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Aujeszky's disease (AD)
"Pigs that have recovered from ADV infection may become asymptomatic carriers."
Aujeszkyâ€™s disease, or pseudorabies, is caused by pseudorabies virus (PRV), a herpes virus that affects mainly pigs, the only known reservoir for the virus. 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 system signs and rapidly die, hence the name Pseudorabies.
About Aujeszky's disease in pigs
Importance of Aujeszky's Disease (AD)
AD is an economically-damaging viral disease of pigs. It is prevalent all over the world. Norway, Australia, and the islands of Southeast Asia are the only areas of the world without AD.
Many countries (e.g. the UK, Norway, Denmark, N & S Ireland, the Netherlands, Canada, the USA, and Chile) adopted control policies, which included compulsory vaccination or, in the case of the UK and Denmark, slaughter and eradication policies. Currently, AD has been eradicated from domestic pigs in Austria, the Czech Republic, Denmark, Finland, France, Germany, Hungary, Luxembourg, the Netherlands, Sweden, Switzerland, Slovakia, Cyprus, the UK, Canada, New Zealand, and, in 2004, the United States. AD is still prevalent in Eastern Europe, Central and South America, Africa, and Asia.
- Caused by pseudorabies virus (PRV) - a 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. The regions which encode the TK enzyme (thymidine kinase) and the surface glycoproteins are important in determining virulence, the course of infection, development of antibodies after infection, and diagnosis following infection.
In vaccine manufacturing, marker vaccines were 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.
The virus is transmitted via bodily fluids from infected pigs. This virus is also capable of being transmitted to fetal piglets through the placenta. It first replicates in the epithelial cells of the tonsils and nasopharynx. The virus then spreads to the rest of the respiratory tract and also to the central nervous system (CNS) along nerves until it reaches the spinal cord. The virus tends to infect the ganglia of the central nervous system and often develops a latent infection that can recur during periods of stress. The virus causes inflammation in the respiratory tract and CNS followed by necrosis of these tissues. The virus may also spread to other areas of the body, including the reproductive tract where the associated inflammation can lead to endometritis, vaginitis, and necrotic placentitis.
Clinical signs in pigs depend on age, virulence of the strain, virus dose, and route of infection. The virus is spread through direct contact between pigs or with fomites (bedding, water, boots, and equipment). AD is not very contagious and transmission occurs most easily through the nasal passages. The virus is shed by infected pigs in almost all bodily fluids and excretions including milk, semen, and urine. The incubation period ranges from 1 - 8 days or up to 3 weeks.
Acute infection in a susceptible herd
After infection with a virulent strain of AD, clinical signs are typical as for any infectious organism: high fever, anorexia, shortness of breath, and dullness. Pigs infected with AD also display vomiting and increased salivation. Clinical signs are typically present for 6 - 10 days.
Depending on the age of the pigs, specific signs occur that are determined by the virulence of the infective strain. The most dramatic signs occur in very young piglets that are 2 - 3 weeks old.
Neurological signs include:
- Incoordinated movements
- Sitting on stretched rear legs
- Recumbency and paddling
Morbidity and mortality in young piglets are very high and may reach 100%.
Growing pigs become increasingly less susceptible with age, and mortality rate and incidence of neurological signs decrease.
- Increased regular returns to estrus (due to failure to conceive) or increased irregular returns to estrus if infection occurs during the early stages of gestation
- At any stage of pregnancy, abortions may occur.
- Mummified fetuses, stillborn and weak piglets
- Small litters
- Respiratory symptoms and fever
Boars may develop severe orchitis or fail to mate because they are running a high fever. The high fever following infection can have an important long term effect on the herd's reproductive performance by affecting spermatogenesis.
As pigs get older, secondary respiratory disease becomes increasingly prominent. Porcine pleuropneumonia and Pasteurella multocida are often the cause of secondary bacterial infection. Infection with PRRS virus, Porcine Circovirus (Associated) Disease (PCVD) type 2 (PCV2), or swine influenza virus can lead to the development of fatal pneumonia, called proliferative and necrotizing pneumonia (PNP), in pigs that are being weaned or post-weaning pigs.
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.
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
Transmission of disease between herds
- Movement of carrier pigs
- Airborne - at least 3km
- Infection from feral (wild) pigs
- Contaminated carcasses
- Mechanically on clothing, boots, vehicles and equipment
- Through infected semen via AI or a carrier boar
- From infected slurry
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. However, other diseases, such as rabies and porcine polioencephalomyelitis, produce similar clinical signs, so laboratory tests are required to confirm the diagnosis:
- Fluorescent antibody tests - tissue from tonsils in carcasses is 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 between wild and vaccine type infections.
Control and Eradication
It 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, but should also include changes in management factors:
- 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, which 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