Encephalomyocarditis virus infection of pigs. 1. An outbreak in New South Wales.

Substantial pig mortalities due to heart disease caused by EMC virus infection are described. The majority occurred in 1970 in conjunction with a rodent plague, but retrospective diagnosis of occasional similar mortalities in previous years was possible. Mortality rates in pigs aged 3 to 16 weeks ranged from a few percent to over 50% and the rate was generally highest in younger animals. It is likely that a considerable proportion of cases was sub-clinical. Focal or diffuse pale areas occurred in the myocardium, the wall of the conus arteriosus being most severely affected. In a few animals, resolving lesions were present. Encephalomyocarditis virus was consistently recovered from pigs with acute heart lesions, but not from those in which myocardial fibrosis had commenced. The virus was demonstrated in rodents, although the dependence of pig disease on rodent-derived virus could not be established. Because of similarities between EMC and FMD myocarditides in pigs, specific diagnosis by virus isolation and identification should be obtained whenever possible.