CONTAMINATED heroin may have caused a tetanus outbreak among intravenous drug users that has killed five people, doctors said. A Health Department inquiry showed that 20 men and four women suffering from the disease had been admitted to hospital in the past six weeks. The patients, aged between 31 and 63, had a history of injecting drugs, but did not appear to be linked in any other way. The Health Department said there was no cause for concern among the public, although immunisation against the disease was advised. Tetanus, or lockjaw, was a serious nervous system disorder caused by bacteria in soil and dust which infects wounds and was commonly associated with animal bites. Symptoms included stiff muscles, including the jaw, and muscular spasms throughout the body. The Assistant Director of Health for Special Health Services, Dr Lam Ping-yan said: ''The cause of the infections is believed to be due to contamination of heroin with the bacteria. ''If only one batch of heroin is affected then infections may have peaked, but if there are more batches then there is likely to be more cases.'' The outbreak came to light after Kwong Wah Hospital reported nine cases of tetanus in a few days. The general manager of clinical services, Dr Wan Tai-tak said: ''We have never had such a large number of drug addicts suffering from tetanus at one time. ''We are afraid the bacteria may have come from the drugs themselves and that contaminated drugs are being sold in Hong Kong.'' One patient told the Health Department he bought heroin in Mongkok and Tsz Wan Shan. Most of the surviving patients were in critical condition in intensive care units. Others, who were unconscious, were said to be in poor condition. Intravenous drug users were urged to be immunised and to seek methadone treatment if they could not stop using drugs.