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Blindness-Causing Sexually Transmitted Disease Worries Scientists

Tim Boyer's picture

Scientists recently report that Chlamydia trachomatis—the most common sexually transmitted disease in the world and the leading cause of infectious blindness—is becoming particularly worrisome due to their discovery that undetectable strains of Chlamydia are evolving faster than previously believed.

Chlamydia is a common sexually transmitted disease (STD) caused by the bacterium Chlamydia trachomatis, which also just happens to be the same bacterium that causes a type of infectious blindness called “trachoma”—a medical condition that occurs primarily in rural settings in developing countries, but can appear in crowded cities in impoverished neighborhoods where poor hygiene is present. The ease of spread of trachoma through casual contact with contaminated towels and clothing, or making hand to eye contact with secretions from the nose or throat makes it especially infectious.

However, the most common route of infection of the bacterium is through vaginal, oral and anal intercourse. In 2010, 1,307,893 chlamydial infections were reported to the CDC from 50 states and the District of Columbia. However, this number is believed to be an under-representation of the true incidence of disease because most people with chlamydia are not aware of their infection. Experts estimate a more accurate number of up to 2.8 million chlamydia infections annually in the U.S.

The lack of awareness of a chlamydial infection is partly due to mild or absent symptoms as well as some level of personal denial that an individual may be carrying a sexually transmitted disease. For a woman infected with chlamydia, symptoms can range from none experienced to abnormal vaginal discharge, painful urination, lower abdominal and back pain, nausea, fever, pain during intercourse or bleeding between menstrual periods. A chlamydia infection can result not only in pelvic inflammatory disease (PID) in women, but also in infertility.

Men with chlamydia might have a discharge from their penis or a burning sensation when urinating, but can also be asymptomatic and unknowingly spread infection to others.

The direness of risk of future chlamydia infections is highlighted in a recent study published in the scientific journal Nature Genetics where scientists report that the chlamydia bacterium is evolving very quickly and could become increasingly harmful. According to the researchers, new strains are rapidly evolving due to the fact that different strains are able to swap DNA while inside an infected individual.

This swapping of DNA is called recombination. In bacteria, recombination occurs in one of three ways referred to as transformation, conjugation and transduction where bits of DNA from one strain is transferred to another. In some cases, an entire genome from one bacterium can be transferred to another in what is called lateral gene transfer.

According to Dr. Simon Harris, lead author of the study, "Scientists recently discovered that if two Chlamydia strains co-infect the same person at the same time, they can swap DNA by a process called recombination. This was originally thought only to affect a few 'hotspots' within the genome. We were very surprised to find recombination is far more widespread than previously thought."

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Part of the researchers’ concern about their findings is that currently there are no diagnostic tests to differentiate between multiple stains of chlamydia a person might be infected with. Current diagnosis is limited to a positive or negative test result; therefore, a physician is unable to determine whether antibiotic treatment has failed, or if a second or third strain of bacteria is present, or if a patient has picked up a new infection after successful treatment of the first infection.

"Until now a person treated with antibiotics with a reoccurring infection of C. trachomatis was assumed to have been re-infected," says Dr. Nicholas Thomson, senior author of the study. "The current gaps in our understanding of the population makeup of Chlamydia limit our ability to implement health policies, because we do not fully understand how Chlamydia spreads within our population."

The ease of spread and heightened promiscuous genetic nature of the Chlamydia trachomatis bacterium makes it especially likely that eventual antibiotic resistance will evolve in some new future strains of chlamydia. Furthermore, scientists are concerned that strains of African chlamydia that cause blindness are evolving into new strains that can avoid detection and response from a person’s immune system.

"For many years various groups have observed co-circulating strains of Chlamydia causing trachoma. In our study we have shown that some strains appear to have swapped only their surface coat," says Dr. Martin Holland from the London School of Hygiene & Tropical Medicine. "This provides real clues as to how this bacterium is able to avoid the human immune system and cause disease."

The researchers are currently expanding their understanding of the Chlamydia trachomatis bacterium by working on deciphering its entire genomic sequence. By being able to identify and monitor new strains, cases of infectious blindness and sexually transmitted disease infection from chlamydia in the future can be effectively prevented and/or treated.

Image Source: Courtesy of Wikipedia


“Whole-genome analysis of diverse Chlamydia trachomatis strains identifies phylogenetic relationships masked by current clinical typing” Nature Genetics (2012) DOI: doi:10.1038/ng.2214; Published online 11 March 2012; Simon R. Harris et al.

Centers for Disease Control