Streptococcus dysgalactiae is a Gram-positive bacterium belonging to the Streptococcus genus, which includes many well-known human and animal pathogens. This organism is spherical in shape and typically grows in chains, which gives the genus its name, meaning “twisted grain.” S. dysgalactiae is recognized as a common pathogen that can affect a wide range of mammalian species, including both livestock and humans. The organism is particularly notable for its two distinct subspecies, which have different primary roles in disease.
Identification and Primary Reservoir
The species Streptococcus dysgalactiae is divided into two subspecies: S. dysgalactiae subsp. dysgalactiae (SDSD) and S. dysgalactiae subsp. equisimilis (SDSE). These subspecies are primarily differentiated by their preferred host and genetic makeup, though both often fall into Group C or Group G streptococci. SDSE strains cause the vast majority of human infections and are often found as a commensal organism in the human alimentary, genital, and respiratory tracts. The subspecies SDSD is nearly exclusively an animal pathogen, with only rare reports of human infection.
The primary reservoir for the species is livestock, where it is a significant veterinary concern. SDSD is widely known as a major causative agent of bovine mastitis, an infection of the udder in cattle. The name dysgalactiae itself refers to “bad milk,” alluding to this prominent role in dairy health. This bacterium is considered both an environmental and a contagious pathogen, spreading from cow to cow, as well as from the environment, such as bedding and manure.
In dairy farming, the control of S. dysgalactiae is a continuous challenge due to its dual nature as a pathogen. Poor udder cleanliness, environmental hygiene, and inadequate milking procedures are major factors in its spread. The organism can also be isolated from other animal species, including sheep and goats, in which it causes various infections.
Manifestations in Human Health
The primary subspecies responsible for disease in humans is S. dysgalactiae subsp. equisimilis (SDSE), which has become increasingly recognized as an important human pathogen. The clinical spectrum of diseases caused by SDSE closely resembles infections caused by the more widely known Streptococcus pyogenes (Group A Strep). Non-invasive infections are the most common manifestations and include conditions like pharyngitis, or strep throat.
SDSE is a frequent cause of superficial skin and soft tissue infections, such as cellulitis and erysipelas. The skin is often the initial site of entry for more serious disease. More concerning are the severe, invasive infections that can occur when the bacteria enters the bloodstream. These invasive diseases include:
- Bacteremia (bloodstream infection).
- Septic arthritis.
- Pneumonia.
- Endocarditis.
- Life-threatening conditions like necrotizing fasciitis and streptococcal toxic shock syndrome.
Invasive SDSE infections are opportunistic, primarily affecting individuals with underlying health conditions or compromised immune systems. Risk factors for severe disease often include advanced age, malignancy, diabetes mellitus, and cardiovascular disease. The increasing incidence of invasive SDSE infections worldwide has raised concern, as the organism shares many virulence factors with S. pyogenes. Furthermore, SDSE has been linked to post-streptococcal sequelae, such as post-streptococcal glomerulonephritis.
Treatment and Control Measures
In human medicine, the preferred treatment for infections caused by S. dysgalactiae is penicillin, which remains the drug of choice. The organism is highly susceptible to beta-lactam antibiotics. For patients with a penicillin allergy, alternative antibiotics such as clindamycin or vancomycin may be used.
Despite the high susceptibility to penicillin, there are occasional reports of strains exhibiting reduced susceptibility or resistance to other common antibiotics. Resistance to macrolides, such as erythromycin, and tetracyclines can be variable and requires proper susceptibility testing. Prompt diagnosis through culture and timely initiation of antibiotic therapy are important for managing invasive infections and improving patient outcomes.
Control measures for this bacterium involve addressing both human and animal health aspects to reduce transmission risk. For human infection, general hygiene practices, such as proper wound care and handwashing, are important for preventing skin and soft tissue infections. In the agricultural setting, controlling the animal reservoir is important for biosecurity and public health. This involves rigorous veterinary management, including:
- Maintaining a clean, dry environment for livestock.
- Using post-milking teat disinfectants.
- Implementing antibiotic dry cow therapy to control mastitis.