The Epidemiology Of Asthma Biology Essay


It is difficult to give a specific definition for asthma because some people with asthma may just have chest tightness that can be treated easily while with others it becomes a life-treating disease with continuous obstruction of the airways. Asthma is common chronic inflammatory condition of the lung airway in childhood. The lung airways are the tubes that carry the air or exchange the air into and out of the lungs. The cause of asthma is incompletely understood and there are several factors that may contribute to the condition.

Asthma appears to be one of the most important health problems in the Western world. It is described by episodic reversible obstruction of the airway, inflammation of airway and increased bronchial reactivity which cause the lungs to get less air. It involves complex interactions between many cells (e.g. eosinophils, mast cells) and inflammatory mediators (e.g. interleukins, leukotrienes). The interaction consequently results in inflammation, airflow obstruction, increased airway hyperresponsiveness, and episodic asthma symptoms, including wheezing, productive cough, and the sensations of breathlessness and chest tightness (Kumar, et al., 2005).

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Asthma severity may vary from person to depending on the sign and symptoms severity .if the symptoms become worse it can lead to asthma attach which can be indicated laboratory investigations.

Figure A shows the location of the lungs and airways in the body. Figure B shows a cross-section of a normal airway. Figure C shows a cross-section of an airway during asthma symptoms.

Figure (1): shows the effect of asthma, A shows the location of the lungs and airways in the body. Figure B shows a cross-section of a normal airway. Figure C shows a cross-section of an airway during asthma symptoms, adapted from


Along time the airways of asthmatic patients become narrower, swollen and filled with mucus leading to the appearance of the symptoms. Asthmatic patients usually show signs of breathlessness, shortness of breath, wheezing, chest tightness and cough. The severity of symptoms varies from patient to another and symptoms may flare up from time to time with no clear reason. It may be possible to avoid certain triggers which may help to reduce the symptoms (Kelly, 2006).

The prevalence of asthma

The prevalence of a disease means the proportion of the population who suffer from the disease at a specific point in time. Asthma prevalence has increased markedly in the world in recent decades and continues to increase. The cases of asthma have been reported even in countries with only mirror cases in the past. Given the rapidity of the increase in asthma's prevalence, environmental factors rather than increased genetic susceptibility are more likely to be responsible for the trend. It is noticed that asthma is distributed differently in the world being more common in the developed countries. The statistics show about 300 million people worldwide have asthma and approximately 50 percent of them live in developing countries (Pawankar, et al., 2008). In 2006, the report by Asthma UK states that 5.2 million people in the UK have asthma( . Rates of asthma in Wales are among the highest in the world, there are over 300,000 people in Wales with 1 in 12 adults and 1 in 10 children living with the condition ). In addition, asthma is one of the highest reasons for hospital admissions in the world, with more than 4,000 admissions every year. Although asthma cannot be cured and it may cause death in severe cases, however management as well as the available treatment, usually with inhalers, has effect on easing and preventing the symptoms. ( The highest rate was reported in New Zeland and lower rate at far Eastern countries such as China and Malaysia and in the Central and Eastern Europe (Kumar, et al., 2005)

Aetiology and pathogenesis

Both genetic and environmental factors play a major role in the regulation of IgE production during allergic disorders and asthma. In 2002 A studies done by researchers had found a connection between asthma and a specific novel gene bronchial hyperresponsiveness located on chromosome 20p 13. The ADAM33 gene is a complex molecule which its expression is restricted to mesenchymal cells including fibroblasts and smooth muscle, which related to asthma as it causes the airways to over-respond and constrict airway passage (Van Eerdewegh et al., 2002). This gene will never be expressed because of lack of environmental stimuli which is the interactions between environment and gene interactions. Further studies of this gene have established several facts one of them is that, there are no significant different in the level of ADAM33 in mRNA or protein expression in asthmatic compared to normal airway (Holgate et al., 2006).

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The pathogenesis of asthma is not totally understood however it is known that the inflammatory response involves a variety of inflammatory cell types including mast cells, alveolar macrophages, neutrophils, eosinophils, lymphocytes and a variety of inflammatory mediators. Th1 and Th2 cells are associated with asthma and are derived from a common naive precursor T cell. Th1 have a key role in macrophage activation in delayed-type hypersensitivity reactions. Th2 cytokines (IL-4, IL-5, IL-10, IL-13 and GM-CSF)  have the ability to change or modify the inflammation of airway, specifically IL-13, which prompt hyperresponsiveness of airway independently of IgE and eosinophilia (Barnes, 2001).


Figure (2) : shows the pathogenesis of asthma, adapted from The Lancet, 368, Holgate ST, Polosa R. The mechanisms, diagnosis, and management of severe asthma in adults, 780-93. Copyright (2006),

The effects of modern life in the epidemiology of asthma

The increase of asthma and it is morbidity during the second half of twentieth century correlates with many different aspects of modern life that could be reversed risk to asthma. Sensitization to allergens is now known to be the most important risk factor for asthma (Byrene, et al., 1995).The changes that have been suggested as causes of the epidemic are described in the following points.

Diet: Several types of food cause allergic reactions leading to triggering of asthmatic symptoms so they are best avoided. The hypothesis behind food allergy is that body become sensitive to certain type of food and so produce IgE antibody. Then mast cells and basophiles become involved in this process which leads to the production of histamine and other substances when they become into contact with the IgE.

There are wide range of food that can cause allergy e.g. shellfish, milk, egg and soy ect. Omega-6 fatty acid, mostly present in animal fat, found to have relation with asthma even though it has wide range of benefits to the body. This is because Omega-6 is metabolised to arachidonic acid which cause inflammation. Instead Omega-3 fatty acid found to have less relation to inflammation reactions (

Common food sources of sulfites include:

Dried fruits or vegetables


Wine, beer

Bottle lemon or lime juice


Pickled foods, such as pickles, relishes or peppers.

Obesity: many studies suggested that there is association between obesity and asthma. This relation is found with asthmatic people either children or adult. It is found that the lungs of obese people are under-expanded and the size of breaths is smaller therefore, the airways become narrower. In addition, smooth muscles are affected by chronic inflammation and the changes in blood hormones, derived from fat tissue, causing the airways to narrow excessively. In addition, British researchers showed that obese children are 77% more likely to get asthma (Carrell, 2002).

Housing: the allergen of house-dust mites (focal droppings that contain allergens) appear to be among the most powerful of the common aeroallergens. In a place where the humidity (greater than 55%) and temperature of homes as well as work/social environments are the most source of indoor allergen. There is evidence that the number of house-dust mites have risen in Australia. This increase can be a part of responsible for the increased prevalence of AHR in sensitized asthmatic people. Trough some particles are small enough to penetrate to the lower respiratory tract (Naspitz et al., 2001).

Air pollutants (indoor & outdoor): at high concentrations, sulphur dioxide, and to a lesser extent a nitrogen dioxide and ozone, act as respiratory irritants and can be trigger factors for acute episodes of symptoms. There is no evidence that air pollutants induce asthma or AHR. In recent study (in Japan) found that diesel exhaust partials potentiated the IgE production in mice. These data have not corroborated by large epidemiology studies in which the severity of asthma is well characterized (Byrene. et al., 1995). Other study of children in Germany, there was a higher prevalence of bronchitis and a lower prevalence of allergic disorder in the eastern with the high level of sulphur dioxide and air pollutants compared with the western part of the country. In Australia suggesting that living conditions are responsible for allergic illness in the country (Byrene. et al., 1995).

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Allergies to animals Some people become allergic to some types of animals such as pet cats and dogs. The irritation is not caused by the fur or feathers of the pets but by proteins secreted by oil glands and shed as dander, proteins in saliva (which stick to fur when animals lick themselves) or by aerosolized urine from rodents and guinea pigs. With the presence or absence of animals their allergens will be present as they are carried on very small particles and remain circulating in the air or carpets and furniture for weeks and months after a pet is gone (

Figure (3): shows the asthma triggers, adapted from

Family size: Children from smaller families seem to be more likely to develop an allergy and asthma. This is thought to be because they are exposing to viral infections in early life than children in larger families resulting in low developed in immunity. In addition, frequent infections in early life promote a switch to the non-allergic state (Gershwin, et al., 2001).

Antibiotics: the increasing use of antibiotics in early life and their inclusion in foodstuffs via animal feed may alter the balance of the germs found on the skin. This could rise of developing allergic asthma (Byrene, et al., 1995).

Passive smoking: there is increasing evidence that passive smoking is involved, to a small extent in the aetiology of childhood asthma. Children with mothers who smoke are at increased risk of having elevated cord IgE level at birth, which may have an effect on the later development of atopy. The risk conferred by passive smoking is higher I children of a low socio-economic status. The possibility that passive smoking is involved is important because positive interventions are possible help to reduce the prevalence of the disease in future generations (Challem‏, 2003).

Infections: There is no evidence that early childhood infection actually bring the first attack of asthma however early respiratory infection, particularly coughs and common cold viruses especially rhinoviruses, considered to be among the most common known cause for wheezing and brochiolities during the first two years of patient's life (Holgate, et al, 2006). Infections with hepatitis A sero-positivity, measles and other stimuli which increase production of interferon-gamma and IL-12 may inhibit the TH2 allergic response and also lead to asthma (Kelly, 2006).

Exercise-induced Asthma

Exercise as a common cause of bronchospasm in patients with asthma was first reported in 1962. Exercise induced asthma (EIA) occurs in up to 90% of individual with asthma. EIA is defined as a condition in which strong physical activity triggers acute airway narrowing in individuals with increased airway responsiveness, occurring several (6-8) minutes after strenuous exercise with a pace sufficient to increased an approximate heart rate of 170/min. The usual symptoms are wheezing, coughing, tightness of the chest and breathlessness. The severity of EIA depends on number of variables related to the type and pattern of exercise; its severity and duration, and environmental condition (McFadden, 2001).

In exercise-induced asthma, it is proposed that bronchoconstriction result from hypertonicity (increased tension of the muscles) of the airway epithelial lining fluid. Hyperventilation leads to airway heat, water loss, Co2 loss, causing a release in mast cell mediators causing the bronchospasm to occur. However, the precise mechanism responsible for EIA is unknown. Several of theories presented as possible but unsubstantiated mechanisms. Research has documented those asthmatics during exercise breathing dry, cold air through the mouth stimulates bronchospasm. Extremely sensitive patient should avoid exercising in subfreezing temperatures (

There are now a number of studies showing that, intensive exercise training programmes help people with asthma to take part in physical activities that were not open to them. It is important to treat each child individually and it is recommended that children with severe asthma to avoid exercise during the coldest parts of day and in times of high pollution (McFadden, 2001). Doing exercise is very helpful to dilate the lungs and bronchial tubes, which reduce the resistance to breathing. The exercise has positive effects which may help preventing asthma symptoms. The lack of exercise or doing less exercise may also contribute and help to develop obesity, which affects the anatomy of the lungs and airways as it is one of the risk factors in asthma. Exercise might trigger attacks for those patients who already have developed asthma, but they can use an inhaler before they start the activity or the exercise. So asthma that is trigger by exercise should certainly not stop a person from having an active lifestyle or participating in sports (McFadden, 2001).

Morbidity and Mortality of Asthma:

Asthma may occur at any age with higher incidence in children under 6 years and the chance of its development is found to be higher in boys than girls. It is suggested that boys under the age of 14 are at greater risk for asthma compared with girls while it is the opposites after the age of 40. Generally the trend of asthma is increasing in all ages and for both genders (Holgate, et al, 2006)..

Even though asthma and its treatment are understood to certain points, the mortality is still increasing specially with children who live in inner-city areas. There is not many cases of death due to asthma among children, however the rate increased by 3.4% per year from 1980 to 1998. The number of deaths annually decreased from 5067 (1960-1962) to a low of 1870 (1975-1978) and then increased to 5429 (1993-1995) (Kelly, 2006). The mortality rate found to be high is United Kingdom and Australia and lower in Iceland and Finland (Massoli, et al., 2004).

Hygiene Hypothesis of asthma

The asthma hygiene hypothesis is the infection that occurs naturally and the exposure of allergens may immunize against the development asthma, autoimmune diseases and allergic. Attention to hygiene of modernity or "safe environment" could be reduced by this natural immunity over the last century, and perhaps it is a factor in the rise these conditions in the world (Liu, et al., 2003). The idea was first suggested in the mid 1970s from studies of the Métis Indians in Saskatchewan. However, the term "hygiene hypothesis" is coined it in 1989 by David Strachan, who explain his observation that in children who grew up in large families, the allergy was less common. Strachan's original observation that exposure to other children reduces the risk of being allergic such as asthma (Riedler, et al., 2001).

The mechanism assumed is that exposure to microbial pathogens is able to protect children from allergic sensitization. The proposal is that less hygienic environments that susceptible to infections in early in childhood help to product against asthma. In the domestic environment, Childhood reduces the risk of acquiring allergic disease due to exposure to animals, this exposure may occur on farms. The findings related to exposure to domestic pets (dogs or cats) are complex (Von Essen, 2001). Other exposures have also been proposed as relevant to the hygiene hypothesis such as enteric infections, use of antibiotics, and mycobacterium exposure in the form of (BCG) vaccination.


It is clear that the cases of asthma have increased during the second half of the twentieth century to a level that at least three-quarters of the cases that present to clinics or to hospitals would not have required treatment in the 1950s. This increase correlates with the changing in the life style of the people around the world. The association of both environmental as well as genetic factors with the occurrence of the serious asthma disease is known. However, their exact effect on changing the expression of the involved genes need to more studied.