It is the second episode of multi-drug resistant TB (MDR TB) in the capital in less than a year, and has renewed concern about the emergence of this form of the disease, extremely rare in the UK, but which has caused havoc in some American cities.
The first victim, a long-term female in-patient, was identified with MDR TB last year at St Thomas's Hospital. In March a second patient, who had Aids and has since died, was found to have the same strain of the bacterium. The patients were on the same ward, although in separate side- rooms, and the dead man is believed to have caught TB from the woman.
Two more Aids patients, identified in April and May, are now thought to have contracted the disease, and tests are under way to establish if the strain is the same as that isolated from the first two cases.
As an extra precaution, public health doctors are contacting by letter 650 in-patients and out-patients at the hospital who may have come into contact with the infected individuals. They will be offered appointments for tests and X-raysas appropriate. Relatives and friends of the TB patients have been informed, and 700 hospital staff are being screened.
Dr John Rees, a consultant chest physician at St Thomas's, said last night: "We have been seeing the emergence of [MDR TB] in the past year. It is worrying and we are treating it very seriously." An inquiry was under way at the hospital to see if there was some breakdown in infectious disease procedures, Dr Rees added.
The three surviving patients have been moved to another hospital with special isolation facilities. Patients diagnosed with MDR TB have to be nursed in isolation in special rooms with negative pressure, which enables the safe and continuous exchange of air. MDR TB is defined as TB which is resistant to at least two of the "first line" TB drugs, isoniazid and rifampicin. There are "second line" alternatives which are usually effective but they are more toxic and difficult to administer to those most vulnerable to TB, such as cancer patients and people with HIV who have compromised immune systems.
A spokeswoman for the hospital said that there was no danger of a serious TB outbreak. "If you are a healthy person the chances of being infected are very small."
The St Thomas's case is believed to be the second-biggest hospital outbreak in the UK. In August last year one patient infected at least four others on an Aids/HIV ward at the Chelsea and Westminster Hospital.
TB is transmitted through airborne droplets expelled in coughing or sneezing. MDR TB has caused serious problems in New York among those with HIV, drug addicts and the homeless.