LONDON — A new, deadly strain of tuberculosis has killed 52 of 53 people infected in the last year in South Africa, the World Health Organization said, calling for improved measures to treat and diagnose the bacteria.

The strain was discovered in the KwaZulu-Natal region of South Africa, and is classified as extremely drug-resistant. Drugs from two of the six second-line medicines, used as a last line of defense against TB, proved ineffective against the new strain.

"We are extremely worried about the issue of extreme drug resistance," said Paul Nunn, coordinator of the WHO's drug resistance department. "If countries don't have the diagnostic capacity to find these patients, they will die without proper treatment."

Though even the most drug-resistant strains of TB have proven to be treatable with three classes of drugs, those drugs are more expensive and are toxic to the human body.

The WHO and its partners, including the South African Medical Research Council and the U.S. Centers for Disease Control and Prevention, planned a two-day meeting next week in South Africa to discuss the new TB strain in Africa and better ways to diagnose and treat it, Nunn said.

Tuberculosis is a respiratory illness spread by coughing and sneezing. Nearly 2 billion people worldwide are thought to be latently infected.

High mortality rates among TB patients in South Africa, however, prompted medical researchers to survey the cases, and ultimately to find the new strain.

Drug resistance is a common problem in TB treatment, but the new strain appears particularly virulent: 52 of the 53 patients infected all died within about three weeks of being tested for drug resistance.

"Genetic processes are constantly throwing up mutations of tuberculosis viruses, so this may have arisen due to some particular quirk of the environment or the way they were treated or their genetic background," said Paul Fine, a professor of communicable diseases at the London School of Hygiene and Tropical Medicine.