Porcine salmonellosis

M-L PENRITH, J A NESER AND M M HENTON

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

Introduction

Infection of pigs with certain serovars of Salmonella results in clinical disease with important consequences for pig production. Pigs can also act as reservoirs for a wide variety of salmonellas with zoonotic potential, leading to their introduction into the human food chain.¹⁴

Porcine salmonellosis usually manifests as septicaemia or enterocolitis in weaned pigs, and can result in serious production losses. Salmonella was first isolated from diseased pigs in 1886 by Salmon and Smith.⁵² The pigs proved to be suffering from classical swine fever (hog cholera), and it was only when that disease was eradicated from the USA that it was recognized that the bacterium isolated, named Salmonella Choleraesuis, was capable of causing septicaemia in pigs.^{14, 52} Enterocolitis caused by S. Typhimurium is also well known in pigs, and at present is the most common form of porcine salmonellosis in pigs in Europe, while S. Choleraesuis remains an important cause of disease in the USA.^{14, 42, 43} Salmonella Choleraesuis was isolated from pigs in outbreaks of suspected African swine fever in South Africa in the first half of the last century,¹² but was subsequently, until 2000, only rarely diagnosed in South Africa. Although on average several thousand isolates from all domestic animal species are received annually by the Onderstepoort Veterinary Institute for typing, numbers from pigs are small (Table 153.1). This suggests that porcine salmonellosis as a disease is uncommon in South Africa, and in recent years until 2000 S. Typhimurium has been the most frequent isolate (Table 153.1). However, in 2000, four isolates of S. Choleraesuis were obtained from three farms. Two were separated by a river, but with road access from different areas, and the third was 50 km distant, with ostensibly no contact with the other two. The source of this pig-associated Salmonella infection is unknown, but its occurrence demonstrates the unpredictability of outbreaks and emphasizes the need for laboratory examination of suspect cases.

Salmonellosis in humans, caused by numerous serovars, the most notable of which is S. Typhimurium, is an important food-borne disease. It usually manifests as a severe gastro-intestinal disturbance that may be fatal in older people and children. Increasing concern about the potential of animal products to cause disease in humans has resulted in the institution of quality control programmes that include routine on-farm and abattoir sampling for pathogenic organisms. Salmonellas are commonly isolated from both healthy and diseased pigs, and, unless associated with characteristic clinical signs and lesions, are unlikely to be significant to the health of the pig. However, outbreaks of human salmonellosis have been traced to pork products.^{10, 14, 38} The problem is compounded by the demonstration of multiple antibiotic resistance among some of the strains commonly isolated.^{2, 14, 22, 38}

Aetiology

For general information on the morphology, physicochemical and other properties of salmonellas, consult the introduction to Salmonella sp. infections.

Most cases of porcine salmonellosis have been associated with S. Choleraesuis or S. Typhimurium. Salmonella Choleraesuis is associated mainly with septicaemia, but pneumonia also occurs as a separate entity.^{26, 43, 46} Meningoencephalitis may occur in a limited number of cases.^{43, 50} Infection with S. Typhimurium usually presents as enterocolitis, and rectal strictures, generally as a complication of colitis, have been reported and experimentally confirmed.^{48, 49} Outbreaks of disease caused by S. Typhisuis have been reported in Europe, Asia and the USA.^{5, 17, 52} Salmonella Typhisuis does not grow well under standard conditions for culture of salmonellas, and it has therefore been suggested that this organism may be underdiagnosed.⁵² Salmonella Dublin and S. Enteritidis have been reported from cases of meningitis in suckling pigs.^{29, 32, 40, 52} Numerous serovars have been isolated from healthy pigs.¹⁴

Table 153.1 Salmonella isolates from diseased pigs typed annually at the Onderstepoort Veterinary Institute, 1994 to 2000

Epidemiology

Salmonellas are ubiquitous organisms that have been isolated from virtually every species of vertebrate from which culture has been attempted.⁵² They are also relatively resistant in the environment, which may consequently remain...