A kennel consisting of 11 inten-elated Newfoundlands and 1 Rhodesian Ridgeback was evaluated for chronic weight loss and increased appetite of 6 months duration. Physical examination of a representative dog revealed underconditioning and an arrhythmia. Basic bloodwork, urinalysis, and fecal analysis were unremarkable. A 6-lead ECG was diagnostic for atrial fibrillation; 2 kennel mates subsequently screened normal. Analysis of dog food revealed that they were receiving a balanced diet. Treatment with fenbendazole resolved weight loss, suggesting parasitism as the primary cause. Amino acid analyses of 3 most affected kennel mates revealed cysteine deficiencies, due to suspected cystinuria. Subsequent taurine and carnitine deficits may be a cause of the detected arrhythmia. Newfoundlands are predisposed to cystinuria, an inherited disease that prevents reabsorption of cystine. This case study suggests that this condition can exacerbate clinical signs of parasitism and lead to nutritional cardiomyopathies.