ATLANTA (AP) _ The rate of foodborne illnesses in the United States has dropped again, but certain types of salmonella climbed to the highest level since the government started keeping track four years ago.

The Centers for Disease Control and Prevention estimated Thursday that 76 million Americans suffer a foodborne illness each year. The CDC said the rate for the nine disease-causing agents it tracks was 40.7 cases per 100,000 people last year, compared with 51.2 per 100,000 four years ago.

Salmonella infections rose from 12.3 per 100,000 people in 1998 to 14.8 per 100,000 in 1999.

Salmonella, which has more than 2,000 variations, is rarely fatal but can cause severe stomach pain, diarrhea, nausea and vomiting.

The CDC said it was unclear why salmonella infections continued to increase last year. But researchers noted that several 1999 outbreaks were linked to unpasteurized orange juice, mangoes and uncooked sprouts.

``So we think we have not made as much progress in decreasing the contamination of fruits and vegetables,'' said Dr. Patricia Griffin, head of the CDC's foodborne disease epidemiology section.

Despite the salmonella surge, other common foodborne diseases continued to decline as progress was made in controlling outbreaks of campylobacteriosis and shigella infections.

The rate of campylobacteriosis, the most widespread cause of food poisoning, fell from 25.2 cases per 100,000 people in 1997 to 17.3 per 100,000 last year.

And the rate shigella, which causes gastrointestinal symptoms similar to salmonella, has dropped 44 percent since 1996, the CDC said Thursday. Cases of E. coli 0157:H7 were down 22 percent in the same period.

The findings were based on preliminary data from the Foodborne Diseases Active Surveillance Network, which collects data on seven bacterial and two parasitic organisms from Connecticut, Georgia, Minnesota and Oregon. Several counties in California, Maryland and New York also report to the network.

The survey covers about 9 percent of the U.S. population.

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On the Net: CDC site: http://www.cdc.gov