The global asthma epidemic

Public Health Significance of Urban Pests Allergic asthma 13 12

1.1.2. The global asthma epidemic

The global epidemic of asthma, which occurred at the end of the 20 th century, has been well documented and appears to parallel changes in industrialized countries Evans et al., 1987; Sears Beaglehole, 1987. Results from studies around the world clearly demonstrate an overall global increase in the prevalence of asthma; however, some countries have reported a plateau or even a decrease over the past decade Eder et al., 2006 The frequency and occur- rence of asthma varies among countries and communities within countries, but in most industrialized nations about 7–10 of the population are affected. Australia, New Zealand and urban communities in the United States, however, have reported 25–40 of their popu- lation affected Peat et al., 1995; ISAAC, 1998; Nicholas et al., 2005. A study of conscripts from Finland where military service is compulsory over the years 1926–1989 showed an increase in asthma, from less than 0.1 to 1.8, among 18-year-old men, with the increase starting in the 1960s Haahtela et al., 1990. In Charleston, South Carolina, admissions for asthma at the university hospital increased 20-fold among African-American males over a 40 year period, starting in 1960 Crater et al., 2001. This association with industrialized countries can be observed cross-sectionally in low-income countries as well. In Kenya, the rural prevalence of paediatric asthma was found to be 2, increasing to 6 in the more urbanized villages and to 10 in Nairobi Ng’ang’a, 1996; Odhiambo et al., 1998. Much research effort has been focused on understanding the reasons for the global increase in the prevalence of asthma; however, no clear cause has been demonstrated. The central question is: what has changed as a result of or in parallel with industrialization over the last several decades to cause an increase in asthma? In response to the question, there are many hypotheses and propositions. An initial proposition was that the reported increase in disease was not a real increase; instead, it was simply a change in the way asthma was diagnosed or was an increased awareness of the disease. While this may add to the overall increase in the prevalence of asthma, a true increase in the disease is widely accepted and has been documented in the literature Evans et al., 1987; Sears Beaglehole, 1987; Lundback, 1998. Outdoor air pollution has been hypothesized as a cause for the increase in the prevalence of asthma Samet, 1995. Air pollution is known to be associated with the exacerbation of asthma symptoms Sandstrom, 1995; increased ozone has been associated with decrea- sed lung function Kinney et al., 1989; and exposure to diesel exhaust particulates has been associated with increased allergic inflammation Nel et al., 1998; Sydbom et al., 2001. However, one argument against air pollution causing the increase in asthma relates to timing. The rise in the prevalence of asthma has not necessarily occurred in parallel with an increase in outdoor air pollution, since environmental regulations in the 1970s have led to a decrease in overall air pollution in the United States. Specifically, a report by Pope and colleagues 2002 showed a moderate decrease in fine particles, with an aerodynamic diameter smaller than 2.5 µm designated PM 2.5 , in many cities over the past 20 years. Moreover, studies from Germany have shown an association between air pollution and bronchitis, but not asthma von Mutius et al., 1994; Braun-Fahrlander et al., 1997. Several hypotheses focus on the increase in asthma being caused by changes in lifestyle. Trends in conserving energy in buildings in the last half of the 20 th century have resul- ted in people living in houses with less natural ventilation. This has led to an increase in indoor air humidity and to a more stable climate for supporting dust mite growth Platts- Mills de Weck, 1989. Increases in the use of carpets, a common dust mite reservoir, have also contributed to the increase in dust mite allergens Platts-Mills de Weck, 1989. In areas of the world currently reporting some of the highest prevalences of asthma, such as suburban Australia, the United Kingdom and inner-city areas of the southern United States, allergy to dust mites is one of the greatest risk factors for having asthma Call et al., 1992; Peat et al., 1994; ISAAC, 1998; Crater et al., 2001. Thus, an increase in expo- sure to dust mites has been cited as a possible driver of the increase in asthma. However, similar prevalence rates of asthma have been reported in areas of the world where, because of low humidity, dust mites cannot live, such as northern Sweden and Los Alamos in the United States Sporik et al., 1995; Ronmark et al., 1999, and such rates have also been reported in cities in the north-eastern United States, such as New York and Boston, where the prevalence of asthma is high, but where dust mite allergy does not contribute greatly to the profile of allergic sensitization Rosenstreich et al., 1997. The so-called hygiene hypothesis, which recently gained support, is another theory for the increase in the prevalence of asthma. Originally proposed by Strachan to explain increa- ses in allergic rhinitis in the latter part of the 20 th century, this hypothesis has been exten- ded to explain the increase in asthma Strachen, 1989. It proposes that a cleaner living environment for children in 20 th century developed countries, including less exposure to viruses and bacteria, has led to an overall shift in the immune response to the more aller- gic T-helper cell 2. Decreased exposure to bacteria, both in the environment and in the gut, has been proposed as a factor that contributes to the increase in the prevalence of asthma von Mutius et al., 2000; Bjorksten et al., 2001. With respect to the relationship between asthma and pests, centuries ago exposure to allergens of cockroaches, mice and rats was invariably associated with pathogens coming from the human environment. Those exposed to these pests were also exposed to infec- tious pathogens. Now, this link between pests and pathogens in the urban environment has been greatly disrupted, and pests might induce only allergies Tatfeng et al., 2005. Decreased exposure to bacteria in early life may have contributed to the increase in allergy that occurred in the late 19 th century in the United Kingdom, when hay fever became prevalent Emanuel, 1988. In the United States, a cross-sectional study of adults found a lower prevalence of hay fever among older adults. This was associated with an increa- sed prevalence of infections, possibly suggesting a change in the prevalence of hay fever associated with hygiene in the 20 th century Matricardi et al., 2002. Examined as an issue of timing, contact with farm animals a significant source of microbial exposure for the majority of individuals in the United States ended more than a century ago. While some studies have found higher bacterial endotoxin levels in homes with pets, a recent study of inner-city residents n = 301 did not find a significant association Heinrich et al., 2001; Park et al., 2001; Wickens et al., 2003; Perzanowski et al., 2006. Public Health Significance of Urban Pests Allergic asthma 15 14 Conversely, it is debatable whether increased cleanliness resulted in the post-1950s increase in asthma. To correspond with the increase in asthma, the proposed change to cleaner living would have to have occurred within one generation. While it seems doubt- ful that parents of today’s asthmatic children grew up in much dirtier environments than those of their offspring, Matricardi, Bouygue Tripodi 2002 have proposed a theory whereby the increase in inner-city asthma is associated with increased hygiene, brought on by delayed class-driven urbanization in urban poor communities. Although many of the recent studies are compelling, the relevance of the hygiene hypothesis to the asthma epidemic is still not well established scientifically. Recently the trends towards a more sedentary lifestyle in industrialized countries have been scrutinized as a possible cause of the increase in asthma Crater Platts-Mills, 1998. Technological advances in indoor entertainment, such as television, became common in households in developed countries in the 1960s, which along with today’s cable televi- sion, video games, computers and video players, have resulted in children today spen- ding a larger proportion of their time in sedentary activities than did prior generations. A study of asthmatic children in inner-city Atlanta, Georgia, found that 82 of the chil- dren had a television in their bedroom Carter et al., 2001. Sedentary entertainment could have the additive effect of both lack of exercise and longer exposure to indoor allergens from sources such as dust mites, cockroaches, cats and dogs, which are known to be stron- gly associated with asthma Crater Platts-Mills, 1998. It is likely that causes for the increase in asthma include multifactorial features of wes- ternized countries, with some contributing more than others in different communities. What is clear is that there must be some commonality in the causes of the increase obs- erved in different communities, given the similar time frames of the increases Evans et al., 1987; Sears Beaglehole, 1987; Aberg, 1989; Haahtela et al., 1990; Crater Platts- Mills, 1998; ISAAC, 1998.

1.1.3. Public health impact of urban asthma