Government scientists said that tracing a 1985 food poisoning outbreak in Los Angeles to antibiotic use on farms demonstrated that use of such drugs in livestock contributes to salmonella infections in humans. Dr. John S. Spika, an epidemiologist at the Centers for Disease Control in Atlanta, said the studies began when Los Angeles reported a nearly fivefold increase in salmonella cases in 1985, including two victims who died. Most of the cases involved a form of salmonella that resisted antibiotics, and researchers traced the strain from hamburger eaten by some of the victims back to three dairies in California. They then conducted a survey of 75 dairies and found a strong association between the use of the antibiotic chloramphenicol and the presence of the resistant bacteria. Three of seven farms that had chloramphenicol-resistant salmonella admitted using the antibiotic in the previous 18 months, even though it was illegal, Spika said.