The emergence of deadly drug-resistant tuberculosis

Tuberculosis was once a scourge in Ireland and I'm old enough to remember the sanatoria that dotted the country for convalescing TB patients. The tide turned on TB in the 1950s with the introduction of new antibiotic drugs and Dr Noel Brown, Minister for Health (1948 - 1951), is fondly remembered for effectively tackling TB in Ireland. Unfortunately, a new global TB threat looms again. Expanded details can be read in an article by Krista Mahr, Time , March 4th, 2013.

TB is an infectious disease caused by the bacterium Mycobacterium tuberculosis . It usually affects the lungs (pulmonary TB) but it can also affect other parts of the body (brain, kidney, bones and joints). Only pulmonary TB is infectious. It spreads easily in overcrowded and unsanitary conditions. The disease is transmitted by coughing up infectious airborne droplets and transmission usually requires long-term close contact with infectious cases. TB is curable with a course of antibiotics, but drug-resistant forms can develop if the course is not completed or if the wrong drugs are used. Untreated TB can prove fatal.

The symptoms of pulmonary TB include coughing (sometimes expelling blood), night sweats, fever and chills. Undernourished people and those with HIV are particularly susceptible to TB. Most TB infections are kept dormant by the immune system, showing no symptoms, but the mortality rate is high when the infection is active.

TB has been tackled successfully with antibiotics for decades. The global TB mortality rate declined by 41 per cent per cent since 1990. However, TB remains one of the world's deadliest diseases, killing 1.4 million people in 2011. Unfortunately, poor diagnosis, shoddy management and misuse of TB drugs have allowed strains of M tuberculosis to develop that are much harder to fight. Multi-drug-resistant strains have arisen against which most drugs are ineffective and in 2011 the first case of totally drug-resistant TB was reported in Mumbai, India. Drug resistant cases of TB intermediate between these two categories are classified as extensively drug-resistant.

The WHO reports almost 4 per cent of new TB cases are multi-drug-resistant and 20 per cent of patients who were treated for TB before now have this form of TB. Some 84 countries are infected with extensively drug-resistant TB and worrying rates of drug-resistant TB occur in east and southeast Asia, South Africa, eastern Europe, India and Russia. Outward migration of drug-resistant TB bacteria from these hot spots could cause a global problem.

Some 25 per cent of the world’s TB cases occur in India, where about two million people develop TB annually and two people die from TB every three minutes. City slums, where people live under conditions that encourage the spread of TB and the development of drug-resistant TB, exacerbate the problem. Slum dwellers live in close quarters, without running water or clean food. Slum TB patients must attend government clinics three times a week to get TB drugs. This poses a problem for day labourers, many of whom stop treatment before the standard treatment is completed.

New drugs are urgently needed to treat TB cases resistant to the existing drugs. However, big drug companies have developed few new antibiotics in recent years, preferring to concentrate on more profitable products such as statins. The standard vaccine against TB is the BCG vaccine. This prevents severe TB in childhood but is less protective against pulmonary TB (the commonest form) in adults. A promising vaccine developed by the biotech firm Aeras recently proved ineffective in trials. About a dozen new vaccines are now in clinical trials.

TB incidence is also on the increase in Ireland and the UK. In the 1980s Britain had 5,000 cases a year. There were 9,000 cases in the UK in 2012, up 5 per cent on 2011. The main risk area is London, with 3,588 cases of TB reported in 2011. Nearly 75 per cent of the cases were not in those born in the UK (Mark Hennessy, The Irish Times , October 16th, 2012). The incidence of TB in Ireland has been slowly increasing since 2001 and about 33 per cent of TB cases are foreign born. The increase associated with the immigrant population does not seem to have affected the incidence among the Irish-born population (Dennis Pringle, Social Science and Medicine , Vol. 68, pp 620-624, 2009). HSE 2010 guidelines recommend new entrants to Ireland from countries with a high incidence of TB should be screened for the disease.

William Reville is an emeritus professor of biochemistry and public understanding of science officer at UCC. http://understandingscience.ucc.ie