Group B Strep Screening
Common Tests During the Third Trimester of Pregnancy
Group B streptococcus (GBS) is a type of bacteria that can exist in the female reproductive tract without causing symptoms. Up to 30% of healthy women can carry group B strep, and it usually does not cause problems. However, sometimes it can lead to serious infection of the bloodstream, infection of the placenta, or urinary tract infection. Group B strep can also have serious consequences for the baby, causing potentially life-threatening infections in the newborn, including meningitis, pneumonia, and sepsis. The test is usually done between the 35th and 37th weeks of pregnancy and should be done in subsequent pregnancies even if you test negative in your first pregnancy.
What is group B strep?
Group B Streptococcus (GBS) is a type of gram-positive streptococcal bacteria also known as Streptococcus agalactiae. This type of bacteria (not to be confused with group A strep, which causes strep throat) is commonly found in the human body (this is termed colonization), and it usually does not cause any symptoms. However, in certain cases, it can be a dangerous cause of various infections that can affect nonpregnant adults, pregnant women, and their newborn infants. In the United States, approximately 30,800 cases of invasive GBS disease occur annually across all age groups. Group B strep disease is the most common cause of neonatal sepsis and meningitis in the United States.
Group B streptococcal infection can also afflict nonpregnant adults with certain chronic medical conditions, such as diabetes, cardiovascular disease, obesity, and cancer. The incidence of group B streptococcal disease in adults increases with age, with the highest rate in adults 65 years of age and older (25 cases per 100,000). Although the incidence of neonatal group B strep infection has been decreasing, the incidence of GBS infection in nonpregnant adults has been increasing.
What causes group B strep infection?
Group B strep bacteria can normally be found in about 25%-30% of all healthy pregnant women. Group B strep is commonly found in the intestine, vagina, and rectal area. Most women who are carriers of the bacteria (colonized) will not have any symptoms; however, under certain circumstances, perinatal group B strep infection of both the mother and/or the newborn can develop. In newborns, if the GBS infection develops in the first week of life, it is termed early onset disease. If the GBS infection develops from 1 week to 3 months of age, it is referred to as late onset disease. On average, approximately 1,000 babies in the United States develop early onset disease each year, with similar rates for late onset disease. According to the U.S. Centers for Disease Control and Prevention (CDC), the rate of early onset infections decreased from 1.7 per 1,000 live births in 1993 to 0.22 cases per 1,000 births in 2016.
How do people get group B strep?
In newborns, group B Streptococcus infection is acquired through direct contact with the bacteria while in the uterus or during birth; thus, the gestational bacterial infection is transmitted from the colonized mother to her newborn. Approximately 50% of colonized mothers will pass the bacteria to their babies during pregnancy and vaginal birth. However, not all babies will be affected by the bacteria, and statistics show that about only one of every 200 babies born to a GBS-colonized mother will actually go on to develop GBS infection.
Group B strep infection is more common in African Americans than in whites. There are also maternal risk factors that increase the chance of transmitting group B Streptococcus to the newborn leading to early onset disease:
- Labor or membrane rupture before 37 weeks gestation
- Membrane rupture more than 18 hours before delivery
- Urinary tract infection with GBS during pregnancy
- Previous baby with GBS infection
- Fever during labor
- Positive culture for GBS colonization at 35-37 weeks
Late onset GBS infection occurs more commonly in babies who are born prematurely (<37 weeks) and in those babies whose mother tested positive for GBS during pregnancy.
Group B strep infection is not a sexually transmitted disease (STD).
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What are group B strep infection symptoms and signs?
In newborns with early onset GBS infection, the signs and symptoms usually develop within the first 24 hours after birth. Those babies who develop late onset GBS infection are often healthy and thriving, with the signs and symptoms of the disease developing after the first week of life. Signs and symptoms that may be observed with GBS infection in babies include
Adults who develop invasive GBS infection may develop
- bloodstream infection (sepsis),
- skin and soft-tissue infection,
- bone and joint infection,
- lung infection (pneumonia),
- urinary tract infection, and
- rarely, an infection of the fluid and lining tissues surrounding the brain (meningitis).
The exact source of group B streptococcal disease in nonpregnant adults is often not determined.
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What types of health care professionals treat group B strep infections?
Group B strep infections may be diagnosed and treated by different specialists, depending on the patient's age and the potential complications encountered by the patient. In pregnant women, a GBS infection may be diagnosed and treated during labor by the patient's obstetrician/gynecologist or by a family practitioner if they are delivering the baby. Babies who develop a GBS infection will be treated by a pediatrician or a neonatologist, and sometimes an infectious disease specialist may be involved. Nonpregnant adults who develop GBS infection will frequently be treated by their family physician, an internist, or an infectious disease specialist, and rarely by a general or orthopedic surgeon if a skin or bone infection requiring surgery is present.
What tests do health care professionals use to diagnose group B strep infection?
In newborns and in adults, isolation of the GBS bacteria is necessary for a definitive diagnosis. Laboratory studies that isolate the organism from certain body fluids, such as blood, cerebrospinal fluid (CSF), and urine, help establish the diagnosis. For screening of pregnant women, a health care professional will obtain a swab of a woman's vaginal and rectal area to screen for GBS infection at 35-37 weeks of gestation. Test results can take a few days. A lumbar puncture (spinal tap) may need to be performed when meningitis is suspected. A health care professional may order imaging studies such as a chest X-ray to evaluate whether pneumonia is present.
What is the treatment for group B strep?
For women who test positive for GBS during pregnancy and for those with certain risk factors for developing or transmitting GBS infection during pregnancy, intravenous antibiotics are recommended at the time of labor. The administration of antibiotics to women before labor who are known to be colonized with GBS is not effective in preventing early onset disease, as it has been found that the GBS bacteria can grow back quickly. Pregnant women with GBS isolated from the urine at any time during the pregnancy and those women who have had a previous infant with invasive GBS infection should receive antibiotics during labor, and thereby do not require the routine GBS screening at 35-37 weeks of gestation. Pregnant women who are not aware of their group B strep status should be given antibiotics during labor if they develop preterm labor (less than 37 weeks gestation), if they have membrane rupture for 18 hours or longer, or if they develop fever during labor. Penicillin or ampicillin are the recommended antibiotics, while individuals who are penicillin-allergic may be given either cefazolin, clindamycin, or vancomycin. The administration of antibiotics has been shown to significantly decrease GBS early onset infection in newborns; however, it does not prevent late onset GBS infection. If a pregnant carrier of GBS receives intravenous antibiotics during delivery, her baby has a one in 4,000 chance of developing GBS infection. Without antibiotics, her baby has a one in 200 chance of developing GBS infection.
In neonates and nonpregnant adults who develop invasive group B streptococcal disease, intravenous antibiotics are also the mainstay of treatment. There are certain conditions associated with invasive GBS infection that may require surgical intervention, for example surgical debridement in certain patients with soft tissue/skin/bone infections.
What are the complications of group B strep infection?
Invasive infection with GBS in babies may result in sepsis, pneumonia, meningitis, or occasionally death. In some babies who survive, long-term sequelae of the disease include deafness, blindness, or developmental disabilities.
In pregnant women, infection with GBS may cause urinary tract infection, infection of the uterus and placenta, as well as preterm delivery, fetal death, or miscarriage.
In nonpregnant adults with chronic medical conditions who develop invasive GBS infection, complications may include pneumonia, urinary tract infection, sepsis, skin and soft-tissue infection, bone and joint infection, and rarely meningitis.
What is the prognosis of a group B strep infection?
The prognosis of a GBS infection varies depending on various factors, including the patient's age and the presence of any underlying medical conditions. In the United States, the death rate (mortality rate) for babies with GBS infection is about 5%. Pregnant women who develop symptoms of GBS infection also have low mortality rates, as they tend to be otherwise healthy. Nonpregnant adults who develop invasive GBS infection tend to have higher mortality rates, ranging from 5%-47% depending on the study, as this group of patients tend to be older and they frequently have serious underlying medical conditions.
Is it possible to prevent group B strep infection?
At this point in time, the best measure for preventing GBS infection is through routine screening during pregnancy. This testing has served to decrease the overall number of early onset GBS infections in newborns by about 80% since aggressive preventive measures were instituted in the 1990s. In pregnant women, routine screening for colonization with GBS is strongly recommended. This screening test is performed between 35-37 weeks of gestation. The test involves using a sterile swab to collect a sample from both the vaginal and rectal areas, with results usually available within 24-72 hours.
Antibiotic administration during labor to pregnant women colonized with GBS and for those with the risk factors outlined above can help decrease the transmission of GBS infection, and thus decrease the incidence of early onset GBS disease in newborns.
Although there is currently no licensed vaccine available for the prevention of GBS infection, there is research under way to try to develop one for use in the future. Further information about GBS can be found at the U.S. Centers for Disease Control and Prevention web site (https://www.cdc.gov/groupbstrep/index.html).
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Medically Reviewed on 12/3/2018
United States. Centers for Disease Control and Prevention. "Group B Strep (GBS)." May 29, 2018. <http://www.cdc.gov/groupbstrep/index.html>.
Verani, J.R., and S.J. Schrag. "Group B streptococcal disease in infants: Progress in prevention and continued challenges." Clin Perinatol 37.2 June 2010: 375-392.c.gov/groupbstrep/index.html>.