Septic Arthritis


By Dr. Dave Mullins: Equine Veterinary Services 


It’s a classic situation. “Hey Doc, can you come and look at my new colt? I think the old mare stepped on him ’cause he’s a bit lame today.” When the foal is evaluated, you find a swollen joint or two, and the colt also has a slight fever. At this point, you’ve most likely got your diagnosis–septic arthritis. In foals, it often is called joint ill, and it is seen commonly in neonatal foals less than one month of age. This is not to say that mares don’t step on their foals, because some do. However, the most common cause of lameness that veterinarians see in neonatal foals is caused by septic arthritis, not careless mares. Septic arthritis also can be seen in older foals and adult horses, but the cause usually is quite different from that in neonatal foals.

What Is Septic Arthritis?

Septic arthritis is infection within the joint. Simplified, arthritis means joint inflammation, and septic means infection caused by microorganisms. Therefore, septic arthritis is joint inflammation caused by microorganisms. Bacteria almost always causes this infection in foals although there have been some cases where yeasts are involvlved. Joint infection in foals leads to progressively worsening lameness and joint swelling. The infection within the joint often is secondary to a primary infection somewhere else in the foal’s body, such as the gastrointestinal (GI) tract, lungs, or umbilicus. If left untreated, the foal can develop not only severe arthritis (which can lead to permanent lameness), but the foal can die from the unbridled infection.

How Do Foals Get It?

The bacteria gain entrance to the foal’s body usually through the umbilicus, but also can enter through the GI tract via oral ingestion, or by inhalation through the lungs. If the foal’s immune system is not effective in destroying the invading bacteria, then infection results. The infection can set up shop in the umbilicus, gastrointestinal tract, lungs, etc. The bacteria then spreads throughout the foal’s body via the bloodstream.

This disease process–bacteria in the bloodstream–is called septicemia, and without proper treatment it is life-threatening to the young foal. After the bacteria have gained access to the foal’s bloodstream, then the bacteria can become trapped within small capillaries near the foal’s joint. This can lead to infection within the physis (the growth plate) or directly in the joint itself. Infection within a growth plate can extend into the joint, since they are in close proximity, also leading to septic arthritis. This process can occur in multiple joints at one time, so it is not unusual for a foal with septic arthritis in one joint to rapidly develop infection in other joints.

Failure Of Passive Transfer

Foals which develop septicemia and/or septic arthritis commonly are those which have had either partial or complete failure of passive transfer. Remember, there is passive transfer of antibodies from the mare to the newborn foal via the colostrum (first milk). The mare concentrates antibodies from her bloodstream late in pregnancy (usually during the last two to four weeks). The concentrated antibodies are found in the mare’s colostrum. The newborn foal must drink the colostrum shortly after birth in order to absorb the antibodies and be protected from many infections.

Clinical Signs And Diagnosis

Foals with septic arthritis have several classic clinical signs. A sudden onset of lameness along with joint swelling are two of the most important and earliest signs. There also might be swelling or edema around the joint, and the foal might have a fever at the time you notice the joint swelling. Some foals which have concurrent infection in the GI or respiratory tracts might have additional clinical signs related to that disease process, such as diarrhea and decreased nursing (GI tract) or coughing and nasal discharge (respiratory). Some umbilical infection symptoms are obvious swelling or drainage from the umbilicus. However, even if the external umbilicus appears normal, it does not preclude a diagnosis of umbilical infection–umbilical structures within the abdomen that cannot be seen or palpated can have infection present.

The ultimate diagnosis will come from your veterinarian after a sample of fluid is obtained from the swollen joint. Samples of joint fluid from infected joints will look similar to pus and will have a greatly increased white blood cell count and protein level. Microscopically, however, the offending bacteria are rarely seen within the joint fluid.

Tests such as a complete blood count, ultrasonography of the umbilical structures, and radiographs of the affected joint(s) are very important. If infection is found within the umbilical structures, surgery often is recommended to remove the umbilicus and the source of infection. Radiographs might reveal infection involving the bony structures of the joint or physis, and they can help your veterinarian decide on the best treatment approach. To determine the exact organism causing the infection, samples of the joint fluid (and often blood) are cultured in an attempt to grow the offending bacteria. If bacteria are grown, then their sensitivity to antibiotics can be determined so that the correct antibiotic(s) can be used. Although this process can take several days, prompt treatment is imperative for a successful outcome, so antibiotic treatment usually is begun before the results of the cultures are known, but after the samples have been obtained.


The treatment for septic arthritis involves the use of broad-spectrum antibiotics. This means that before the exact organism and its antibiotic sensitivity are known, antibiotics that cover the broadest spectrum of likely organisms are used. It is extremely important to begin treatment immediately after the first clinical signs are noted in a horse. Delays in treatment can lead to a permanently damaged (arthritic) joint.

In addition to the antibiotics, treatment of the joint itself is paramount. Initial therapy of the joint usually includes joint lavage. This procedure involves “flushing” the joint with sterile saline (often by adding a small amount of the anti-inflammatory drug dimethyl sulfoxide, or DMSO). The joint lavage helps remove bacteria and harmful by-products of the infection (such as en-zymes) that can damage the cartilage surface of the joint.

The joint lavage is performed while the foal or horse is under sedation or general anesthesia. Large-gauge needles are used to introduce the fluid under pressure. The fluid quickly fills the joint, then exits via a separate needle.

After the lavage, antibiotics are deposited directly into the joint to achieve very high concentrations. This procedure might be performed several times–usually every other day–until the white blood cell count and protein levels within the joint decrease. If the joint infection is re-solving, then the swelling and lameness will improve. If after several treatments with joint lavage there is no improvement, or if the radiographs reveal more extensive problems (such as bony lysis or disintegration, or a bone cyst), then arthroscopy often is recommended (see next section).

In addition, foals which are being treated for septic arthritis also require anti-ulcer treatment. Because of the stress involved in unaccustomed frequent handling and illness, the young horse is very prone to gastric ulceration.


Arthroscopy can be used in septic arthritis cases to evaluate the joint surface for any abnormalities and remove large debris (such as fibrin clots) that cannot be removed via joint lavage. In cases of septic arthritis, bacteria prefer to live and hide within the synovial tissue of the joint capsule. This makes them difficult to remove with just a simple joint lavage.

Arthroscopy also can be used to remove the enlarged synovial tissue (a procedure called synovectomy) to help decrease bacterial numbers. In some cases where infected bone is present, arthroscopy can be used to remove the infected bone tissue that antibiotics might have difficulty reaching, and can encourage healing.

Other Treatments

In some cases, long-standing infection cannot be cured with joint lavage or even arthroscopy. In those cases (which do not respond favorably within a few days to one week with systemic antibiotics and joint lavage), then open drainage or an arthrotomy is necessary, possibly following arthroscopy. An arthrotomy is a long incision or incisions made into the joint to allow for continuous drainage. These large openings also allow for large-volume lavage and the removal of the large amount of debris that accumulates in an infected joint.

The disadvantage of this procedure is that it requires extreme care in maintaining the cleanliness of the wound. The joint must be bandaged using sterile bandages, and it must be monitored frequently. If the bandage slips or comes off, the site must be cleaned and re-bandaged immediately to prevent any contamination of the incisions.

Other treatment options for refractory cases of septic arthritis include regional limb perfusion. In this method of treatment, a tourniquet is applied above the affected joint on the horses leg, and antibiotics are infused into a vein in the horses leg below the tourniquet. In this way, antibiotics are allowed to be in a very high concentration in a very specific and small area (instead of throughout the entire horses body). This large increase in antibiotic concentrations helps destroy the infection within the joint or physis. The tourniquet is only applied for a short period of time, usually not longer than 20-30 minutes.

After The Infection

One of the most important questions that should be asked before beginning treatment for any disease is: What is the prognosis? Are we just prolonging the inevitable, and what is the level of pain involved? What will be the horse’s quality of life afterward?

Unfortunately, these questions can be difficult to answer with certainty at the beginning of this disease process, but should be discussed during the course of the treatment. Luckily for horses with joint infections, the chance of returning to the same level of performance and even for young horses to survive and still perform at the same level as their peers has been increasing. If caught early and treated aggressively, the prognosis for septic arthritis patients is favorable.


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