Introduction

Septic arthritis occurs in one of three ways: direct inoculation of the joint via trauma or iatrogenic injection, bacteremia due to a distant infection, or extension of a locally septic process into the joint. The latter frequently presents in adult cattle that acquire a septic distal interphalangeal joint via extension of a sole abscess. In calves, however, septic arthritis secondary to bacteremia is most typical. The three leading causes of bacteremia resulting in joint sepsis in calves are umbilical infection, pneumonia, and eneritis.¹ Of the forms of umbilical infection, omphalophlebitis is the most likely to result in bacteremia and septic arthritis due to its communication with the liver and subsequently systemic circulation.

Failure of transfer of passive immunity (FTPI) is a compounding factor in septic arthritis; FTPI firstly predisposes the calf to diseases such as pneumonia and infection of the umbilicus, and further allows bacteremia to more readily occur and seed a joint. For Holstein calves, 4 L of good quality colostrum should be fed within the first 12 hours of life to ensure appropriate transfer of immunity, keeping in mind that calves born in dystocia or prematurely may not nurse or absorb IgG from the gut well. A serum total protein of ≥5.5 or Brix refractometer plasma value of ≥10% indicates appropriate IgG absorption. Unfortunately, if a calf did not absorb enough IgG within the absorption window, additional colostrum PO will not result in an appropriate increase in IgG. After this point, the only option to increase serum IgG would be through a plasma or whole blood transfusion. Plasma may be given at a dose of 20–40 mg/dL IV. Whole blood has more diluted IgG, and thus the dose should be increased. As with any transfusions, a small volume (5 ml/kg) should first be given over 15–20 minutes while monitoring for a reaction. After this point, the remainder may be given at a higher rate.

Pathophysiology

In the case of septic arthritis caused by bacteremia, the joint is first seeded as bacteria cross the synovial membrane through periarticular capillaries. After bacteria has been introduced, it colonizes and proliferates. Proliferation of bacteria leads to an acute inflammatory response during which neutrophils and macrophages are recruited and proinflammatory mediators are released. The proinflammatory mediators such as TNF-α, IL-1, IL-6 result in bone resorption along with bone and cartilage damage caused by matrix metalloproteinases. Furthermore, the joint effusion accumulated due to the inflammation results in greatly increased articular pressure which in turn impedes blood supply and nutrient diffusion from the periarticular vasculature. Meanwhile, fibrin deposition impedes nutrient diffusion and harbors bacteria by protecting it from the immune system and preventing resolution.

Clinical Findings

Calves that develop septic arthritis often present with a history of an acute onset of moderate to severe lameness. The effected joint(s) are usually visibly swollen and painful on palpation. In descending order, the most frequently effected joints in calves are the carpus, stifle, and tarsus.^1,2 In more chronic cases, breakdown of the fetlock and/or varus deformity of the contralateral limb due to over weightbearing is observed. Radiographs may be useful to rule out other causes of lameness such as fracture but can also be useful prognostically. One should not expect to see osseous changes on radiographs unless the disease has been present at least 10 days, and radiographic evidence of decreased joint space and subchondral bone resorption were associated with negative outcomes in one study.²

Arthrocentesis can be helpful, especially in acute cases, to confirm the diagnosis. Although recommended sites for optimal arthrocentesis are published, selecting the point of maximal distention is typically adequate. Diagnosis can usually be made presumptively based on macroscopic examination of the joint fluid. Normal joint fluid should be clear to slightly yellow-tinged, viscous, and transparent. In the case of septic arthritis in calves, the joint fluid becomes white, loses its viscosity, and opaque. If cytologic examination is performed, a protein of >4.5 g/dL, nucleated cell count of >25,000 cells/µL, and neutrophils of >20,000 cells/µL, 80% of which are degenerate, is indicative of septic arthritis.

Ultrasound may be a beneficial diagnostic for determining effected joints, especially those surrounded by more soft tissue such as the glenohumeral or coxofemoral joints. It may also be of use in identification of fibrin within the joint, and for guidance of an arthrocentesis.

Culture of the joint fluid is not always rewarding, but one recent study found that gram-positive cocci (strep spp.) were most isolated, followed by Mycoplasma bovis. In more chronic cases, Trueperella pyogenes is more represented.¹

Treatment

Treatment of septic arthritis should focus on 3 goals: (1) reduction in bacterial load, (2) decreasing inflammation, and (3) to provide analgesia.

Reducing the bacterial load is accomplished by physically removing the bacteria, as well as providing antimicrobial therapy. When selecting the appropriate antimicrobial, label and food safety considerations, delivery into the joint, treatment of concurrent disease, and dosing route and frequency are all factors which must be accounted for. Treatment of food animals narrows the list considerably due to drug residue concerns. If concurrent disease is still present for which a specific drug has a label and will also treat the joint, this drug should be selected. In the absence of concurrent disease, no drug has a label for treatment of septic arthritis. In this case, an educated decision must be made to select an appropriate drug for extra-label use. Drugs such as florfenicol and tulathromycin are good choices, as they have been shown to accumulate in joints and maintain MIC90 for longer than systemic levels.³ Local administration of antimicrobials, i.e., intraarticular injection or regional limb infusion may be considered as these methods deliver a high dose to the region of interest while still using a much smaller dose than would be used systemically. Ampicillin sulbactam may be a good option for regional use.⁴ Regardless of drug selection, antimicrobial therapy may be necessary for up to 3 weeks to appropriately treat septic arthritis in calves.

Physical removal of the bacteria and fibrin is necessary to clear the infection. Fibrin will harbor the bacteria and protect it from the immune system and antimicrobials. Flushing the joint can be accomplished by inserting 1 needle, infusing the joint with isotonic fluids, and pulling the fluid back out. This is referred to as tidal lavage. Two or more needles can be placed for “through and through” lavage. This allows for simultaneous infusion and drainage of fluid. Massage of the joint and allowing pressure to build up before release will help to break up fibrin and ensure that the entire joint is being accessed. In many cases, arthrotomy is necessary. This can be performed as a needle-guided stab incision into the joint and allows for removal of fibrin that would otherwise be too large to exit a needle and in many cases causes repeated obstruction of drainage. Fibrin can be expressed via massage of the joint, but the use of hemostats is of benefit. Instruments can be inserted through the arthrotomy, the jaws closed, and gentle traction applied. If resistance is felt, the hemostats should be released, as it is likely that synovium or another articular structure has been inadvertently grasped. During this process, periodic or continuous lavage through a needle or into the arthrotomy site will assist with dislodgement of more fibrin. Arthroscopy may also be used in select cases, though advanced equipment and training are required, and the cost increases. The benefits of arthroscopic management of septic arthritis include improved visualization of the joint and higher volumes of lavage achieved. Following joint flushing, any arthrotomies should be left open and a bandage placed. Two subsequent flushes at 24-hour intervals are ideal. Flushing the joint not only helps in removing the bacteria causing the infection, but also clears many of the hosts inflammatory mediators, decreasing inflammation and improving comfort.

In addition to flushing, which removes many of the inflammatory mediators, anti-inflammatory drugs should be provided to limit joint damage and provide analgesia. This can be provided in the form of non-steroidal anti-inflammatories (NSAIDs) such as flunixin meglumine or meloxicam.

Analgesia is provided in part by flushing of the joint and reducing inflammation with the use of NSAIDs. In especially painful animals, opiates, epidurals, and other methods may be useful.

In some cases, joint destruction may be so severe that treatment is impossible. In these cases, humane euthanasia should be considered. If the animal is particularly valuable due to genetics, sentimental attachment, or other reasons, further treatment may be desired. Ankylosis or arthrodesis of the joint can be pursued in these cases. The prognosis for such procedures is guarded to fair, and in successful cases, the patient will lose use of the joint and maintain a functional lameness.

Sequestra

A sequestrum is a piece of necrotic, most commonly infected, bone which is no longer attached to the main column of healthy bone. The etiology in farm animals is either sharp trauma which exposes the bone, damages the periosteum, and causes ischemia, microthrombi which cause ischemia to a piece of bone, or a fracture that leaves a fragment of bone which is not incorporated with the callus during healing. Animals with sequestra often have moderate to severe lameness. Many sequestra, especially those with a penetrating traumatic etiology, have a chronic draining tract associated with them which has been unresponsive or partially responsive to antibiotic therapy and wound care. Sequestra secondary to a septic microthrombus may have no such tract. Regardless of etiology, radiographs of the effected region will provide a diagnosis.

The sequestrum is surrounded by a lucent pocket in the parent bone known as the involucrum. The involucrum is lined with a glycocalyx biofilm which harbors bacteria and protects it from the immune system. The communication from soft tissue into the involucrum is called the cloaca. Successful treatment is possible with long-term administration of antibiotics, but surgical removal of the sequestrum is often necessary. A sequestrectomy is initiated by incising directly over the cloaca, using the draining tract to guide the incision if present. To remove the sequestrum, the cloaca may be widened with osteotomes, chisels, or rongeurs, but bone should be removed conservatively. Once the sequestrum is removed, the lining of the involucrum should be debrided with a curette. If a large amount of parent bone was removed in the process, the animal should be recovered in a cast or splint to prevent fracture.

References

1.  Constant C, Nichols S, Desrochers A, et al. Clinical findings and diagnostic test results for calves with septic arthritis: 64 cases (2009–2014). J Am Vet Med Assoc. 2018;252:995–1005.

2.  Constant C, Masseau I, Babkine M, et al. Radiographic study of haematogenous septic arthritis in dairy calves. Vet Comp Orthop Traumatol. 2018;31:252–260.

3.  Jones ML, Washburn KE, Fajt VR, et al. Synovial fluid pharmacokinetics of tulathromycin, gamithromycin and florfenicol after a single subcutaneous dose in cattle. BMC Veterinary Research. 2015;11:26-015-0346-0344.

4.  Depenbrock SM, Simpson KM, Niehaus AJ, et al. Pharmacokinetics of ampicillin-sulbactam in serum and synovial fluid samples following regional intravenous perfusion in the distal portion of a hind limb of adult cattle. Am J Vet Res. 2017;78:1372–1379.