Bovine salmonellosis

S G FENWICK AND M G COLLETT

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

Introduction

Bovine salmonellosis usually manifests clinically as a syndrome of septicaemia, acute or chronic enteritis, or abortion. One or more of these syndromes may occur simultaneously in the same animal or herd. Calves less than three months of age are most commonly affected, while animals exposed to stressful situations are particularly prone to contract the disease. Although salmonellosis in cattle is caused by a number of different serovars, Salmonella Dublin and S. Typhimurium are by far the most common. The source of infection is usually an environment contaminated by the faeces of either clinically affected or carrier animals. Carrier animals play an important role in the epidemiology of Salmonella infections. Salmonellosis is a zoonosis.

A disease of calves that was subsequently considered to be salmonellosis was first diagnosed in South Africa by Hutcheon in 1893.⁴⁸ It was similar to the condition described by O. Henning in 1894 as ‘lewerziekte’ (liver disease) or ‘yellow liver’.⁴⁸ Salmonella Dublin as a cause of abortion in cattle in South Africa was first reported in 1943.¹⁰

Salmonellosis in both calves and adult cattle occurs in most countries of the world and has been shown to be economically important in Europe and North America.^{61, 102, 150} In any outbreak, the economic impact will depend on the rates of morbidity and mortality.⁹⁷ In South Africa, the disease is frequently diagnosed in calves, but it is only rarely encountered as a clinical problem in adult cattle.^{48, 122} Although an accurate estimate of the occurrence of salmonellosis in southern Africa is not available, its importance (especially in calves) is likely to be similar to that in many other parts of the world.⁵⁶

Aetiology

Serovars of the genus Salmonella are identified on the basis of specific combinations of somatic and flagellar antigens and, to a lesser extent, by their biochemical reactions (see the introduction to Salmonella spp. infections).

Salmonella Dublin and S. Typhimurium are the serovars most frequently associated with bovine salmonellosis;^{49, 83, 102} other serovars (e.g. S. Newport, S. Bovismorbificans) are incriminated only rarely.^{5, 17, 150} Henning in 1939⁴⁷ and 1953⁴⁸ reported that more than 95 per cent of cases of salmonellosis in calves in South Africa were due to S. Dublin, while Botes in 1965¹¹ recorded that S. Typhimurium infection was responsible for up to 19,1 per cent of cases. Recently in New Zealand, S. Brandenburg has been reported increasingly as a cause of enteritis and abortions.²⁰

In the UK, epidemics due to S. Typhimurium phage types DT104, DT193 and DT204C, which are resistant to a range of antibacterials (such as chloramphenicol), have emerged since 1977.^{55, 128, 152} The phage type DT104 possesses in vitro resistance to ampicillin, chloramphenicol, streptomycin, sulphonamides and tetracycline (R-type ACSSuT), as well as apramycin,⁷⁶ and has also been detected in France⁶ and the USA.^{8, 9} Of even more concern is the increasing emergence of trimethoprim and fluoroquinolone resistance in phage type DT104,^{25, 130} and that resistant clones can spread rapidly.¹

For information on the general characteristics of the salmonellas, consult the introduction to Salmonella spp. infections.

Epidemiology

The diseases in cattle caused by S. Dublin and S. Typhimurium are clinically indistinguishable, but they differ epidemiologically (Table 151.1).^{2, 112, 151} The host preferences¹⁴⁵ and the ability of the two serovars to establish carriers in animals following initial infection also differ.^{2, 13, 16, 46, 105, 145, 151, 156}

Salmonella Dublin is regarded as a serovar specific to cattle and only rarely infects other species of animals and humans.^{13, 116, 141}On the other hand,most other serovars, including S. Typhimurium, are not host-specific,¹⁵⁶ and direct or indirect transmission of S. Typhimurium between cattle as well as from other domestic (e.g. sheep) or wild animals to cattle, and vice versa, may occur.^{13, 110, 156}

Table 151.1 Important epidemiological differences between Salmonella Dublin and Salmonella Typhimurium infections in cattle

Calves aged between 2 and 12 weeks are the most frequent victims of salmonellosis, but mature cattle may also suffer from clinical disease.^{2, 3, 11, 49, 105, 110, 145} In calves infected with S. Dublin, salmonellosis usually occurs at between 6 and 12 weeks of age, whereas disease caused by S. Typhimurium is most common in the first three weeks of life.^{11, 158} The possible explanation for this difference is that S. Dublin infection is more prevalent in adult cattle than S. Typhimurium;¹⁵⁶ consequently, the presence of...