The deadliest strain of tuberculosis is XDR-TB, the “extensively drug-resistant” type, meaning that many antibiotics do not work against it.
XDR-TB is rare but very lethal. Curing it takes up to two years, and the drugs prescribed can cause permanent deafness, nerve damage, vomiting and rashes. Between 50 percent and 80 percent of those who start treatment die.
XDR-TB has been reported in 117 countries, according to the World Health Organization. Most cases are isolated, but thousands of hidden cases are likely to exist because it takes multiple lab tests to detect each one.
The worst known outbreak is in South Africa, in the rural Zulu-dominated mountains behind Durban, where H.I.V. is also extremely common.
In 2006, researchers there reported an alarming finding: 52 out of 53 patients with both H.I.V. and XDR-TB had died, and half died within a month of getting a diagnosis.
Initially, doctors thought most patients had developed XDR-TB because of “treatment failure” — that they had had regular TB or a slightly drug-resistant version, and because they had either not been prescribed the right drugs or had not taken them all, their infections became resistant to multiple drugs.
Instead, they found many patients had never been treated, implying that the deadliest strain was being transmitted between people.
Now a study, published in The New England Journal of Medicine by South African and American scientists, has shown the problem there is much bigger than previously realized.
More than two-thirds of the patients had never been treated for drug-resistant TB, the scientists found, and 84 percent had strains closely related to at least one other patient’s. There was even one cluster of 212 related cases.The investigators looked into 404 cases of XDR-TB detected over four years, interviewing patients and their relatives to see who had been treated before and who had contact with whom. The researchers also genetically typed patients’ strains. Almost 80 percent of the patients also had H.I.V.
That suggested, the authors concluded, that most victims contracted TB not through treatment failure, but rather directly from another victim through coughing, spitting or other close contact. Family contacts were the most dangerous; clusters were spread across generations and houses.
Hospitals were also dangerous. Sixty-one percent of the patients who had spent time in hospitals before they received a diagnosis were linked to another patient in the same hospital. Suspected transmission to co-workers was also found.
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