Page 181 - Small Animal Internal Medicine, 6th Edition

CHAPTER 7 Myocardial Diseases of the Dog 153

sudden death, many affected dogs live a normal life span West Nile virus is uncommon in dogs but has been
with stable disease and minimal to no clinical signs. There reported to cause severe lymphocytic and neutrophilic myo-
VetBooks.ir also are reports of young dogs (most commonly terrier carditis and vasculitis, with areas of myocardial hemorrhage
and necrosis. Vague clinical signs can include lethargy, poor
breeds) in which dynamic LV outflow tract obstruction, sys-
tolic anterior motion of the mitral valve, and LV hypertrophy
histochemistry, reverse transcription polymerase chain reac-
are documented at a young age (less than 1 year), but echo- appetite, arrhythmias, neurologic signs, and fever. Immuno-
cardiographic changes spontaneously regress after the dog tion (RT-PCR), serology, and virus isolation have been used
reaches adulthood. Some of these dogs were treated with in diagnosis.
β-blockers (usually atenolol), yet others received no treat- Bacterial myocarditis
ment. It is unknown whether these cases represent a variant Bacteremia and bacterial endocarditis or pericarditis can
of congenital HCM in dogs or a transient physiologic process cause focal or multifocal suppurative myocardial inflamma-
associated with aging. tion or abscess formation. Localized infections elsewhere in
the body may be the source of the organisms. Clinical signs
MYOCARDITIS include malaise, weight loss, and, inconsistently, fever.
Arrhythmias and cardiac conduction abnormalities are
A wide variety of agents can affect the myocardium, although common, but murmurs are rare unless concurrent valvular
disease manifestations in other organ systems may over- endocarditis or another underlying cardiac defect is present.
shadow the cardiac involvement. The heart can be injured by Serial bacterial (or fungal) blood cultures, serology, or PCR
direct invasion of the infective agent, by toxins it elaborates, may allow identification of the organism. Organisms reported
or by the host’s immune response. Noninfective causes of to be implicated in bacterial myocarditis include Staphylo-
myocarditis include cardiotoxic drugs and drug hypersensi- coccus, Streprotococcus, Citrobacter, Bacillus, Moraxella, and
tivity reactions. Myocarditis can cause persistent cardiac others. Bartonella vinsonii subspecies have also been associ-
arrhythmias and progressively impair myocardial function. ated with cardiac arrhythmias, myocarditis, endocarditis,
and sudden death. Serology and PCR using an enriched
INFECTIVE MYOCARDITIS growth medium specific for Bartonella (Bartonella α Proteo-
bacteria Growth Medium, BAPGM) are used for diagnosis.
Etiology and Pathophysiology Lyme carditis
Viral myocarditis Lyme disease (infection with the spirochete Borrelia burg-
Lymphocytic myocarditis has been associated with acute dorferi) is often mentioned as a cause of myocarditis in dogs,
viral infections in experimental animals and people. Car- although definitive diagnosis is rarely proven; systemic man-
diotropic viruses can play an important role in the patho- ifestations related to immune-complex deposition usually
genesis of myocarditis and subsequent cardiomyopathy predominate (polyarthritis, glomerulonephritis, meningo-
in several species, but this is not recognized commonly encephalitis). The prevalence of this disease is higher in
in dogs. The host animal’s immune responses to viral and certain geographic areas, especially the northeastern, western
nonviral antigens contribute to myocardial inflammation coastal, and north central United States, as well as in Japan
and damage. and Europe, among other areas. Lyme disease was the most
A syndrome of parvoviral myocarditis was recognized in common cause of canine myocarditis in a recent case series
the late 1970s and early 1980s. It is characterized by a per- from Ireland. Borrelia is transmitted to dogs by ticks (espe-
acute necrotizing myocarditis and sudden death (with or cially Ixodes genus) and possibly other biting insects (see
without signs of acute respiratory distress) in apparently Chapter 69). High-grade AV block is the classic finding in
healthy puppies about 4 to 8 weeks old. Cardiac dilation with dogs with Lyme disease. Syncope, CHF, reduced myocardial
pale streaks in the myocardium, gross evidence of CHF, large contractility, and ventricular arrhythmias are also reported
basophilic intranuclear inclusion bodies, myocyte degenera- in affected dogs. Pathologic findings of Lyme myocarditis
tion, and focal mononuclear cell infiltrates are typical nec- include infiltrates of plasma cells, macrophages, neutrophils,
ropsy findings. This syndrome is uncommon now, probably and lymphocytes, with areas of myocardial necrosis. These
as a result of maternal antibody production in response to are similar to findings in human Lyme carditis. A presump-
virus exposure and vaccination. Parvovirus may cause a tive diagnosis is made on the basis of positive (or increasing)
form of DCM in young dogs that survive neonatal infection; serum titers or a positive SNAP test and concurrent signs of
viral genetic material has been identified in some canine myocarditis, with or without other systemic signs. Endo-
ventricular myocardial samples in the absence of classic myocardial biopsy with immunohistochemical staining can
intranuclear inclusion bodies. confirm the diagnosis. Resolution of AV conduction block
Canine distemper virus may cause myocarditis in young may or may not occur in dogs despite appropriate antimi-
puppies, but multisystemic signs usually predominate. His- crobial therapy, and temporary or permanent artificial
tologic changes in the myocardium are mild compared with pacing may be required.
those in the classic form of parvovirus myocarditis. Experi- Protozoal myocarditis
mental herpesvirus infection of pups during gestation also Trypanosoma cruzi, Toxoplasma gondii, Neosporum cani-
causes necrotizing myocarditis with intranuclear inclusion num, Babesia canis, Hepatozoon americanum, and Leishma-
bodies leading to fetal or perinatal death. nia spp. are known to affect the myocardium (see p. 1525).