Topics of Discussion

Septic arthritis/osteomyelitis is a manifestation of septicemia (widespread infection) in foals.  Approximately one-fourth of foals presented to Tufts Hospital for Large Animals with a suspicion of sepsis also present with or develop evidence of septic arthritis. Cohen and colleagues recognize infectious arthritis as a cause of death in 12.5% of foals (8-31 days of age) in Texas. The problem arises from hematogenous (through the bloodstream) seeding of the synovium (joint lining) and the physis/epiphysis (actively growing bones in the joints) with bacteria. Foals may present with one or multiple joints affected.

The younger foals tend to present with signs of septicemia in conjunction with a joint swelling. These foals usually have complete failure of transfer of maternal antibodies or developed infection in utero. Older foals may appear normal the first week of life and then present with lameness. If there is obvious joint effusion and a fever present then the diagnosis of a septic joint should be high on the differential diagnosis list until proven otherwise.  Sometimes, as in this case, joint effusion is not obvious because the joint affected is too deep to feel the swelling or because the infection is isolated to the growth plate and has not extended into the joint itself. In early growth plate (physeal) infections, the first signs may be lameness and swelling at the growth plate region.

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Figure 6 – Normal viscosity of synovial fluid.

Arthrocentesis (sampling of the joint fluid) of the affected joint is confirmatory for septic arthritis. The synovial fluid of an affected foal will have a high cell count with a predominance of neutrophils and an elevated protein. Synovial fluid becomes serous (thin) in nature, losing its normal viscosity due to inflammation which results in depolymerization of hyaluronan, an important component of joint fluid . Bacteria are occasionally seen on gram stain of the fluid.  A quick on-farm analysis of septic synovial fluid may include the appearance of cloudy joint fluid, a protein > 2.0 gram/dl on a refractometer reading and a decreased viscosity as evidenced as loss of “stringy” nature when placed between two fingers and pulled apart. (Figure 6)

Radiographic imaging is helpful in the presence of bone involvement, but there is a lag period in which bone demineralization goes unrecognized. Computed tomography has been shown to demonstrate osteomyelitis in some foals that have no radiographic changes. CT is also better at delineating the extent of the bone involvement because it negates the problem of overlapping bone seen with radiographs, and makes the diagnosis clearer. If CT is not an option then radiographs should be repeated every 3-5 days to look for bone involvement. With ’11 Royal Reign, a 10-day-old crossbred filly which was referred to the internal medicine service at Tufts Hospital for Large Animals at the Cummings School of Veterinary Medicine, with a 24-hour history of sudden hind limb lameness,  we had the opportunity to perform an initial and a follow-up CT which helped us determine the length of necessary treatment.

Treatment of a septic joint is directed at providing immunologic support in the form of a plasma transfusion for complete or partial failure of passive transfer of colostral antibodies from the mare, broad spectrum antimicrobial coverage and joint health maintenance. Antimicrobial coverage should be initiated before culture results are available.  Bacterial organisms involved in septic arthritis mirror those that are seen in neonatal sepsis. Some of the most prevalent bacterial include E. coli, Klebsiella, Enterobacter, Salmonella and Streptococcus sp. Broad spectrum coverage with an aminoglycoside, such as amikacin, and an antibiotic effective against gram positive bacteria, such as penicillin or ceftiofur, should be the initial protocol. Fluoroquinolone antibiotics should be reserved as backup drugs because of the potential cartilage damage that is seen in foals in association with this class of drugs. Marbofloxacin (3.5 -4 mg/kg, orally, once/day) was used in this foal because anecdotal reports are that it causes fewer cartilage problems than enrofloxacin. Cefpodoxime (10 mg/kg q 6 hours) was added to the regimen to provide coverage against possible gram positive bacteria. Strict stall rest was important for added protection in preventing cartilage damage.

Joint health maintenance involves decreasing the tension (swelling) on the joint capsule and diluting the inflammatory products of the white blood cells in the synovial fluid which damage cartilage. This is best accomplished by joint lavage. Joint lavage in the foal is accomplished under general anesthesia or heavy sedation.  The joint is aseptically prepared and a 14 gauge needle is inserted into one side of the affected joint.  The joint is distended with sterile fluid and a second needle is inserted in the opposite side of the distended joint. One to 2 liters of sterile saline fluid should be used in the larger joints and less in the joints such as a fetlock.  Intermittent distension of the joint by covering the exit needle hub is helpful in achieving a thorough flush.  We lavage the affected joint every other day until improvement is seen. For non-responsive cases, arthroscopy is recommended to provide even larger volume flush and removal of any fibrin tags.  Lavage can be coupled with regional limb perfusion in cases of osteomyelitis in lower joints.

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Figure 7 – Fracture at a growth plate secondary to osteomyelitis of the growth plate of the distal metacarpal (lower cannon) bone.

The survival rate of the neonatal foal with septic orthopedic disease at Tufts Hospital for Large Animals is about the same as that seen in the overall neonatal population – 75%.  The cause of death in non-surviving foals is usually due to other concurrent problems such as septicemia and pneumonia or they have fractured at a growth plate secondary to infection. (Figure 7)  The long term prognosis for racing in the foals with septic arthritis has been looked at by several researchers and found that 50% of affected foals as adults went on to race at least one race. These foals made significantly less earnings when compared with unaffected siblings. No one has published the effect of neonatal septic arthritis on the athletic future of non-racing horses.  An unpublished follow-up survey of 11 foals which presented to Tufts Hospital for Large Animals with septic arthritis reported 8 foals were sound at discharge, 10 were sound at the time of follow-up and 9 of the owners felt that the foals were meeting their athletic expectations. Seven of these foals had been diagnosed with osteomyelitis (bone infection) and 4 with just synovitis (joint lining infection).