Salmonella spp. infections

Salmonellosis in livestock is caused by infection with both host-specific and non-host-specific Salmonella serovars, and results in enteritis, septicaemia or abortion. It is an economically important disease of cattle in many parts of the world, but may also be responsible for serious sporadic outbreaks of disease in sheep, horses and pigs. Carrier animals and contaminated environments are important sources of infection. The most common serovars associated with disease in livestock are listed in Table 1.

The genus Salmonella is classified in the family Enterobacteriaceae, whose members are Gram-negative cocco-bacilli.³ With the exception of Salmonella Gallinarumpullorum, all salmonellae are motile, as they have peritrichous flagella.

The genus Salmonella consists of two species, S. enterica, which comprises six subspecies (enterica, salamae, arizonae, diarizonae, houtenae, and indica), the distinction between subspecies being based on biochemical reactions, and S. bongori. ^{8, 17–24, 27} Salmonella enterica subsp. enterica contains all the serovars found in warm-blooded animals — those formerly assigned to subgenus 1.

The members of the genus Salmonella are typed into serovars which are differentiated from each other by the combinations of their somatic (O) and flagellar (H) antigens and, to a lesser extent, by their biochemical reactions. The serovars, of which over 2000 have been identified, do not have a species status allocated to them in the current classification of the genus, and the names of the serovars (such as Typhimurium or Dublin) should be used without italicization or underlining, and with the first letter capitalized, e.g. S. enterica subsp. enterica ser. Typhimurium. As it is tedious and impractical to adopt this long, formal nomenclature for everyday use, it is common to refer to serovars as, for example, Salmonella Typhimurium, or as serovar Typhimurium.^{8, 18}

 It should be noted, however, that the use of the shortened nomenclature prohibits abbreviation of the name of the genus (Salmonella = S.) because the abbreviation of the name of a genus is only authorized if it is followed by the name of a species. Nevertheless, as this format is used by many authors, it is also used in this book.

Antigenic composition

The O antigen is part of the lipopolysaccharide component of the cell wall that also contains lipid A and a core portion. The O antigen, or O-specific side chain consists of repetitive oligosaccharide units of which the type, order, and repetition of sugar moieties differ between serovars. These differences and those in the flagella antigens are used to type Salmonella into serovars. At least 67 different O antigens are currently known and they are identified by the Arabic numerals 1 to 67. Some of these occur singly (e.g. 11), while others occur in combination (e.g. 1, 4, 5, 12; and 6, 7) (Table 2).^{17, 24}

Some Salmonella mutants have defects in the synthesis of the oligosaccharide O-specific side chain, with the result that the oligosaccharide is not fully synthesized when cultured. The colonies of such mutant strains have a ground-glass appearance and are therefore referred to as ‘rough’ strains. Colonies of strains which possess the complete oligosaccharide O-specific side chain are smooth and the strains are therefore referred to as ‘smooth’. Rough strains are untypable as they do not agglutinate with O typing antisera.²⁴

Table 1 Most common serovars of Salmonella that infect livestock and the syndromes they induce

Table 2 Antigenic composition of common Salmonella serovars

The complete O antigen is not only the major immunogen of bacteria in this genus, but it also possesses virulence properties (see Virulence factors, below). It elicits both humoral and cellular immune responses to infections by salmonellas. Strains with incomplete O antigens (rough strains) are therefore used for the production of live attenuated vaccines, such as the calf paratyphoid (Salmonella Dublin) vaccine.

The H antigens are heat-labile and are an integral part of the flagella in those serovars which possess them. The antigens are designated by a combination of letters of the alphabet and numerals (e.g. a to z, z1 to z32 and 1 to 7).⁸ Two antigenic forms (also referred to as ‘phases’) of the flagella may occur in culture. A culture may therefore contain cells in which the flagella are all in the same phase, or cells which...