Condoms are the best defence. (CBC)

Capital Heath is launching a new campaign to try to stop the spread of chlamydia, a sexually transmitted infection on the rise in Halifax.

The number of cases climbed 20 per cent in five years. Rates in the city are now 30 per cent higher than the national average.

Marie-Rose Lesperance, a public health nurse, said women in their early 20s make up most of the cases, and most of them don't have symptoms.

"They can go a very long time with the infection and not know it," Lesperance told CBC News.

Chlamydia is usually treated with a single dose of antibiotics. But if left untreated, it can lead to pelvic inflammatory disease or infertility.

Health officials say they don't know why rates are increasing. To stem the tide, they're launching a campaign to let women know about a simple test.

"It's a self swab that you can do when you're in the doctor's office," said Lesperance.

Women swab themselves, then pass the sample to their doctor to forward to the laboratory. Until now, a doctor had to collect the sample — a process similar to a pap smear.

Dr. Todd Hatchette, director of the virology and immunology lab, said the new test is more effective, though it's more expensive.

"It allows us to test more people," he said, adding it can also screen for gonorrhea, another sexually transmitted disease.

The new test has been available since the start of the month. Posters and information packages will be available at doctors' offices beginning next week, as well as at universities.

Health officials say the best form of prevention is the use of condoms.