Porcine pasteurellosis

S S BASTIANELLO AND J W NESBIT

GENERAL INTRODUCTION: FACULTATIVELY ANAEROBIC GRAM-NEGATIVE RODS Pasteurella and Mannheimia spp. infections

Introduction

Porcine pasteurellosis may manifest as septicaemic or pneumonic forms.^{5–7, 11, 12} Septicaemic pasteurellosis occurs in both neonatal and juvenile pigs while pneumonic pasteurellosis is a disease of predominantly juvenile animals. The infection, in conjunction with other infectious agents, Bordetella bronchiseptica in particular, is also implicated in the aetiology of atrophic rhinitis (see Bordetella bronchiseptica infections); a syndrome peculiar to pigs.^{2, 3, 5, 8, 13}

Aetiology

In pigs, infection by Pasteurella multocida is more prevalent than that by Mannheimia (Pasteurella) haemolytica. Of the isolations made from the specimens submitted to the Onderstepoort Veterinary Research Institute, about 92 per cent were P. multocida, ⁶ per cent M. haemolytica and 2 per cent comprised P. pneumotropica or untypable species of Pasteurella. ⁹ Pasteurella multocida serotypes A and D predominate in isolates from specimens obtained from pigs that suffered from septicaemia or pneumonia in South Africa.⁹ Haemorrhagic septicaemia caused by P. multocida serotype B has not been recorded in pigs in the region. It has, however, been implicated in an outbreak of the disease in India.¹¹ Whilst of rare occurrence in pigs, M. haemolytica (biotype and serotype undetermined) has been isolated from aborted foetuses and neonatal piglets suffering from septicaemia^{5, 6} and from pneumonic lesions in juvenile pigs.^{8, 15}

Toxigenic strains of P. multocida serotype D are also associated with atrophic rhinitis in pigs.^{2–5, 8, 13}

Septicaemic pasteurellosis in pigs

Septicaemic pasteurellosis caused by P. multocida has been reported in juvenile and adult pigs.^{5, 11} Although specific pathogen-free animals are apparently at greatest risk,⁵ the prevalence of this form of the malady, irrespective of whether caused by P. multocida or M. haemolytica, is higher in neonatal pigs.^{5–7, 15} The mode of transmission is unknown although infection via the umbilicus and/or the respiratory tract have been mooted.^{7, 10}

The course of the disease is peracute, death supervening after a period of illness that lasts no more than 12 hours in most cases, but rarely 24 to 48 hours.^{5, 6, 11} Clinical signs usually include bluish-red discoloration of the skin of the snout, ears and ventral regions of the neck, flanks and abdomen,lowered habitus, fever, dyspnoea and reluctance to move.^{6, 11} Generalized congestion, cutaneous lividity, subcutaneous oedema and petechiation, serosanguineous effusions in the body cavities and extensive ecchymotic haemorrhages in the lungs and epicardium are often present at necropsy.^{5, 6, 11} Acute catarrhal to fibrinous pharyngitis accompanied by peripharyngeal oedema, severe congestion of the cranial cervical lymph nodes,¹¹ and rarely, also acute fibrinous meningitis, particularly in neonatal pigs, may be encountered.⁵

The diagnosis is confirmed on the basis of isolation of the bacterium in pure culture from specimens of blood, brain, or other suitable tissues (spleen, lymph nodes and liver). The differential diagnosis should include other diseases associated with sudden death, such as African swine fever, colibacillosis, salmonellosis, erysipelas, Haemophilus septicaemia, streptococcal septicaemia, porcine pleuropneumonia and leptospirosis.

Specific antibacterial therapy is hampered by the sporadic nature and acute course of the disease. Control may be best achieved by separating diseased animals from healthy susceptible pigs and improving in the standards of housing, nutrition and hygiene.

Pneumonic pasteurellosis in pigs

Epidemiology

Pneumonic pasteurellosis is the most prevalent form of Pasteurella spp. infection in pigs.^{2, 5, 7, 12, 15} Fattening pigs in overcrowded conditions are at greatest risk.^{7, 15}

As in other species, the causative organism is a normal constituent of the flora of the upper respiratory tract, and stress factors (such as overcrowding, transportation, commingling, handling, exposure to excessive amounts of environmental or feed dust, and poor hygienic conditions), sometimes in association with concomitant pulmonic infection, probably trigger dissemination of P. multocida to the lung parenchyma. Intercurrent viral and bacterial infections, such as hog cholera, Aujesky’s disease and swine influenza,andinfections with Bordetella bronchiseptica, Actinobacillus pleuropneumoniae and Salmonella Choleraesuis have been implicated as predisposing factors in pig-producing countries elsewhere in the world.^{2, 5, 7} However, enzootic mycoplasmal pneumonia is the most important predisposing infectious condition on a worldwide basis, including South Africa.^{5, 7, 12, 15} In South Africa pneumonic pasteurellosis occurs very rarely, except as a complication of mycoplasmal pneumonia.¹⁴

Pathogenesis

The available information on the pathogenesis of pneumonic pasteurellosis in pigs is incomplete and refers mostly to the infection as a complication of mycoplasmal pneumonia.^{5, 7} The infection is acquired through aerosol transmission.^{7, 15} Injury to the lungs that has been induced by prior infection by Mycoplasma hyopneumoniae presumably provides not only a portal of entry for P. multocida but also the environment for local proliferation of the bacteria. Adherence to ciliated epithelial cells and evasion of...