Parvovirus Infection Is Associated With Myocarditis and Myocardial Fibrosis in Young Dogs

Abstract

Perinatal parvoviral infection causes necrotizing myocarditis in puppies, which results in acute high mortality or progressive cardiac injury. While widespread vaccination has dramatically curtailed the epidemic of canine parvoviral myocarditis, we hypothesized that canine parvovirus 2 (CPV-2) myocardial infection is an underrecognized cause of myocarditis, cardiac damage, and/or repair by fibrosis in young dogs. In this retrospective study, DNA was extracted from formalin-fixed, paraffin-embedded tissues from 40 cases and 41 control dogs under 2 years of age from 2007 to 2015. Cases had a diagnosis of myocardial necrosis, inflammation, or fibrosis, while age-matched controls lacked myocardial lesions. Conventional polymerase chain reaction (PCR) and sequencing targeting the VP1 to VP2 region detected CPV-2 in 12 of 40 cases (30%; 95% confidence interval [CI], 18%-45%) and 2 of 41 controls (5%; 95% CI, 0.1%-16%). Detection of CPV-2 DNA in the myocardium was significantly associated with myocardial lesions ( P = .003). Reverse transcription quantitative PCR amplifying VP2 identified viral messenger RNA in 12 of 12 PCR-positive cases and 2 of 2 controls. PCR results were confirmed by in situ hybridization, which identified parvoviral DNA in cardiomyocytes and occasionally macrophages of juvenile and young adult dogs (median age 61 days). Myocardial CPV-2 was identified in juveniles with minimal myocarditis and CPV-2 enteritis, which may indicate a longer window of cardiac susceptibility to myocarditis than previously reported. CPV-2 was also detected in dogs with severe myocardial fibrosis with in situ hybridization signal localized to cardiomyocytes, suggesting prior myocardial damage by CPV-2. Despite the frequency of vaccination, these findings suggest that CPV-2 remains an important cause of myocardial damage in dogs.

Keywords: canine parvovirus; cardiomyopathy; dogs; heart; in situ hybridization; myocardial fibrosis; myocarditis.

Figures 1–4.. Heart, dog.

Figure 1. Myocarditis, dog No. 24. A cardiomyocyte with a large intranuclear inclusion body (arrow) characteristic of canine parvovirus 2 (CPV-2) myocarditis. Hematoxylin and eosin. Figure 2. Myocarditis, dog No. 24. Labeling of cardiomyocyte nuclei and cytoplasm for parvoviral nucleic acid (brown). In situ hybridization (ISH). Figure 3. Myocardial fibrosis, dog No. 38. Marked left ventricular fibrosis replaces and expands the tissue between remaining cardiomyocytes. Masson’s trichrome. Figure 4. Myocardial fibrosis, dog No. 38. Parvoviral nucleic acid (granular brown staining) within cardiomyocyte nuclei (arrow). ISH.

Figures 5–7.. Heart, dog No. 36.

Figure 5. Myocarditis. There are multifocal to coalescing infiltrates of lymphocytes and plasma cells separating cardiomyocytes. Hematoxylin and eosin. Figures 6 and 7. Serial sections comparing labeling of myocardium for parvoviral antigen by immunohistochemistry (Fig. 6, arrows) to more extensive and prominent labeling for parvoviral nucleic acid by in situ hybridization (Fig. 7, arrow).