LOS ANGELES, Mar. 11 (Xinhua) -- A new study has found that at least one in four teenage American girls has a sexually transmitted disease (STD), prompting calls for better screening, vaccination and prevention.

    The overall STD rate among the 838 girls in the study was 26 percent, which translates to more than 3 million girls nationwide, according to the study conducted by researchers at the U.S. Centers for Disease Control and Prevention (CDC).

    The results of the study were released Tuesday at an STD prevention conference in Chicago.

    Only about half of the girls in the study acknowledged having sex. Some teens define sex as only intercourse, yet other types of intimate behavior including oral sex can spread some diseases.

    Among those who admitted having sex, the rate was even more disturbing -- 40 percent had a STD, according to the study.

    Some experts attributed the shocking rate to both the abstinence-only sex education and teens' own sense of invulnerability.

    "This is pretty shocking," said Dr. Elizabeth Alderman, an adolescent medicine specialist at Montefiore Medical Center's Children's Hospital in New York.

    "To talk about abstinence is not a bad thing," but teen girls -- and boys too -- need to be informed about how to protect themselves if they do have sex, Alderman said.

    The study was based on an analysis of nationally representative records on girls aged 14 to 19 who participated in a 2003-04 government health survey.

    The teens were tested for four infections: human papillomavirus, or HPV, which can cause cervical cancer and affected 18 percent of girls studied; chlamydia, which affected 4 percent; trichomoniasis,2.5 percent; and genital herpes, 2 percent.

    Dr. John Douglas, director of the CDC's division of STD prevention, said the results are the first to examine the combined national prevalence of common sexually transmitted diseases among adolescent girls. He said the data, now a few years old, possibly reflect current prevalence rates.

    Disease rates were significantly higher among black girls -- nearly half had at least one STD, versus 20 percent among both whites and Mexican-Americans, according to the study.

    The second most common infection after HPV was Chlamydia, found in 4 percent of the young women.