Wall-July PV-Review - VetLearn.com

616 Small Animal/Exotics Compendium August 2002
can obliterate the infection. 8
Chronic, low-grade microbemia predisposes dogs to
IE. 7,13,14 Infective microorganisms in the blood must be
capable of adhering to a platelet-fibrin matrix. 7,8,15
Within this matrix, not only are white blood cells
unable to phagocytize bacteria, but the bacteria are very
concentrated. 7,8,15 Very few granulocytes are within the
vegetation; and layers of fibrin, platelets, and RBCs
protect the microorganisms. 7,8
VALVE INVOLVEMENT
Heart valves are subject to constant trauma as they
open and close. 7,8 The incidence of IE appears to be
directly related to the force placed upon each valve. 16,17
The mitral and aortic valves are most commonly
infected, and the tricuspid and pulmonic valves are seldom
involved. 2,4,6,9–16,18,19 In a study of 45 dogs, 22 had
aortic involvement, 16 had mitral, 1 had both aortic
and mitral, and only 1 had tricuspid involvement. 6
During the past 10 years, our experience is that approximately
90% of infections involve the aortic valve.
EPIZOOTIOLOGY
The prevalence of IE is much lower than that of
dilated cardiomyopathy and myxomatous mitral valve
degeneration. The incidence rates at two referral institutions
were both approximately five cases per year. 6,10
During the last 10 years, two to five dogs with IE have
been recognized annually at the University of Georgia.
Typically, affected dogs are 4 to 8 years of age and
medium to large sized (less than 10% weigh less than
10 kg). 6,10 Affected dogs are seldom younger than 2 or
older than 10 years of age. German shepherds, boxers,
golden retrievers, Labrador retrievers, and rottweilers
may be overrepresented. 1–4,6,9–11 Males are affected more
often; a 2:1 ratio, possibly due to chronic bacterial prostatitis,
has been reported. 6
PREDISPOSING FACTORS
There are numerous factors predisposing dogs to the
development of IE. A common misconception is that IE
is associated with structural heart disease. It is interesting
that dogs with the highest incidence of advanced
myxomatous mitral valve degeneration (older, smallbreed
dogs) have a very low incidence of IE. Furthermore,
IE is seldom associated with congenital heart diseases
in dogs, with the exception of subvalvular aortic
stenosis (SAS). 1–4,6,19 With SAS, turbulent, high-velocity
blood flow in the left heart may produce microtrauma
on the valve leaflets and facilitate microbe colonization.
9,10,19,20 Boxers, rottweilers, and golden retrievers
have relatively high incidences of both IE and
SAS. 2,3,6,19,20 Less than 10% of dogs with SAS develop IE,
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and possibly less than 10% of dogs with IE have
SAS. 2,3,6,19,20 Establishing or eliminating an ultrasonographic
diagnosis of SAS with aortic valve IE can be difficult
because the vegetation may obscure the SAS lesion.
Predisposing factors for platelet–fibrin–RBC deposition
on the endocardium include procedures involving
the gingiva, oropharynx, gastrointestinal tract, or
infected tissues and intravenous (IV) catheters. 6,8,9,12,21–23
Although gingivitis/stomatitis may be associated with
bacteremia and IE, dogs with the highest incidence of
this condition (older, small-breed dogs) seldom develop
IE. 2,3,6 Hypercoagulability associated with malignancies,
disseminated intravascular coagulation, hypercortisolemia,
or immune-mediated diseases may predispose
dogs to IE. 7,8,24 Any abnormality that either is associated
with chronic bacteremia (e.g., gingivitis, urinary or prostatic
infection, pyoderma, wounds) or impairs host
defenses (e.g., neoplasia, diabetes mellitus, hyperadrenocorticism,
exogenous corticosteroids or other
immunosuppressive therapies) increases the risk for
IE. 5,8,12,13 However, dogs most prone to hyperadrenocorticism
have a low incidence of IE, and we have rarely
identified the two conditions in the same dog. The
source of infection may be suspected but is proven in
less than 50% of cases. 6,12,13
Damaged endothelium exposes the extracellular polysaccharide
dextran, which facilitates the attachment of
some bacterial species. 25,26 Because of the limited species
of microbes that account for the vast majority of IE
cases, it can be deduced that certain organisms (e.g.,
coagulase-positive staphylococci and some streptococci)
have an increased ability to colonize the endocardium.
2,3,6–8,15 Fibronectin can be produced by
endothelial cells and platelets in response to vascular
injury, and receptors for fibronectin are present on the
surface of some bacteria, including coagulase-positive
staphylococci and some streptococci. 26 Fibronectin may
even facilitate the adherence of some bacteria, including
staphylococci, to intact endothelium. 15,26
Corticosteroids
Dogs affected with IE often have a history of receiving
corticosteroids. 6 In some instances, corticosteroids
were administered in the weeks preceding the development
of IE to treat an illness associated with unrecognized
bacteremia, skin disease, or presumed immunemediated
disease. Survival rates of dogs with mitral
valve IE that received corticosteroids were lower than
those that did not receive them. 6
Infective Microbes
The vast majority of IE cases are the result of bacteria;
rickettsial agents are occasionally identified, but fungal