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FlockChek* Newcastle Disease Antibody Test Kit:
Avian Newcastle disease (NDV) is a highly contagious viral
disease of chickens, usually manifested as a respiratory condition,
and may cause high mortality rates in young flocks or poorly
protected flocks. An assessment of immune status, as well as
serological identification of NDV, requires a measurement of
antibodies to NDV in serum.
Conventional profiling was often done using haemaglutination
inhibition (HI). Some of the drawbacks to using HI are that it can
be cumbersome, time consuming and inconsistent in results.
Additionally, results vary from lab to lab, and the interpretation
is dependent upon how the NDV antigen is prepared. The FlockChek*
Newcastle Disease Antibody Test Kit provides a sensitive and
specific method of antibody quantification. The assay can be
performed in less than two hours, greatly reducing the time required
for serological detection of NDV vaccine immune response or field
challenge exposure. The vaccine response in broilers can be
monitored by establishing baseline titer ranges based on the decay
of maternal antibodies sampled at three days of age, and live
vaccination titers at 35 days of age. The use of paired samples with
blades taken at 35 days of age and at the processing plant can be
used to help define the dynamics of a field exposure and the
interaction with the current vaccination program.
The vaccine response in broiler breeders and commercial layers
can be monitored by routine bleeding of flocks to the decay of
maternal antibodies and the vaccination program. Following live
vaccination, flocks should be sampled at 5–6 weeks of age, 10
weeks of age, and then 4–5 weeks post-vaccination with the
inactivated vaccine. Further testing dates are at 25–30 weeks
of age, 45 weeks of age and 55–60 weeks of age. The duration of
the immunity should be determined, as well as the coefficient of
variation (CV) in titer for the flock and the interactions of field
NDV exposure. These three parameters correlate with production
performance and clinical conditions.
xChek* is an effective tool to store flock information in a
database format when the initial assay is performed. Data can be
retrieved and explained in different graphic and report formats
using flock identification, farm, etc. Data from a flock that has
been monitored for vaccine immune response to NDV can be presented
as a histogram, which reports flock identification, test date,
number of samples tested, mean titer and CV, along with a bar graph
representing the number of birds whose titers fall within each of
the particular titer groups. Testing flocks that receive the same
vaccination program, after the vaccine has been applied, and
comparing the level and variability in titer from flock-to-flock can
also be done to check the efficacy of vaccine administration. A
homogenous immune response to vaccination typically shows a CV of
less than 35–40%. Improper administration of vaccines has been
shown to increase the CV in the flock. A flock with higher than 40%
CV indicates that there are birds in the flock with little or no
response, in addition to birds with a high or adequate response to
vaccination. Using xChek, you can compare on a flock-to-flock basis,
or with previous flocks using the same vaccination program, or any
other variations in vaccine methodology, such as drinking water vs.
spray or different droplet sizes (coarse spray vs. fine spray). The
use of printed reports and histograms makes the site-by-site and
vaccine variable comparisons easy. Establishing baselines is easily
accomplished using the baseline feature in the xChek software. The
software facilitates the choice of flocks to be included in your
baseline, and automatically calculates the normal mean titer for all
of the flocks selected, the minimum and maximum titer within flocks
selected, and the variability between flocks.
For maternal antibody decay, we suggest using at least 23 baby
chicks that have hydration levels of at least 35 grams (40 grams is
ideal), no yolk sac retention, and good absorption of the egg yolk
with no hemorrhagic or yellowish-green discharge. These chicks
should be selected from different breeder ages (~35 weeks, ~45 weeks
and ~55 weeks). With a bigger sample size and higher quality of
chicks, you can establish a reliable baseline for your biosecurity
programs. At the processing plant, flocks should be sampled randomly
(15 samples per flock) each day and identified by locations,
complex, farm and house. At the end of the week, these samples
should be screened for NDV antibody. Over a period of six months to
one year, a baseline is established for what is a "normal" or
expected NDV titer in a geographic area, complex, farm, etc. Changes
in flock data as testing continues will flag potential problems that
may be developing in particular farms or broiler-complexes. This can
help in updating the biosecurity measures to reduce field challenge
pressure, as well as any changes in the vaccination programs
regarding vaccine strains—day of vaccination, adding a booster,
changes in the vaccine application methodology, etc. Seasonal
changes in titer can also be tracked using processing-plant
monitoring.
The Newcastle Disease Antibody Test Kit correlates well to
traditional HI testing. Graph 1 shows the correlation between the
Newcastle Disease Antibody Test Kit and HI titers.
Graph 2 shows the Newcastle Disease Antibody Test Kit high,
medium and low titers. Low NDV titers are common in low
NDV-challenged areas, where the B1-B1 Hichner NDV strain is used for
the control of Lentogenic NDV strains. These mean titers need to be
analyzed for the coefficient of variation (CV), where low CVs are
associated with a good uniform application of vaccination, and high
CVs with low titers are suggestive of a need to improve the vaccine
application methods. Medium titers with a high CV can indicate a
reaction factor to the vaccines themselves (i.e., "rolling factor").
However, medium titers with a low CV may also be an indication of
NDV-challenged situations, where the flocks were properly
vaccinated, but biosecurity measures allowed repeated exposure on
the farm. A low CV with these types of titers can also be associated
with an early challenge, but the titer will often be observed in the
upper level of the medium titer ranges.
High titers are usually related with field challenged or endemic
NDV areas, where an NDV strain is likely present. When broiler
operations and commercial layers use inactivated vaccines and face
continuous field NDV pressure, titers are observed to be high and
the CV will change according to the timing of the field challenge
and the age that the samples were taken. Recent studies from Dr.
John Smith, DVM, MS, MAM, have shown that high NDV ELISA titers were
related to rolling reaction situation caused by B1-B1 NDV vaccine
strain, due to the interference with the infectious bronchitis virus
(IBV) fraction of combined live NDV+IBV vaccines. Titers of
12,000–16,000 at the processing plant indicated the vaccine
rolling.1
Graph 3 shows the performance of different commercial ELISAs for
NDV. They vary in range of titers and accuracy to indicate and
differentiate vaccine immune response vs. field NDV challenge. The
IDEXX Newcastle Disease Antibody Test Kit nicely distinguishes
between low, medium and high titers, adding value for production
veterinarians in terms of recognizing when a farm is heating up
under field challenge. This allows them to apply preventive medicine
tool(s) to minimize the economic consequences of the clinical
disease and secondary bacteria contamination in the farm. Serum
samples with known low, medium and high titers for NDV were
analyzed.
To summarize, this study showed that:
- Every ELISA for NDV has a different definition of medium and
high titers. This means that baselines between these four products
can be different, especially to interpret medium and high titers.
- The IDEXX FlockChek* Newcastle Antibody Test Kit has a good
separation between low, medium and high titers. This is an
important aspect in the sensitivity of detecting real field
challenge and not misleading the production veterinarian in the
interpretation of results.
- The Newcastle Antibody Test Kit has good linear relationship of
low, medium and high titers
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In Poultry Testing
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