Acute haematogenous osteomyelitisis an acute, closed-space infection of the bone marrow with active involvement of the endosteum and the periosteum. It is a suppurativenecroticinflammatory process caused by pyogens. Since this is an infective process it can involve any bone. However, the most frequent sites are the metaphyseal regions of long bones of growing subjects; in adults the site most commonly affected is the vertebral column, giving rise to discospondylitis. Haematogenousosteomyelitis usually affects a single bone. Although rare in dogs, acute haematogenousosteomyelitis should be included in the differential diagnosis of severe lameness of acute onset in a growing animal.
AETIOPATHOGENESIS
Infection of bone tissue is caused by bacterial growth from septic foci, with transient bacteraemia and localisation in the bone. Haematogenousosteomyelitis is not secondary to either trauma or surgery but is rather a systemic infection. The primary focus of infection is often unknown but pyodermititis, tonsillitis and maternal mastitis have all been described as possible causes.
Staphylococcus aureus is the most frequently implicated pathogen although various other bacterial strains, such as Streptococci, Diplococci, and Corynebacterium, are not to be excluded. Bone infection is associated with blood vessel occlusion which causes bone necrosis and local spread of the infection. The characteristic metaphyseal blood supply of subjects during the growth period is a predisposing factor. The nutrient artery of the metaphysis creates a vascular network with sinusoidal capillary loops which slow down the blood flow and, therefore, provide a preferred site for bacterial growth.
The extent and depth of bone necrosis and of the formation of bone sclerosis in the surrounding tissue all vary according to the severity of the infection, the size of the area involved and the time elapsed before treatment. Bone sequestration takes place when the necrotic bone is surrounded by a productive bone reaction which isolates the primary lesion. Since the osteomyelitic focus develops in a growth plate, the septic process and the consequent inflammatory reaction can cause premature closure of the physis involved, with a subsequent impact on alignment of the limb.
EPIDEMIOLOGY
Haematogenous osteomyelitis is found in all breeds of dogs, usually between 2 and 8 months of age, with no gender prevalence. Although no predisposition for a particular breed has been reported, most cases involve large and giant breeds, most frequently Boxers and molosser breeds in general.
CLINICAL SIGNS
The clinical signs vary greatly in intensity and the form of onset is also varied. The most typical form is characterized by hyperthermia, pain in the affected area and variable degrees of lameness, but usually grade III or IV. Before the development of orthopaedic symptoms there is sometimes a history of episodes of apathy and loss of appetite, indicating a febrile form which has often disappeared by the time of the clinical visit. Severe forms of osteomyelitis with widespread sepsis can result in pathological fractures because of the associated weakening of bone trabeculation. It has been reported that, in humans, when the osteomyelitis involves the proximal metaphysis of the femur or the humerus, the septic process can spread to the joints because of the partially intracapsular location of these metaphyses, causing further complications, such as septic arthrosynovitis.
DIAGNOSIS
In the acute phase, palpation of the involved bone is painful and there is marked lameness. If the osteomyelitic focus of infection involves a superficial bone metaphysis, such as that of the distal radius/ulna or distal tibia, localised bone swelling, although rare, can be observed. The diagnostic suspicion can be confirmed by X-rays. In the very early stages of infection, the bone tissue may appear normal on X-rays. However, as previously mentioned, in some sites such as superficial metaphyseal areas, swelling of perilesional soft tissue may be seen. In these cases, the recommendation is to start therapy and take further X-rays after a few days.
X-rays show a focal reduction in bone density which is later delimited by a sclerotic halo (Fig. 1a [3]and 1b). The radiographic appearance of bone sequestration is characterized by a central sclerotic area, the sequestrum, surrounded by a radiolucent bone reaction, delimited by a further sclerotic halo, the “sarcophagus” (Fig. 2). Although in the early stage conventional radiography is not useful (at this stage bone scintigraphy is more sensitive), the general clinical rule is that every case of sudden onset of bone pain in a subject during the growth period, on a background of a compromised general clinical condition, even before orthopaedic signs are present, should raise a strong suspicion of osteomyelitis (Maroteaux). Comparison with X-rays of the contralateral bone segment can help to identify initial lesions.
TREATMENT
The management of acute haematogenous osteomyelitis is usually medical and is based on the administration of antibiotics. Blood cultures and an antiobiogram are recommended to identify an appropriate antibiotic, above all when the diagnosis has been made very early; however, bacteraemia is generally no longer present at the time of the clinical examination. Nevertheless, even in cases of negative blood cultures or while awaiting identification of the microbe, initiation of treatment with broad-spectrum antibiotics is recommended. The severity of the lesion and the clinical profile present could make it necessary to administer the chosen antibiotic parenterally for the first few days of treatment, after which the more convenient oral administration can be adopted. The duration of antibiotic therapy should be based on X-ray monitoring of the lesions and the clinical picture; however, therapy should last at least 3-4 weeks. In the case of an osteomyelitic abscess, surgery could be required to drain the septic fluid. Medical management may also be unsuccessful in the case of extensive bone sequestration. Although this is not a common pathology in dogs, the recommended management is surgical removal of the sequestrum together with medical treatment in order to clean the site completely.
PROGNOSIS
The prognosis of osteomyelitis varies depending on the timing of the diagnosis and treatment. Early, targeted antibiotic therapy can resolve the infection and allow normal bone structure to be restored (Figs. 3a and 3b). When the diagnosis is made late, the growth plate could be compromised, leading to its premature closure and consequent deformity of the axial bone, which could subsequently require surgical correction.
Suggested readings
- Caywood DD, Wallace LJ, Braden TD “Osteomyelitis in the dog: a review of 67 cases” J Am Vet Med Assoc 172(8): 943-6, 1978
- Emmerson TD, Pead MJ “Pathological fracture of the femur secondary to heamatogenous osteomyelitus in a weimaraner” J Small Anim Pract 40(5): 233-5, 1999
- Bigham AS and Shafiei Z “Unilateral premature closure of distal radial growth plate due to hematogenous osteomyeliyis in a German shepherd dog” Comp Clin Pathol 17: 275-277, 2008
- Cabassu J and Moissonnier P “Surgical treatment of a vertebral fracture associated with a haematogenous osteomyelitis in a dog” VCOT 3: 227-30, 2007