Reportable and Transboundary Diseases
Pseudorabies, also known as Aujeszky’s Disease is a disease of the nervous, respiratory, and reproductive tracts that causes a wide range of symptoms across all age groups. It affects a multitude of other mammalian species such as cattle, sheeps, dogs, cats, rodents…. However, higher primates are not sensitive to the virus, so pseudorabies is not a zoonotic disease. In non-porcine species, the virus causes rabies-like symptoms and quick death, hence the name “pseudorabies”. Pigs are considered the natural host of the virus because mortality tends to be low with the exception of pre-weaning piglets. After a 5 year government-ran program, the disease was completely eradicated from US commercial herds in 2004. It is important to note that feral pigs are still positive and represent a risk of reintroduction.
Etiology and transmission
Pseudorabies Virus (PRV) is a member of the Herpesvirus family. Only one serotype is known, but different isolates can vary in virulence. The infectious dosage in pigs is quite high, so the disease is not highly contagious. Direct contact between infected pigs, usually nose-to-nose, is the main route of transmission. Indeed, pseudorabies virus is a herpesvirus, meaning that even when an infected pig recovers from the disease, it becomes a life-long latent carrier that can be reactivated and start shedding again. Sows and boars can shed the virus in milk and semen respectively. Pigs that are less than two weeks post infection tend to shed the highest amount of PRV. The virus is quite resistant in the environment surviving up to 2 months in lagoons or humid fecal matter. Pseudorabies virus is sensitive to heat and desiccation but indirect infection through contaminated environment should be considered a viable route. The virus can spread via aerosol transmission within a barn but long distance transmission has not yet been demonstrated.
Pseudorabies virus usually enters the pig through the nostrils or the mouth and replicates in the tonsils and pharyngeal area. It infects local nerves, and moves through the axon to the cell body of the neurons. Viruses can then spread from one neuron to the next through synapses and eventually reach the brain. Viremia is also possible, infecting many other systems.
All US pigs are free of pseudorabies virus. True or false?
Different clinical presentation are observed based on the age of the pig.
The most severe symptoms occur in naive piglets born to infected and non-vaccinated sows. Clinical presentation is characterized by paddling, dog sitting, foaming at the mouth, lameness, and shaking. Before weaning, mortality due to pseudorabies is extremely high, up to 100%. However, the older the affected pigs are, the lower the mortality rate. Indeed, once the pigs reach one month of age the mortality rate is halved and by the time they are five months old, less than 5% of pigs will die. Clinical presentation is less severe in those pigs: neurological symptoms such as anorexia, tremors, and some shakiness of hind limbs can be noticed.
In growing pigs, respiratory issues such as coughing, sneezing, and nasal discharge, similar to a flu syndrome can be seen. Pigs will usually recover within ten days.
Clinical signs associated with pseudorabies infection in sows is dependent upon time of infection. Early in the gestation, embryonic death and resorption of fetuses are common whereas abortions and mummified fetuses are typical of mid to late gestation.
Necropsy examination in pseudorabies cases is often frustrating because few lesions can be observed macroscopically. The most consistent lesions are small areas of necrosis on the tonsils, liver, and spleen. Edema, hemorrhage, and consolidation of the lungs can be seen in older pigs.
The most characteristic microscopic lesion of pseudorabies is a nonsuppurative meningoencephalomyelitis in the white and grey matter of the brain. Neuronal degeneration is commonly seen in the central nervous system. In the epithelial cells of other organs, such as the liver, spleen, and tonsils, small foci of necrosis can be noticed.
Necropsy and clinical signs alone are not enough to make a diagnosis of pseudorabies. In clinical cases, PCR testing of either infected tissues such as tonsils, or of oral fluids and nasal swabs is the preferred method to detect the virus. PCR is very sensitive and provides results in less than a day.
To assess if a herd is infected or not, laboratory tests targeting antibodies are used. Serum neutralization used to be the standard test but it has been replaced by ELISA. Indeed, pseudorabies ELISAs can discriminate between an immune response due to natural infection or due to vaccination.
Neurological signs and mortality in young pigs: African swine fever, Classical swine fever, teschovirus, Streptococcus suis, salt toxicity
Respiratory signs: PPRS, Influenza, PCV2
Reproductive signs: Parvovirus, PRRS, PCV2
What is the best method to diagnose a clinical case of pseudorabies?
- Clinical presentation and necropsy findings are enough to make a diagnosis
- PCR testing of infected tissues such as tonsils, brain or fetuses
- ELISA testing on serum samples
Treatment, Prevention and Control
There is no treatment available for pseudorabies-infected pigs. Pseudorabies has been eradicated from US commercial by vaccinating the sows and then implementing the test and removal method. The goal is to identify all of the breeding animal infected with pseudorabies, that are latent carrier and to remove them from the herd. Blood samples are taken from all of the animals and tested by ELISA. The sow is then identified as naturally exposed or vaccinated. Infected animals are then removed from the herd. The process is repeated several times to ensure that no animal has been missed: between the incubation period and the time to develop antibodies, several weeks are necessary. ONce all of the animals are deemed negative, naive sentinels gilts are introduced. If they do not seroconvert, then the herd is considered negative.
US herds are still negative by preventing contact between domesticated and feral pigs, and ensuring strict biosecurity measures. In countries where pseudorabies is endemic, vaccinating sows helps protect the piglets from the most severe symptoms, and prevent mortality. All-in all-out practices with disinfection between groups can help prevent a contaminated environment.
How was pseudorabies eradicated in the US?
- Depopulation-repopulation of the herds
- Herd closures during 5 months
- Test and removal of infected animals