Laboratory diagnostics: Parvovirus

Assays available:

Immunofluorescence antibody (IFA)

• Detects presence of viral antigen
• Sample types: tissues (aborted or mummified fetuses)
• Pros:
  • Detects virus in aborted or mummified fetuses (good proof of causation).
  • Any positive result is considered significant.
• Cons:
  • Some mummified tissues are too decomposed to detect virus.
  • Requires significantly more virus to be present than PCR.
  • Often virus infection occurred several weeks prior to abortion or delivery of mummified fetuses.

Polymerase chain reaction (PCR)

• Detects presence of specific sequence of viral nucleic acid (DNA)
• Sample types: tissues
• Pros:
  • High sensitivity.
  • Moderate cost
    • Can often do pooling of tissue samples to lower cost while minimizing loss of sensitivity (especially regarding clinical relevance).
• Cons:
  • Requires tissue samples from aborted or mummified fetuses for best results.
  • Some mummified tissues are too decomposed to detect virus.

Enzyme-linked immunosorbent assay (ELISA)

• Detects presence of antibodies
• Sample types: serum
• Pros:
  • Animals remain positive for several months to years
  • Able to test large number of animals
  • Vaccinated animals produce significantly lower antibody levels compared to field infections
  • Only takes 6-10 days to become seropositive
• Cons:
  • Seroprevalence is high as pathogen is common
  • Some ELISAs can differentiate vaccine from wild type virus exposure
    • Inactivated vaccines only stimulate antibodies to structural proteins (VP1/VP2)
    • Differential ELISA targets non-structural proteins (NS)

Hemagglutination Inhibition (HI)

• Detects presence of antibodies
• Sample types: serum
• Pros:
  • Animals remain positive for several months to years
  • Higher sensitivity than ELISA (can detect lower concentration of antibodies)
  • Vaccinated animals produce significantly lower antibody levels compared to field infections
    • Vaccine usually ≤ 1:500
    • Field infection ≥ 1:2000
  • Animals test seropositive as soon as 6 days post infection
• Cons:
  • Not feasible for large number of samples.
  • Reliability is highly dependent on technician skills.
  • Reactivity is dependent on incubation temperature.

Result interpretation:

IFA

• Positive: Virus is present in fetus
• Negative: negative or infection occurred early in pregnancy and too late or tissue too decomposed to detect virus

PCR:

• Positive: Virus is present in fetus
• Negative: negative or infection occurred early in pregnancy and too late or tissue too decomposed to detect virus

ELISA (differential ELISA)

• Positive:
  • Low values past exposure (>6 days) to vaccine
  • High values: recent exposure to wild-type virus
• Negative: Negative or wild-type infection too early to detect.

HI

• Positive:
  • Low values past exposure (>6 days) to vaccine
  • High values: recent exposure to wild-type virus
• Negative: Negative or wild-type infection too early to detect.

Scenarios:

Sow herd with infertility and mummified fetuses

• Collect 4-8 fetuses (target mummified fetuses) and pool samples for PCR testing.
• Aborted sows/gilts: Collect serum from affected
  • If HI titers are ≥ 1:2000 – confirm recent viral infection
  • Otherwise retest in 2-4 weeks and compare titers over time looking for > two-fold increase

Sow herd with infertility and NO mummified fetuses:

• Collect serum from affected and normal sows today and retest same animals in 2-4 weeks
  • If HI titers are ≥ 1:2000 – confirm recent viral infection
  • Otherwise compare titers over time looking for > two-fold
• Evaluate parity distribution of affected animals
  • Parvovirus infection is more common in 1st or 2nd parity animals as older parity animals are more likely to already have protection.