Page 1123 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness in the Young Horse 1089

INFECTIOUS CAUSES OF LAMENESS myelitis. The region is usually warm to hot and is pain­
ful on manipulation. Occasionally, the area may be cold
VetBooks.ir extremely common, so much so that it is a safe to assume compromise, or there may be vascular thrombosis
Lameness in foals caused by infectious agents is
if the swelling is excessive enough to create vascular
that all lameness in foals is septic in origin until proven
associated with the sepsis.
otherwise. Foals may be affected shortly after birth or up
Hemogram is often normal on presentation because
to 4 months of age, although such lameness may sporadi­ of the localized and acute nature of the infection. When
cally occur in older foals; approximately half the foals are hemogram alterations occur, there is an elevation in the
affected by 60 days of age. It is perplexing that foals white blood cell count, with or without a left shift and
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appear to be plagued with these conditions more than an increase in fibrinogen of 500–800 mg/dL or higher.
other species with similar housing and hygiene practices. Plasma fibrinogen concentration of more than 900 mg/
The most common infectious conditions causing lame­ dL may be useful as an indicator of physeal or epiphy­
ness include the complex of osteomyelitis, septic arthritis, seal osteomyelitis, and values may reach 1500 mg/dL.
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septic physitis, and/or septic tenosynovitis. 15,18,19,32–34,41,45 Serum amyloid A (SAA) has more recently proven ben­
Although there are numerous pathways by which these eficial as a marker to identify inflammation associated
conditions may occur, such as external injury, the most with sepsis. 31
common is believed to be hematogenous spread from a If joint involvement is suspected, diagnosis is con­
primary source. Such sources include the urachus or firmed through arthrocentesis, joint fluid analysis, and
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urachal remnants, gastrointestinal tract, respiratory tract, culture and sensitivity. Unfortunately, culture results are
or a distant site in the musculoskeletal system. 22,36 It is often negative because of the bacteriostatic properties of
commonly thought that there is a propensity for bacterial synovial fluid and previously administered antimicrobi­
colonization in the extensive vascular plexus composed als. 4,36 Fluid analysis typically reveals more than 20,000
of venous sinusoids, metaphyseal loops, and epiphyseal, white blood cells and an elevation in total protein.
metaphyseal, and physeal vessels in growing long bones. Diagnosis may be more challenging without articular
The slow and sluggish blood flow and relative vascular involvement; however, ultrasound evaluation coupled
stasis in this region allow bacteria to proliferate at or with aspiration and analysis of any fluid pockets may
close to the cartilage interface. 6,15 Cartilage matrix degra­ facilitate accurate diagnosis. Radiographs are helpful
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dation and proteoglycan loss may occur within 2 days, in establishing a diagnosis, prognosis, and treatment
and loss of collagen within 9 days after inoculation by plan. Initial radiographs are often normal but begin to
bacteria. 4,6,35 The chance of infection is increased in high‐ show areas of lysis within a few days of onset. Aspiration
risk individuals such as a foal with failure of passive of a lytic area may be performed with a needle; however,
transfer or exposure of a naïve immune system to an if this technique is not productive, a sample may be
aggressive pathogen. acquired via passage of a drill into the lesion and shav­
Hematogenous osteomyelitis and septic arthritis ings from the drill may be cultured. 22,32 This portal may
have been classified into five types based on the struc­ also be used for therapy by instillation of antimicrobial
ture involved and location. 15,46 Infectious synovitis, or agents through a needle or through a cannulated bone
S‐type, most commonly affects younger (under 2 weeks) screw placed in the hole.
foals and involves only the synovial structure. Treatment of septic arthritis includes systemic broad‐
Epiphyseal type (E‐type) involves the joint and adjacent spectrum antimicrobials while waiting on culture results.
epiphysis. Involvement of subchondral bone results in Pain control, protection against gastric ulceration, and
eventual radiographic changes, although these may not overall support for the foal regarding hydration and
be evident for a week or more. P‐type involves the phy­ nutritional intake are important, especially in young
sis and is seen in foals 1 week to 4 months of age. foals. In cases with joint involvement, local treatment by
Lesions are seen radiographically as lytic changes in the joint lavage using sterile polyionic fluids is important;
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physis, metaphysis, or epiphysis. T‐type involves the alternatively, constant rate infusion therapy with antimi­
tarsal or carpal cuboidal bones and may result in col­ crobials has recently proven beneficial. 1,28 Joint lavage is
lapse of these structures. These most commonly occur not necessary if the lesion is isolated to bone. Regional
in neonates that suffer from dysmaturity, although nor­ perfusion has proven beneficial, as has intraosseous
mal foals may be affected. I‐type involves periarticular antimicrobial administration. 22
soft tissue abscess infecting a physis or joint and often Anecdotally, the use of chondroprotective agents par­
involves the hip or stifle, or more commonly the distal enterally, orally, and intra‐articularly is believed to be
metacarpus or metatarsus. Foals older than 6–8 weeks beneficial in restoring and maintaining joint health,
are most often involved. The clinical appearance of the especially in systemically ill foals. Hyperbaric therapy
involved region is diffuse pitting edema, with eventual for treatment of osteomyelitis has likewise gained favor
abscess development and drainage. for the benefits of revascularization of ischemic tissue
The common presenting complaint of osteomyelitis and increasing the effectiveness of antimicrobial ther­
and/or septic arthritis is acute onset of lameness, and apy. In debilitated or cachectic foals, the common clin­
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there is often a pyrexic episode at presentation or in the ical impression is that immediately following hyperbaric
recent past. The degree of lameness varies from mild to treatment, the animal is usually brighter and has an
non‐weight‐bearing and largely depends on the severity improved appetite.
of the infection and structures involved. The foal may be With early diagnosis and appropriate treatment, the
noted in recumbent positions more often than normal. prognosis for the survival of foals with septic osteomyeli­
With joint involvement there is synovial distension as tis is favorable, and a return to function in Thoroughbred
opposed to the diffuse swelling typically seen with osteo­ racehorses reported between 37% and 48.3% 22,36,42,43 ;