Study Finds No Gender Difference In Chlamydia Transmission Rates

December 04, 1996

Using sensitive new diagnostic techniques, National Institute of Allergy and Infectious Diseases (NIAID) researchers have shown that men and women are equally likely to contract Chlamydia trachomatis from infected sex partners. Previous studies had suggested that male-to-female transmission of chlamydia, the most common sexually transmitted pathogen in the United States, occurred more often than female-to-male transmission.

"Within heterosexual partnerships, the transmission efficiency of chlamydia appears to be bidirectional and equal," says Thomas C. Quinn, M.D., M.Sc., of NIAID's Laboratory of Immunoregulation and The Johns Hopkins University School of Medicine in Baltimore, Md.

Dr. Quinn and colleagues from Johns Hopkins and the Baltimore City Health Department studied chlamydial infection and transmission among couples attending sexually transmitted disease (STD) clinics in Baltimore. The study results appear in the Dec. 4 issue of the Journal of the American Medical Association.

Over a four-year period, 958 individuals -- STD clinic visitors and their sex partners --enrolled in the study. The researchers tested each individual for chlamydial infection using two different diagnostic techniques: cell culture, which involves incubating tissue samples from a man's urethra or a woman's cervix for several days, and a polymerase chain reaction (PCR) test designed to detect minute traces of chlamydial DNA.

As previous studies have shown, cell culture revealed a significant gender difference in chlamydial infection - approximately 13 percent of women and 8 percent of men in the study tested positive for the pathogen. PCR testing of the same samples, however, detected chlamydia in about 15 percent of both men and women. More than 40 percent of infected men and nearly 80 percent of infected women had no symptoms of infection.

Further evidence that chlamydia is an equal opportunity pathogen came from analyzing the infection status of sex partners of individuals who tested positive for chlamydia by PCR. The researchers found that two-thirds of male sex partners of infected females and two-thirds of female sex partners of infected males were themselves infected with chlamydia. Women younger than 20 and those who had greater than 25 percent cervical ectopy, a condition in which the lower cervix is covered by a type of tissue susceptible to chlamydial infection, were most likely to contract chlamydia from infected male sex partners. In contrast, the researchers identified no variables associated with men's risk for contracting chlamydia from infected female sex partners.

"These data agree with what we already know about chlamydia - it tends to affect women in their young adult or teenage years, when ectopy is most common," explains Dr. Quinn. "The degree of ectopy varies among young women. The more ectopy you have, the more susceptible you are to chlamydia. We know that women over 20 have less ectopy and less chlamydial infection. The big unknown in this relationship is the role that immunity plays in susceptibility to chlamydial infection. By the time some women have reached age 20, have they acquired sufficient immunity to resist chlamydia? That will be the focus of the next study."

"The similar rates of infection in men and women in this study suggest that our thoughts about women being more susceptible to infection may be incorrect," adds Penny Hitchcock, D.V.M., chief of NIAID's sexually transmitted diseases branch. "And the fact that many infected individuals were asymptomatic underscores the need to prospectively screen for chlamydial infection, especially to prevent pelvic inflammatory disease (PID) in women."

PID, an infection of the upper reproductive tract that often leads to infertility, tubal pregnancy and other serious complications, affects more than 1 million women in the United States each year. Most cases are caused by chlamydial infection, which is diagnosed in 4 to 5 million people in this country each year. The actual number of cases probably is much higher, since, as the current study shows, chlamydial infections often produce no symptoms and thus frequently go undiagnosed. Earlier this year, NIAID-supported researchers reported that women who were screened and treated for asymptomatic chlamydial infections were nearly 60 percent less likely to develop PID than women who received treatment only after they showed clinical signs of chlamydial infection.

The U.S. Centers for Disease Control and Prevention (CDC) recommends widespread testing and treatment for chlamydial infections among sexually active individuals and their partners. In a recent report, the Institute of Medicine, a private, nonprofit organization that provides health policy advice to the federal government, also called for comprehensive screening for chlamydia and other STD pathogens.

Dr. Quinn notes that diagnostic tests that use PCR or ligase chain reaction (LCR), a similar method for amplifying and detecting trace amounts of genetic material, could prove to be important tools for controlling the spread of chlamydia. Since the current study was begun, two such tests for detecting chlamydia in urine samples have become available commercially. Because they do not require patients to undergo an invasive tissue-sampling procedure, urine-based tests could lead more people to be tested for chlamydial infection.

"It's clear that PCR and LCR tests are significantly more sensitive compared to culture," says Dr. Hitchcock. With these new, non-invasive tests, coupled with single dose antibiotics for treating infected individuals, we have an extremely effective approach to chlamydia prevention and control."

Dr. Hitchcock adds that NIAID is committed to supporting research aimed at applying PCR and LCR technology to the development of even better non-invasive and inexpensive diagnostic tests for chlamydia and other STD pathogens.

NIAID is a component of the National Institutes of Health. NIAID conducts and supports research to prevent, diagnose and treat illnesses such as AIDS and other sexually transmitted diseases, tuberculosis, asthma and allergies. NIH and the CDC are agencies of the Public Health Service, U.S. Department of Health and Human Services.
NIAID press releases, fact sheets and other materials are available on the Internet via the NIAID home page at

NIH/National Institute of Allergy and Infectious Diseases

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