Reported chlamydia cases in Maryland jumped by 57 percent from 1997 to 2006, and one state health official called it "the tip of the iceberg" for the often symptomless sexually transmitted disease.
Diagnosed chlamydia cases increased in all but Talbot County, and the rate per capita grew in all but Talbot and Worcester counties, according to the Department of Health and Mental Hygiene.
But state and local health officials attribute the increase largely to improved screening methods.
"To me, chlamydia is one of these tip-of-the-iceberg things," said Barbara Conrad, the state health department's director of the sexually transmitted diseases program. "If there were more testing, I think we would find more of it."
The number of cases statewide grew from 13,965 in 1997 to 21,859 last year, while the rate grew from 273 cases per 100,000 people to 389 per 100,000 in the same period.
Baltimore City had the state's highest 2006 rate at 989 per 100,000 population, followed by Somerset, Dorchester, Prince George's and Wicomico counties. Carroll County had the lowest rate, at 86 per 100,000 people.
"It's prevalent. It's probably one of the most frequent STDs we encounter with our patients," said Dr. Donald Shell, the Prince George's County health officer.
Shell said reported cases are up primarily because new urine testing methods are less invasive than the other type of screening, a genital swab. It is easier to get people, particularly men, to agree to the voluntary urine test, Shell said.
Prince George's tests people in sexually transmitted disease clinics and during routine physical exams.
Chlamydia is the most commonly reported bacterial STD in the country, but it remains underreported because three-quarters of infected women and half of infected men show no symptoms, according to the Centers for Disease Control and Prevention.
Health experts estimate that as many as 3 million new cases occur nationwide every year, but only one-third are diagnosed.
Shell declined to estimate the number of undiagnosed cases in Prince George's, but said a "rule of thumb" is that half of sexually active people become exposed to sexually transmitted diseases.
Fred Wyand of the American Social Health Association described the onset of noticeable chlamydia symptoms in women as a "domino effect." The infection can spread from the vagina throughout the reproductive tract, causing painful inflammation, abscesses and, if left untreated long enough, infertility.
Pregnant women with chlamydia can pass it to their babies, causing eye disorders and even blindness in newborns.
For men, complications such as pain and eventual sterility are rare. Health officials say it can be challenging to convince men of the need to get tested when they are not exhibiting symptoms.