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Serology

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Testing Schedule

Our goal is to provide accurate results and timely service. However, many diagnostic assays are not performed daily and/or testing schedules may change. If your case involves testing animals for export, testing show animals, testing animals to be sold, and/or testing a large number of samples, please contact the VDL (515-294-1950) prior to submission to verify the availability and turnaround time of specific assays. A phone call will often shorten turnaround time by allowing us time to prepare for testing prior to receipt of samples.

Serology for Export Cases

Please use the following guidelines for submission of sera for export cases:

  1. To minimize testing delays, contact the VDL at least one week before sending sera for testing. Provide the date sera will be delivered to the VDL, the number of samples, the specific assays to be run, and the date results are needed.
  2. Indicate on the Serum Submission Form that the testing is for export purposes (check the box) and indicate the importing country.
  3. On the Serum Submission Form, clearly indicate the specific tests needed, including the agent, test type, and dilution required. Include a copy of the importing country's requirements, if available, to avoid confusion. Example: Vesicular stomatitis virus (VS) SVN test at 1:8 dilution.
  4. If export requirements specify a minimum change in antibody titer, the sera from different bleed dates must be tested as paired sera on the same test date. This information must be communicated to the VDL prior to sending in sera from the initial bleed dates.
  5. Never assume test results will be done by a certain date. Follow up your written test requests with a phone call to verify that the correct tests are being performed and that results will be available in the required time frame.

Reporting Serology Results

Serology results will be reported as requested on the Serum Submission Form. The fastest methods of reporting results are by email, telephone, and/or FAX.

Interpretation of Selected Serologic Tests

ASSAY COMMENTS
Actinobacillus pleuropneumoniae CF

The CF is a highly specific test for detecting ant-APP antibodies. Samples with antibody titers 1:4 are suspect; 1:8 are considered positive.

Antibodies develop 7 to 10 days after infection, peak at 1:4 to 1:256, and last for several months. Maternal antibody may be detected until 8 to 9 weeks of age.

Anaplasma marginale CF Positive results indicate exposure. Sample quality is important to avoid non-specific reactions (AC).
Bovine respiratory syncytial virus VN Vaccine antibody titers are usually 1:128. To aid in interpretation, compare titers in clinically affected vs. unaffected animals, or acute vs. convalescent.
BVD virus VN BVD type 1 and type 2 are run in separate assays, but comments apply to both.
Antibody response is highly variable, in part because strains vary antigenically. Field strains generally produce high neutralizing antibody titers (1:2,048). Titers from inactivated-virus vaccines are often 1:256 and against MLV vaccines 1:2,048. Exposure to vaccine vs. field virus cannot be reliably differentiated on the basis of antibody titiers. Compare titers from clinically affected vs. unaffected animals to aid in interpretation. Titers 1024 in unvaccinated calves >6 months of age suggest exposure to field virus.
Haemophilus somnus CF 1:64 to 1:128 suggestive of vaccinated animals.
1:64 to 1:256 suggestive of animals vaccinated more than once.

1:256 to 1:512 suggestive of early active or chronic infection.
1:1040 to 1:4096 suggestive of recent active infection.
For interpretation, use paired samples or compare affected vs. unaffected.
IBR virus (BHV-1) VN

Inactivated vaccine antibody titers are usually <1:16 and MLV vaccine antibody titers are usually <1:64.

Serology is of limited diagnostic use in bovine abortions caused by IBR, especially in well-vaccinated herds. Cows generally abort 23 - 53 days post exposure, i.e., cows reach maximum IBR antibody titers prior to aborting.

Leptospira interrogans MAT Diagnostically significant antibody titers are usually 1:800.
Titers have usually developed by the time abortion occurs, so consider comparing samples from clinically affected vs. unaffected animals to aid in interpretation.
Interpretation requires vaccination history. Vaccine antibody titers may be 1:400 to 1:800. In dogs, if icterohaemorrhagiae and canicola both react, it is probably vaccination. Dogs infected with grippotyphosa may have titers of 1:200 to 1:800. In cattle with reproductive failure associated with hardjo, hardjo antibody titers may be 1:100 to 1:200.
Mycoplasma hyopneumoniae
ELISA
The Serology Section of VDL has implemented a USDA-licensed ELISA: Mycoplasma hyopneumoniae antibody test manufactured by IDEXX Laboratories, Inc. Serum samples with S/P ratios <0.3 are considered negative; 0.3 to 0.4 are classified as suspect; and >0.4 are considered positive. Cross-reaction is possible with M. hyorhinis, or M. fluocculare. M. hyopneumoniae resides on the surface of the respiratory tract and may colonize without stimulating an immune response. When the organism does begin to proliferate and induce disease, the immune response is slow to develop.
Parainfluenza virus type 3 VN A ubiquitous virus. Positive results indicate exposure. Compare titers in clinically affected vs. unaffected animals, or acute vs. convalescent.
Porcine circovirus IFA The VDL assay utilizes PCV Type 2 as the antigen in the IFA.
For screening, samples are run at dilutions of 1:20 and 1:40. The sample is considered positive if reactive at either dilution.
Porcine parvovirus HI Vaccination titers usually 1:256. Maternal antibody may persist for months. Although considered ubiquitous, dams in commercial herds frequently have not been exposed to field virus.
PRRS virus ELISA S/P values 0.40 are positive; <0.40 are negative. The manufacturer estimates sensitivity of 100% (90 to 100); specificity of 99.5% (98.3 to 99.9). The commercial ELISA detects antibody against either North American or European strains of PRRS virus. Antibodies are detectable within 14 days of infection or vaccination with modified-live vaccine.
PRV gI ELISA The gI ELISA differentiates field virus infection from vaccination.
Serum samples with S/N values 0.60 are positive for antibodies against field virus; samples >0.60 to 0.70 are suspect, and samples with S/N >0.70 are negative.
PRV latex agglutination assay The PRV latex agglutination assay does not differentiate between field infection and vaccination. A latex agglutination index of <4.5 is negative for PRV antibodies; > 8.0 is positive; and intermediate values are suspect.
PRV screen ELISA Serum samples with S/P values <0.4 are negative; samples 0.40 are positive.
The PRV screen ELISA does not differentiate between field infection and vaccination.
Swine influenza HI Interpretation of HI results is the same for SIV H1 and SIV H3. Antibody titers 1:40 are positive, although clinically relevant titers are usually 1:80. Titers rise to 1:320 by 14 days post-infection and persist for approximately 4 weeks before declining.
Antibody 1:40 interfers with inactivated vaccine. Maternal antibody in pigs from unvaccinated sows will last up to 8 weeks; in pigs from vaccinated sows up to 20 weeks of age.
TGE virus VN Neutralizing antibody titer of 1:4 is considered positive, although recent infection usually results in titers 1:32. Vaccination titers are usually 1:16. Antibodies against TGE virus and PRC virus are indistinguishable by VN.
TGE virus/PRC virus differential ELISA Antibodies against TGE virus can be differentiated from antibodies against PRC virus using ELISA. The test is not accurate for recent infection (<28 days).