Outline of Veterinary Skeletal Pathology

contents Ch 2, p 5 Chapter 2, Page 6 Ch 2, p 6 

Outline of Veterinary Skeletal Pathology

Chapter 2 - Bone, Pathologic Conditions

E. Infections of Bone (osteomyelitis)
1. Definitions.
a. Osteomyelitis. Strictly speaking, osteomyelitis is inflammation of the marrow spaces in contact with the endosteum or cancellous bone (figs. Ib5-1, Ib5-2). Osteomyelitis is more often used as a general term indicating inflammation of bone caused by pyogenic organisms.
b. Osteoperiostitis. Inflammation confined to the connective tissue spaces on outer surfaces of cortical bone.
c. Spondylitis. Inflammation of one or more vertebra (fig. Ib5-3). Rarely does infection of the abdominal or pelvic viscera spread via the vertebral venous plexuses.
d. Diskospondylitis. Inflammation of the vertebral body and adjacent intervertebral disk (fig. Ib5-4).

2. Predisposing Factors.
a. Age.
(1) acute osteomyelitis of hematogenous origin is seen almost exclusively in young animals.
(2) because the physis is still open, infection is localized to regions supplied by end arteries, either in the epiphysis adjacent to the articular-cartilage growth zone or in the metaphysis adjacent to the physis (figs. Ib5-5, Ib5-6).
b. Sepsis.
(1) an extra-osseous septic focus is the most common cause of hematogenous spread of bacteria to bone and joints.
(2) infection can spread to the bone from an adjacent septic arthritis or periarticular soft tissue abscess.
c. Penetrating trauma.
d. Complications of fractures or surgery.

3. Types.
a. Acute osteomyelitis.
(1) causative organisms.
(a) Staphylococcus and Streptococcus sp. are the most frequent aerobic bacteria causing hematogenous bone infections in dogs.
(b) less pathogenic organisms such as E. coli or Fusobacterium necrophorum can be opportunists and cause acute osteomyelitis.
(c) Actinomyces pyogenes is frequently isolated from osteomyelitis in cattle, sheep, and swine, and Salmonella sp. are commonly isolated from foals.
(d) anaerobic bacteria, frequently Peptostreptococcus anaerobicus or Bacteroides asaccharolyticus, or mixed aerobic and anaerobic infections are also frequent causes.
(2) pathogenesis.
(a) effect of age.
(i) in young animals that still have a physeal plate, arterioles within the metaphysis form a loop at the growth cartilage-bone junction and then enter into the spaces between trabeculae of the primary spongiosa without forming a capillary bed.
(ii) during endochondral ossification, vascular sprouts form blind-ended vessels that are lined by an attenuated endothelium with no underlying basement membrane.
(iii) bacteria lodge in this region, and certain types of bacteria can attach to the surfaces of trabecular bone or cartilage matrix, but not to adjacent vascular endothelium or erythrocytes.
(b) effect of bacterial type.
(i) certain bacteria such as Staphylococcus aureus or Pseudomonas aeruginosa contain fibronectin-binding surface proteins, which have the ability to interact with collagen, or the surface proteins contain binding sites that attach to sialoprotein, a noncollagenous bone matrix protein.
(ii) adherence stimulates the organisms to exude a glycocalyx that becomes progressively more dense.
(iii) this biofilm buries the organism and serves as a barrier against host defense mechanisms and antibiotics.
(c) sequence.
(i) initially organisms lodge in the marrow space in the epiphysis or metaphysis as indicated above.
(ii) in the marrow spaces of the spongiosa, the bacteria soon give rise to one or several small abscesses.
(iii) liberated bacterial toxins and ischemia lead to bone necrosis.
(iv) the necrotic zones may coalesce into an avascular zone permitting further bacterial proliferation.
(3) pathology.
(a) sequestrum (fig. Ib5-5). In the case of certain infections such as Salmonella sp. in foals, a sequestrum, a piece of dead bone that has become isolated during the process of necrosis, may form.
(b) involucrum (fig. Ia-41).
(i) this is a tissue reaction that forms around the sequestrum.
(ii) the appearance may range from a microscopic reaction, composed of either a sheath of pyogenic fibrous tissue or reactive bone, to a gross lesion where the whole cortex is involved.
(c) Brodie's abscess. Brodie's abscess is a sharply delimited focus of infection seen in the metaphysis of growing animals.
(i) it occurs when the bacterial infection and inflammatory reaction become localized and chronic.
(ii) after the inflammation is contained, continuation of longitudinal bone growth increases the distance between the metaphyseal inflammatory focus and the physeal plate (fig. Ib5-6).
(iii) grossly, the lesion appears as a small cystic space.
(iv) the suppurative metaphyseal focus has a wall lined by inflammatory granulation tissue. The surrounding metaphyseal, cancellous bone forms wide compact trabeculae of the sclerotic margin.
(d) puriform softening (fig. Ib5-2).
(i) lysis of bone results when lysosomal enzymes are liberated from polymorphonuclear leukocytes and macrophages, but in inflammation there is also a regional acceleratory phenomenon with activation of remodeling by osteoclasts.
(ii) tissue destruction within the metaphysis may be so extensive that the vascular supply to the physeal plate is interrupted. Because the local inflammatory lesions prevent normal vascular invasion of the mineralized hypertrophic cartilage, focal rachitic-like lesions with thickening of the physis develop.
(e) secondary arthritis.
(i) suppurative exudate may escape from the bone's medullary spaces to the metaphyseal surface at the bone-physeal junction.
(ii) once the suppurative exudate is outside of the bone, it may dissect through synovial tissue to invade the joint leading to a secondary suppurative arthritis.
(ii) the exudate may also form a draining sinus through the skin.
(f) cortical bone necrosis.
(i) the exudate also may flow between the periosteum and the cortex and isolate bone from its blood supply in the periosteum by shearing off the perforating arteries and causing ischemia to a portion of the cortex (fig. Ia-43).
(ii) some animal species have a dual blood supply to the cortex.
(iii) the outer one third of the cortical bone and much epiphyseal and metaphyseal bone is supplied by vessels from the periosteum, the rest comes from the endosteal surface via the nutrient artery.
(iv) the cortical bone may undergo complete necrosis if it is isolated from its dual blood supply by periosteal and marrow exudate.
(v) as the suppurative exudate dissects between the surface of the cortex and the periosteum, it causes reactive bone to be formed (fig. Ia-43).
b. Chronic osteomyelitis.
(1) causative organisms.
(a) chronic osteomyelitis is characteristic of certain infections that have a propensity to affect bone. Many of the organisms causing chronic osteomyelitis produce pyogranulomatous or granulomatous inflammation.
(b) Diseases include brucellosis (Brucella canis and Brucella suis), tuberculosis in young animals (Mycobacterium tuberculosis (fig. Ib5-7) and Mycobacterium bovis), actinomycosis (Actinomyces bovis) (fig. Ib5-8), and coccidioidomycosis (Coccidioides immitis)(fig. Ib5-9).
(c) Chronic osteomyelitis is also seen with aspergillosis (Aspergillus fumigatus and Aspergillus terrens and others)(figs. Ib5-10, Ib5-11), blastomycosis (Blastomyces dermatitidis), histoplasmosis (Histoplasma capsulatum), cryptococcosis (Cryptococcus neoformans), and mycetoma (diverse etiology of actinomycetes and higher fungi).
(2) pathology.
(a) chronic osteomyelitis may result from transition of acute osteomyelitis in severe cases.
(b) healing of bone abscesses progresses with formation of granulation tissue and a fibrous wall.
(c) larger abscesses are transformed into cystic cavities.
(d) the inflammatory response caused by certain organisms is pyogranulomatous or granulomatous.
(e) granulomatous osteomyelitis without a specific distribution is often associated with hematogenous spread of fungal infections to bone.
(f) actinomycosis occurs as mandibular periostitis and osteomyelitis in cattle and other species (fig. Ib5-8). Brucella canis and Brucella suis are the causes of diskospondylitis in the dog and pig respectively.

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