Cellulitis is a painful bacterial infection of soft tissues under the skin.
You can commonly find it in one of your horse’s hind legs but it can occur anywhere in the body.
Cellulitis can rapidly progress from a little swelling around a small cut to massive swelling of the entire limb. Closely observing your your horse and good communication with your vet is key to detecting cellulitis early and successfully treating it.
(This information was adapted from an article (PDF, 0.9 MB) by UQ lecturer and equine veterinary specialist Dr Allison J. Stewart BVSc(hons), MS, DACVIM, DACVECC and Dr Georgina Johnston BVetMed BSc (Hons1) MRCVS.)
What are the signs of cellulitis?
A horse with severe cellulitis and myositis in the right thigh region with a secondary oedema over the lower limb. No initial wound on the limb was identified.You may first notice cellulitis in your horse as sudden swelling that is hot and painful to the touch.
As the infection worsens, your horse may develop fever or become lame and not want to bear weight on the affected leg.
Your horse's swelling can spread from the initial infection site to affect their entire leg, as the infection spreads throughout the tissue it can cause massive swelling.
Your horse’s lymphatic system (the fine vessels that return tissue fluid or ‘lymph’ back to the circulatory system) can become overwhelmed or compressed by swollen tissue which leads to more swelling from non-infected tissue fluid called oedema.
The more severe and extensive the cellulitis is, the more difficult and expensive it is to treat and the more scar tissue will result. Scar tissue from cellulitis can lead to your horse having a permanently enlarged limb which is unsightly and can lead to mechanical lameness.
Is your horse's limb swollen due to cellulitis or oedema?
You may find it's difficult to distinguish which parts of your horse's swollen limb is due to cellulitis or oedema.
You can diagnose oedema (which is not painful to your horse) by making an impression of your finger in your horse's swollen tissue. Your horse will give a pain response if it is cellulitis.
In severe cases, your horse's skin can become so stretched that it splits open and drains fluid or pus.
Secondary facial oedema from cellulitis and clostridial myositis.Cellulitis and oedema of the head.
What are the symptoms?
Your horse may become depressed and lose their appetite because of fever or pain.
In rare cases, your horse could suffer life-threatening complications such as sloughing of stretched skin due to loss of blood supply, gangrene (if infected with Clostridial organism or laminitis) or death.
How is cellulitis caused?
We don't know exactly what causes cellulitis. Risk factors for humans include trauma, obesity, poor hygiene, diabetes mellitus, poor circulation and damage to lymphatic drainage.
We don't fully understand the risk factors in horses but cellulitis can develop due to:
certain environmental conditions, including prolonged exposure to deep mud or sand
contributing factors such as external parasites (e.g. mites)
damage to lymphatic vessels in the leg
a wound (the size doesn't matter even a tiny scratch can act as an entry point for infection)
bacteria entering your horse from a tiny penetrating object (blackberry prickle, thistle or rose thorn) if there is no visible wound or scratch.
Staphylococcus and Streptococcus are the most common bacteria species cultured from equine cellulitis.
Chronic cellulitis or repeat episodes
Some horses can have chronic cellulitis or repeat episodes. A University of Pennsylvania study in 2007 found that 23% of cellulitis cases have at least one reoccurrence. Horses with non-healing wounds or with compromised blood or lymphatic supply from scar tissue formation or trauma may be prone to repeated episodes of cellulitis.
Other causes of limb swelling
Clostridial myositis (gangrene)
A horse being treated for clostridial myositis.
Clostridial myositis is a similar but more severe condition to cellulitis where the muscle is infected by a soil dwelling clostridial bacteria.
It is usually caused by a deep penetrating wound much like tetnus which is also caused by clostridial bacteria known as Clostridium tetani. Intramuscular injections of anti-inflammatory drugs and vitamins are common causes.
Clostridial organisms only survive and proliferate without oxygen but often produce their own putrid smelling gases.
They cause massive muscle tissue degeneration and the condition is often life threatening.
For centuries it was known as gas gangrene and killed many horses and humans during wartime before antibiotics were available.
Treating clostridial organisms
It is sometimes necessary for the veterinarian to surgically incise into a horse's muscle to help control the infection because clostridial organisms can't survive and grow in air.
Gangrene cases are best managed by equine internal medicine and critical care specialists at referral hospitals such as University Veterinary Schools.
White tailed spider bites
White tailed spider bites in Australia and rattlesnake bites in the USA can appear similar to cellulitis.
If there is swelling of all four legs then an allergic reaction to an insect or spider bite or vaccine or drug is possible.
These conditions usually resolve very quickly with appropriate veterinary care.
Purpura
Purpura is a more severe condition. It is an immune reaction that occurs several weeks after recovery (or vaccination) from Strangles or less commonly, various respiratory viruses.
It is important to perform an endoscopy and diagnostic testing of a fluid sample from the guttural pouches to diagnose or rule out Strangles and chronic carrier animals.
Equine Viral Arteritis (EVA)
EVA is a virus that can cause swelling of multiple limbs.
It is frequently spread as a respiratory virus causing fever, coughing and nasal discharge in a group of in contact horses.
It can also cause abortions if spread from a stallion (who becomes a life-time carrier).
Treating cellulitis
It's ideal if you're able to identify and treat the source of cellutis and remove a foreign object from your horse but finding the exact cause may not always be possible.
Most cases are treated with a combination of antibiotics and non-steroidal anti-inflammatory drugs (NSAIDS).
Additional therapies such as cold hosing, topical osmotic agents (strong salt solutions to draw out excess fluid) and pressure bandages may also be helpful.
The protocol your vet decides on depends on several factors including the severity of the infection and the type of bacteria involved.
Pressure bandages are expensive to apply, especially if cold water therapy is required three times per day.
Reusable pressure stockings are available for horses and are extremely useful to manage cellulitis.
What can I do to reduce the risk of my horse getting cellulitis?
You can take preventative measures to reduce the risk of your horse getting cellulitis by:
maintaining a regular exercise program (if your horse is fit to do so) helps to improve circulation and lymphatic drainage from the legs.
keeping your horse’s legs clean and dry and disinfecting all abrasions and cuts.
avoiding turning your horse out in areas with standing water, mud or deep sand, or thorns.
