Acute respiratory infections

Acute respiratory infections are divided into two groups according to their anatomical location:

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infections affecting the airways above the epiglottis, designated upper respiratory tract infections (AURI), which also include otitis media;

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those affecting the airways below the epiglottis, called lower respiratory tract infections (ALRI), which include laryngitis, tracheitis, bronchitis, pneumonia and, in young children, bronchiolitis.

However, upper and lower parts of the respiratory tract are often affected simultaneously or consecutively during an acute episode, and there are also disseminated forms such as the influenza syndrome. The incidence of ARIs is seasonal almost everywhere, with the highest annual peak in either the cold or the rainy season.

AURI and acute bronchitis are very common in all populations, the incidence being similar in both low/middle- and high-income countries. On average a young child has 4–6 episodes and an adult 2–4 episodes of AURI a year, counting every episode, even the mildest ones. However, there is a striking difference in the incidence rates of community-acquired pneumonia depending on the country income level (Table 2.5).

Table 2.5

Estimated annual incidence of community-acquired pneumonia, by age, in low/middle- and high-income countries.

Pneumonia rates are high in very young children and decline with increasing age until age 60 years. The lowest rates are usually observed in young adults: the estimated annual incidence among young adults is 0.6% in high-income countries and 1.0–2.0% in low/middle-income countries. In persons aged 60–74 years, the rates are 1.5% in high-income countries and 3.0–4.0% in low/middle-income countries. The highest rates in adults are observed in persons of 75 years and above. Pneumonia rates in adults can double during epidemics of influenza A. People who are HIV-seropositive have an ALRI incidence almost 10 times higher than who are HIV-seronegative, a risk of bacterial pneumonia 4–20-fold higher and a rate of invasive pneumococcal infection between 40- and 1000-fold higher.

Pulmonary tuberculosis

Pulmonary TB refers to disease affecting the lung parenchyma and is by far the most frequent type of TB. Pulmonary TB is classified as direct AFB smear microscopy positive or direct smear microscopy negative. Extrapulmonary TB tends to occur more frequently in HIV-infected than in non-HIV-infected persons, but pulmonary TB remains the most common type of TB in both groups worldwide. Smear-negative pulmonary TB is more common among HIV-positive patients than among HIV-negative TB cases. Among the extrapulmonary TB forms, pleural, laryngeal and bronchial TB are counted as respiratory TB locations.

In 2006, there were an estimated 9.2 million new cases of TB in the world, of which 4.1 million were smear-positive (Table 2.6). Eighty per cent of the cases are estimated to occur in 22 high-burden countries, which represent 63% of the world population.

Table 2.6

Estimated TB incidence, all forms and smear-positive pulmonary TB, 2006.

Even though the DOTS strategy has been widely adopted at global level and substantial progress has been made in the implementation of effective TB control programmes in a growing number of countries worldwide, the burden of TB remains enormous. Co-infection with HIV is a major contributing factor in many countries, mainly those of sub-Saharan Africa; in 2006, for example, the estimated incidence rate in sub-Saharan Africa was 363 cases per 100 000 population. TB control has also been complicated by the emergence of multidrug-resistant TB, and to some extent extensively drug-resistant TB, in many countries, particularly those of the former Soviet Union.

After increasing at a rate of 1% per year until 2004, the incidence of TB became stable or declined in all the six WHO regions in 2005 and 2006. However, the total number of new TB cases continued to rise slowly.

Asthma

Asthma is a chronic, inflammatory disorder of the airways that causes recurrent episodes of wheezing, breathlessness, chest tightness and cough, particularly at night and in the early morning. The airflow limitation is variable and partially or totally reversible, either spontaneously or with treatment. The total number of patients suffering from asthma worldwide has been estimated at 300 million, most of whom live in low- and middle-income countries.

Asthma occurs at all ages in most countries, with higher prevalence rates in urban than in rural areas, in children than in adults, and in adult females than in adult males. Reports on asthma prevalence have shown huge variations within and between countries, even between high-income countries, as confirmed by surveys carried out using similar methodology and standard research protocols (Table 2.7). Some variations may be due to different interpretations of the definitions used, but the most important reasons for variation are not clearly established. They are probably linked to differences in exposure to environmental risk factors, either for the development of asthma or for exacerbations of this variable disease.

Table 2.7

Asthma prevalence in age groups 13–14 and 20–44 years in several regions of the world.

Since 1960, asthma prevalence has gradually risen in most high-income countries as well as in many low/middle-income countries. In several areas, there has been a 100% increase in the overall prevalence of asthma in children. Severe asthma is an emerging public health issue among the poorest people, especially minorities, living in degraded areas of big cities of both developed and developing countries. The increase is linked to changes in exposure to environmental factors that may exacerbate asthma:

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at home, indoor pollutants such as second-hand tobacco smoke and smoke from the combustion of solid fuels, as well as allergens;

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at the workplace, allergens and irritants;

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outdoors, allergens and air pollution.

Asthma prevalence has increased in most developing countries , particularly in Africa and Latin America. The epidemic of asthma observed in low- and middle-income countries may continue in the future with increasing urbanization and adoption of western lifestyles, which are factors that have been associated with the increasing trends.

Chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease, or COPD, is a nonspecific term developed to describe chronic lung disease characterized by airflow limitation that is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles and gases. The pathological conditions that contribute to COPD are chronic bronchitis and emphysema.

Many previous definitions of COPD have emphasized the terms “emphysema” and “chronic bronchitis”; these are no longer included in the definition. Emphysema is a pathological term and describes only one of the several structural abnormalities present in patients with COPD. Chronic bronchitis, defined as the presence of cough and sputum production for at least 3 months in each of 2 consecutive years in a patient in whom other causes of chronic cough have been excluded, remains a useful clinical and epidemiological term. However, it may or may not be associated with airflow limitation, which is the essential characteristic of COPD.

By far the most important cause of COPD is tobacco smoking. Other important factors reported to be associated with the condition include indoor air pollution, occupational exposure to irritants, and childhood respiratory infections. Cigarette smoking continues to increase in all low- and middle-income countries and will substantially increase the global COPD prevalence, particularly among certain subpopulations in Asia, who are becoming early and heavy smokers. In addition, increasing life expectancy is likely to be followed by higher COPD prevalence. COPD is an important cause of restricted activity and chronic disability, with a consequent reduction in quality of life from adulthood to old age.

In some high-income countries, such as the United States of America, the prevalence of COPD has shown a progressive decline in men during the past decade but a progressive increase in women. Unfortunately, different survey methodologies and variable definitions for COPD make inter-country comparison of epidemiological data difficult. Available data are likely to underestimate the total COPD burden.

The estimated prevalence of COPD worldwide in 2001 was 1013 per 100 000 population – 1206 males and 810 females. These estimates include people of all ages and therefore underestimate the frequency of disease in adults, because COPD rarely occurs in young age groups. The highest prevalence (1675/100 000 population) was found in WHO's Western Pacific region, particularly because smoking is a very common habit in China (60% of the adult male population smoke). The lowest prevalence was in sub-Saharan Africa (179/100 000 population) probably because of Africa's young population (only 3.2% are over 65 years) and the low prevalence of smoking. Recently, WHO estimated that COPD affects 210 million people globally. The disease is currently the fourth leading cause of death globally and may become the third by 2030.

Lung cancer

Lung cancer was a relatively uncommon disease at the beginning of the twentieth century. Since then, its incidence in the world has been steadily growing, more rapidly after 1960 than before, in both developed and developing countries. Global incidence has been rising at 0.5% per year in recent years; a major contribution to this trend comes from eastern Europe and developing countries. Lung cancer is the most common cancer in males. There were 1.3 million new cases in 2000, of which 939 900 were in men (30.9 per 100 000) and 365 700 in women (11.9 per 100 000). The estimated incidence of lung cancer varies greatly with region and depends on age and population structure, prevalence of tobacco smoking and other risk factors, and opportunities for detection and treatment.

Table 2.8 presents the estimated age-standardized incidence rates of lung cancer by sex and mortality stratum by region. Incidence in males is highest in the countries of Europe and North America, ranging from 43.3 to 52.2 per 100 000 population. In females, the highest rate, 30.5 per 100 000, was estimated in North America. Lung cancer incidence is lowest in Africa.

Table 2.8

Global and regional age-standardized incidence rates of trachea/bronchus/lung cancer by sex and region: estimates for 2002.

Epidemiological studies have consistently shown that the majority of lung cancer patients have a history of cigarette smoking, and the highest mortality attributable to smoking corresponds to lung cancer. There are other risk factors, however, particularly exposure to asbestos. The falling incidence observed in developed countries in recent years seems to be related mostly to decreased cigarette smoking.