TETANUS has reached epidemic proportions in Hong Kong, killing more than 20 drug addicts in just four months. Of 41 intravenous drug addicts infected with the disease, 22 have died in an epidemic which an expert says may be one of the worst in the world. Dr Yuen Kwok-yung, a microbiologist at the University of Hong Kong, said: ''Tetanus has a high mortality rate and is extremely serious. The patient has nearly a 50 per cent chance of dying. ''I have never seen or heard of such a concentrated outbreak of tetanus before. The epidemic in Hong Kong could be one of the biggest cases ever.'' The first case was reported in November and, despite free tetanus jabs on offer at the territory's 23 methadone clinics, the death toll continued to rise. So far, nine of the 41 reported cases have been discharged but 10 are still being treated at hospitals. Although a contaminated batch of heroin is believed to be the cause, a team of forensic experts has still not managed to trace the source. The Department of Health has indicated that more people than those officially documented have the disease. More than 3,000 addicts have been immunised since the department started the free vaccinations. The Government has also launched a poster and leaflet campaign to send a clear message to Hong Kong's estimated 20,000 intravenous drug addicts that they need to be vaccinated. Dr Yuen stressed the Department of Health was doing everything in its power to stop the outbreak. ''But this is a near impossible task at the moment,'' he said. ''Drug addicts are the biggest worry. Many have not been immunised and can contract tetanus quite easily, especially if they are injecting. ''We will not know how serious this is for some time because tetanus can remain dormant for many months and we do not know how many drug addicts are harbouring spores in their body. ''When those spores are activated, more and more cases will come to our attention,'' Dr Yuen said. ''This is why it is so important to be immunised. People must look at these jabs as life-savers.'' Last month Coroner Warner Banks warned the death toll could rise unless the Government devised an emergency plan of action. Tetanus derives from the Greek word tetano, which means tension. It is a disease characterised by uncontrollable muscle spasms caused by bacteria called clostridium tetani. The bacteria are often found in soil and animal or human faeces. They also produce spores in a dormant form which can survive high temperatures and chemical disinfection. These spores, together with other foreign bodies, are often introduced into damaged tissue intravenously. Once the spores become active, toxin is produced and carried along nerve fibres, deadening nerve cells as it travels. This creates a loss of balance and results in muscle failure. Symptoms are varied and agonising and include lockjaw, neck stiffness, facial and back muscle spasms, breathing difficulties, convulsions, irregular heart beats, fever and blood pressure problems. The treatment for the disease is human tetanus immunoglobulin (HTIG) which neutralises any free neurotoxin. Antibiotics such as Penicillin G are used to kill growing bacteria and surgery is often needed to clear pus collection or dead tissue. Intensive care, sedation, muscle relaxation and other supportive treatments are essential for the recovery of tetanus victims. Even with specialised treatment, the fatality rate is still high, varying from 25 per cent to 50 per cent. A full course of tetanus toxoid vaccination protects most people for up to 20 years.