What causes asthma?

Asthma is believed to result from a complex interplay
between a person’s genes and various environmental factors
at a specific time in his or her life. It can thus be
viewed as the result of interactions that occur between
internal (genetic) elements and external (environmental)
exposures.
 Environmental factors that have been studied
include viruses such as RSV (respiratory syncytial virus),
cockroaches, cigarette smoke, exhaust, farm animals, medications
(including acetaminophen), pesticides, pets, and
wood smoke. The data are intriguing and raise further
questions that continue to stimulate research both into
genetic and environmental features. Recent attention
has, for example, focused on factors that may affect the
unborn child’s immune system. Given the elevated rate
of childhood asthma and the observation that more than
half of children with asthma are diagnosed by the age of
3, researchers have also sought to examine the possible
influence of prenatal environments on the developing
fetus’s risk of developing asthma early in life. Some studies
in particular have suggested the possibility of (but not
proved) a link between lower-than-normal vitamin D
levels in mothers during pregnancy and the development
of asthma in their children. More research into why some
persons, but not others, go on to develop asthma in the
course of their lifetimes is desperately needed. In the case
of vitamin D’s importance, for example, a clinical trial
was scheduled to begin in late 2009 to investigate if adequate
maternal vitamin D supplementation in pregnancy
leads to a decrease in asthma during the child’s first
three years of life. The trial is titled, “Maternal Vitamin
D Supplementation to Prevent Childhood Asthma
(VDAART)”; details are available on the Web at
 http://www.clinicaltrials.gov/ct2/show/NCT00920621. Vitamin
D is also thought to perhaps play a role in asthma symptom
severity. John Brehm and colleagues published
“Serum Vitamin D Levels and Markers of Severity of
Childhood Asthma in Costa Rica” in the May 2009
American Journal of Respiratory and Critical Care Medicine
and provided evidence of an inverse relationship between
vitamin D levels and measures of allergy and asthma
severity in Costa Rican children with asthma. The
authors also point out that the “Results of some, but not
all, epidemiologic studies suggest that vitamin D deficiency
is associated with an increased incidence of asthma
symptoms . . . higher maternal intakes of vitamin D
during pregnancy are associated with decreased risks for
recurrent wheeze in young children suggesting that vitamin
D may play a role in the development of asthma.”
Clinicians have long noted that certain viral infections
seem to be related to the development of asthma in predisposed
individuals, both children and adults. Physicians
refer to particular “asthmagenic” viruses (such as RSV) that
cause typical respiratory infection and symptoms at first,
only to leave the patient with an asthma-like condition.
Not all infectious agents are viruses, however, and attention
has also been directed to other infectious organisms
and their possible role in asthma development.
Lung infection with a common bacterium (not a virus)
called Chlamydia pneumoniae (recently renamed Chlamydophila
pneumoniae) has in particular been suggested (but
not proven) as a possible cause of reversible asthma in
adults.