Asthma is a common chronic disease that affects millions of people of all ages in all parts of the world. It is a cause of substantial burden often causing a reduced quality of life. New surveys are needed to update asthma trends.
GLOBAL BURDEN OF DISEASE DUE TO ASTHMA
Asthma, a disease of the airways, occurs in people of all ages, and wheeze is the most common symptom. The most recent revised global estimate of asthma suggests that as many as 334 million people have asthma, and that the burden of disability is high. The historical view of asthma being a disease of high-income countries no longer holds: most people affected are in low- and middle-income countries, and its prevalence is estimated to be increasing fastest in those countries. Ongoing monitoring is needed to follow the epidemic of asthma and its management.
What is asthma?
Asthma is a disease of the bronchial tubes in the lungs (the “airways”). People with asthma typically experience “wheezing”, a high-pitched whistling sound heard during breathing, especially when breathing out. However, wheezing does not always occur, and asthma can also involve breathlessness, chest tightness or coughing. The underlying process includes chronic inflammation of the airways, reversible obstruction of the flow of air in and out of the airways, and the tendency of the airways to over-react to stimuli. Asthma most commonly develops in early childhood, and more than three-quarters of children who develop asthma symptoms before age 7 no longer have symptoms by age 16. However, asthma can develop at any stage in life, including adulthood.
How many people have asthma?
The number of people with asthma in the world may be as high as 334 million. This figure comes from the most recent comprehensive analyses of the Global Burden of Disease Study (GBD) undertaken in 2008-2010. A lower figure of 235 million used in the Global Asthma Report 2011 came from the most up-to-date GBD information available at that time based on analyses from 2000-2002. These numbers are not precise, rather they are estimated from the best data available. However, as the following paragraphs illustrate, there are many gaps in asthma statistics. There is no evidence that the number of people with asthma in the world has increased from 235 to 334 million between our 2011 and 2014 reports; rather this situation illustrates the need for high quality data on asthma to be collected in an ongoing way.
Much of the information on which the later estimate is based is already out of date, as the last global surveys of the proportion of the population who have asthma (that is, prevalence) were carried out about 10 years ago. Unfortunately the World Health Organization (WHO) is not undertaking any future global asthma monitoring work; however the Global Asthma Network (GAN) plans to continue this work with worldwide studies to find out how the pattern of asthma is changing in children and adults (See Summaries/Global Asthma Network).
To make comparisons of the prevalence of asthma between different parts of the world, and changes over a period of time, standardised measurements are needed (that is, measurements done in the same way at different places and times). The most common way of doing this is by questionnaire, which is feasible for large scale surveys. Using this approach The International Study of Asthma and Allergies in Childhood (ISAAC) undertook its latest survey between 2000 and 2003.
ISAAC found that about 14% of the world’s children were likely to have had asthmatic symptoms in the last year and, crucially, the prevalence of childhood asthma varies widely between countries, and between centres within countries studied (Figure 1). These conclusions resulted from ISAAC’s ground-breaking survey of a representative sample of 798,685 children aged 13-14 years in 233 centres in 97 countries. (A younger age group of children (6-7 years) was also studied by ISAAC and the findings were generally similar to the older children). These adolescents were asked whether they had experienced wheeze in the preceding 12 months. Prevalence of recent wheeze varied widely (Figure 1). The highest prevalence (>20%) was generally observed in Latin America and in English-speaking countries of Australasia, Europe and North America as well as South Africa. The lowest prevalence (<5%) was observed in the Indian subcontinent, Asia-Pacific, Eastern Mediterranean, and Northern and Eastern Europe. In Africa, 10-20% prevalence was mostly observed.
In this same survey, the prevalence of symptoms of severe asthma in the preceding 12 months, defined as 4 or more attacks of wheeze, waking at night with asthma symptoms one or more times per week, and/or any episodes of wheeze severe enough to limit the ability to speak, also varied substantially, but was > 7.5% in many centres (Figure 2).
The prevalence of asthma in younger adults varies widely as it does in children. Overall, 4.3% of respondents to WHO’s World Health Survey aged 18-45 in 2002-2003 reported a doctor’s diagnosis of asthma, 4.5% had reported either a doctor’s diagnosis or that they were taking treatment for asthma, and 8.6% reported that they had experienced attacks of wheezing or whistling breath (symptoms of asthma) in the preceding 12 months (Figure 3). The highest prevalence was observed in Australia, Northern and Western Europe and Brazil. The World Health Survey, which was conducted about the same time as ISAAC, used a different survey method which may contribute to some of the differences in the findings within a region. The prevalence of asthma was measured by questionnaire administered to 177,496 persons aged 18 to 45 years living in 70 countries.
Much less is known about the prevalence of asthma in middle-aged and older adults. This reflects both a paucity of survey data and the greater difficulty of distinguishing asthma from other respiratory conditions, such as chronic obstructive pulmonary disease (COPD) in older age groups. There are no internationally standardised comparisons of asthma prevalence in the elderly.
Is asthma becoming more or less common?
Asthma symptoms became more common in children from 1993 to 2003 in many low- and middle-income countries which previously had low levels, according to ISAAC. However, in most high-prevalence countries, the prevalence of asthma changed little and even declined in a few countries. Factors responsible for increasing asthma rates are not fully understood, but environmental and lifestyle changes play the key roles (see Causes and Triggers). What has happened to the prevalence and severity of asthma since 2003? We do not know because there have been no surveys.
What is the impact of asthma on rates of disability and premature death?
The burden of asthma, measured by disability and premature death, is greatest in children approaching adolescence (ages 10-14) and the elderly (ages 75-79) (Figure 4). The lowest impact is borne by those aged 30-34. The burden is similar in males and females at ages below 30-34 years but at older ages the burden is higher in males. This sex difference increases with increasing age. Figure 4 shows the GBD’s measure of health loss attributable to specific diseases, for asthma. The GBD used mortality statistics and health survey data, where available, to estimate, for many countries of the world, two components of disease burden: years of life lost due to premature death, and years of life lived with disability. The latter quantifies both the extent of disability and its duration. The years of life prematurely lost, and the years of life lived with disability are added together and expressed as disability adjusted life years (DALYs), which is the measure of burden of disease.
Among people aged less than 45 years, most of the burden of disease is disability. The GBD estimated that asthma was the 14th most important disorder in terms of global years lived with disability. However, for people in older age groups, premature death due to asthma contributes more to the burden of disease (Figure 5).
Asthma has a global distribution with a relatively higher burden of disease in Australia and New Zealand, some countries in Africa, the Middle East and South America, and North-Western Europe (Figure 6).
The global burden of disease due to asthma has become better understood through standardised measurement of the proportion of the population who have asthma, severe asthma, disability due to asthma and/or who have died from asthma. Little is known about asthma in the many countries where it has not been studied, and little information is available about asthma in adults over the age of 45.
Governments should commit to research, intervention and monitoring to reduce the burden of asthma in the world. Global surveillance of asthma requires standardised measures of asthma implemented in large scale surveys of both children and adults in diverse settings worldwide.