Septic Arthritis in the Newborn Foal
Dr. Mary Rose Paradis
The presumptive diagnosis of septic arthritis/osteomyelitis in the neonatal foal is made on the physical appearance of lameness in the foal, joint swelling and a history of possible failure of passive transfer of colostral antibodies. Certain diagnostic tests will help to confirm the diagnosis. Complete blood count. A newborn foal with signs of septicemia will usually have a lower than normal White Blood Cell (WBC) count. This is generally due to the infection overwhelming the immune system and using up the white blood cells faster than they can be produced. If the foal is slightly older (2 to 3 weeks old) and has septic arthritis, the WBC count is usually higher than normal. The slower onset of the problem allows for the body to recruit white blood cells to fight the infection.
Chemistry profile. A chemistry profile is a series of blood tests that check various organ systems, electrolytes and protein levels. Foals with septicemia may have abnormalities in renal function and protein levels but there is nothing specific to indicate septic arthritis.
Immunoglobulins. Most foals with septic arthritis/osteomyelitis have the infection because of inadequate protection from colostral antibodies. This is called failure of passive transfer (FPT). The immunoglobulin (antibody) levels in the foals blood can be measured by many different methods. One of the most common tests is called the CITE test. This is a semi-quantitative test that can be performed stall side in approximately 20 minutes. The ease and speed of the test makes it valuable in obtaining a quick answer to the foal immunologic status. Foals that receive adequate colostrum generally have levels of greater than 800 mg/dl. Foals with septic arthritis/osteomyelitis generally have levels less than 400 mg/dl and sometimes less than 200 mg/dl.
Arthrocentesis and fluid analysis. If a foal has a swollen, painful joint, it is important to obtain a sample of the joint fluid. The joint is clipped and scrubbed clean (surgically prepared). A sterile needle is then inserted into the joint and synovial fluid (joint fluid) is aspirated. Normal synovial fluid is pale yellow, clear and viscous. Synovial fluid from an infected joint is cloudy and watery. Fluid analysis of normal synovial fluid generally has less than 250 WBC per microliter, and the protein is less than 2 mg/dl, while the cytology from an infected joint has a WBC count in the thousands and a protein level greater than 2. These results indicate a severe inflammatory response to the infection.
Joint fluid and blood cultures. Joint fluid culture of a septic joint will only grow bacteria in approximately 50 percent of the cases. You are more likely to have a positive culture from foals with osteomyelitis and just synovitis. A negative culture does not negate that diagnosis of a septic joint. Bacterial growth may be inhibited by the inflammatory response or by previous antibiotic administration. Blood cultures are sometimes helpful in determining the cause as well. Organisms cultured from these foals (from blood, synovial fluid, umbilicus, etc.) include E. coli, Klebsiella, Streptococcus, Enterobacter, Actinobacillus, Staphylococcus epidermis and Pseudomonas. Sensitivity patterns are important in deciding the antibiotic that would be most effective against the causative organism. Because cultures may take several days before they grow bacteria it is important to start the foal on antibiotics that have a wide range of effectiveness (broad spectrum antibiotics), no matter what.
Radiographs of the affected joint. It is important to take radiographs of the affected joints to determine the presence and extent of any bone involvement. Early in a bone infection (e.g. first 21 days) you may not see any bone destruction but it will give you a baseline by which you can compare future radiographs. If the swelling and pain persist, radiographs should be repeated every 4 to 5 days to evaluate progression of the lesions. If osteomyelitis bone infection) is present, your treatment may become more aggressive than if you are dealing with a synovitis alone.
Ultrasound of the umbilical structures. Fifty percent of the foals with septic arthritis/osteomyelitis have an infection of the umbilical structures suggesting that this may be a common port of entry for the bacteria. Usually, the external umbilicus looks normal, despite the fact that the deeper structures such as the umbilical arteries and vein, are to be enlarged and infected. The only way to evaluate these structures is through the use of ultrasound. Enlargement of any of the structures is highly suggestive of infection.
Supportive care of the foal with septic arthritis includes fluid therapy, adequate nutrition, environmental temperature control and plasma transfusion to increase immunoglobulin levels.
The antibiotic choice usually consisted of penicillin or ampicillin combined with gentocin or amikacin. This provides protection against the most common pathogens - E. coli, Klebsiella and Streptococcus. Because of an increasing resistance to gentocin, our drug of choice to treat gram negative is amikacin. Antibiotic therapy should be continued for a minimum of 7 days after the resolution of clinical signs. This is generally 3 to 4 weeks.
Joint lavage. When infection occurs in or around a joint, inflammation of the synovial membrane takes place. There is an increase of fluid, white blood cells and protein within the joint. Degradative enzymes are released and cartilage degeneration can then ensue. Joint drainage and lavage are important adjuncts to the effective therapy of the infected joint. It allows for the removal of the enzymes from the joint and decreases the pressure of the effusion. Several methods of joint drainage have been suggested. Simple drainage alleviates the high intra-articular pressure but does not effectively remove all of the inflammatory products. Joint lavage can be done by a "through and through" method, by distention irrigation, by arthrotomy or arthroscopy. This is best accomplished under general anesthesia. The affected joint is clipped and sterilely prepped with povidone iodine and alcohol. An 18-16 gauge needle is placed into the joint and connected to a liter bag of normal saline or lactated ringers solution. The joint is distended under pressure and then another needle is placed in the opposite side of the joint and the fluid is allowed to drain out. A 3-way stop-cock can be placed on the egress needle to allow multiple distensions of the joint capsule. This provides a thorough flushing of the infected joint. This procedure should be repeated every other day until the joint fluid becomes more normal with decreased WBC and protein.
Surgical intervention may be necessary in osteomyelitis. This may involve surgical removal of the infected bone.
The foals were confined to their stalls with minimal exercise. Support wraps were utilized on some of the cases, but immobilization of the joint was not done.