Appropriate Use of Indirect ELISA for Actinobacillus
pleuropneumoniae Based on Capsular Polysaccharides
The indirect ELISA for Actinobacillus
pleuropneumoniae (APP) based on capsular polysaccharides
is useful for surveillance of swine herds and detection of
chronically infected animals. It can be used as part of a
continuous health monitoring program. The ELISA detects antibodies
to the capsular polysaccharide of APP serotypes 1, 3, 5 and
7. Since the serotype specificity resides in the capsular
polysaccharides it is also a good test for differentiation
of serotypes.
Cross reactivity between serotypes
that are observed in the same sample or between serum samples
from the same herd could be due to: (1) cross reactivity due
to antibodies against "APP-like" organisms; (2)
cross reactivity with serotypes that do not cause disease
but may produce an antibody response (e.g. serotypes 4 and
7, serotypes 1 and 9).
For the above reasons it is best to
use the test to screen samples from apparently healthy animals.
In addition, it is important to test appropriate age groups
of animals and optimal number of serum samples. Without testing
a sufficient number of animals it is difficult to determine
if a herd is free of APP. Published reports indicate that
a minimum number of 30 serum samples, irrespective of the
herd size, should be tested before a decision is made.
When testing for introduction of animals
into new herds, a sufficient number of animals should be tested
from the herd of origin. Testing only animals that are to
be introduced may not provide you the correct information.
Negative results obtained from quarantined animals do not
guarantee that they are free of APP.
In the case of serological monitoring
for APP, results of the ELISA should be supplemented by information
on bacterial isolation. Deaths due to respiratory illness
should be thoroughly investigated and APP bacterial isolation
attempted. The results should be recorded and used in conjunction
with the ELISA results.
Please consider the following guidelines
when you request serological tests for APP:
- Be sure to select the appropriate target population.
- Be sure to test a representative number of animals.
- Decide which serotype(s) of APP is (are) important for
your producer.
- There is no serological test that is 100% sensitive: remember
that the number of positive results may also depend on the
prevalence of the infection.
- There is no serological test that is 100% specific, even
though this figure was obtained during the validation of
the test. The real and absolute "field" specificity
of a serological test may never be known, since animals
may come in contact with hundreds of different bacterial
species that will never be tested for possible cross-reactions
with APP.
- Use results from the laboratory as a diagnostic tool:
you have to correlate them with other information available.
The final decision on the real health status of the herd
(regarding APP) will be up to you.
- A serologically negative result during the quarantine,
even if it is repeated twice, does not ensure that one specific
animal is not a carrier of APP if the serological status
of the herd of origin is positive, suspicious, or unknown.
*Adapted from, "M. Gottschalk
and R. Bilodeau, 1995, Detecting carrier animals in herds
chronically infected by Actinobacillus pleuropneumoniae:
the detection of antibodies and the detection of the bacteria.
Allen D. Leman Swine Conference".
- by Thiagu Dorairajan
- edited by Ching Ching Wu, DVM, PhD
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