Deadly drug-resistant staph infections – rarely seen in patients a decade ago – have now become the leading type of skin infections treated in emergency rooms, scientists reported Wednesday.

The study in the New England Journal of Medicine was the first to demonstrate the extent to which drug-resistant Staphylococcus aureus has spread throughout the country.

The bacterium accounted for 59 percent of skin infections in the study, researchers said. Local prevalence ranged from 15 percent in New York to 74 percent in Kansas City, Mo. In Los Angeles, drug-resistant staph accounted for 51 percent of skin infections, researchers said.

“The message for doctors is to recognize how prevalent these infections are,” said Dr. Gregory J. Moran of Olive View-University of California, Los Angeles, Medical Center who was the lead author of the study.

The superbug was found to cause painful skin lesions that resembled infected spider bites. It also can cause a deadly lung disease known as necrotizing pneumonia, and toxic-shock syndrome – a type of blood poisoning that can be fatal.

Not long ago, doctors could confidently prescribe tried-and-true antibiotics for skin infections, but Moran said that was no longer the case.

At his hospital, “we assume the infection is resistant and treat it accordingly” with a different set of drugs, Moran said.

What triggered the spread of drug-resistant staph isn’t known. Some years ago, researchers started finding infections in jail inmates, sexually active gay men and professional athletes. Then last year, infections were reported among the general population in Atlanta, Baltimore and Minnesota.

The staph was resistant to antibiotics routinely used to treat skin and soft-tissue infections, such as erythromycin, cephalexin, sold as Keflex, and dicloxacillin, sold as Diclocil, scientists said.

Most of the infections were more or less susceptible to other antibiotics, including, clindamycin, tetracycline, rifampin and sulfa drugs.

In addition, the study said, many drug-resistant infections can be cured by surgically lancing and draining them.