Ovine and caprine salmonellosis

J A NESER, M M HENTON AND S G FENWICK

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

Introduction

Salmonella serovar infections in sheep and goats are manifested by the development of two distinct syndromes: an acute to subacute septicaemic disease with an associated diarrhoea, and abortion. The septicaemic syndrome is most frequently caused by S. Typhimurium and is precipitated by stress. It is characterized by the development of a waterygreen to blood-tinged diarrhoea, the consequence of a severe necrohaemorrhagic ileitis, typhilitis and colitis. Although the septicaemic disease in sheep is less prevalent than bovine salmonellosis, its occurrence and economic importance appear to be increasing in South Africa with the development of the sheep feedlot industry. In the UK, Australia and Canada, Salmonella Abortusovis,^{30, 67} S. Dublin,^{3, 67} S. Typhimurium,^{13, 29} S. Montevideo⁶⁵ and S. Arizonae⁴⁰ infections have been incriminated as causes of abortion in sheep. More recently, S. Brandenburg has also been reported as a cause of large-scale abortions in sheep in New Zealand.⁸ Abortions due to infections by Salmonella serovars have also been documented in South Africa.¹

Aetiology

Salmonella Typhimurium is most frequently isolated from sheep in South Africa while S. Dublin, S. Enteritidis and others are only rarely encountered.¹ Isolations of Salmonella from sheep in the UK from 1991 to 1997 showed that S. Typhimurium, followed by S. Montevideo, S. Derby and S. Dublin, were most commonly found.³⁸ In New Zealand, S. Hindmarsh is the most commonly isolated serovar from enteric disease ⁵ and S. Brandenburg from cases of abortion.⁸ For information on the characteristics of the salmonellas, consult the introduction to Salmonella spp. infections.

Epidemiology

Salmonellosis in sheep and goats occurs under both intensive and extensive farming conditions.

Sheep of all age groups are susceptible, but two- to four-toothed sheep are most commonly affected when they are marketed from pasture and consigned to abattoirs or feedlots.⁶³ Since 1936 when S. Typhimurium and S. Onderstepoort infections of sheep were first reported in South Africa,²⁵ salmonellosis has often been associated with mortality in sheep which have been railed over long distances to the Onderstepoort Veterinary Institute. Salmonellosis is also reported as a major cause of losses to the sheep export industry of Australia during large-scale shipment to the Middle East.^{27, 63} With few exceptions, there is therefore a history of preceding transport lasting several hours.¹ Most outbreaks of the disease in South Africa occur within one to four weeks of the introduction of sheep from pastoral grazing into feedlots where diets comprising high proportions of grain and insufficient amounts of roughage are fed. Other nutritional practices, such as a too rapid change to finishing rations with a high (12,5 per cent) nonprotein nitrogen content which favours the proliferation of salmonellas in the digestive tract,^{56,10, 19, 28}

The disease also occurs under pastoral grazing conditions after sheep have been crowded during dipping and shearing.^{29, 63} Although the source of infection in outbreaks is often obscure, it can sometimes be traced back to animals that have been exposed to poor hygienic conditions along the course of transit. Heavy contamination of holding pens, road or railway vehicles, water troughs, abattoir or feedlot yards, dams and streams by Salmonella-infected faeces of carrier animals and birds has been reported.^{13, 24, 28, 29} In New Zealand, abortions caused by S. Brandenburg have occurred on the South Island since 1996. Spread of the agent between regions was due to transporting animals from drought-affected areas. In a large outbreak in 1998 involving 130 farms, seagulls scavenging on the products of abortion were believed to have contributed to the spread of S. Brandenburg between farms.¹¹

Salmonella Typhimurium has been shown to survive for at least 251 days in soil and for 168 days on pastures.^{29, 32} Under field conditions, infection by the oral route is the most important, but sheep may also become infected by the nasal route.^{30, 60, 61} Although S. Abortusovis has been isolated from the male genital tract on several occasions, venereal transmission has only been demonstrated experimentally.⁷⁶ Stress-inducing factors, such as mobbing, starvation, and a high saline content of drinking water have been associated with abortions due to S. Dublin and S. Typhimurium.^{3, 13, 63} In one study, abortions in twin and triplet-bearing ewes were more common due to S. Brandenburg infection than in ewes bearing single lambs, and poor weather appeared to increase the number of daily abortions during the outbreak.⁶²

Sheep, and possibly goats, may excrete Salmonella serovars in their faeces intermittently.^{1, 36} However, in contrast to infected cattle, which may excrete Salmonella serovars in their faeces for 12 months,²⁹ or even indefinitely^,26 sheep have not been found to excrete S. Typhimurium for longer than 30 days after oral infection, and therefore do not appear to become carriers after this route of infection.⁸² Longer periods of faecal excretion have been observed after intravenous³³ and intranasal infections. Vaginal discharges or products of abortion may also contaminate the environment.^{10, 21, 30, 43, 58, 69} Infected cattle...