Gonorrhea cases jumped 57% in Alameda County, and the number of syphilis cases in Contra Costa County is expected to increase 69% this year, according to health officials in Oakland. Dr. Robert Benjamin, Alameda County chief of communicable disease control, said the increases are mostly among heterosexuals and portend a spread of the acquired immune deficiency syndrome virus to the general population. "The heterosexuals clearly are not taking heed, they're not listening," he said. "There is no question in my mind, locally and nationally, that we're in for trouble." In Alameda County, gonorrhea cases increased from 3,501 during the first nine months of 1986 to 5,456 cases in the first nine months of this year. Cases of syphilis, however, declined from 245 last year to 232 this year. In Contra Costa County, syphilis cases should increase by 69% to 144, and gonorrhea cases are expected to increase as much as 25%, to 2,452.