Meddleton Equine Hospital

A geriatric Quarter Horse gelding was evaluated for a sudden onset of severe right forelimb (RF) lameness. The horse was depressed and would bear almost no weight on the RF. The leg was hot and swollen to approximately twice its normal size from the coronary band up to the axilla; the swelling was extremely painful to the touch. Clipping revealed an open sore over the fetlock.

A presumptive diagnosis of bacterial Cellulitis was made. The leg was cleaned, dressed with DMSO/Nitrofurazone (anti-inflammatory and topical antimicrobial), and bandaged. Oral antibiotics (Trimethprim-Sulfamethoxazole) were started and continued for 15 days. The wound and leg were bandaged and kept clean for 15 days. The leg swelling decreased steadily after antibiotics were started, and the horse became more comfortable over the next few days.

Fifteen days after the initial evaluation, reevaluation revealed that the swelling was completely resolved and there was no more pain. There still remained an open sore, but it was showing all the signs of normal healing. The owner was instructed to stop antibiotics and to continue to bandage the sore until it was closed.

20 days after stopping antibiotics, the horse was again depressed, the RF was very swollen, and he was very lame. He was found to have new sores all along the leg and now had fluid pockets under the skin at the level of the knee. These were lanced and pus was obtained. The fluid was sent to the laboratory for bacteriology. Aggressive antibiotic therapy was initiated in the form of intravenous enrofloxacin and a regional limb perfusion (delivers extremely high concentrations of an antibiotic to a small area of a leg using a tourniquet) with amikacin.

He was bandaged as before. He made steady improvements once back on antibiotics. The laboratory reported a pure culture of Corynebacterium pseudotuberculosis (the causative agent of Pigeon fever) and was found to be sensitive to the current antibiotic and also to Trimethoprim-Sulfamethoxazole (can be given orally). The horse was doing well after 10 days of injectable antibiotics. The catheter was removed and he was switched to oral antibiotics. He continued to do well on them and made an uneventful recovery after more than 1 month on the oral antibiotics.

Case Discussion

Cellulitis is a broad term used to describe an infection of the subcutaneous tissues (underneath the skin). There are many bacteria that can cause this, but the most common is Staphylococcus (multiple species). These infections usually involve a horse’s limb and result from a break in the skin to allow entry of the bacteria. The cut, puncture, or bite can be very small and not observed when it occurs.

The first sign is usually an acute onset of severe lameness (often not bearing any weight on the affected leg), fever, depression, and severe, painful swelling. If not treated rapidly, the infection and swelling usually spread rapidly up the leg causing even more pain. If left untreated a life-threatening blood infection (septicemia) can occur.

Founder in the opposite foot (due to an excessive load when the diseased leg is not used) is another possible life-threatening complication. Treatment will include aggressive debridement and lavage (removal of dead tissue and cleaning) of the wound and antibiotic therapy. In severe cases, intravenous antibiotic therapy and possibly regional limb perfusion have to be employed.

Bandaging of all legs to provide support and decrease swelling along with anti-inflammatory therapy (phenylbutazone or Banamine, and cold therapy) to reduce swelling and pain are also necessary for a good outcome. A bacterial culture can help modify the antibiotic plan later in the course of treatment (resistance or multiple species of bacteria involved), but broad spectrum antibiotics need to be started as soon as the horse is seen. A positive outcome can be expected with prompt action and diligent treatment and monitoring.

Corynebacterium pseudotuberculosis is a bacterium that is very common in the southwestern United States. It is the causative agent of Pigeon fever or Dryland distemper (same disease). The reservoir in the horse population has not been definitively determined (where does it spend its winters?), but during an outbreak, pus from draining abscesses provides a source of the bacterium for flies to transmit from one horse to another. Infection usually occurs when a fly carrying the organism bites a horse on the ventral abdomen (along the midline of the belly). If a horse already has an open sore (cut, bite, summer sore) infection via a fly is even easier.

Most affected horses will develop sores on their belly (usually missed by caretakers). The bacterium then travels along the lymphatics (vessels that shuttle free fluid around the body) and can set up abscesses along the way. The most common site is deep to the muscles of the chest, but the axilla, groin, prepuce, and udder are also occasionally targets for abscess formation; these can be basketball sized. These external (not within a body cavity) abscesses will eventually mature (grow, rupture, and drain) to the outside.

Groin and axillary abscesses can be extremely painful, but most of these external abscesses will resolve without any medical intervention. Antibiotic therapy before rupture has been shown to prolong the course of the disease by an average of 15 days. Once the abscesses rupture (or are lanced surgically), they have to be flushed with antiseptic solution. At this point antibiotic therapy can be initiated, but the value of this is still being debated.

Care must be taken to use fly repellent around all open sores and to dispose of any pus as soon as possible to limit the spread of this bacterium. Corynebacterium will manifest as external abscesses in approximately 91% of cases. In up to 8% of cases (based on 1 large study), internal abscesses will form. This is a life-threatening condition in which infection occurs within body cavities.

The clinical signs will depend on which organ(s) is/are affected; liver, mesentery, mediastinum, lungs, diaphragm, pericardium, and kidneys are the most commonly reported. Untreated, internal abscessation with Corynebacterium is uniformly fatal. With aggressive antibiotic therapy (4-6 weeks minimum), 60% of horses survive this. This illustrates the point that a horse with persistent depression, fevers, and signs that cannot be explained by external abscesses alone should be evaluated for internal abscesses (ultrasound, rectal, abdominal tap).

In approximately 1% of cases Corynebaterium will cause a disease called Ulcerative Lymphangitis. This manifestation is common in sheep, but very rare in horses. The infection usually begins around the coronary band and tracks up the leg using the lymphatics. On its way up the leg it breaks out as multiple abscesses and sets up local cellulitis (discussed above). These horses are very lame and the leg is extremely painful. Initial diagnosis is easy to confuse with another type of bacterial cellulitis.

Adequate treatment of this manifestation usually requires a minimum of 4-6 weeks of antibiotic therapy. This happened with the case reported here. The initial evaluation was suggestive of a cellulitis and treated accordingly. The response was initially good to two weeks of antibiotic therapy and bandaging, but clinical signs returned shortly after antibiotics were discontinued. At this point the diagnosis of a simple cellulitis was no longer appropriate. A bacterial culture was obtained, a pure culture of Corynebaterium pseudotuberculosis was recovered, and a diagnosis of Ulcerative Lymphangitis was made. An adequate course of antibiotics was then prescribed.