Encephalomyocarditis Virus Infection: Introduction

Encephalomyocarditis (EMC) is a specific viral infection of swine and exotic mammals that is caused by members of the genus Cardiovirus in the family Picornaviridae. It has been recognized in many parts of the world, and molecular studies have revealed a genetic homogeneity of viral isolates from some geographic areas but not others.

Swine may die acutely at any age due to associated myocardial failure or may be affected with near-term abortions, fetal mummification, and apparent reproductive failure. A variety of exotic mammals have been fatally afflicted with EMC in zoologic parks in the USA, Australia, and other parts of the world, and have included African elephants, rhinoceroses, hippopotamuses, sloths, llamas, various antelope species, and many types of nonhuman primates (chimpanzees, orangutans, baboons, cynomologous monkeys, lemurs, etc). An episode of lion deaths at a zoo in the USA was associated with the feeding of the carcass of an African elephant that had died of EMC, and a spontaneous outbreak of fatal EMC was reported in free-ranging African elephants at Kruger National Park in South Africa in 1995.

Serologic studies have revealed that subclinical infections are common in EMC outbreaks. Humans appear to be resistant to fatal infections with EMC viruses; although EMC antibodies are frequently detected, no deaths have been confirmed.

Epidemiology:

Cardioviruses are small, nonenveloped viruses that are almost always associated with rodents, and disease in other mammalian species has often been attributed to spillover from populations of mice and rats. These, and presumably other rodent species, shed the viruses in feces and urine, which may contaminate food and water supplies of large mammals. Ingestion of rodents dead or dying of EMC may be another means of infection. Pigs shed virus in nasal secretions and feces during the first 3 days of experimental infection. During this short period, the virus may be transmitted to other pigs by contact. Cardioviruses are resistant to adverse environmental influences and may remain infective for weeks to months under favorable conditions.

Clinical Findings and Lesions:

The disease is named for its predilection for the CNS and cardiovascular systems of experimental mice, and both encephalotropic and cardiotropic strains have been defined. In swine and zoologic species, however, acute and subacute deaths are almost always attributed to the destructive effects of the virus on the myocardium, with resultant cardiac insufficiency, pulmonary edema, and frothy transudation in the respiratory tract. Affected animals often appear to have asphyxiated in their own respiratory fluids. Other clinical signs may include fever, anorexia, listlessness, trembling, staggering, dyspnea, and paralysis. Mortality approaching 100% has been described in suckling swine but becomes successively lower in older age groups. Strains of EMC viruses that target the pancreas and are diabetogenic in experimental mice have been recovered, but the significance of this finding for other mammals has not been established.

EMC viruses are known to cross the placenta in swine and have been recovered from conceptuses in cases of reproductive failure due to near-term abortions (107-111 days of gestation), stillbirths, and mummifications. Reproductive problems often persist in affected herds for 2-3 mo and may affect sows of all parities.

Diagnosis:

Because the pale necrotic heart muscle lesions that may be observed in fatal EMC infections also are seen in septic infarction or vitamin E/selenium deficiency, a definitive diagnosis requires virus recovery and identification. Myocardial tissue and spleen collected from acutely dead animals or abortuses are the specimens of choice for virus isolation. Because EMC viruses are very stable, they may be recovered from frozen tissues.

Serologic diagnosis via virus neutralization, hemagglutination-inhibition, or ELISA is possible if acute and convalescent sera are collected, but the frequency of subclinical EMC infections makes single serum determinations of little value in aborting sows. Detection of antibody against EMC viruses in stillborn or large mummified fetuses is significant for fetal infection, however, because maternal immunoglobulins are not transferred across the placenta in swine.

Treatment and Control:

There is no specific treatment for EMC, but mortality may be minimized by avoiding stress or excitement in animals at risk. EMC viruses appear to cycle in rodents and are most likely to affect swine and zoo animals when rodent populations are high. Rodent control is thus important for minimizing exposure of susceptible species. Prompt and proper disposal of animals that have died of the disease is also recommended. EMC viruses are inactivated by the judicious use of many disinfectants labeled for livestock use.

Killed vaccines for the prevention of myocarditis in weaned swine are available. Pigs are inoculated IM at ≥4 wk of age, revaccinated 2-3 wk later, and semiannually thereafter. These vaccines have been used extra-label in susceptible exotic animal species in zoos and amusement parks where EMC has been problematic.