Laboratory diagnostics: Aujeszky’s disease (Pseudorabies - PRV)

Assays available:

Immunohistochemistry (IHC)

• Detects presence of viral antigen
• Sample types: tissues
• Pros:
  • Detects virus at site of lesion (good proof of causation).
  • Any positive result is considered significant.
• Cons:
  • Correct tissue sample must be submitted.
  • Requires significantly more virus to be present than PCR.
  • Only evaluating a small tissues sample.
  • Latent infections likely will be missed unless trigeminal nerve and/or tonsils are included.

Polymerase chain reaction (PCR)

• Detects presence of specific sequence of viral nucleic acid (DNA)
• Sample types: tissues
• Pros:
  • High sensitivity.
  • PCR quantification is associated with presence of lesions.
  • Moderate cost
    • Can often do pooling of tissue samples to lower cost while minimizing loss of sensitivity (especially regarding clinical relevance).
  • Some PCR assays can differentiate wild-type from gene-deleted vaccine viruses.
• Cons:
  • Requires tissue samples (post-mortem) for best results.
  • Latent infections likely will be missed unless trigeminal nerve and/or tonsils are included.

Enzyme-linked immunosorbent assay (ELISA)

• Detects presence of antibodies
• Sample types: serum
• Pros:
  • Animals remain positive for several weeks.
  • Can be used in chronic cases.
  • Can be used to differentiate exposure to wild-type from gene-deleted vaccine (see Table 1).
  • Any positive result to wild-type virus is considered significant.
  • Only takes 5-7 days to become seropositive after wild-type exposure.

Table 1 helps demonstrate the DIVA (Differentiate Vaccinated from Infected) capabilities of some ELISA assays when using a gene deleted vaccine.

VN Result	Differential ELISA targeting deleted gene gE/g1 or gB result	Interpretation
Positive	Positive	Exposed to wild-type virus
Positive	Negative	Vaccinated animal not exposed to wild-type virus
Negative	Positive	Assay error, result not possible
Negative	Negative	Not vaccinated and not exposed to wild-type virus

• Cons:
  • Need to match the proper assay with the proper gene-deletion in the vaccine.
  • Takes 10-14 days to become seropositive after vaccination.

Virus neutralization (VN)

• Detects presence of antibodies
• Sample types: serum
• Pros:
  • Animals remain positive for several weeks
  • Can be used in chronic cases
  • Higher sensitivity than ELISA (can detect lower concentration of antibodies)
• Cons:
  • Not feasible for large number of samples.
  • Takes 12 days for animals to become seropositive.
  • Unable to differentiate vaccine vs. wild-type infection.
  • Reliability is highly dependent on technician skills.
  • Reactivity is dependent on virus isolate used

Result interpretation:

IHC

• Positive: Virus is present at site of lesion
• Negative: Negative or infection is too late to detect virus

PCR

• Positive: Virus is present. Recent vaccination with a modified live virus can result in positive PCR results
• Negative: Negative or virus could have been missed if testing occurs late after infection

ELISA (target gene deleted)

• Positive: Maternal antibodies or past exposure (>5-7 days) to wild-type virus.
• Negative: Negative or wild-type infection too early to detect.

VN

• Positive: Past exposure (> 5-14 days) to wild-type virus or vaccine.
• Negative:
  • Negative to vaccine or wild-type virus
  • Infection too early to detect (<5-7 days for wild-type virus or <10-14 days for vaccination)

Scenarios

Sow herd with abortions

• Collect 6-8 fetuses and pool samples for PCR testing.
• Sows/gilts that aborted: Collect serum from sows/gilts that recently aborted for PCR testing. Can pool in groups of 5.
  • ELISA testing of individual samples can be done if >7 days after abortion.
  • VN testing instead of ELISA is preferred in herds not vaccinated for Aujeszky’s disease due to higher diagnostic sensitivity of assay.

Replacement gilt selection from vaccinated herds with known positive animals

• Collect blood samples from 100% of replacement gilts and test individual samples via ELISA and VN.
  • Only keep animals positive on VN and negative on ELISA (see Table 1 above).

Replacement gilt selection from vaccinated herds with no known positive animals

• Collect blood samples from at least 30 randomly selected replacement gilts and test individual samples via ELISA and VN.
  • If all samples tested are positive on VN and negative on ELISA (see Table 1 above) you can keep entire group of replacement animals.
  • If any sample is negative for VN, you must test 100% of the remaining gilts and only keep gilts which have been vaccinated (VN= positive).
  • If any gilts test positive via ELISA, re-evaluate the merit of keeping any animal from the group as this would suggest the source herd is now positive for Aujeszky’s disease.