Aujeszky’s Disease (Pseudorabies)
Occurrence: Formerly present in many countries, it has been eradicated from some and is Notifiable in many.
Age affected: All ages
Causes: Aujeszky’s Disease virus
Effects: Respiratory signs, abortion, stillbirths, mummies, pre-weaning deaths with nervous signs. In pigs younger than 3 weeks, incoordination, loss of appetite, depression, vomiting, diarrhoea and convulsions may occur followed by death of whole litters. Older pigs often have fever, loss of appetite, sneezing, coughing, pneumonia, convulsions and sometimes blindness. Abortion, stillbirths and mummified pigs can also occur.
Aujeszky’s Disease is caused by infection with a herpes virus, Suid herpesvirus 1 (SHV) which multiplies in the nuclei of the cells of the pig to lyse (kill) them, to form syncytia (merge them) or to produce a characteristic intranuclear inclusion body. It can be grown in tissue culture in the laboratory. It is a DNA virus. The glycoproteins (gp) and thymidine kinase (TK) of the virus are important in the disease, in vaccines and in diagnosis and their genes have been sequenced. Only one major antigenic type of the virus occurs. Aujeszky's disease virus infects the upper respiratory tract and virus travels along the cranial nerves and invades the brain. Strains found in Northern Ireland and Central Europe also produce pneumonia. Invasion of the uterus, maternal and foetal placentas and foetuses occurs and can cause abortion, foetal death, mummification and foetal resorption. The virus can be adsorbed onto the zona pellucida of embryos and can infect recipient sows in embryo transfer and may be shed in the semen for up to 10 days. Virus is shed from 2 days post-infection until 10-14 days post-infection in most cases and exceptionally until 19-20 days. Latent (undemonstrable) infections commonly develop.
Mode of transmission
The main route of infection is by aerosol or by contact with infected pigs. The virus is shed in nasal secretions from clinically-affected and recovered carrier pigs. It spreads from farm to farm by the movement of infected pigs and also in transport and by aerosol. Under favourable conditions disease may be transmitted for up to 70 km in cool moist air. Although cats, dogs and cattle can be infected, they do not transmit the infection to pigs.
Clinical signs in piglets under 4 weeks of age begin within 3-7 days of infection and they may vomit or show diarrhoea and become depressed, with fever (41.5˚C, 107˚F), trembling, in-coordination (including circling) the adoption of a dog-sitting position, spasms followed by prostration and death in 100% of newborn animals after 12 hours but only 40-60% die by 4 weeks of age. The disease lasts 4-8 days in animals aged 1-5 months. Anorexia (inappetence) with occasional vomiting occurs within 3 days of the onset of fever and from the 4th day, tremor and incoordination of the hind limbs progress to muscle spasms and to short convulsions followed by prostration and death in up to 15% of animals. Finishing periods may be extended by 10-14 days. Severe pneumonia has been recorded in weaned pigs in addition to the nervous signs described above. Infection in adults may be inapparent, result in transient inappetence or a rise in temperature accompanied by mild nervous and respiratory signs. Up to 50% of affected pregnant animals abort or give birth to mummified or macerated foetuses. Reproductive failure may follow. Semen quality may decline from 10-14 days post-infection for a period of 1-2 weeks.
Abortion, neonatal deaths, nervous signs in piglets and coughing and listlessness in finishing pigs spreading through a non-immune herd should suggest Aujeszky’s Disease. Clinical signs may be slight or undetectable in vaccinated, partially immune herds, breeding herds or those with finishers only. Pruritis and death in farm cats and dogs, cattle or sheep is a common early sign, as these species may also be affected. Confirmation of disease is by laboratory tests for virus in tissues of infected pigs, especially from tonsillar swabs, or for antibody in serum samples from whole or part of a herd.
Pig with nerve disorders, typical for pseudorabies.
Photo credit: Tan Tao, Marika Genzow & Carlo Maala.
Post-mortem findings such as necrotic (dead) tonsils or nasal septum and turbinates (conchae) may indicate Aujeszky's disease. The characteristic intranuclear inclusions seen by microscopic examination are diagnostic, but occur in less than half the cases. Confirmation of disease is by laboratory tests for virus in tissues of infected pigs or for antibody in sera. Virus can be grown from the brain, spleen and lung of affected pigs, demonstrated in the tonsil or brain using a fluorescent antibody, immunoperoxidase, DNA probes, in situ DNA hybridisation and the Polymerase Chain Reaction, a DNA test which detects tiny amounts of virus especially in latent infections. The origin of virus can be determined and whether it belongs to wild or vaccine types. ELISA tests for serum antibody are used widely on recovered pigs and in herd diagnosis. They, too, can distinguish wild and vaccine type infections.
Treatment and prevention
There is no treatment although hyper-immune serum may protect piglets less than 4 weeks old in some countries. Control is by herd vaccination in countries where the disease is widespread and by prevention of re-infection where eradication has taken place. Vaccination is with live or killed vaccines made from virus lacking one of the glycoproteins or the thymidine kinase. These protect, and antibody to them can be distinguished in ELISA tests from that to wild type virus. Sow vaccination prevents abortion and disease in young pigs, but maternal antibody may prevent successful vaccination of piglets and 3 vaccinations, the last at 14 weeks of age may be needed to prevent disease in finishers. Serum antibody profiles determine the correct time to vaccinate a herd when antibody is absent. Aujeszky's disease has been eradicated by slaughtering herds containing infected animals or by vaccinating herds to reduce disease and then slaughtering pigs with antibody to wild type virus. Wind can spread virus for 2-17 km on land and 70 km over water, so eradication is best carried out on an area basis. Disease free stock, embryos and semen must be available and depopulated units must be disinfected or left empty for 10-12 weeks.
This disease is not transmissible to humans, but is notifiable in many countries, especially in the EU.