Parotid salivary drainage in horses—Moving in the right direction

The accompanying case report by Jimenez et al. (2024) describes a novel treatment approach for parotid salivary drainage in the horse. In each case, the salivary drainage was iatrogenic following surgical intervention to remove neoplastic masses. Case 1's surgery site was located over the left parotid salivary gland. Case 2 involved the proximal parotid duct in the caudal–medial mandibular region. Both cases were treated the same—multiple ultrasound-guided intraglandular injections using 200 IU botulinum toxin A diluted to 5 IU per 0.1 mL. One case required two treatments. There were no other complications, and cosmetics were excellent. The successful management of parotid salivary gland and duct injury is to either repair the defect surgically or eliminate salivary secretions (Dixon & Gerard, 2012). This principle holds true for iatrogenic surgical complications. The buildup of parotid saliva at surgical sites is likely due to the leakage of a gland radical, which can lead to chronic fistula formation. Many direct injuries to the parotid salivary gland can heal by granulation and second intention healing, with salivary leakage ending within 21 days (Dixon & Gerard, 2012; Schumacher & Schumacher, 1995). The goal of the cases reported here was to eliminate salivary secretions so second intention healing could take place without further complication. Based on many reports in human literature, the application of botulinum toxin A for equine parotid salivary leakage is logical (Ellies et al., 2004; Laskawi et al., 2013). The mechanism of action of botulinum toxin A suggests that the parotid salivary gland may regain function following the irreversible inhibition of acetylcholine release through the regeneration of affected nerve terminals over time (Sostaric et al., 2022). An extended report using botulinum toxin in 33 human patients to control drooling, salivary fistulas and sialadenitis found a 79% reduction in salivary flow after the first injection. A dramatic drop in salivary flow occurred within 1 week and an increase in salivary flow was noted between 12 and 16 weeks. Follow-up sonography did not identify any major changes to salivary gland parenchyma and no other side effects were observed (Ellies et al., 2004). While the side effects of intraglandular botulinum toxin injections seem innocuous by follow-up sonography in humans, a German pathology review reports that intraglandular botulinum injections have been shown to induce salivary gland atrophy in animal experiments (Wittekindt et al., 2009). In Case 1, salivary leakage was greatly reduced in 2 days, and retreatment for minor leakage 23 days later was successful. The authors acknowledge the possibility that the wound had ample time to heal by second intention on its own. The authors also suggest that horses could be much more sensitive to botulinum toxin than other species. Is there a possibility that the equine parotid gland is more sensitive to botulinum toxin A than in humans, particularly after two injections? Parotid gland ablation was shown to occur with retrograde infusion of a relatively non-irritating iodinated contrast material (Schmotzer et al., 1991; Schumacher & Schumacher, 1995). Case 2 described the iatrogenic damage to a section of the proximal parotid duct. The photograph in figure 4 suggests a complete section of the duct was removed. Left to heal by second intention, there is little to no chance or expectation that the duct would regenerate, reconnect and become functional without surgical intervention using a stent (Lempe et al., 2012). Since additional surgery was not an option, the specific objective would be to stop parotid salivary secretion permanently. Did the intraglandular injection of botulinum toxin A cause a temporary secretory blockade, or was there glandular ablation? If the parotid gland did return to a functional state as experienced in humans, the healed surgery site likely obstructed salivary flow, which would eventually cause glandular atrophy, as the authors suggested. Chemical ablation of the parotid gland using 10% formalin has been the historical method of choice to stop salivary secretions when surgical repair is not possible, when repairs fail, or when cost is a factor (Dixon & Gerard, 2012; Schumacher & Schumacher, 1995). Complications are rare and mostly associated with manageable soft tissue inflammation. In one case, abscessation followed multiple direct injections of formalin in the parotid duct and ventral parotid gland of a foal with congenital parotid atresia. The abscess was drained, and healing progressed with good cosmetics (Williams & Nickels, 2020). Another report described a possible association between parotid salivary injury and ipsilateral dental caries in two horses (Jackson et al., 2021). Chemical ablation with formalin is best carried out when a suitable length of parotid duct is available to cannulate. When a ligature is secured around the duct and cannula, retrograde infusion of 35 mL of formalin is infused, held in place for 90 seconds, then allowed to drain freely. Salivary drainage typically stops within 15 days. Cosmetic results are excellent (Schmotzer et al., 1991). More aggressive chemical irritants are not recommended or necessary. The author has accomplished chemical ablation of the parotid gland in practice using 5% formalin in this manner without complication. Direct injection of chemical irritants into the parotid gland is not recommended. The two case reports in this study present the first real innovation in the treatment of non-surgical equine parotid salivary drainage in over 30 years. As with most newly applied treatments being adapted to a new species, while the benefits become established, more questions are naturally generated. The rapidity with which botulinum toxin A reduces equine parotid salivary secretions is impressive, and patient comfort is improved compared to patient comfort using chemical ablation techniques. The use of botulinum toxin A as an adjunct to primary parotid gland surgery is clearly demonstrated and has the potential to replace many clinical chemical ablation procedures. While the short-term effect on salivary secretions is significant, optimal dosages and administration protocols should be explored. The long-term effect of botulinum toxin A injection into the equine parotid salivary gland and salivary secretion is yet to be determined and should be encouraged. None. The author declares no conflicts of interest. Not required for this clinical commentary.