Streptococcus dysgalactiae is a group of bacteria distinct from Streptococcus pyogenes, which is more widely known as Group A strep and commonly causes strep throat. S. dysgalactiae includes strains classified under Lancefield serogroups C and G, which are carbohydrate antigens. Specifically, Streptococcus dysgalactiae subspecies equisimilis is the subspecies most frequently associated with human infections and typically expresses Lancefield group C or G antigens. S. dysgalactiae can be found as part of the normal bacterial populations in humans and animals.
Human Infections Caused by Strep Dysgalactiae
Streptococcus dysgalactiae can cause a range of human illnesses, from superficial skin conditions to severe systemic infections. Skin and soft tissue infections are frequently observed, including cellulitis, which is a deeper skin infection characterized by redness, swelling, warmth, and pain. Erysipelas, a more superficial skin infection, also presents with similar symptoms. In some instances, it can lead to more severe conditions like necrotizing fasciitis, a rapidly progressing infection that destroys soft tissue.
The bacterium can also cause serious invasive infections when it enters normally sterile body sites. These include bacteremia, where bacteria circulate in the bloodstream, and septic arthritis, an infection within a joint that causes pain and swelling. Osteomyelitis, an infection of the bone, and endocarditis, an infection of the heart valves, are other severe manifestations that can occur. While less common than with Streptococcus pyogenes, S. dysgalactiae can also be a cause of pharyngitis, presenting as a sore throat.
Sources of Infection and At-Risk Populations
Streptococcus dysgalactiae commonly resides on human skin, in the throat, and gastrointestinal tract. Infections often arise endogenously, meaning they originate from the individual’s own body. This typically occurs when the bacteria gain entry through a breach in the skin, such as a cut, surgical wound, ulcer, or even an insect bite. The integrity of the skin barrier plays a role in preventing these opportunistic infections.
Certain populations face an elevated likelihood of developing Streptococcus dysgalactiae infections. Elderly individuals are particularly susceptible, as are those with chronic underlying health conditions. Such conditions include diabetes, various forms of cancer, and pre-existing heart or liver diseases. People with compromised immune systems are also at increased risk.
The Connection to Animals
Streptococcus dysgalactiae is recognized as a significant pathogen in animals, reflected in its name, which means “bad milk.” It is a well-known cause of bovine mastitis, an udder infection in dairy cows, causing economic losses. It also causes infectious arthritis in lambs, neonatal mortality in puppies, and infections in piglets, calves, goats, and fish.
While S. dysgalactiae is prevalent in animal populations, direct transmission from animals to humans is considered less frequent than infections arising from a person’s own bacterial flora. Most human cases are caused by S. dysgalactiae subspecies equisimilis, which is largely human-adapted. Despite the potential for zoonotic transmission, human infections are more commonly linked to the bacteria already present on or within the individual.
Diagnosis and Medical Management
Diagnosing an infection caused by Streptococcus dysgalactiae requires laboratory testing. A healthcare provider will typically collect a sample from the affected area, such as a swab from a wound or a blood sample if a systemic infection is suspected. This sample is then sent to a laboratory for bacterial culture, where the bacteria are grown and identified. Rapid antigen tests for strep throat, designed for Group A Strep, do not identify S. dysgalactiae.
Infections caused by Streptococcus dysgalactiae are generally treated with antibiotics. Penicillin is considered the preferred medication for these infections due to its effectiveness. For more severe or widespread infections, intravenous antibiotics may be administered in a hospital setting to ensure higher drug concentrations reach the site of infection. While S. dysgalactiae is typically susceptible to penicillin, healthcare providers will select the most appropriate antibiotic based on the specific infection, the patient’s individual circumstances, and any known allergies. Completing the prescribed course of antibiotics is important to eradicate the infection and prevent antibiotic resistance.