Actinobacillus pleuropneumoniae (App) is the bacterial cause of porcine pleuropneumonia. App continues to be present on a world wide basis. In Ontario, the availability of serological testing for App allowed for a significant reduction in the number of breeding herds infected with the more problematic types of App. The disease, however, is still present in Ontario. In Canada, a serious outbreak caused by a non-traditionally highly virulent serotype 8 has been described. A recent outbreak of App serotype 15 in the midwest USA caused some concern with respect to rapid spread between grow-finish operations. Ultimately suboptimal biosecurity compliance was found to be the biggest cause of spread in the App 15 outbreak. Of the 19 serotypes of App that have been described so far, serotype 12 is generally considered to be of low virulence and is primarily a subclinical disease. The most important factors involved in the pathogenesis of the infection are the secreted protein RTX-toxins, ApxI, ApxII, and ApxIII.
In this investigation the researchers described four atypical serotype 12 field strains recovered from severe clinical App outbreaks in Chile.
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These Chilean strains exhibited an unusual toxin gene profile (apxIICA, apxIIICA, apxIBD, apxIIIBD), suggesting the ability to produce both ApxII and ApxIII toxins.
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Comparative genomic analyses revealed that these atypical strains carry the capsule genes of serotype 12 but the LPS biosynthesis genes of serotype 15, indicating a hybrid genomic structure.
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Phylogenetic analysis confirmed that they cluster separately from classical serotype 12 strains and are closely related to other atypical strains from Japan, USA, and Canada.
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Experimental infection in pigs demonstrated significantly increased virulence of the atypical Chilean strain compared to the reference strain of serotype 12, with higher clinical scores and severe lung lesions.
Take Home Messages:
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While most serotype 12 isolates continue to be low-virulence strains there are a few App serotype 12 isolates that are also highly virulent.
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When a herd is experiencing significant clinical signs of App and only App 12 is being detected then additional diagnostic approaches that incorporate both capsule and LPS profiling may be required to sort out the situation.
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The existence of these atypical hybrid App strains is an example of the gradual evolution of pathogens and this change certainly keeps us humble.
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Finding an App serotype 12 in a herd may seem to be an innocuous finding but we should always be wary of “the potential for a wolf in sheep’s clothing”.
Reference: Antony T Vincent , Sonia Lacouture , Guillaume St-Jean , Rodrigo Tapia , Servane Payen , Kon Michiha , Joachim Frey , To Ho , Marcelo Gottschalk Atypical Actinobacillus pleuropneumoniae serotype 12 strains with a higher virulence potential Vet Res . 2025 Jul 13;56(1):149. doi: 10.1186/s13567-025-01579-9.
