Proposal of a new serovar of Actinobacillus pleuropneumoniae: serovar 15

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Abstract

We report on the re-examination of nine Australian isolates of Actinobacillus pleuropneumoniae that have been previously assigned to serovar 12. In the ring precipitation test, none of the nine isolates reacted with antisera to serovars 1–14 of A. pleuropneumoniae. Antiserum prepared against one of the Australian isolates gave no reaction with any of the 14 recognised serovar reference strains, except serovar 7. This reaction of the HS143 antiserum with serovar 7 antigen could be removed by adsorption with serovar 7 antigen. The adsorbed antiserum remained reactive with HS143 and the other eight Australian isolates. The nine Australian isolates were all shown to express ApxII and ApxIII, found in serovars 2, 4, 6 and 8, as well as the 42 kDa outer membrane protein found in all serovars of A. pleuropneumoniae. The nine Australian isolates were found to possess the following toxin associated genes apxIBD, apxIICA, apxIIICA, apxIIIBD and apxIVA. The toxin gene profile of the Australian isolates is typical of A. pleuropneumoniae serovars 2, 4, 6 and 8. On the basis of the serological characterisation results and the toxin gene profiles, we propose that these isolates represent a new serovar of A. pleuropneumoniae — serovar 15 — with HS143 being the reference strain for the new serovar.

Introduction

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia, a severe respiratory disease of pigs that is of economic importance wherever pigs are raised (Nicolet, 1992). Two different biovars are recognised within the species — with biovar 1 being dependent upon nicotinamide adenine dinucleotide (NAD), V factor, for growth in vitro while biovar 2 is NAD-independent (Nicolet, 1992). Within biovar 1, 12 serovars have been recognised to date (Nicolet, 1992). Initially, two serovars were proposed within biovar 2 (Fodor et al., 1989). However, later reports indicated that some biovar 2 isolates shared type-specific antigens with biovar 1 serovars and have been allocated to serovars 2, 4, 7 and 9 within biovar 1 (Nicolet, 1982, Beck et al., 1994, Kamp et al., 1994). Recently, Nielsen et al. (1997) proposed an integration of the serotyping schemes for biovars 1 and 2 — with two new serovars, serovars 13 and 14, belonging to biovar 2 being added. Hence, A. pleuropneumoniae isolates, regardless of NAD requirement are now allocated to one of 14 recognised serovars.

A knowledge of the serovars prevalent in a region or on a farm is important as inactivated vaccines can protect only against those serovars present in the vaccine (Nielsen, 1976, Nielsen, 1984). In Australia, three serological characterisation studies have been performed to date (Eaves and Blackall, 1988, Blackall and Pahoff, 1995, Blackall et al., 1999). In the 1995 study, almost 50% of isolates could not be confidently assigned to any of the 12 recognised serovars (Blackall and Pahoff, 1995). The 1999 study reported that those isolates which were difficult to serotype were related to serovar 12 (Blackall et al., 1999).

In recent studies, we have accumulated evidence that those Australian isolates previously assigned to serovar 12 show properties that are distinctly different from the accepted properties of true serovar 12 isolates. In this paper, we present this evidence and formally propose the creation of serovar 15 of A. pleuropneumoniae.

Section snippets

Bacteria

The nine Australian field isolates used in this study were originally isolated in the period from 1990 to 1992 and were derived from pigs in located in two eastern states (Queensland and New South Wales) and one western state (Western Australia). As well, the reference strains for all 14 of the currently recognised serovars of A. pleuropneumoniae (strains 4074, S1536, 1421, M62, K17, FemΦ, WF83, 405, CVJ 13621, 22009, 56153, 1096, N273 and 3906, serovars 1–14, respectively) were used in this

Results

In the RPT, the nine Australian isolates showed no reaction to antisera raised against the 14 recognised serovars of A. pleuropneumoniae. In the RPT, a serum raised against Australian isolate HS143 reacted with the serovar 7 type strain but none of the other 13 serovar reference strains. After adsorption of the HS143 antiserum with antigen prepared from the serovar 7 reference strain, there was no reaction in the RPT between the adsorbed HS143 antiserum and antigens of any of the 14 recognised

Discussion

An important point in resolving the status of HS143 was to examine the serological reaction between this isolate and the previously proposed serovars of A. pleuropneumoniae. While A. pleuropneumoniae serovars 1–12 have been recognised for some years, serovars 13 and 14 have only recently been proposed. Hence, it was important to ensure that valid specific antisera to all 14 currently recognised serovars was available. We found that the serovar 13 and 14 antisera showed some cross-reactions —

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