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One of the ways the HerdChek 2XR PRRS Antibody Test Kit has been
used by the swine industry is for the detection of the level of
antibodies in positive herds that have been vaccinated or exposed to
PRRSV. The peak level of seroconversion was observed to occur 4–6
weeks after vaccination with the previous format of HerdChek PRRS
Antibody Test Kit, with the peak between 1.5 to 2.5 S/P.
Preliminary results have shown that the new format HerdChek PRRS
2XR Antibody Test Kit demonstrates a different seroconversion level,
occurring at the same time-interval post-vaccination. As reported in
recent studies, 4–6 weeks after vaccination the animals can reach
S/P levels between 2.5 to 3.5 in general.4 Therefore,
animals that have been vaccinated and then face a field challenge can
show a higher level of seroconversion due to the field challenge and
the previous immune response from the herd resulting in an anamnestic
response.
Based on duplicate testing of several different sets of
experimental sera from animals inoculated with either field virus or
vaccine virus, the Boehringer Ingelheim Vetmedica Health Management
Center (Ames, Iowa) has diagramed the composite ELISA serologic
detection curves for the original ELISA format and 2XR format as
depicted in Graph 1.
This shows the new trend of ELISA seroconversion with the HerdChek
PRRS 2XR Antibody Test Kit in vaccinated animals. The geometric mean
S/P of a vaccinated herd can be 1.0–1.5 S/P units higher, as
compared to the previous format.
The HerdChek ELISA test is the ideal tool for monitoring
populations on a herd basis, rather than on an animal-by-animal basis.
We recommend making the baseline analysis with samples from a herd
assessed by S/P distribution, and not by analyzing the data sample by
sample. You should consider the geometric mean S/P, the coefficient of
variation and the distribution of the histogram as a population, as
the analysis of individual samples can mislead your interpretations.**
The first step is to build the serologic profile of your herd based
on the different physical locations that may have different levels of
challenge, strains of field virus, different production systems,
vaccination programs, etc. To initiate a baseline profile, 23 samples
is generally considered an adequate, statistically valid number to
use.1 However, the sample size may vary according to the
size of the herd.3 This first step in profiling can take
6–8 months, and may vary based on the market age or type of
production.
It is important to remember that the number of samples, as well as
the quality of the samples, has to be truly representative of the
herd. Random collection of samples will help ensure that the results
are reliable to create a baseline for the farm.
With this S/P baseline as a reference, you can start using 15 serum
samples from each herd as a statistically valid number for the
accuracy of the interpretations and collecting every eight weeks. If
10 samples are used on a per herd basis, an increase in the frequency
of the testing is needed every four weeks in order to compensate for
the smaller numbers of serum samples tested per sampling event.2
We recommend that swine veterinarians and producers follow these
specific indications:
- Build a baseline with at least 23 serum samples (or adapt the
number according to the herd's size).
- Continue monitoring with 15 serum samples every eight weeks.
- Analyze the data on a herd or population basis.
- Look at the baseline and then at the group of sample results
compared to the baseline.
- Use the geometric mean S/P, the population CVs and the graphic
distribution for histograms. Analyze these data looking for trends
within herds and compare different locations, geographic
distribution, seasons and production systems.
ELISA technology is the ideal tool for monitoring herds that have
been vaccinated or field-exposed to any type of field PRRS virus. An
effective ELISA test for PRRSV antibodies should detect a response to
both the European type and the American type of strains, as well as be
able to detect the majority of field virus with a single test. Some
diagnostic kit companies are recommending the use of ELISA in PRRSV
management for typing purposes, but laboratory technicians need to run
two different ELISAs in order to compare the level of seroconversion
for the European kit versus the American kit, and then "assume" that
the higher seroconversion should be linked with the type of strain
that is more predominant in the herd.
There are tests available with a better level of specificity and
with more accuracy for the typing of PRRS viruses such as RT-PCR and
orf sequence analysis. These tests can be very helpful for performing
epidemiological studies on the farm or in a company's swine
operations. For routine screening to assess the presence of virus or
response to vaccinations and natural exposures, ELISA should play an
important role as a screening tool.5
The HerdChek PRRS 2XR Antibody Test Kit should be considered for
screening and monitoring swine herds, because it can detect the most
predominant European or American type of PRRS viruses that can be
present at the farm, with a good balance of detection for both type of
strains.
Table 1. Sensitivity of four commercially
available PRRS antibody ELISA test kits using a set of
well-characterized serum samples, from pigs known to be exposed to
Lelystad (European strain) and U.S. (North American strain) PRRSV
strains.
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In Swine Testing
