Of the bacterial group of gram-positive cocci comprising the genera Streptococcus, Enterococcus, and Peptostreptococcus, streptococci constitute the most consequential pathogens of swine. Streptococci are also associated with infectious conditions of humans, cattle, sheep, goats, and horses. Relative to pigs, Streptococcus suis (an alpha-hemolytic Streptococcus) is by far the most important agent of infectious diseases in this group, affecting mainly nursing and recently weaned pigs. Septicemia, meningitis, polyserositis, polyarthritis, and bronchopneumonia are associated with S suis infections.
Streptococcus dysgalactiae equisimilis is considered the most important beta-hemolytic Streptococcus involved in lesions in pigs. S porcinus, another beta-hemolytic Streptococcus, has been associated particularly in the US with an infectious clinical entity in growing pigs known as streptococcal lymphadenitis. Streptococcus equi zooepidemicus (S zooepidemicus) sequence type (ST) 194, originally described in Asia, has also emerged in North America and was associated with high-mortality outbreaks affecting pigs of any age after weaning.
Other streptococci have occasionally been isolated from diseased pigs, such as S pluranimalium, S porcorum, S hyovaginalis, S gallolyticus gallolyticus, S plurextorum, and S porci. So far, there are no clear data about the habitat and/or virulence properties of these streptococcal species. In addition, S parasuis (which includes previous S suis serotypes 20, 22, and 26) and S orisratti (previous S suis serotypes 32 and 24) are sometimes isolated from diseased pigs.
Enterococci were previously categorized as streptococci but are now recognized as a distinct genus. Enterococci reside in the intestinal tract and may cause disease in multiple species. In pigs, the Enterococcus faecium species group, mainly E durans and E hirae, are especially associated with enteritis and diarrhea.
Enterococci are known as part of the intestinal flora; however, some strains can extensively colonize the mucosal surface of the small intestine. Some enterococcal species that show typical adhesion to the apical surface of the enterocytes of the small intestine of young animals have been described as associated with diarrhea in different species, including piglets 2–20 days old. Taxonomic studies have shown that most of these enterococci are members of the E faecium species group, mainly E durans and E hirae. Enterococci are usually considered a reservoir for antimicrobial resistance genes.
