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Winter 1997 Newsletter


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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: 

  1. Be sure to select the appropriate target population. 
  2. Be sure to test a representative number of animals. 
  3. Decide which serotype(s) of APP is (are) important for your producer. 
  4. 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. 
  5. 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. 
  6. 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. 
  7. 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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