Etiology

The cause(s) of dilated cardiomyopathy (DCM) in dogs is (are) unknown. Some of the proposed causes of DCM include: genetic defect(s), viral infection, microvascular spasm, chemical toxin(s), dietary deficiency, and immune-mediated processes. There appears to be a familial predisposition to the development of DCM in some breeds of dogs, and many investigators suspect a heritable defect in the metabolic processes of myocardial cells. It is quite possible that DCM is not a single disease, and that there are many etiologies. Taurine deficiency has been convincingly shown to be a reversible cause of DCM in cats and is also a suspected cause of DCM in foxes, but is not an important cause of DCM in dogs-except in Cocker spaniels. A number of chemical toxins (anthracycline antibiotics, gossypol, monensin) have been shown to cause myocardial failure. There is evidence that Adriamycin exerts at least some of its toxic myocardial effects by inducing histamine and catecholamine-mediated microvascular spasm.

One of the most frustrating aspects of attempts to identify the etiology behind DCM is determining if changes in protein expression are primary or secondary in nature. Up-regulation and down-regulation of proteins responsible for cardiac contraction ( 1, 2, and a receptors), ventricular relaxation (SERCA2, phospholamban) and energy production (carnitine transport, creatine kinase) occur to equivalent degrees in volume overload, pressure overload, and cardiomyopathy. "In this respect the intracellular biochemical specificity of the response of the myocyte to a chronic insult appears to be relatively restricted. The foremost question remains, which, if any, are the true pathogenic alterations and which are cellular adaptations."

The exact prevalence of DCM is unknown, but it is believed to be the most common cause of cardiac disability in large and giant breed dogs. Although DCM has been identified with increasing frequency in medium size breeds such as English and American cocker spaniels, this disease remains primarily a disease of large and giant purebred dogs. According to the Purdue VMDB the prevalence rate of DCM was highest in Scottish deerhounds (6.0 percent), Doberman pinschers (5.8 percent), Irish wolfhounds (5.6 percent), Great Danes (3.9 percent), Boxer dogs (3.4 percent), Saint Bernards (2.6 percent), Afghan hounds (1.7 percent), Newfoundlands (1.3 percent), and Old English sheepdogs (0.9 percent). The prevalence of DCM was 0.69 percent in English cocker spaniels and 0.34 percent in American cocker spaniels. Dogs of both sexes and all ages may be affected but the disease is most commonly diagnosed in middle-aged male dogs. The prevalence of DCM increases with age (VMDB). However, because of the lower number of aged dogs examined, the majority of dogs presented for evaluation and treatment of DCM and heart failure are between 4 and 10 years old. The prevalence of DCM in male dogs (0.66 percent) is nearly twice that of female dogs (0.34 percent) according to the VMDB. An autosomal dominant mode of transmission has been reported in the Irish Wolfhound, Newfoundlands, and Doberman Pinschers. In the juvenile Portuguese Water Dog, an autosomal recessive transmission has been documented.

History and clinical signs

Affected dogs are usually presented with rapidly progressing clinical signs that the owner has been aware of for only a few weeks. Weakness and exercise intolerance are often the first signs noticed by the owner. The spectrum of clinical signs exhibited by dogs with DCM is similar in all breeds, but the observed frequency of these signs differs between the various breeds and, to some extent, with the lifestyle of the animal. Right sided heart failure manifested as abdominal distension, anorexia, weight loss, and fatigue often predominates in giant breeds, while signs of left heart failure or syncope are more common in Doberman pinschers and Boxer dogs. More variable signs include anorexia, weight loss, syncope and sudden death. These latter two events appear to be most common in Boxer dogs and Doberman pinschers.