Background and history

This is an important disease of pigs caused by a herpes virus. The virus can remain hidden in nerves of the pig in a carrier state for long periods of time and then be reactivated. Once introduced into a herd the virus usually remains there and it can continually affect reproductive performance at varying levels. The virus can survive for up to three weeks outside the pig. Acute outbreaks of disease occur when virulent strains of the virus first infect an unvaccinated susceptible herd. The virus crosses the uterus and placenta and infects the foetuses.

The pig is the main host. Dogs and cattle may become infected, show nervous signs and die.

Similar diseases

When disease is first introduced into a susceptible breeding herd there are few other diseases except possibly swine fever that would be confused with AD. Once the disease has become chronic it could be confused with PRRS and chronic swine fever. Laboratory tests would be required to differentiate them.

Clinical signs

Sows

• Coughing.
• Fever
• Nervous signs
• Reproductive failure.
• Abortions.
• Mummified piglets.
• Stillbirths.
• Birth weak litters.

Piglets

• Nervous signs.
• Uncoordination.
• Sneezing.
• Coughing.
• High mortality.
• Low/ poor viable piglets.

Weaners and growers

• Fever.
• Sneezing.
• Coughing.
• Pneumonia.
• Nervous signs including uncoordination, fits and meningitis.
• Some strains of the virus can cause severe respiratory disease and others severe rhinitis.
• Usually low mortality.

All other species

• Nervous signs.
• Death.

Acute disease

Acute outbreaks of disease occur when virulent strains of the virus first infect an un-vaccinated susceptible herd. The virus crosses the uterus and placenta and infects the foetuses. Often the first clinical signs are abortions, stillbirths and the birth of weak litters which soon die. Abortions may rise to 5 percent over about 6 weeks followed by reproductive failure at all stages of the cycle. Embryos are killed and absorbed and sows return to heat.

These reproductive problems may occur in up to 20 percent of dry sows. However these are not the only clinical signs seen in the herd. Dogs and cattle may become infected, show nervous signs and die.

Chronic disease

In an un-vaccinated herd, when the early acute phase of the disease is over and the herd has developed an immunity clinical signs are sporadic and milder. The signs may also be difficult to associate with an infertility problem because of their insidious nature. Depression of reproductive efficiency across all parameters is a feature of the chronic infection with increased levels of repeats, mummification, stillbirths and piglet mortality. Young carrier females that are stressed shed virus thus maintaining infection throughout the herd. Spread of infection in the breeding herd is low with immunity and infection waning and rising over one to two year cycles.

Carrier state

After the acute phase clinical signs of disease may die out altogether and this is often seen in small herds of less than 100 sows. Sometimes the virus itself may disappear. However it is more likely to persist in a few animals sporadically.

Diagnosis

When a susceptible breeding herd first breaks down with this disease the clinical signs described above strongly suggest Aujeszky's disease and are almost diagnostic. Laboratory tests are required to confirm the diagnosis. The common ones are as follows:

• Fluorescent antibody tests on dead piglet tissues particularly tonsils. This is reliable and results are available in few hours.
• Virus isolation from the lung and tonsils and its identification. This test is slow taking several days but may be done for added confirmation.
• Blood tests (serology) to demonstrate rising antibodies take too long to be useful.

Causes

• Movement of carrier pigs.
• Airborne - at least 3km (2 miles).
• Infection from feral (wild) pigs.
• The role of mechanical spread by birds is questionable.
• From contaminated carcasses.
• Mechanically on people.
• From contaminated vehicles.
• Through infected semen via AI or a carrier boar.
• From infected slurry.

Prevention

• AD (PR) is a disease you cannot afford to live with.
• Gilts and boars should only be purchased from known free herds and be vaccinated before arrival or in isolation.
• Keep it out of the herd by isolating all purchased breeding stock and blood sampling them before they enter the herd.
• Only buy from AD free herds.
• If your herd is at risk, in other words within a 3km radius of large infected herds then vaccinate it to prevent disease.
• Vaccination helps to prevent the establishment of the virus.
• If you have AD, adopt strategies for eliminating disease as discussed in chapter 12.

Treatment

• There is no treatment available but antibiotic medication to control secondary bacteria in a new outbreak could be considered such as:
  • 600-800g of CTC or OTC in the breeding ration for 3-4 weeks as advised by the veterinarian.
• Vaccination. This is the key action to take. As soon as disease is identified all breeding stock should be vaccinated with a gene deleted vaccine to mitigate the effects and reduce spread of the virus.