Outline of Veterinary Skeletal Pathology

contents Ch 5, p 1 Chapter 5, Page 2 Ch 6, p 1 



Outline of Veterinary Skeletal Pathology

Chapter 5 - Joints, Pathologic Conditions

C. Inflammatory Joint Disease.
1. Infectious Arthritis. Arthritis caused by bacteria progress through stages, and the type that is seen depends upon the duration of the infection and the causative agent.
a. Acute Inflammation.
(1) Types.
(a) Serous arthritis. Synovial fluid is a dialysate of blood plasma into which hyaluronate is secreted by synovial cells. Serous inflammation is equivalent to the formation of excess synovial fluid, which distends the joint capsule and clinically forms a "puffy" swelling. The articular and synovial surfaces show nothing more than a slight hyperemia and edema. This type of inflammation is also seen with mild, and often, repeated trauma.
(b) Fibrinous arthritis. This type of arthritis is seen when the inflammatory exudate contains fibrinogen that forms clots or strands of fibrin (fig. IIb-14).
(c) Suppurative arthritis. When pyogenic organisms infect a joint, the exudate is purulent (fig. IIb-15). The joint capsule and sometimes tendon sheaths are edematous, and there is purulent inflammation of the synovial membrane. In such cases, the articular surfaces may be eroded, and there is extreme pain.
(2) Predisposing factors:
(a) wounds that open the joint cavity to infectious microorganisms.
(b) generalized septicemic or pyemic infections.
(c) pyogenic osteomyelitis that occurs more frequently in young animals than adults.
(3) Cause. Pyogenic arthritis may be a consequence of bacteriemia and direct infection of the synovial membrane.
(a) pyosepticemia neonatorum, "joint-ill." The most common condition is pyosepticemia of the newborn resulting from infection of the umbilicus at birth (see infectious arthritis).
(i) in foals, Actinobacillus equuli is the most common organism, but other organisms such as Streptococcus sp., Salmonella sp., and Escherichia coli are frequently involved.
(ii) polyarthritis also can occur as a result of hematogenous spread from other infected sites.
(iii) in lambs, Erysipelothrix rhusiopathiae, Streptococcus sp. and Hemophilus agni are the most frequent cause. Also, Staphylococcus aureus may cause infection in lambs as a complication of tick-born fever (babesiosis).
(iv) in calves, Streptococcus pneumoniae or other Streptococcal sp. may produce polyarthritis and frequently meningitis. Coliform polyarthritis of calves is caused by Escherichia coli.
(v) polyarthritis and meningitis of swine can be seen with either streptococcal polyarthritis (Streptococcus suis type I or II) or in Glasser's disease (Hemophilus suis).
(vi)Chlamydia psittaci type 2 is a cause of sporadic or endemic arthritis in lambs and calves.
(b) adults. A variety of microorganisms may be associated with infectious arthritis in adult animals.
(i) Erysipelothrix rhusiopathiae is notable for causing a chronic low grade arthritis and periarthritis in swine and turkeys.
(ii) Corynebacterium ovis, Escherichia coli, and Streptococcus sp. are also frequent causes of arthritis in sheep.
(iii) Mycoplasma sp. are important causes of arthritis in swine, sheep, goats, and cattle.
(iv) Brucella sp. and Streptococcus sp. also frequently cause a suppurative arthritis in swine. It should be clear from this list that arthritis is an important manifestation of many infectious diseases, but infectious polyarthritides are noticeably absent in dogs and cats.
(c) Pathogenesis following osteomyelitis.
(i) the epiphyses and metaphyses of long tubular bones are a predilection site for suppurative osteomyelitis prior to growth-plate closure.
(ii) infection may spread from the affected bone to the neighboring joint.
(iii) the physeal plate is a barrier to the spread of infection from the metaphysis to the joint. So, in the large diarthrodial joints of young animals such as the stifle, suppurative exudate spreads from the bone's interior through the cortex and along the periosteum into the joint space at the attachment of the joint capsule to the periosteum.
b. Chronic inflammation. Chronic arthritis may follow trauma or an acute infection.
(1) pathogenesis. This type of arthritis may have an immune-mediated component that augments and prolongs the inflammatory processes. Antibodies may be formed in response to matrix constituents liberated from degenerating articular cartilage, or antibodies may complex with antigens derived from infecting organisms.
(2) granulomatous arthritis is a type of chronic arthritis associated with delayed hypersensitivity. It is uncommon but is seen sporadically with mycotic infections such as those caused by Blastomyces dermatitidis or Cryptococcus neoformans and in tuberculous arthritis (Mycobacterium avium). Tuberculous arthritis may develop in any species, is rare in nonavian animals in North America and when present often localizes in the vertebral column.
(3) Lyme disease
(a) cause. Borrelia burgdorferi, a spirochete, causes Lyme disease.
(b) incidence. It is known to infect wild animals and has been reported to cause arthritis in dogs, cats and a cow.
(c) pathology. A small number of spirochetes are the antigenic stimulus for chronic synovial inflammation, and organisms are found in the vessel walls within the synovial membrane. Villus hyperplasia, lymphoplasmacellular infiltrates, and fibrin deposition are nonspecific inflammatory changes. Obliterative microvascular and fibrinoid lesions in the synovial membrane are specific of arthritis caused by Borrelia burgdorferi.

2. Noninfectious (Immune-mediated) Arthritis. Diseases in this category are sometimes assumed to have an immunologic basis because of the failure to isolate microorganisms.
a. Deforming or erosive arthritis. This type of arthritis leads to erosion of cartilage at articular margins, villus hyperplasia, and lymphoid and plasma-cell infiltrates in the synovial membrane. Examples are rheumatoid arthritis, erosive polyarthritis of Greyhound dogs, and feline chronic progressive polyarthritis.
b. Nondeforming or nonerosive arthritis. The joint disease appears to be cyclic in nature, and there are minimal pathologic changes. Synovium shows sparse mononuclear cell infiltration with moderate to severe superficial acute synovitis with polymorphonuclear leukocytes and fibrin. The condition is seen in lupus erythematosus, secondary infections such as subacute bacterial endocarditis and pyometra, and inflammatory bowel disease in dogs and calves.

D. Synovial sarcomas. Although synovial tumors are uncommon, many different types of tumors may arise. Tumor types include synovial cell sarcomas, histocytic sarcomas, malignant fibrous histocytomas, fibrosarcoma (fig. IIb-16), or myxoma. Synovial cell sarcomas are locally destructive tumors that arise from the synovial membrane of a joint or less commonly from tendons or synovial sheaths.
1. Incidence. Synovial cell sarcomas are rare in dogs and cats and occur less frequently in cattle and horses.

2. Clinical features. Tumors cause lameness, and a nonpainfull, palpable mass is found on a major weight bearing joint.


3. Pathology.
a. Macroscopic appearance. A sarcomatous mass usually invades and replaces bone (fig. IIb-17).
b. Microscopic appearance. The classic tumor is made up of a biphasic population composed of epithelioid and fibroblastic cells. Some cells may show immunohistochemical staining with antibodies to cytokeratin. While areas with clefts and tubular spaces (fig. IIb-18) lined by epithelioid cells are considered characteristic diagnostic features, these are rare in most canine synovial sarcomas. Solid masses of cells are more typical (fig. IIb-19). Variable numbers of giant cells may be present.
c. Biologic behavior. The tumor growth may be slow at first and then accelerate. Metastasis is rare, but with time there is aggressive local invasion.

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