May 9 (HealthDay News) -- Instances of Group B streptococcus, a major cause of serious infections, have dropped by about 25 percent among week-old infants, but rose by almost 50 percent among most adults during a recent six-year period, according to a new study.

Group B strep is the leading cause of sepsis and meningitis in the first week of life. Prevention strategies put in place during the 1970s have helped quell the condition, called early-onset disease. However, an estimated 21,500 cases of invasive disease and 1,700 deaths were traced to the disease during 2005, according to the study, published in the May 7 issue of Journal of American Medical Association.

Group B streptococcus can also cause invasive disease in older infants, pregnant women, children and young adults with underlying medical conditions and older adults. An increase in disease incidence among non-pregnant adults has been previously documented in past decades.

The new study, which examines data on laboratory-confirmed invasive group B streptococcal disease in 10 states from 1999 to 2005, found 14,573 cases of the disease overall. Of those, 1,232 cases where early-onset disease.

Incidence decreased among week-old infants by 27 percent after the early-onset disease prevention guidelines were revised in 2002. Incidence remained flat among babies age 7 days to 89 days and pregnant women.

Among those age 15 to 64 years, Group B strep increased 48 percent. Occurrence of the disease increase by 20 percent among those 65 years or older experienced a 20 percent increase.

The proportion of patients who died was highest in the oldest age groups.

All strains of Group B strep tested were vulnerable to the antibiotics penicillin, ampicillin, and vancomycin, but 32 percent and 15 percent were resistant to erythromycin and clindamycin, respectively.

The study's authors concluded that group B streptococcus vaccination trials for mothers should become a priority, followed by vaccine development to help the elderly and younger adults with chronic underlying conditions.

SOURCE: JAMA Archives journals, news release, May 6, 2008