|Fact sheet No 122: Cities and Emerging or Re-emerging Diseases in the XXIst Century - June 1996 (WHO, 1996, 3 p.)|
Twenty years ago, many health specialists thought that public health measures could soon limit the importance of infectious diseases. Yet today they remain the worlds leading cause of death, killing at least 17 million people a year. One problem is the emergence of new infectious diseases, at least 30 previously unknown diseases having emerged since 1973. Another is the re-emergence of infectious diseases previously considered as under control. Epidemic cholera, for instance, was again reported in the Americas in January 1991 after being absent for many decades; between 1991 and December 1995 more than 1.3 million cases were reported and upwards of 11,000 deaths. For nearly half a century yellow fever has been considered a rural disease. But many cities, especially in the Americas, have become infested with the Aedes aegypti mosquitos which transmit the disease and are once again at risk.
Other diseases are re-emerging because the pathogen or the vectors that transmit them have become resistant to control measures. Malaria still kills more than a million people each year (mostly children) with a total of 250-450 million clinical cases annually. Although malaria has long been considered mainly a rural disease, it has become one of the most serious health problems in many urban centres where the anopheline mosquito can find standing water in which to breed. Dengue and dengue haemorrhagic fever, also spread by Aedes mosquitoes, are growing problems, the number of outbreaks and epidemics having increased significantly in the last decade. Tens of million of cases occur each year, most of them unreported, causing 500 000 hospitalizations and more than 20,000 deaths. Some 2 billion people worldwide are at risk, mostly in urban areas.
Urbanization concentrates large numbers of people particularly vulnerable to infection. The numbers of older people and of persons with HIV, two categories who are more vulnerable to many infectious diseases, are also increasing in towns. People are constantly moving in or out of cities, which can mean the arrival of newcomers who bring new infections to which the city population has little or no immunity. The rapid increases in travel and trade over the last few decades have increased opportunities for pathogens and vectors to spread to new areas. The mosquito responsible for transmitting dengue fever in Asia has become established in the United States, Brazil and parts of Africa. Anopheles stephensi, the principal vector for urban malaria, has adapted to the urban environment in India and the Eastern Mediterranean region, while other species have adapted to breed in swamps and ditches around urban areas in Nigeria and Turkey.
In the early decades of antibiotic use, public health specialists did not foresee the ability of many agents of infectious diseases to rapidly develop resistance to the drugs or chemicals that had previously killed them. The problem has been exacerbated in countries where antibiotics are overused and misused and where governments have not sufficiently prioritized disease control programmes. One example is tuberculosis, which is responsible for some 3 million deaths each year and is the single largest cause of adult death in the world, mainly among populations living in the poorest areas. In urban areas, a combination of overcrowding and poor ventilation often means that one person with tuberculosis, if not detected and cured, will transmit the infection to 10-15 other people each year -- and often to family members. The burden of tuberculosis has increased rapidly over the past decade or so, partly linked to the spread of HIV/AIDS. Most deaths from tuberculosis occur in the South, but they have also increased in many wealthy nations during the 1980s and early 1990s after decades of steady decline. WHOs directly observed treatment, short course (DOTS) strategy aims at curing tuberculosis patients in all settings, including urban areas, and if adopted will effectively prevent the emergence of drug resistant strains of tuberculosis.