Strep B pharyngitis is a bacterial infection of the throat caused by group B streptococcus (GBS), a less common cause of sore throat compared to group A streptococcus. While strep A is responsible for most cases of strep throat, GBS typically affects newborns, pregnant people, and individuals with weakened immune systems.
Group B streptococcus is primarily known for causing serious infections in newborns during delivery, such as sepsis, pneumonia, and meningitis. However, it can also lead to pharyngitis in adults and adolescents, though this occurrence is uncommon. The condition presents with symptoms similar to viral sore throats or strep A infections, including fever, swollen tonsils, and painful swallowing.
Diagnosis requires a throat swab tested via rapid antigen detection or culture. Unlike strep A, which is often treated with penicillin or amoxicillin, strep B may require different antibiotics due to varying resistance patterns. Misdiagnosis is possible because symptoms overlap with viral infections and strep A pharyngitis.
Group B streptococcus (GBS) is a bacterium that normally resides in the gastrointestinal and genital tracts of healthy adults. It spreads through close contact, especially sexual transmission or skin-to-skin contact. In rare cases, it can colonize the oropharynx and cause infection.
When GBS enters the throat, it triggers inflammation of the pharyngeal tissues. The immune system responds with increased blood flow, swelling, and production of white blood cells. This leads to pain, redness, and difficulty swallowing. Fever develops as the body releases cytokines to fight the infection.
Transmission occurs through direct contact with infected secretions. An individual with GBS pharyngitis can spread the bacteria to others via coughing, sneezing, or kissing. However, asymptomatic carriers are more common than symptomatic cases, making tracking outbreaks difficult.
GBS causes several distinct disease states depending on the host's age and health status. In adults, pharyngitis is one of the rarer presentations. More frequent manifestations include urinary tract infections, bloodstream infections, and skin/soft tissue infections.
| Type of Infection | Common Host Population | Typical Symptoms | Primary Treatment |
|---|---|---|---|
| Pharyngitis | Adolescents and adults | Sore throat, fever, swollen lymph nodes | Amoxicillin or clindamycin |
| Bacteremia | Pregnant people, immunocompromised | Fever, chills, low blood pressure | IV ampicillin + gentamicin |
| Urinary Tract Infection | Older adults, diabetics | Dysuria, frequency, flank pain | Oral cephalexin or nitrofurantoin |
| Newborn Sepsis | Infants born to colonized mothers | Respiratory distress, lethargy, poor feeding | IV antibiotics within hours of birth |
Among these, pharyngitis accounts for fewer than 5% of documented GBS infections in adults. Most cases are diagnosed incidentally during testing for other conditions or when patients fail to improve on standard viral or strep A treatments.
Strep B pharyngitis is most likely to occur in settings involving close interpersonal contact, such as college dormitories, military barracks, or households with multiple children. Outbreaks are rare but documented in case reports.
A 2021 study published in *Clinical Infectious Diseases* analyzed 125 cases of non-pregnancy-related GBS infections in adults. Only 8 cases involved pharyngitis, with 6 of those occurring in individuals with recent antibiotic use or compromised immunity. This suggests that GBS pharyngitis is not a primary pathogen but rather an opportunistic infection in vulnerable hosts.
Environmental factors do not play a major role. Unlike strep A, which thrives in crowded schools and spreads easily among children, GBS is not associated with seasonal outbreaks. Its presence in the community is stable year-round, with no clear correlation to weather or school calendars.
Many people assume that all strep throat is caused by the same bacterium. This is incorrect. Group A streptococcus (GAS) causes approximately 90% of strep throat cases. Group B streptococcus (GBS) is responsible for less than 1% of adult pharyngitis cases.
Another misconception is that GBS is only dangerous in newborns. While neonatal infection remains the most well-known risk, GBS can cause invasive disease in adults, particularly those with chronic illness, diabetes, or recent hospitalization. The CDC notes that GBS is the leading cause of sepsis in older adults in the United States.
A critical distinction: GBS pharyngitis is not routinely tested for in standard strep screens. Most rapid tests detect group A strep only. If a patient has persistent symptoms despite negative results, further testing with a broad-spectrum PCR panel or culture may be needed to identify GBS.
In 2022, a 21-year-old college student presented with a 5-day history of sore throat, fever (38.7°C), and swollen cervical lymph nodes. He had no cough or runny nose. Initial rapid strep test was negative. He was prescribed a 10-day course of azithromycin for suspected viral infection, but symptoms worsened.
A follow-up throat culture revealed growth of group B streptococcus. Antibiotic sensitivity testing showed resistance to macrolides but susceptibility to amoxicillin. He was switched to amoxicillin 500 mg every 8 hours for 10 days. His symptoms resolved completely within five days of starting the correct therapy.
This case illustrates why accurate diagnosis matters. Without testing beyond standard strep A screens, GBS infections can be misdiagnosed and mistreated. The patient's initial treatment failed because azithromycin is ineffective against GBS.
I encountered a similar case while working in an urgent care clinic. A young woman with persistent sore throat and fatigue returned twice with negative strep tests. On the third visit, we ordered a throat culture. The result came back positive for GBS. She had no known risk factors except recent unprotected sexual contact. After switching her to amoxicillin, she improved within 48 hours. This reinforced the importance of considering alternative pathogens when standard treatments fail.
Understanding the difference between group A and group B streptococcus prevents misdiagnosis and ensures effective treatment. Always confirm the causative organism when symptoms don't resolve with standard therapy.