node on the floor of the medial compartment into the guttural pouch lumen.


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