Genus Characteristics

The streptococci and enterococci are Gram-positive cocci that occur singly, in pairs, or in chains of varying lengths. Each coccus is less than 2 µm in diameter and can appear as spherical or ovoid. They are facultative anaerobes, catalase-negative, oxidase-negative and non-motile with the exception of some of the enterococci. The streptococci are fastidious and require the addition of blood or serum to media for growth. The enterococci tolerate the bile salts in MacConkey agar and appear as small pin-point colonies on this medium. Strictly anaerobic streptococci of veterinary importance, previously classified in the genus Peptostreptococcus, have been reclassified in the genus Peptoniphilus.

Natural Habitat

Streptococci are worldwide in distribution. Most of the streptococci of veterinary interest live as commensals on the mucosa of the upper respiratory and lower urogenital tracts. They are susceptible to desiccation and do not usually survive for long away from the animal host. The enterococci are opportunists and can be found in the intestinal tract of many animals.

General Differentiation of the Streptococci

Generally the beta-haemolytic streptococci tend to be the most pathogenic for animals. Lancefield Group C streptococci from horses usually produce large zones of clear, beta-haemolysis (Fig. 8.3).

Pathogenesis and Pathogenicity

Streptococci are pyogenic bacteria that are commonly associated with suppuration and abscess formation while enterococci are opportunistic pathogens. The polysaccharide capsules of S. pyogenes, S. pneumoniae (Fig. 8.5) and some strains of S. equi subsp. equi and S. agalactiae are anti-phagocytic. The principal diseases and associated virulence factors of the streptococci and enterococci of veterinary importance are given in Tables 8.1 and 8.2.

Streptococcus pyogenes is a rare agent of mastitis in cattle and lymphangitis in foals but is an important human pathogen causing serious respiratory and soft tissue infections. These include necrotizing fasciitis and toxic shock-like syndrome. Streptococcus agalactiae is an obligate parasite of the mammary gland of cattle and a well-known agent of chronic contagious mastitis. It enters through the teat orifice and adheres to the mammary epithelium, following which colonization of the lactiferous ducts occurs. Inflammation and fibrosis of the gland leads to fibrin plug formation in ducts, involution of glandular tissue and agalactia. Infection may become chronic if not treated with antimicrobial agents. This microorganism may also cause occasional cases of neonatal septicaemia, kidney and uterine infections in dogs and cats. Streptococcus dysgalactiae subsp. dysgalactiae is an opportunistic pathogen and causes sporadic cases of acute or subclinical mastitis. Insect bites or other injury to the epithelium of the teat or the gland are reported to facilitate infection. This streptococcal organism has also been associated with cases of suppurative polyarthritis in lambs, bacteraemia in dogs and disease in fish. Streptococcus uberis is a commensal organism, found in the tonsils, gastrointestinal and genital tracts of the cow and can cause mastitis via an ascending infection of the teat canal. It is also found in the environment and is associated with subclinical and clinical intramammary infections of both housed animals and those at pasture.

Streptococcus equi subsp. equi is the agent of equine strangles, a contagious upper respiratory tract disease with abscessation of the local lymph nodes. Clinical signs include fever, nasal discharge and swelling of the mandibular and retropharyngeal lymph nodes and intermandibular region. Dissemination of the organism may occur with abscesses forming in the lungs or other locations. The source of the infection is usually pus from abscesses or nasal discharges from infected horses. However, feed and/or water can also be contaminated. A small proportion of animals fail to clear the infection from the guttural pouches and these healthy carriers are likely to be responsible for spread of infection between epidemics. Depending on the immunity status, the disease can take an acute or mild course with most horses recovering relatively quickly. Streptococcus equi subsp. zooepidemicus is a commensal organism with a large host range, which opportunistically causes disease following viral infections or tissue injury. It is a frequent cause of pneumonia and joint infections in horses. There have been several reports of haemorrhagic pneumonia in dogs associated with this organism in recent years (Priestnall & Erles 2011). It is also a zoonotic agent with infection following close contact with horses or contaminated milk products. Streptococcus dysgalactiae subsp. equisimilis may be a more significant pathogen of horses than previously thought as a recent study showed that it was frequently isolated from cases of infection of the equine genital tract (Erol et al. 2012).

Streptococcus suis is associated with septicaemia, meningitis, polyarthritis and pneumonia in pigs. It is also a zoonotic pathogen capable of causing severe invasive disease in humans exposed to pigs or pork products. Most pigs are healthy carriers with S. suis located in their tonsils, nasal cavities, genital or digestive tracts. Disease development is incompletely understood but is believed to be related to the strain and immune status of the host. Some virulence factors appear to be associated with more severe, invasive disease (Beineke et al. 2008, Wei et al. 2009). Stress and intensive management practices are also likely to predispose to disease occurrence. Most cases of disease in both pigs and humans are caused by S. suis serotype 2. In both species the main clinical manifestations of S. suis are meningitis and septicaemia although some serotypes are commonly associated with pneumonia. Streptococcus suis outbreaks in pigs are common while most reports in humans describe sporadic cases (Gottschalk et al. 2007). However, S. suis has recently emerged as an important zoonotic agent in some Asian countries, and has been reported as the primary cause of adult meningitis in Vietnam (Mai et al. 2008). Streptococcus porcinus causes a variety of pathological conditions in pigs including contagious cervical lymphadenitis, ‘porcine strangles’, endocarditis, and abortion. It is believed that the organism enters its host through the mucosa of the pharyngeal or tonsillar surfaces. Organisms are then carried to the lymph nodes, primarily of the head and neck region, where abscesses are formed. Streptococcus porcinus is also an opportunistic agent in many other hosts. It has rarely been implicated as a human pathogen with only a few cases reported. Streptococcus dysgalactiae subsp. equisimilis is a commensal of the porcine tonsil and a cause of suppurative athritis in piglets. Acquisition of this microorganism is usually via the tonsils, umbilicus or a skin abrasion.

Streptococcus canis is an opportunistic pathogen found as part of the normal flora of the oropharyngeal, anal and genital mucosa in both dogs and cats. A variety of infections have been reported associated with this organism. Contagious lymphadenitis is possibly the most frequent manifestation in cats. In dogs, sporadic necrotizing fasciitis and toxic shock syndrome have been associated with S. canis. This syndrome may be related to fluoroquinolone usage, which was found to induce a lytic bacteriophage encoding a homologue of pokeweed mitogen (Ingrey et al. 2003).

Streptococcus pneumoniae is an important human pathogen which causes invasive disease in adults and children but is rarely isolated from clinical disease in animals. It has been reported to infect and cause pneumonia in horses. Equine isolates of S. pneumoniae are reported to represent a tight clonal group and to be genetically distinct from human isolates (Whatmore et al. 1999).

The agent of streptococcosis in pigeons is Streptococcus gallolyticus subsp. gallolyticus, which was formerly classified as S. bovis biotype I (Devriese et al. 1998). Streptococcosis is an important septicaemic disease in pigeons. Clinical signs include lameness, emaciation, inability to fly, polyuria, production of green and slimy droppings and sudden death. Extensive and well-circumscribed areas of necrosis in the pectoral muscles and arthritis of the knee, hock and shoulder joints are observed at necropsy. Streptococcus gallolyticus is an opportunistic pathogen of humans and S. gallolyticus subsp. gallolyticus is the subspecies most often linked with human endocarditis-associated colonic cancer.

Enterococci are opportunistic pathogens found in the intestinal tracts of animals and humans. There are many species of enterococci but E. faecalis and E. faecium are the species most frequently isolated from infections in animals. Types of infection include wound infections in all species, mastitis in cattle and infections of the ears and urinary tract in dogs. Enterococci are frequently intrinsically resistant to many classes of antimicrobial agents and vancomycin-resistant enterococci are important nosocomial pathogens in human medicine. Vancomycin resistance in animal isolates of enterococci was associated with avoparcin use but resistance levels have decreased following a ban on the use of this antibiotic in animals since 1997.

Finally, the anaerobic Streptococcus, Peptoniphilus indolicus has been described in association with Trueperella pyogenes in ‘summer mastitis’ of cattle in Europe. Anaerococcus tetradius and Peptostreptococcus anaerobius have been associated with abscesses in dogs and cats respectively.