Subcutaneous Pythiosis in a Horse

.A 10 year-old warmblood gelding presented to PHD veterinary services for the complaint of a swollen left front fetlock that was associated with a chronic draining wound. The recent history included a biopsy of the wound and diagnosis of pythiosis per histopathology. The gelding was treated with a single dose of a systemic pythiosis vaccine. In Figures 1 and 2 the yellow arrows are pointing to the firm swelling that is directly on the backside (palmar aspect) of the fetlock joint. In addition, the red arrow in Figure 2 corresponds to the chronic draining wound. The gelding was sensitive to direct pressure over the swelling, passive flexion of the fetlock and was mildly lame at the trot.
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Figure 1

Figure 2

An ultrasound exam was performed of the area in question. The firm swelling consisted of multiple fluid filled structures along with focal areas of apparent mineralization. The structure appeared embedded within the distal annular ligament; however the superficial flexor tendon (SDF) and the deep digital flexor tendon (DDF) appeared to be normal. In Figure 3 the yellow arrows are highlighting the fluid filled structures. This case is a very unusual presentation for pythiosis in a horse. The majority of cases present with an external lesion as seen in Figure 4. As external lesions there are several treatment options which involve aggressive surgical de-bulking and various topical medications. The pathogen responsible for this condition is found in stagnant water and invades superficial abrasions that might be present any where on the horse. Hence the lower limbs are most commonly affected but it has been diagnosed in the naso-pharynx and GI track of horses.

Figure 3

Figure4

Because of the complicated location of the lesion in this horse, we elected to treat with a potent anti-fungal medication known as Amphotericin B. The medication was administered through a vein in the lower limb using regional limb perfusion. This technique maximizes the local perfusion of the soft tissues with the medication and minimizes the systemic effect of the medication. 

Figure 5

The lesion was re-evaluated 30 days after the initial treatment and there was approximately 40-50% reduction in size of the lesion. However, after 60 days, the lesion appeared to resume growth and a second treatment with Amphotericin B was recently administered. In addition, the gelding as received 2 vaccines against the pythiosis and will likely receive another. Because of the location, surgery is not an option for this horse hence the systemic therapy is our best option at this point. The case is on -going and the blog will be updated as we go!!

Figure 6

Recurrence of Pharyngeal Pythiosis in a Horse

The image in Figure 1 is that of a "normal" larynx and pharynx in a horse. In Figure 2-4, the pharynx is filled with granulomas that are nearly completely obstructing the larynx. The images in Figures 2-4 belong to a middle-aged quarter horse mare that has been treated several times over 2 years for pythiosis of the pharynx. After each treatment with systemic anti-fungal medication, the granulomas shrink in size and the hemorrhagic discharge stops (Figure 4) . However, once the medication stops, the granulomas slowly begin to re-grow and the discharge returns. The image in Figure 4 was taken approximately 12 months prior to Figures 2 and 3. Although the granulomas are present and there is some drainage, the over-all condition of the pharynx is better than what we see in Figures 2 and 3!

Figure 1

Figure 2

Figure 3

Figure 4

 The most likely scenario in this case is that the pythiosis pathogen resides within the granulomas and although they shrink and appear to become inactive, the pathogen persists. Once the medication is stopped, the pathogen is allowed to multiply and the granulomas grown unabated!  Pythiosis was once believed to be a fungi put is now described as an algae that lives in stagnant water throughout the southeastern United States. The pathogen gains access to the horse through small wounds in skin or mucosa. I have diagnosed pharyngeal pythiosis in over a dozen horses in the past 10 years. Every case has involved horses with access to free standing water and all have responded favorably to the systemic anti-fungal. The case imaged in the figures above has been refractory and proven difficult to completely resolve!!

Pythiosis!

The endoscopic images below are of the pharynx of a horse with a 30 day history of a bloody nose. The nasal discharge consisted of bright red blood mixed with some purulent debris. The middle-aged gelding lives in a pasture that includes multiple low-lying areas that have been flooded with stagnant water. In addition, the gelding has suffered from failure to thrive most likely due to a poor appetite.

Figure 1

The endoscopic exam revealed bleeding tissue that surrounded the pharynx but was most severe along the dorsal wall of the pharynx. There was purulent debris mixed with the bleeding tissue and multiple small, white granuoles were noted within the bleeding tissue (Figure 2) .  Possible disease processes for this condition include fungal granuloma, cancer, bacterial abscess, and pythiosis.  Pythiosis is caused by the invasion of tissues by the pathogen pythiosis insidiosum and may affect external and internal tissues. The pathogen typically gains access to the horse via exposure to stagnant water that contains p. insidiosum.  As such the lower limbs (pasterns, coronary bands, and fetlocks) are most commonly affected. Once infected, the wound becomes increasingly inflamed with persistent bloody discharge. In addition, the lesions are very puritic resulting in additional inflammation via the horse biting and scratching at the wounds. Within the large areas of inflammation are firm, yellow granuoles also known as "kunkers".  This condition can be very deadly and requires aggressive treatment. For external cases, surgical removal and topical treatment are common. In addition a vaccine is available that has proven effective for cases of equine pythiosis.

Figure 2

However, pythiosis of the pharynx is not treatable with surgical resection or the common topical medications. Fortunately, systemic antifungals have proven effective if treated early and for the appropriate amount of time. The endoscopic images below are from the same horse approximately 4 weeks into the systemic anti-fungal treatment protocol. I recommend a specific protocol that involves  6 weeks of a tapering dose and a follow up endoscopic exam. In the past 8 years I have treated nearly a dozen cases of pharyngeal pythiosis. In every case there was a body of stagnant water available to the horses and all but 1 case responded to the systemic anti-fungal medication. For more information regarding the condition and the vaccine check out the following link: http://pythium.pavlab.com/subpage5.html

Figure 3

Figure 4

Core Biopsy Please!!

The only way to know for sure would require a core biopsy. This procedure is quite simple to perform in the field and will save significant time with regards to treatment and resolution of clinical signs. The first image was treated unsuccessfully as a "summer sore" with medication that actually worsened the clinical signs. The biopsy results were consistent with Pythiosis and aggressive medial/surgical treatment was instituted. The prognosis for full recovery is not good once there has been significant disruption of the soft tissue structures and in a case that involves the lower pastern/coronary band area. Early diagnosis is key!

The case below was treated as a suspect "summer sore" with minimal improvement. Biopsy results indicated that it in fact it was a case of Habronemiasis that was not responsive to the treatment provided (systemic and topical ivermectin). Habronemiasis or "Summer Sores" is a condition that results from flies depositing larvae within the skin causing a significant allergic reaction. Treatment involves medication to kill any living fly larvae and most importantly medication to suppress the local allergic reaction. Treating with ivermectin systemically and topically is often not enough!! Yet that is how many try to treat this condition! For small lesions, topical treatment with concentrated cortisone can be effective, however, for more aggressive lesions, systemic administration with corticosteroids is necessary. Importantly, systemic treatment with corticosteroids is the treatment you definitely want to avoid if you are dealing with Pythiosis! Hence the importance of a core biopsy.