How to Use Sterile Maggot Debridement Therapy for Foot Infections of the Horse

Sterile medical grade maggots can be a useful tool in treating established infections of the equine foot. Larval therapy is a non-traumatic, minimally invasive way to debride high-scale infections without disturbing the normal architecture of the foot.

1. Introduction

Hoof disease and injuries are common and often serious problems for the horse. The horse's foot is constantly in contact with and interacting with its domesticated environment. Therefore, puncture wounds by foreign bodies and ascending infections from defects in the white line are not uncommon. For an infection to become established within the horse's foot, bacteria must breech the barrier of the protective hoof capsule (hoof wall, sole, and frog) and gain access to the inner sensitive tissues. When an infection has occurred, we can classify the infection as deep or superficial [1]. Superficial abscesses only involve the tissue directly beneath the hoof capsule of the dermis/corium. Superficial infections are effectively treated by establishing drainage, soaking the foot in an Epsom salt solution, poulticing the foot until drainage has ceased, and protecting the foot until the hoof capsule defect has healed. Deeper infections are more serious and may involve the coffin bone, distal interphalangeal joint, navicular bursa, tendon sheath, collateral cartilages, and digital cushion. Most deep infections, if treated early, are effectively treated with light debridement, lavage, systemic antibiotics, regional limb perfusions, and foot soaks. Infections that have become more established and diffuse can become more problematic and difficult to treat effectively. Tissue damage from the infection and extensive surgical debridement can cause structural damage to the foot and, in some cases, permanent lameness; prolonged healing time can be expected. Occasionally, the opposing digit experiences a supporting limb laminitis from the chronic lameness. Non-traumatic removal of necrotic tissue and bacteria is imperative for the most optimal outcome. Maggot debridement is a non-traumatic, minimally invasive method to remove necrotic tissue from an extensive foot infection. This therapy is often used in conjunction with and after light surgical debridement. Maggot therapy decreases healing time in post-surgical coffin-bone debridements and is useful in treating chronic, reoccurring non-healing foot ulcers, canker, quittor (necrosis of collateral cartilage), navicular bursa sepsis, chronic osteomyelitis/septic arthritis of distal interphalangeal joint (DIP), chronic soft-tissue abscess and osteomyelitis secondary to digital instability such as chronic laminitis, and reverse rotation secondary to ruptured deep digital flexor tendon.

2. Materials

The maggot therapy laboratory at the University of California at Irvine has made disinfected fly larvae (Phaenicia sericata) available to requesting physicians and veterinarians for clinical use since 1995. Eggs collected from gravid females are disinfected. The eggs are then transferred to sterile vials; 500 - 1000 larvae are hatched per vial and embedded in sterile gauze. The vials are available for overnight shipment for clinical use. The procedure for egg collection, disinfection, and quality assurance is described by Sherman and Wyle [2]. In the years from 2003 to 2005, maggot debridement therapy has been used on 108 podiatry cases at Rood and Riddle Equine Hospital. This therapy most often followed a light surgical debridement. However, in some instances, it was used as the primary method of debridement.

3. Patient Preparation

Necrotic tissue from the foot wound is usually lightly surgically debrided [3] and packed with saline-soaked gauze. It is important not to pack the site with disinfectants for 24 h before maggot application because the residue may kill the larvae. All bleeding must be stopped before the larvae are applied to the wound. One vial of 500 - 1000 larvae embedded in gauze are applied lightly to the external surface of the lesion. A stack of dry, sterile gauze is gently bandaged over the larvae and wound to absorb any exudate. Bandages are changed daily to remove any discharge/excretions created by the maggots and the suppurating wound. A dose of maggots usually lasts 5 - 7 days before the larvae are satiated and can no longer remove necrotic tissue. A new batch of maggots is applied to the wound every 5 - 7 days until the wound is completely debrided. Depending on the location of the wound, maggots can be placed underneath a hospital plate of a shoe, into a small window cut into a foot or leg cast, or underneath a bandage (Fig. 1). If the infection is deep within a puncture wound, placement of the Penrose drain can ensure a constant open portal for the larvae to maintain access to the necrotic tissue. Repeated applications of maggots are used on the wound until all necrotic tissue is debrided and a healthy bed of granulation tissue is evident. In cases in which the area is unable to be visualized, such as a deep puncture wound, therapy should continue until the patient's comfort level is significantly improved for at least 1 wk and the larvae persistently exit the wound.

Figure 1. Window bar shoe with treatment plate. Maggots were placed into the surgical site 3 days after a minor surgical debridement.The infection involved the lateral wing of the third phalanx, extensive solar corium, laminae, and the collateral cartilage.The maggots seen here were hatched 5 days previously and have been in this site for 3 days.

4. Results

Coffin Bone Osteomyelitis

Maggot therapy significantly speeds the recovery time in post-operative coffin-bone debridements. Of 43 cases, 41 healed uneventfully with only one light surgical debridement followed by maggot therapy. In two cases, the maggots died in the wound. Both of these cases grew a proteus on culture. Maggot therapy eliminated the need for any second debridement.

Chronic Laminitis

Of the 222 chronic laminitic horses studied at Rood and Riddle Equine Hospital from 2003 to 2005, 38 had severe reoccurring abscesses that were unresponsive to surgical debridement. All 38 cases had severe digital instability that was addressed with either special shoeing, foot casts, resections, and/or transection of the deep digital flexor tendon. Seventeen of thirty-eight cases were euthanized because of the severe untreatable digital instability. Of the remaining 18 cases, the secondary infections were resolved with larval therapy alone. Three cases are still under treatment.

Septic Navicular Bursa

Puncture wounds to the navicular region and contamination of the bursa which are not treated promptly and appropriately generally carry a poor prognosis for return to soundness [4-7]. Maggot therapy combined with systemic antibiotics and regional limb perfusion cured the infection in all eight cases. On admission, all cases received light debridement of the puncture tract, navicular bursa lavage with a needle placed into the navicular bursa at the palmar pastern and lavaged out the entry tract, regional limb perfusion, intravenous penicillin and gentocin, and special shoeing with heel elevation. A .25-in Penrose drain was fed through the entry tract and directed just palmar to the deep digital flexor tendon; this was secured to the skin at the palmar pastern. The purpose of the drain is to keep an open tract to the vicinity of the infection for the maggots to maintain access to the diseased tissue. On days 2 and 3, sterile maggot therapy was initiated. One batch of 500 - 1000 larvae were applied to the tract and Penrose drain site. After 10 - 14 days, the drain was removed, and any remaining maggots were flushed from the tract. Some of the cases received Na-hyaluronic acid injections into the navicular bursa after the infection had resolved. However, mechanical damage (adhesions) to the tendon and damage to the flexor surface prevented four cases from returning to athletic soundness (all four cases were admitted later than the optimal 7 - 14 days after puncture wound). One unsound horse received a palmar digital neurectomy because of persistent lameness. The other three horses are pasture sound (sound at the walk). Four cases are sound for athletic use. All cases were admitted in the chronic stage of the disease (after 72 h). Cases admitted before this (before extensive tissue necrosis) are usually successfully treated with light debridement, lavage, regional perfusions, special shoeing, and systemic antibiotics, and there is no need for larval therapy.

Chronic Distal Interphalangeal Joint Sepsis

Four cases were treated with maggot therapy for chronic DIP joint sepsis and P-1 and P-2 osteomyelitis. Four cases have been treated with systemic antibiotics, Penrose drain placement into DIP joint, and maggot therapy into drain tracts. In three cases, the infections resolved, and two are pasture sound with no reoccurrence of infection. One case is still lame at the walk and currently under treatment. One case was euthanized for massive necrosis and digital instability.

Canker

Of the six canker cases, three cases were treated with maggots after surgical debridement followed by lasering of the surrounding tissue. These cases had diffuse areas affected with canker. The remaining three cases were treated with light surgical debridement of the focal area of canker followed by maggot application after the bleeding had stopped (1 - 2 days after debridement). In all six cases, the canker lesions resolved with no known reoccurrences. One horse developed a supporting limb laminitis and is currently pasture/breeding sound. The other five cases returned to their intended use.

Acute Caudal Coffin Bone Rotation

Six cases with chronic advanced navicular disease had secondary ruptured deep digital flexor tendons and subsequent acute reverse rotation. Five cases were euthanized because of chronic digital instability and secondary osteomyelitis of the wings of the P-3; however, maggot therapy was useful in debriding the secondary infection. One case is pasture sound with special shoeing and no reoccurrence of the infection after maggot therapy of the extensive heel infection.

Non-Healing Foot Ulcers

Two pregnant mares had non-healing ulcers in the palmar region of the foot. Radiographs and fistulogram revealed no bone involvement. Inspection/probing of the wound showed ulceration to the underlying dermis. Lesions were resected on both mares several times by different clinicians and packed with antibiotics and antiseptic dressings at different times. Lesions in both mares never epithelialized over and were non-healing lesions that appeared similar to avascular necrosis or non-healing foot ulcers seen in humans [8]. Both lesions were quickly healed with 10 - 14 days of larval therapy.

Necrosis of Collateral Cartilage (Quittor)

Trauma to the collateral cartilage with subsequent necrosis is usually caused by interference by another foot. Treatment is usually surgical removal of the necrotic cartilage above the level of the coronary band and the establishment of drainage distally, usually through a resected area of distal hoofwall. Systemic antibiotic therapy, regional perfusion, lavage, antibiotic impregnated beads, and antiseptic dressings are usually the treatment. In the one case seen over the past 2 yr, maggot therapy placed in the drainage tract has resolved the condition with no reoccurrence.

The only side effect of maggot therapy is that the occasional patient may experience irritation or itching at the wound site. The movement of the larvae within the wound may be a cause for this. Some bacterial infections may be resistant to maggot debridement. Two cases had the larvae die in the wound. Both of these wounds grew proteus mirabilis on culture. Other than the minor irritation/itching experienced by the rare case, there are no other known side effects experienced in the 104 podiatry cases to date.

In all but two cases, maggot therapy has been useful in treating the disease process; however, predominately in cases in which infection is secondary to digital instability, special shoeing, surgery, casting, and braces are imperative for a successful outcome. None of the cases described were euthanized for failure to treat the infection. However, horses were euthanized because the primary problem of digital instability could not be treated. When stability is achieved, maggot therapy can effectively treat the infection.

5. Discussion

Maggot therapy has long been used to treat chronic infections in humans [9-11] but rarely has it been reported in the treatment of infections in animals [12-15]. Maggot therapy is believed to not only debride necrotic tissue but also stimulate fibroblast activity and angiogenesis [13]. Thus, the healing effect may be more than just aiding the wound in the debridement phase. The most useful cases for maggot therapy seem to be deep-penetrating wounds that have a lot of hard to reach soft-tissue necrosis, such as infections of the palmar regions of the foot. Most wounds to the anterior regions of the foot are easily debrided surgically and usually heal uneventfully. This is probably because of the tight and compact architecture of the anterior foot. Maggot therapy is not practical for removing large necrotic areas such as a large area of osteomyelitis or a sequestrum, and the author does not recommend larval therapy as an alternative to surgery in these cases. However, larval therapy can be useful in debriding residual necrotic tissue after a surgical debridement in these cases. Chronic infections to the palmar regions of the foot (osteomyelitis of the palmar process and penetrating wounds to the navicular apparatus, digital cushion, and coffin joint) often have diffuse soft-tissue involvement and multiple tracts. These cases seem to benefit the most from larval therapy after light surgical debridement and lavage. This combination of treatment is an effective way to debride diseased tissue without disturbing the normal architecture of the foot. Regional limb perfusions and systemic antibiotics can be used along with maggot therapy and seem to have no harmful effects on the larvae within the wound. Using maggot therapy in cases with digital instability such as chronic laminitis or acute reverse rotation secondary to ruptured deep digital flexor tendon is useless unless the primary condition is addressed and the pedal bone is stabilized. If a lower limb or foot cast is required to provide stability, such as in a case with a complete hoof wall ablation, maggot therapy is unique in its ability to debride necrotic tissue within the cast. In these cases, the larvae are applied through a window cut into the cast and replaced as needed.