Equine salmonellosis

M G COLLETT AND T D MOGG

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

Introduction

Salmonellosis is an important infectious disease of horses of all ages, usually caused by non-host-specific salmonellas (especially Salmonella Typhimurium). The disease occurs in most countries and is diagnosed sporadically. Depending on the age group, the disease manifests as peracute septicaemia, acute enterotyphlocolitis, chronic diarrhoea, failure to thrive or, more rarely, polyarthritis, orchitis, fistulous withers or abortion. Salmonellosis may affect individual horses or, at times, it occurs in extensive epidemics in groups of horses that are at risk. Outbreaks in foals on stud farms have been reported in several countries.^{7, 9, 45, 87} Foals and aged, debilitated or stressed horses are particularly susceptible.^{27, 87} Asymptomatic carriers play an important role in the epidemiology of the disease. Nosocomial Salmonella infections are common in large equine clinics or hospitals.^{21, 49} Severe outbreaks in such institutions may force their temporary closure in order to control the disease.⁸

More than a century ago, the most common manifestation of equine salmonellosis was abortion due to the hostspecific Salmonella Abortusequi.^{6, 27} Since then the prevalence of abortion has declined markedly to the extent that isolations of S. Abortusequi in the USA virtually ceased by the 1970s.^{49, 50} Abortion is still a problem in India,^{26, 72} where other Salmonella serovars are also implicated in the syndrome,⁴² as well as in Croatia,⁴⁴ Italy¹⁷ and Argentina.²⁵ Although Henning in 1956³¹ reported S. Abortusequi as the cause of purulent tendovaginitis, bursitis and pneumonia in horses in South Africa, it has not been isolated in the country since 1970.³²

Aetiology

For the general morphological and physicochemical properties of the salmonellas, consult the introduction, Salmonella spp. infections.

At least 40 of the known serovars of Salmonella have been isolated from horses. Several serovars may occur in contaminated environments at the same time, and the simultaneous infection of an animal with two or even more serovars is not uncommon. When this does occur, it is usually reason for a poor prognosis.^{50, 68}

Apart from S. Typhimurium, other serovars isolated from cases of equid salmonellosis include S. Enteritidis, S. Anatum, S. Newport, S. Heidelberg, S. Typhimurium var. Copenhagen, S. Kottbus, S. Dublin, S. Saint-paul, S. Hadar, S. Ohio, S. Agona, and S. Krefeld.^{7, 8, 22, 27, 50, 67, 77} Some investigators regard S. Typhimurium and S. Typhimurium var. Copenhagen to be the most pathogenic,⁸ while others believe that S. Anatum infection has the gravest prognosis.⁵⁰ Numerous phage types of S. Typhimurium have been shown to be involved in equine infections. However, in most cases no clear conclusions can be reached regarding the origin of the various types, even of phage types known to originate from wild birds, such as finches, sparrows and pigeons.^{7, 27} It appears that the susceptibility of horses when exposed to sufficient numbers of salmonellas — irrespective of sero- or phage type — is similar to that in other animal species.²⁷

Epidemiology

The prevalence of salmonellosis in horses on stud farms has been reported to range from 0,36 to 27 per cent. It has been estimated that up to 10 per cent of horses in the USA may be infected with the organism at some time or another.⁵⁰ In a recent study in the USA the prevalence of faecal shedding of a Salmonella serovar in normal horses was 0,8 per cent: isolates from faeces were from eight serogroups and represented 14 serotypes, the most common being S. Muenchen.⁸¹

Horses most frequently become infected via the oral route, although infection may also take place through the mucous membranes of the eyes and the nose via aerosol droplets.⁷⁵ Experimental infection of horses can be achieved with oral doses ranging from 9,5 × 10⁶ to 8,8 × 10¹¹ organisms.⁶⁸

There are many environmental sources of infection, but particular problems are posed by contamination of water by effluent, contaminated feed (meat or bone meal and milk products) and dietary additives, as well as carrier birds, rodents, horses and other farm animal species that excrete the bacteria.^{27, 87}

In a recent study in the USA, the prevalence of salmonellas in grain and other concentrates used for horse feed was 0,4 per cent. Isolates from feed represented three serotypes, none of which were also identified in the faeces of shedding horses.⁸¹ An infected foal can excrete 3 × 10⁵ organisms per gram of faeces, resulting in significant environmental contamination.⁵⁰

In a population of horses, up to 24 per cent may be asymptomatic carriers, based on faecal cultures.^{67, 76, 81} In one Australian study, 1,65 per cent of the normal non-hospitalized horse population were found to be shedding salmonellas in their faeces, while 23,8 per cent of horses hospitalized in a veterinary teaching hospital were shedders.⁶⁷ The prevalence of asymptomatic carriers in healthy adult horses, as assessed by culture of mesenteric lymph nodes at a slaughter facility, was 71,4 per cent.⁴⁷ On the other hand, another study based on the culture of mesenteric lymph nodes from horses submitted for necropsy at a veterinary teaching hospital found a Salmonella prevalence of only 1,96 per cent.³⁸ On studs experiencing outbreaks of salmonellosis in...