Aujeszky's disease (AD)

Aujeszky’s disease or pseudorabies is caused by a herpes virus and affects mainly pigs which are the only known reservoir of the disease. It is an important disease of pigs causing severe economic losses. Once introduced into a herd the virus usually remains there and it continues to affect reproductive performance. It is sometimes transmitted naturally from pigs to individual cattle, horses, dogs and cats which develop nervous signs and rapidly die, hence the name pseudorabies.

About Aujeszky's disease in pigs

• Importance
• Etiology
• Clinical Signs
• Transmission
• Diagnosis
• Control and Eradication

Importance of Aujeszky's Disease (AD)
AD is an economically-damaging viral disease of pigs. It is prevalent all over the world. Only two countries, with intensive pig industries, have escaped A.D.: Canada and Australia.

Many countries (e.g. the UK, Norway, Denmark, N & S Ireland, the Netherlands, Canada, the USA, and Chile) adopt control policies, which may include compulsory vaccination or in the case of the UK and Denmark, slaughter and eradication policies.

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Etiology

• Herpes virus, Suid herpesvirus 1 (SHV)
• Double stranded DNA virus
• Only one major antigenic type of the virus occurs

Large parts of the Aujeszky's Disease Virus genome have been sequenced. Of importance are the regions which encode the TK enzyme (thymidine kinase) and the surface glycoproteins. These regions are important in determining virulence, the course of infection, development of antibodies after infection and diagnosis of infection.

In vaccine manufacture, marker vaccines are developed by deleting gE and Thymidine Kinase (TK). The gE deletion makes it possible to differentiate between vaccinated and field infected animals. TK deletion ensures a safe non virulent vaccine strain.

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Clinical signs
Clinical signs in pigs depend on age, virulence of the strain, virus dose and route of infection.

Acute infection in a susceptible herd
After infection with a virulent strain of A.D.V., clinical signs are typical as for any infectious organism: fever, anorexia and dullness.

Depending on the age of the pigs, specific signs occur determined by the virulence of the infective strain. The most dramatic signs occur in very young piglets.
Neurological signs are seen:

• trembling
• inco-ordinated movements
• sitting on stretched rear legs
• recumbency and paddling.

Morbidity and mortality in young piglets is very high and may reach 100%.

Growing pigs become increasingly less susceptible with age, mortality rate and incidence of neurological signs decrease.

Breeding Sows

• Increased returns to service (i.e. regular or irregular returns to oestrous) if infection occurs during the early stages of gestation
• At any stage of pregnancy abortions may occur
• Mummified foetuses, stillborn and weak piglets
• Small litters

Boars
Boars may develop severe orchitis, or fail to mate because they are running a high temperature. The high temperatures following infection can have an important longer term effect on the herd's reproductive performance by affecting spermatogenesis.

Chronic disease
As pigs get older secondary respiratory disease becomes increasingly prominent. Actinobacillus pleuropneumoniae and Pasteurella multocida are often the cause of secondary bacterial infection.

Morbidity in older growing pigs and adult pigs will be high. Susceptible animals will become infected easily but may not necessarily show clinical signs of disease. The mortality rate is low 1 to 2 %) but is dependent on the virulence of the strain involved.

Carrier state
Pigs that have recovered from ADV infection may become asymptomatic carriers. They are able to transmit the virus to susceptible pigs and may transmit virus to their offspring either in utero or after birth.

Figure 1: Summary of clinical signs

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Transmission of disease between herds

• Movement of carrier pigs
• Airborne - at least 3km
• Infection from feral (wild) pigs
• Birds may spread the virus, not yet proven
• Contaminated carcasses
• Mechanically on clothing, boots, vehicles and equipment
• Through infected semen via AI or a carrier boar
• From infected slurry

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Diagnosis
When a susceptible breeding herd first breaks down with this disease the clinical signs described above strongly suggest Aujesky's disease and are almost diagnostic.

Laboratory tests are required to confirm the diagnosis:

• Fluorescent antibody tests – tissue from tonsils in carcasses often used. This is reliable and results are available in few hours.
• Virus isolation from the lung and tonsils.
• ELISA tests for serum antibody detection are used widely on recovered pigs and in herd diagnosis. Tests are available to distinguish wild and vaccine type infections.

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Control and Eradication
lt is important to distinguish between AD control (suppression of clinical signs and preventing the losses in production) and eradication (elimination of AD from a farm, region, country or continent).

For disease control, vaccination is the first step.
AD control should not be based on vaccination alone. Management factors are very important:

• Good housing conditions with adequate climate control
• All-In All-Out management systems
• Good hygiene and disinfection of empty areas
• Good biosecurity to keep farms as a closed system, restricting contact with other farms
• Only buy from AD free herds

Eradication of AD
There are several ways disease can be eradicated. The classical way is a test and slaughter policy or depopulation followed by repopulation. This approach was used to eradicate AD from Great Britain and Denmark. Eradication based on this system should only be attempted in areas where disease prevalence is low (e.g. < 10%), otherwise it is economically unfeasible.

In all countries where disease prevalence is high AD eradication is viable with marker vaccines (See Vaccines). Marker vaccines allow the distinction between naturally infected and vaccinated animals via serology. Vaccination must be combined with good management practices including:

• Reliable individual animal identification systems
• Movement restrictions
• Replacement stock must be serologically negative for AD
• Quarantine facilities
• Continuous screening of blood samples should be compulsory so that at all times the health status of all animals is available

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