Reported cases of gonorrhea and syphilis have taken a sharp and sudden drop nationwide this year in what health officials say may be a major change in sexual activity sparked by fears of herpes and acquired immune deficiency syndrome (AIDS).

Joe Blount, a statistician for the federal Centers for Disease Control in Atlanta, said syphilis cases dropped about 3 percent in the first 24 weeks of this year, reversing a 7.5 percent increase for 1982. Gonorrhea dropped 6.6 percent in the first 24 weeks of this year, accelerating a 3 percent decline in 1982.

"Herpes has had a considerable impact on sexual activity," said Dr. Michael Langer, assistant chief of venereal disease control in Los Angeles County, where the number of reported gonorrhea cases fell 19 percent in 1982.

Fear of contracting genital herpes, a painful virus infection with no effective cure, appears to have forced a cutback in sexual activities that also lead to more traditional venereal diseases such as gonorrhea and syphilis, Langer said.

Los Angeles County officials first suspected that their 19 percent drop in gonorrhea had been a statistical fluke caused by a change in reporting practices or a switch to a new computerized compilation system. But a small study showed that statistical factors could account for no more than 5 percent of the change.

Health officials reported significant drops last year in venereal disease rates in other major cities, where there had been widespread publicity about genital herpes and the danger of its being passed on by promiscuous sexual contact.

Blount said reported cases of gonorrhea in 1982 dropped 18 percent in San Francisco, 17 percent in Columbus, 8.6 percent in Chicago, 2 percent in New York City and 2 percent in Atlanta.

Blount said that although no link between the fear of herpes and AIDS and the decline in reports of venereal disease has been proven, no other national developments, such as major new treatment and detection programs or information-gathering methods, would explain the change.

He said his office has been receiving "anecdotal information" on "significant changes in life-style patterns," such as declining use of bath houses frequented by homosexuals looking for partners.

In the District of Columbia, reported cases of syphilis dropped from 255 to 168, or 34 percent, in the first 24 weeks of this year compared with last year. Gonorrhea increased 7 percent in the District last year and there are no statistics yet to gauge whether that rate has begun to decline.

Cases of both gonorrhea and syphilis reported in Virginia dropped about 9 percent in the first half of 1983, state health officials said. In Montgomery County, reported gonorrhea cases dropped 3.7 percent and syphilis cases 30 percent in 1982. In Prince George's County, gonorrhea cases increased 5.8 percent in 1982 while reported syphilis cases dropped 20 percent.

Dr. Martin Levy, D.C. administrator for preventive health services, said a connection between the recent decline in venereal diseases and fear of herpes "is certainly logical . . . , but I'm not sure there's been enough to judge yet."

Langer said publicity about a national herpes epidemic began more than a year ago, and probably had the greatest impact on the venereal disease rate. He also pointed to recent widespread media reports about AIDS, an often fatal disorder first identified in homosexual men.

Scientists don't know exactly how AIDS is spread, but publicity about the disease may increase popular apprehension about any intimate contact with strangers, which also can lead to exposure to other venereal diseases.

Both syphilis and gonorrhea are sexually transmitted bacterial diseases easily treated with antibiotics. Reported cases of syphilis have been increasing in recent years, Blount said, and this year's early drop was the first reported quarterly decline in the disease in five years.