Porcine pleuropneumonia

M-L PENRITH AND M M HENTON

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS Actinobacillus spp. infections

Introduction

Porcine pleuropneumonia, caused by Actinobacillus pleuropneumoniae, is a highly contagious and often fatal disease that most often affects pigs aged between 12 and 15 weeks, although animals of all ages are susceptible.^{27, 55, 69, 81, 82} It may manifest clinically as a peracute, acute, subacute or chronic disease, the more acute forms being characterized by the presence of a fibrinous to fibrinonecrotic bronchopneumonia, and the chronic by the development of pulmonary sequestra and reduced weight gain. Pigs that survive acute disease, as well as healthy animals that have never developed clinical signs of the disease, may be carriers of the infection.^{17, 55}

A Haemophilus-like organism was first described from cases of porcine pleuropneumonia in England in 1957.⁶¹ Since the late 1960s, porcine pleuropneumonia has been of major economic importance in European countries, particularly Scandinavia,⁶⁹ and has also become increasingly important to the pig industry in many countries throughout the world. In South Africa the organism was isolated for the first time in 1985. In contrast to most countries where it initially caused explosive outbreaks of disease with high mortality, the first cases diagnosed in South Africa were characterized by the development of an insidious form of chronic pneumonia complicated by simultaneous infections with Pasteurella multocida, Streptococcus dysgalactiae var. dysgalactiae, Streptococcus suis or Trueperella pyogenes. Subsequently, the more typical peracute, acute, subacute and chronic forms of the disease have occurred in South Africa. Most outbreaks have almost exclusively involved pigs aged between two and six months.

Outbreaks of porcine pleuropneumonia are the cause of severe economic losses as a result of the high mortality rateand high cost of chemotherapy. The subclinical and chronic forms of the disease may result in reduced herd performance.^{55, 66, 82}

Aetiology

The genus Actinobacillus belongs to the family Pasteurellaceae, which comprises short, Gram-negative, non-motile,non-spore-forming, pleomorphic rods, with a tendency to occur mainly in coccobacillary forms. The other two members of the family are Haemophilus and Pasteurella.

The genus Haemophilus is defined by growth factor requirements, which have also been used as a criterion for speciation within the genus.⁴⁰ These, possibly plasmid-controlled, factors²¹ are Factor X (protoporphyrin IX or protoheme) and Factor V (nicotinamide adenine dinucleotide or NAD).

As most strains of A. pleuropneumoniae require Factor V, or Factor V-related compounds, the species was initially placed in the genus Haemophilus. However, on the basis of DNA:DNA hybridization data, it is now classified as an Actinobacillus⁶² and is divided into two biotypes. Biotype 1 requires Factor V and occurs worldwide. Biotype 2 is Factor V-independent (although it still appears to require Factor Vrelated compounds) and has been isolated in Switzerland, the USA, and Canada.^{17, 38, 50}

Actinobacillus pleuropneumoniae is furthermore Factor X-independent and CAMP-positive, ferments mannitol and is β-haemolytic on 5 per cent calf- or sheep-blood agar (Table 161.1).⁴⁰ It is urease-positive, but a urease-negative variant of A. pleuropneumoniae has been described.⁸ Actinobacillus pleuropneumoniae forms small, whitish, opaque colonies (1 to 2mm in diameter) within 48 hours of incubation on chocolate blood agar. Two types of colonies have been described: a rough, round, hard, waxy type that adheres to wire inoculating loops, and a flatter, soft and more readily serotypable, glistening colony.⁴⁰ Most virulent strains are heavily capsulated and produce iridescent colonies on Levinthal agar.³⁹

The antigenic properties of A. pleuropneumoniae have been extensively researched.^{12, 19, 49, 54, 58} There are antigenic, serotype-specific, membrane lipopolysaccharides and capsular polysaccharides.⁴¹ Resistance to complement-mediated host defences is due to the capsule,¹⁷ and nonencapsulated mutants are generally avirulent and rapidly cleared.³² Based on differences in the capsular polysaccharides, A. pleuropneumoniae biotype 1 is divided into 12 serotypes. ⁵⁵ Common antigens (lipopolysaccharide and outer membrane proteins)^{32, 46, 56} exist that complicate diagnostic and epidemiological studies based on serology.⁵⁹ Cross– reactions are common between serotypes ^{1, 9 and 11,} between 4 and 7, and between ^{3, 6} and ^{8,41, 46, 56} but cross-reactivity between virtually all serotypes can occur, depending on the antigen and test method used.²⁴ The use of monoclonal antibodies yields more specific results.⁴⁸ Isolates that cannot be typed have been reported.^{7, 17}

Table 161.1 Simplified bacteriological differentiation criteria of Actinobacillus spp. that may be isolated from pigs³¹

v = variable

Epidemiology

Actinobacillus pleuropneumoniae is a primary pathogen of the respiratory tract of pigs. It rarely affects other animals. The only reports of isolation from lesions in other species are a case of arthritis...