Background and history

The main reservoir host for the EMC virus is the rat, although mice may also spread it. It infects and causes disease in a wide range of vertebrate animals but pigs appear to be the most susceptible of farm animals.

One strain, type A, causes reproductive problems; a second strain, type B, causes heart failure; and other strains are mild or non-pathogenic.

The virus is world-wide but differs in pathogenicity and virulence in different countries and regions. In most countries of Europe it tends to be relatively mild or non-pathogenic and disease in pigs is rarely diagnosed. In Australia the strains appear to be much more virulent for pigs than those in New Zealand. Virulent strains in Florida, the Caribbean and Central America damage the heart and cause death, whereas those in the Mid-West of the US tend to cause reproductive problems.

Clinical disease in pigs tends to occur when rat numbers increase to plague levels. The disease does not seem to spread very readily between pigs.

In affected herds there are usually no clinical signs in weaned and growing pigs.

EMCV could be confused with Aujeszky’s disease (AD), Porcine parvovirus infection (PPV) and Porcine reproductive and respiratory syndrome (PRRS) although there are distinguishing signs between these four. EMCV would be the last on the list of diagnostic priorities in Europe but would be more concerning in the Mid-West, USA.

Clinical signs

Sows

In gilts and sows first signs are often:

• A few abortions near the end of pregnancy.

Then over a period of about 3 months:

• The numbers of mummified foetuses and stillbirths increase and pre-weaning mortality rises.
• The farrowing rate worsens.
• Affected females may show signs of fever and lack of appetite.
• Embryo death.

Piglets

• Poor viability.
• Usually none.

In affected herds there are usually no clinical signs in weaned and growing pigs.

Diagnosis

To make a definitive diagnosis the virus has to be isolated and identified or rising antibodies demonstrated in blood samples taken two weeks apart.

EMCV should be distinguished from AD, PPV and PRRS.

Causes

• Pigs can be infected from rats or from rat-contaminated feed, water or bedding.
• Incoming breeding stock with pathogenic strain.

Prevention

• Check the source of incoming breeding stock for pathogenic strains.
• General hygiene.
• Control of vermin.
• Reports of killed vaccines being effective have been documented.

Treatment

There are no methods of treatment.