Aujeszky's Disease (AD), or Pseudorabies (PR) as it is also known, is an important worldwide disease. Some countries are free from the disease, while it is a major problem in others. Control and eradication remain the goal for this disease and biosecurity has a pivotal role.
The pig is the natural host reservoir for AD/PR virus. It can infect other species including cattle, sheep, dogs, cats and rats but in all of these the infected animal dies, and these are known as end hosts. The virus can survive in the environment for up to 3 weeks.
Clinical signs of pig infection vary depending on the type of herd and its immune status. Acute outbreaks in naïve breeding herds are characterised by inappetence in a few sows, quickly followed by abortions. Then there are stillbirths and weak pigs born. Weak pigs and neonates become unthrifty, febrile, uncoordinated, and may convulse and die. Under 7 days mortality can reach 100%. Later on returns and mummified pigs are seen.
In naïve growing herds, pigs are febrile (41°C/106°F) with inappetence, sneezing, nasal discharge and coughing. Nervous signs and vomiting occasionally occur. Secondary infections (e.g. Enzootic Pneumonia, Actinobacillus pleuropneumonia or Pasteurella multocida) are common. Mortality can reach 8%, but is often lower.
Herds with some immunity, or infected by less virulent strains of AD/PR, show much less obvious signs, and it can be difficult to detect or differentiate from mild reproductive or respiratory disease of other causes. This is especially the case in finishing farms.
Importantly, following natural infection, a few animals develop a carrier state. These show no clinical signs and only shed virus when stressed or exposed to other disease (e.g. PRRS or PMWS).
How is AD/PR Spread?
The main causes of spread are listed below:
Infected pigs - including carriers
Airborne spread - at least up to 3 km
Passive carriage on people or other animals
End case hosts (cattle, dogs etc,)
Slurry and slurry aerosols
Of these the most important are pigs, airborne transmission, semen and then vehicles.
Factors in Control
One thing is certain about AD/PR in the long term - it is a disease that cannot be afforded in modern pig production.
As there are so many ways in which AD/PR can spread, biosecurity measures to help control it have to be broad-based and thorough. Typically farmers need to use a comprehensive biosecurity programme such as the Chemours Animal Health Solutions Swine Biosecurity Programme. A number of disinfectants are active against AD/PR virus, but if we consider the secondary role of other agents in the clinical syndrome, the triggering of shedding of AD/PR virus in the carrier stage by such as PRRS virus and PMWS, and the fact that we want to protect against other diseases, then it is best to use a proven broad spectrum disinfectant such as Virkon® S. As always prior to disinfecting, a heavy-duty detergent such as Biosolve® Plus should be used to aid proper cleansing.
An important aid to control has been the development of gene-deleted vaccines. These protect against the worst of the disease and reduce multiplication of the virus (and hence challenge to other pigs). However, since serological tests can differentiate between pigs vaccinated with these vaccines and those infected with wild virus, a cull of the latter will eliminate the virus in a herd provided that re-infection is prevented.
Control on an Infected Farm
Antibiotics will not work against AD/PR virus but may help reduce secondary infections. Provided we can prevent re-infection on a farm, the aim should be to eliminate the virus. This can be done by depopulation - re-population followed by extensive cleaning and disinfection (Biosolve® Plus / Virkon® S) and by restocking with known free stock. This technique is generally used where multiple diseases are present.
The other main approach is to remove weaners and growers to another site, weaning before 21 days. Then vaccinate the breeding herd with the gene-deleted vaccine. Clean and disinfect all accommodation. Then test a few weeks later with the differential test and cull any sows positive for wild virus. Repeat testing once or twice until all sows are negative. Vaccination may be continued if there is a fear of reinfection from local farms. Weaners and growers can then be kept on the breeding farm, or remain in 2 or 3 site production. External biosecurity must be maximised to reduce the chances of reinfection (see below).
Control on a Free Farm
Controls on a free farm are most important in a heavily infected area, but are vital in all free farms.
First buy stock only from known AD/PR-free farms with good biosecurity and an effective testing programme. Isolate at least 3 km away from the unit (and other pigs) for 6 weeks and test at least once for AD/PR before transferring to the main unit. Biosecurity of the isolation unit must be as good as on the main farm and personnel and equipment should be kept separate. Semen should be purchased from AD/PR-free studs with a valid testing programme and good biosecurity. Vaccination of breeding stock may be done to reduce the chance of infection with AD/PR becoming established, and to reduce the effects if it does.
Weaner, Grower and Finisher farms should buy from AD/PR-free breeding farms if they are free from the disease.
External biosecurity must be maximised on all types of farms. Perimeter fences are required to keep out wild boars, other animals and people. Biosecurity of transport is an important area. Vehicles used must be cleaned and disinfected prior to use (Chemours Animal Health Solutions Vehicle Biosecurity Programme). One area of concern is truck washes. Never use the same site for both positive and negative hauling trucks due to the risk of cross-contamination. Disease breakdowns are more common in the winter and much of this is attributed to poorer vehicle biosecurity, when washes are not weatherproofed. All vehicles should pass through a disinfectant wheel dip or spray (Virkon® S) prior to reaching the pig unit. They should deliver from outside the perimeter. Drivers should change overalls and boots on arrival.
Visitors to the unit should be kept to a minimum. Downtimes should be enforced on units that have them. All personnel entering the unit should shower and change clothes if possible, but at least have clean overalls, hats and boots and use foot dips (Virkon® S) and wash their hands with a suitable antibacterial hand wash prior to entry.
Control in a Given Area
In AD/PR-free areas, control ingress of pigs and semen. Wild boar should be controlled. Biosecurity of transport is important, as is transit through the area. External biosecurity of farms should be maximised.
In infected areas vaccination will decrease challenge to uninfected farms. Often one of the difficult things is to get finishers, who experience fewer cost penalties from the disease, to vaccinate to keep the challenge in an area as low as possible. Once the financial resources for testing and control of movements are established, an individual farm-by-farm eradication plan using vaccination, cleaning and disinfection and culling should be undertaken. Vaccination is maintained until at least the main challenge in the area has passed. Provided that movement controls and biosecurity of farms is good enough, AD/PR can progressively be eradicated from the area. Even then it is essential to maximise biosecurity of all farms to help prevent re-infection of individual farms or the whole area.
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