What is cellulitis?

Cellulitis is a diffuse (widespread) inflammation/infection, generally associated with bacterial infection of the skin and the soft tissue directly underneath the skin. In horses, it generally involves only one limb.

What causes the condition?

The disease can be divided into cases with a known cause, or secondary cellulitis, and those with no obvious underlying cause, named primary cellulitis.

Causes of secondary cellulitis include infections that occur following surgery, joint injections, wounds, or blunt trauma.

Primary cellulitis is thought to arise from a break in the skin that could be as small as the tiny puncture wound created by an insect or mite bite, and not discernible even with careful inspection of the area. In most of the cases I see, the break isn’t obvious, and it appears as though the limb simply swelled up for no reason. In most cases it is assumed that the infection is caused by staphylococcal organisms, since staph is the chief inhabitant of equine skin surfaces. In many instances, we can’t confirm the actual pathogen involved but certain environmental conditions - including prolonged exposure to deep mud or sand (both can cause drying and irritation of the skin, which allows bacteria to break through its protective barrier) - can promote cellulitis development.

Are some horses more prone to develop it than others?

Primary cellulitis seems to be particularly common in thoroughbred racehorses and in mares but can occur in any breed or sex of horse.

Are affect horses more susceptible to a repeat bout?

Yes, an episode of cellulitis can become a chronic or recurring problem. You may get it under control initially but have episodic flare-ups later. It is important to treat the condition early and aggressively to help prevent this situation, but there are many instances in which the horse does get treated promptly and the inflammation still returns later.

How would an owner identify cellulitis?

Typical signs are heat, swelling and pain in a limb, sometimes centred around a given area such as a hock or pastern, and sometimes a more diffuse swelling extending up and down much of the limb. The swelling itself is usually hot, painful, and pits when firm pressure is applied. The horse is lame and sometimes unable to bear weight on the affected limb (more commonly a hind limb than a front limb). The lameness develops acutely and may precede the marked swelling that follows within a few hours. Owners often suspect that the horse has a fracture due to the severity of the lameness (i.e. horses can be toe touching to non-weight bearing lame). The horse may develop a fever (temperature > (greater/higher than) 101.5°F or 38.6°C).

What could an owner do to alleviate the symptoms?

Physical therapy measures are very important to help maintain circulation during and after the infection. The swelling that occurs due to oedema accumulation is not only painful in the initial stages, but is often hard to get rid of even once the infection has resolved and this can contribute to continued lameness issues. Principles of therapy aimed at to minimising inflammation, oedema accumulation, and swelling include compression and cryotherapy. Compression has been shown to be effective in stimulating tissue healing, minimising oedema, and increasing blood flow. Cryotherapy reduces the inflammatory response in the tissue, reduces the metabolic demand of the tissue, and provides a short-term analgesic effect.

Compression can be provided by bandaging the limb, or by boots or other devices that provide intermittent pneumatic compression. Cryotherapy can performed via a variety of ice boots or whirlpool systems. Minimally, hydrotherapy and hand walking (if the horse is comfortable enough to walk) two or three times daily helps decrease oedema and improve circulation and comfort. After walking, a topical preparation can be applied to the leg and then a compressive full-limb bandage is applied.

When would it be appropriate to call your veterinary surgeon?

The veterinary surgeon should be called in all cases of non-weight bearing lameness or if the horse has a fever (temperature > 101.5°F or 38.6°C).

How would the veterinary surgeon diagnose cellulitis?

On clinical signs of lameness, heat, swelling and painful to palpate the affected leg. Also sometimes serum may seep through the skin. Horses may occasionally have an elevated body temperature and be off their food.

How it is treated?

Antibiotics given intravenously if severe or orally if horse is willing to eat. Anti-inflammatories (painkillers) given IV initially for immediate relief then orally e.g. Bute. Also cold hosing in the early stage, when leg is warm, then drying well before applying a stable bandage. A support bandage may be applied to the opposite leg. Forced hand walking or on a horse walker. Massage of the affected leg to assist circulation and encourage fluid drainage.

How might cellulitis be prevented?

Preventative measures include:

Maintaining a regular exercise program or plenty of turnout on a daily basis as long as the horse is fit to do so. This helps improve fluid drainage through the lymphatic system from the legs.

Keeping the legs clean and dry. Loss of skin integrity is an important risk factor for cellulitis.

Bathing equipment may act as a carrier for the causative bacteria, or the act of bathing may predispose the skin to drying and chapping. The hind limbs may be more difficult to adequately wash and dry after exercise because horses are often hosed down by the same individual who is holding them, making access to the plantar aspect of the hind limbs difficult. Therefore hygienic handling of bathing equipment and careful drying of limbs after bathing is highly recommended. Avoiding the use of harsh disinfectants like chlorhexidine is also good practice.

Avoiding turning horses out in areas with standing water, mud, and sand, if possible. Observing your horse closely and communicating with your veterinary surgeon for early detection of reoccurrence and prompt initiation of treatment.

Using protective leg boots if horse prone to self-trauma while turned out or on exercise.

Are there potential complications?

Yes. Potential complications include laminitis, dermal necrosis, vascular thrombosis, persistent lameness and colic.

Laminitis may be severe enough to progress to founder and loss of the horse’s life. It can be a factor during the acute phase of the infection when it is still developing. The horse may become septic and bacteria, bacterial toxins, or both, can enter the circulation and injure the laminar tissues in the feet. And, if the affected limb stays very painful for a protracted length of time, the horse may develop a mechanical laminitis in the unaffected foot because of excessive weight-bearing.

Dermal necrosis (death of the skin overlying the cellulitic area) may develop as a result of compromise of the skin’s blood supply secondary to severe swelling, as a direct result of toxin production, or as a result of a combination of these factors.

Vascular thrombosis often occurs in the jugular veins of the neck due to either the presence of a long term catheter or repeated injections of antibiotics and anti-inflammatories into the vein.

Persistent lameness may occur due to permanent damage to the limb, which often does not return to its normal contour. Colic can occur when the horse does not eat or drink enough as a consequence of being in pain either due to the cellulitis itself or conditions related such as fever or laminitis.

Have there been any recent developments in treating cellulitis?

While hand walking, cold water hosing and bandaging of the leg is a mainstay of treatment for horses with cellulitis, new rehabilitation tools that have shown promise as adjunctive treatments are becoming more readily available. The Game Ready® system has specific boots designed for equine use and provides both intermittent pneumatic compression and cryotherapy. Typically the system is applied to the limb one or two times daily for 20-30 minutes at a time. Cold salt-water spas provide cold, hypertonic water with aeration that combines the efficacy of cryotherapy with the osmotic action of salt water in order to decrease soft tissue inflammation and provide analgesia.

In the USA, hyperbaric oxygen (HBO) chambers are becoming increasingly available for equine patients; they increase the oxygen content delivered to the tissues by having the horse breathe 100% oxygen within a pressurised hyperbaric chamber. The proposed therapeutic effects of HBO therapy that would be beneficial for treatment of cellulitis include: hypoxia reversal, reduction in oedema, modulation of nitric oxide production, acceleration of microbial oxidative killing, improvement of antibiotic exchange across membranes, and decreasing ischemia-reperfusion injury.

Padded compressive full-limb bandage/boots/other devices providing intermittent pneumatic compression.

For cases where systemic antibiotics have failed then intravenous regional limb perfusion can be tried. This involves injecting antibiotics directly into a vein as close as possible to the affected area. Another method of delivering antibiotics directly to the cellulitic area is to implant antimicrobial impregnated PMMA beads or collagen sponges.

Aoife Byrne Dr.Med.Vet, MRCVS Cert AVP, qualified from Szent István University Faculty of Veterinary Science in Budapest in 2007 and did an equine internship on the Curragh for 12 months before working for an equine ambulatory practice. She followed this with a stud season at the Beaufort Embryo Transfer Centre. Aoife then worked at Rowe Equine & the Equine Eye Clinic for a few years before moving to Norfolk where she now works for Chapelfield Vets equine clinic. From a horsey family in Carlow, Aoife enjoys hunting side saddle in the winter and showing in the summer.