Experimental Viral Myocarditis: Parvoviral Infection of Neonatal Pups

Myocarditis was produced in seronegative five-day-old pups by oral and by intraperitoneal inoculation of canine parvovirus. The disease was subclinical. Histologic lesions were compatible with, but less extensive than, those seen in naturally occumng canine parvoviral myocarditis. In pups necropsied 23 days after inoculation, scattered cardiac myocytes contained intranuclear inclusion bodies, and virus-infected myocytes were demonstrated by immunofluorescence. Degeneration and loss of cardiac myocytes usually was not associated with a cellular infiltrate. At 51 days after inoculation, the myocardium contained an extensive lymphocytic infiltrate which was sometimes associated with fragmented myocytes, and was often contiguous with areas of interstitial fibrosis. At 108 days after inoculation, inflammatory lesions had regressed, and there were multifocal areas of myocardial fibrosis. The appearance in the late 1970's of a severe canine enteric disease caused by canine parvovirus was tem- porally associated with a syndrome of sudden death of 2- to 16-week-old puppies in which the major changes were cardiac rather than enteric.'-''* Mortality var- ied within litters from 20 to 100%2',29 with no apparent hereditary, breed, or sex predisposition. The most com- mon clinical presentation was sudden death of pups between three and eight weeks of age without premon- itory signs.2.8-10 Pups older than eight weeks of age usually exhibited a sudden onset of severe dyspnea followed by collapse and death within 24 ho~rs.~'.~~ The most consistent pathologic finding in affected pups was lymphocytic interstitial myocardi- tis.2*6*8. 10*2s329 There were scattered intranuclear inclu- sions in cardiac myocytes which were shown by electron microscopy to contain spherical particles consistent in appearance with a parvoviru~.~~'~~~~~~~'.~~ I nclusion bod- ies were found with greatest frequency in younger pups, and sometimes were unassociated with an inflamma- tory i11filtrate.6.~~ In subacute cases, necrotic muscle fibers were interspersed among a mixed inflammatory infiltrate consisting of neutrophils and lymphocytes.2*25 In older pups intranuclear inclusion bodies usually were absent, but single or multiple groups of muscle fibers were replaced by fibrous connective tissue at different stages of mat~rity.~.~~ Initial attempts to reproduce the disease were unsuc- cessful.8.26 However, virus isolated from affected myo- cardium caused mild enteritis' or severe enteritis and death when inoculated intravenously into four- and seven-week-old pups.27 A few myocardial cells con- tained intranuclear inclusion bodies in some pups. These results suggested that both the myocardial and enteric disease were caused by the same virus. The disease was successfully reproduced by the in utero inoculation of puppies eight days before parturi- tion.16 The pups were clinically normal at birth, but two of four died during the third week of life. Both had acute lymphocytic myocarditis and the virus was reiso- lated from the heart of each pup. Two other clinically normal littermates which were killed painlessly 87 and 13 1 days after inoculation had multifocal interstitial fibrosis accompanied by a mild lymphocytic infiltrate. Others7 reported finding lesions of myocarditis in clin- ically normal littermates of pups which died of the disease, and the sera of surviving littermates contained antibody to canine parvovirus.2' It is apparent that the clinical manifestations of canine parvoviral myocarditis are highly variable. In a preliminary experiment, subcutaneous inocula- tion of a bitch at 45 days gestation with virulent canine parvovirus resulted in the transplacental transfer of virus-specific immunoglobulin G, but evidence of fetal viral infection was not found. Antibody titers were augmented by colostral transfer and had waned by 12 weeks of age (Meunier, unpublished observation). This report describes the results of an experiment in which