Myocarditis

Chapter 48 Myocarditis



Meg Sleeper, VMD, DACVIM (Cardiology)



KEY POINTS



• Myocarditis is an inflammatory process involving the heart. Inflammation may involve the myocytes, interstitium, or vascular tree.

• Myocarditis has been associated with a wide variety of diseases. Infectious agents (viral, bacterial, protozoal) may cause myocardial damage by myocardial invasion, production of myocardial toxins, or activation of immune-mediated disease.

• Myocarditis can also be associated with physical agents (doxorubicin), underlying metabolic disorders (uremia), toxins (heavy metals), or physical agents (heat stroke).


INTRODUCTION


Myocarditis is a rare cause of heart failure in dogs and extremely rare in cats. Clinical pictures vary, including asymptomatic patients who may have electrocardiographic abnormalities and patients with or without heart enlargement, systolic dysfunction, or even full-blown congestive heart failure. The patient’s history (i.e., environment and exposure) is often critical in determining likely risk and suggesting appropriate diagnostic tests. Clinical reports of canine myocarditis are most common in immunocompromised or immunonaïve patients.



INFECTIOUS MYOCARDITIS



Viral Myocarditis


Numerous viruses have been associated with myocarditis in humans. In dogs, viral myocarditis appears most commonly in immunonaïve patients, and the virus most commonly associated with the disease is parvovirus. However, at this time the entity appears to be very rare. In the late 1970s and early 1980s, when the parvovirus pandemic first was recognized, puppies did not receive maternal antibodies and very young puppies developed a fulminant infection with acute death due to pulmonary edema when exposed to the virus. Older puppies (2 to 4 months) often died subacutely from congestive heart failure, but others developed a milder myocarditis and later developed dilated cardiomyopathy (DCM), usually as young adults. Basophilic intranuclear inclusion bodies are found in the myocardium of acutely affected younger puppies, but may be absent in older puppies.1 Older dogs typically have gross myocardial scarring. Rare cases of parvovirus-induced myocarditis have been reported since the early to mid-1980s.


Rarely, other viruses have been associated with myocarditis in dogs. In 2001, Maxson and others evaluated myocardial tissue from 18 dogs with an antemortem diagnosis of DCM and 9 dogs with a histopathologic diagnosis of myocarditis based on a polymerase chain reaction analysis to screen for canine parvovirus, adenovirus types 1 and 2, and herpesvirus. Canine adenovirus type 1 was amplified from myocardium of only one dog with DCM and none of the dogs with myocarditis, suggesting these pathogens are not commonly associated with DCM or active myocarditis in the dog.2 Distemper virus–associated cardiomyopathy with a mild inflammatory infiltrate has been produced by experimental infection of immunonaive puppies.3 Natural infection with West Nile virus was associated with myocarditis in a wolf and a dog in 2002, the third season of the West Nile virus epidemic in the United States.4 Viral genomic deoxyribonucleic acid has also been identified in feline myocardial tissue from patients with hypertrophic cardiomyopathy, DCM, and restrictive cardiomyopathy, suggesting that viral myocarditis may be a factor in these feline-acquired diseases.1



Protozoal Myocarditis



Chagas Disease


Chagas disease is caused by Trypanosoma cruzi, a protozoal parasite. Chagas disease is the leading cause of DCM in humans of Latin America, but it is rare in North America. In North American dogs, Chagas disease occurs most commonly in Texas and Louisiana. There have been no reported feline cases in North America. The organism is transmitted by an insect vector (Reduviidae), and reservoir hosts include rodents, raccoons, opossums, dogs, cats, and humans. The trypomastigote is the infective stage, but on entering host cells the organism enters the reproductive stage and becomes an amastigote. Amastigotes multiply until the host cell ruptures.1,8


Dogs with clinical Chagas disease have an acute or a chronic syndrome. In the acute stage, circulating trypomastigotes may be seen in a thick blood smear, and most dogs are brought for treatment because of sudden development of signs of right-sided heart failure (ascites, tachycardia, lethargy). Dogs with chronic Chagas disease may enter a quiescent stage free of clinical signs for months or even years. Nervous system damage often causes ataxia and weakness in these patients.1,8



Bacterial and Other Causes of Myocarditis


Bacterial myocarditis is possible whenever bacteremia or sepsis is present, with the most common agents being staphylococcal and streptococcal species.1 Myocarditis associated with Citrobacter koseri, an opportunistic pathogen of immunosuppressed human patients, has been described in two 12-week-old sibling Boxer puppies.5 Tyzzer disease (infection with Bacillus piriformis) was associated with severe necrotizing myocarditis in a wolf-dog hybrid puppy.6


Myocarditis has also been recognized secondary to rickettsial organisms such as Rickettsia rickettsii, Ehrlichia canis, and various Bartonella species.1 Myocarditis has been noted in 2 of 12 dogs diagnosed with endocarditis, 11 of which were seroreactive to Bartonella vinsonii subspecies.7 Lyme disease (secondary to infection by the spirochete Borrelia burgdorferi) has been implicated as a cause of myocarditis in dogs, but documented cases are rare. Clinical signs are often vague and nonspecific, and serologic testing is not a reliable method to determine active infection.1 In humans, Lyme myocarditis may be due to direct toxic effects or immune-mediated mechanisms, and the disease is usually self-limiting.11 Fungal infections of the myocardium are extremely rare but have occurred in immunocompromised patients.1


A group of cats was described with transient fever and depression that appeared to be infectious in nature. Postmortem examination revealed microscopic lesions consistent with myonecrosis and an inflammatory cell infiltrate. A viral etiology was suspected, but no organism was identified.8 In a retrospective study reviewing 1472 feline necropsies over a 7-year period, 37 cases were diagnosed with endomyocarditis. The cats with endomyocarditis had a mean age at death of 3.4 years, and 62% of them had a history of a stressful event 5 to 10 days before being brought for treatment. Interstitial pneumonia was present in 77% of the cats at postmortem examination. Special stains for bacteria and fungi were negative.9


Parasitic agents can also lead to myocarditis. Toxoplasma gondii bradyzoites can encyst in the myocardium, resulting in chronic infection. Eventually the cysts rupture, leading to myocardial necrosis and hypersensitivity reactions.1 Neospora caninum can infect multiple tissues, including the heart, peripheral muscles, and central nervous system. Clinical signs associated with noncardiac tissues typically predominate; however, collapse and sudden death has been reported in affected dogs.1 Infestation with Trichinella spiralis is a common cause of mild myocarditis in humans.11 The parasite has been associated with at least one case of canine myocarditis complicated by arrhythmias (Figure 48-1).12


image

Figure 48-1 Electrocardiogram from a mixed-breed dog with trichinosis involving the heart. The dog was brought in for collapse due to complex arrhythmias. Note the ventricular escape beats. An underlying supraventricular tachycardia is likely as well.

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  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Myocarditis

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