Bacterial Pathogens: Streptococci Overview

Bacterial Pathogens: Streptococci

• Streptococci are gram-positive cocci, often occurring in chains or pairs (diplococci).
• Over 40 species exist.
• Most species are host-specific.
• Classified into groups based on characteristics like haemolytic activity, surface antigens, and fermentation properties.

Haemolysis

• Haemolysis (horse blood agar) testing differentiates streptococcal species.
• α-haemolysis results in partial haemolysis, β-haemolysis in complete haemolysis and γ-haemolysis indicates no haemolysis.
• β-haemolytic streptococci (pyogenic) possess Streptococcal pyrogenic exotoxins (SPEs) and other virulence factors like SLO and SLS.

Grouping of Streptococcal Species

• Lancefield groups categorize species based on cell wall polysaccharides.
• 21 groups are identified, but practical applications are limited.
• Additional groupings exist based on M-proteins (~120 subgroups).

Streptococcus and Host Preference

• Specific streptococcal species exhibit preferential tropism for particular animal hosts.
• Lancefield groups and their associated species, and host preferences are shown in a table.

Equine Strangles

• Equine strangles is a contagious respiratory disease prevalent in horses.
• Usually affects horses younger than 5 years.
• A widespread issue causing significant farm disruptions and potential mortality.
• Streptococcus equi is the primary pathogen involved.

Equine Group C Streptococci

• Str. equisimilis, Str. zooepidemicus, and Str. equi are common in this group.
• All are nasopharyngeal commensals and opportunistic pathogens, causing sepsis, wound infections, and other conditions.

Clinical Symptoms of Strangles

• Early symptoms: abrupt onset of fever (103°F or higher), dysphagia (difficulty swallowing), reluctance to eat, extended neck, and listlessness/depression.
• Later symptoms: lymphadenopathy (swollen lymph nodes, often submandibular and retropharyngeal), painful oedema that develops within 1 week of infection, possible serum oozing and later pus discharge.

Bastard Strangles

• Metastasis to other systems occurs occasionally, including those unrelated to the primary infection.
• Bacteraemia, abscesses can be found in lymph nodes and surrounding organs.
• Haemorrhagic purpura, and swellings/bruises on the legs and head, and mouth.
• Brain infection may occur occasionally.
• Bastard strangles is often fatal.

Pathogenicity Factors of Pyogenic Streptococci

• Adherence factors: many surface proteins bind to host cells, and some enable uptake.
• Degradative enzymes: proteases and hyaluronidase break down host tissues.
• Anti-opsonic and anti-phagocytic factors: capsule, M-protein, and other proteins to circumvent immune defense mechanisms.
• Cell and tissue damage: hemolysins (SLO,SLS) and other exotoxins cause harm/damage.

Pathogenesis of Strangles

• Colonization/adherence: S. equi enters the mouth/nose, attaches to cells in the tonsils and produces MSCRAMMs (adhesins) for better bonding to host tissues.

M protein/M-like Proteins

• Hypervariable region accounts for 124 M types.
• Antiphagocytic effects include binding to Factor H, preventing opsonization and phagocytosis.
• M protein binds to fibrinogen and other proteins for host-specific interaction.
• ZAG protein binds the FC portion of antibodies, interfering with opsonization.

Streptolysin S and Other

• Streptolysin S lyses cells, damaging tissues.
• Superoxide dismutase (SOD) interferes/counters action of phagocytes.
• Degradative enzymes like cysteine protease and hyaluronidase break down host proteins and tissues.

Streptococcal Pyrogenic Exotoxins (Superantigens)

• Activate cytokines.
• Modulate immune response
• Cause shock.
• Fever can be caused by superantigens.

Diagnosis of Strangles

• Traditional methods include cultures from nasal swabs, washes or pus from abscesses using specific media.
• PCR methods are more sensitive, particularly for early stages of the infection..

Epidemiology of Strangles

• Purulent discharges from horses, horse-to-horse contact, contaminated housing, and indirect transmission spread the infection.
• Affected animals may shed bacteria even after recovery possibly contaminating.
• Guttural pouch can lead to prolonged carriage and result in chondroid formation.
• ~10% of animals become carriers.

Vaccination

• High rate of long-lasting immunity after natural infections.
• Recombinant vaccines have been tested and developed
• Bacterins and adjuvanted extracts have had varying efficacy.
• Multivalent M-protein candidate immunogen and attenuated live vaccine are other possible approaches. Intervet vaccine uses mutations in certain genes.

Treatment

• Choice of treatment includes antibiotics like penicillin, but treatment of all isolates might not be always effective. Aminoglycosides often pose a problem (e.g., streptomycin).

Control of Strangles Outbreaks

• During infection; restricts movement of horses in and out of affected areas;quarantine of infected horses and contacts; antibiotic treatment.
• After clinical recovery; repeat swabs for 1 week to 2;strict hygiene protocols are vital.

Aetology and Pathogenesis of Streptococcal Mastitis

• Str. agalactiae: udder commensal, acute/chronic mastitis
• Str. dysgalactiae: mouth and udder commensal, summer mastitis
• Str. uberis: skin and gut commensal, environmental mastitis; faecal contamination.

Streptococcus suis

• Primarily affects pigs with few or no symptoms.
• Can cause infections like septicaemia and meningitis in piglets.
• Can affect humans through zoonotic transmission; outbreak may follow outbreaks in pigs.
• No human-to-human transmission has been reported.
• Causes streptococcal toxic shock syndrome in susceptible humans.

Bacterial Pathogens 3: Summary and Learning Outcomes

• Understanding streptococcal characteristics and host tropism is crucial.

• Early and advanced clinical signs of equine strangles.

• Molecular pathogenesis including key virulence factors crucial in strangles control/prevention.

• Understanding control and prevention strategies for strangles.

This quiz covers the essential aspects of Streptococci, including their classification, haemolytic activity, and the significance of Lancefield groups. Learn about the different types of haemolysis and how they aid in identifying streptococcal species. Test your knowledge on the host-specific preferences and virulence factors of these pathogens.