Discussion
This was the first study to investigate the difference in the prevalence
and patterns of allergen sensitization in patients with allergic
symptoms from all the seven geographic regions of mainland China. Based
on this large study, the most common allergen for patients with allergic
diseases in mainland China was house dust mites. The results were
consistent with those of previous studies 6. Studies
have also reported that house dust mites were the most common
aeroallergen in many Asian countries 7, 13. Another
study also reported a high prevalence of 90% in Taiwan for house dust
mite allergies. This is quite different from other continents of the
world. For example, cat and dog allergies are the most common allergies
in Europe and the United States 7, 14, while in
Africa, the most common allergies are cat and cockroach. The differences
may be caused by lifestyle factors, socioeconomic factors and genetic
predispositions 15. In European and Western countries,
people usually dry laundry with clothes dryers, which is helpful for
killing house dust mites. Although the United States and China have
similar longitudes and latitudes, which means that they may have similar
climates, the populations of these regions have different genetic
makeups. Therefore, genetic predisposition may also explain the
difference in the prevalence of house dust mites 13.
The statistical tests showed a
significant difference in the prevalence of allergens among different
regions except for mold mix. The prevalence of house dust mite
sensitization had obvious regional characteristics. The further south
and the more coastal an area was, the higher the prevalence. Humility
and temperature are the primary two determinant factors that affect dust
mite reproduction and growth 16. It is easy for them
to grow in warm and humility places 8. A study16 concluded that few house dust mites were found in
houses with ample sunshine. Studies also found that long lint-covered
and handmade carpets, wool and synthetic blankets, and synthetic quilts
could result in more mites 17. Studies16, 17 also found that the age of homes and mattresses
are also positive factors affecting the number of house dust mites.
Regions in South and coastal are in have subtropical and tropical
climates with higher temperature and relative humidity than other
regions. The higher indoor number of house dust mites in southern
coastal region than those from North region may also can explain the
higher prevalence of house dust mite sensitization in the southern
regions than in the northern and eastern regions. Furthermore, people in
the north are provided with heating for at least 5 months a year, which
results in low indoor humidity. Therefore, this lifestyle can also be
the reason for the lower prevalence in the north. In regions other than
northern regions, which do not have heating supplies, people usually
need long and thick carpets, blankets and quilts to keep warm indoors,
all of which contribute to the proliferation of house dust mites. The
buildings in the cities in the northern regions in our study had the
characteristics of fewer floors and larger spacing, which enables a
greater supply of sunlight for the indoor rooms, resulting in fewer
house dust mites in the north.
German cockroaches were the second most common allergen affecting
allergic patients in mainland China, with a prevalence of 24.5%.
Previous studies reported positivity rates of 11.5%-18.7%6, 18 for SPT and 20.9-26.5% 9 for
sIgE. Cockroaches have been reported to be one of the most common indoor
allergens worldwide 19. Cockroach sensitization has
been reported as 30% in Africa, 30% in India, and 6–29% in European
countries. The high prevalence may be caused by immune regulation,
genetic determinants 20 and cross-reactivity between
cockroaches and house dust mites 18. The prevalence in
Southwest and Southern coast China was higher than those in the East and
North China. However, Southwest China had higher prevalence than
Southcoast. This was different from the prevalence pattern of house dust
mite sensitization. The climate in the south may be more comfortable for
cockroach growth than that in the north. However, compared with American
cockroaches, German cockroaches are commonly found in dry and cold
conditions 21, which may be the reason for the higher
prevalence in Southwest than in Southcoast China. Southcoast is more
humid and hotter than west.
The prevalence of tree pollen mix sensitization varied among regions.
This may be attributed to different plant species in different areas22.
Among the four food allergens in our study, their prevalence fell into
the range of 10%-20%. The prevalence of cow’s milk and egg whites in
North China was higher than those in other regions. The difference may
be caused by the low ethnicity-dependent genetic polymorphisms in
vitamin D-binding protein, which evoke differential effects on vitamin D
bioavailability. On the other hand, dietary patterns may also be the
reason for variation in the prevalence of food allergens in different
regions. Studies have concluded that infant diets with high levels of
fruits, vegetables, and home-prepared foods are associated with fewer
food allergies 23. People in provinces such as Hubei,
Hunan, and Jiangxi eat more peppers than people in other regions, which
may lead to high sensitization to eggs. The high prevalence of cow’s
milk in East China may be caused by a family history of food allergies
and atopy 24. However, further evidence is required.
Geographical variations in the patterns of aeroallergens also influence
food allergy distributions where cross-reactivity exists between
aeroallergen and food allergen components 25, 26. In
our study, the prevalence patterns of crab and shrimp sensitization in
all regions were quite similar to that of cockroaches, which may be
caused by cross-reactivity. Furthermore, the higher prevalence may also
have been because of high exposure to crabs and shrimp because people
living near the coast were more likely to eat seafood. Different regions
have different allergen sensitization patterns, which will provide a
favorable basis for local allergen diagnosis and treatment.
Sex-based differences in the prevalence of allergies have long been
recognized. However, conclusions about this topic are inconsistent among
questionnaire studies and the detection of specific IgE antibodies or
skin prick tests 27. In our study, the prevalence of
all nine common allergens was significantly higher in men than in women.
This suggests that being male is one of the risk factors for allergen
sensitization. The results may be connected to sexual hormones and
differences in the lifestyles, such as diet, sports, doctors and
hormonal medications 28. Further studies need to be
conducted.
Age had an influence on the prevalence of sensitization for all
allergens except the tree pollen mix. However, a previous study found
that in young and middle-aged patients with rhinitis and/or asthma, the
prevalence of pollen varied in different age groups8. Our results
were different from others, possibly because of the small sample size.
The patterns for the prevalence of egg white and cow’s milk
sensitization both typically presented a falling curve with increasing
age. The prevalence of egg whites and cow’s milk among individuals
younger than 6 years old was obviously much higher than that of milk in
individuals older than 6 years old. The patterns were quite similar to
the findings in a previous study 8. Egg whites and
cow’s milk were the two major food sources of sensitization for younger
children, who have immature immune systems. The results also showed that
the prevalence patterns of crabs and shrimp were higher in the
middle-aged groups than in the other groups. This may be attributed to
different lifestyles, such as high exposure in crabs and shrimp, and may
also be regarded as cross-reactivity 8. The
identification of major allergens in different age groups can help to
improve the pertinence and accuracy of allergen detection for clinical
diagnosis, which can reduce the burden of medical expenses for patients.
The peaks of German cockroach and house dust mite sensitization
prevalence were both located in summer, which may be explained by the
fact that house dust mites and cockroaches grow more easily and are more
common in warm and humid conditions. The prevalence of sensitization to
the tree pollen mix was significantly higher in April and October, which
was consistent with the flowering seasons of most plants22. The prevalence of egg white and cow’s milk
sensitization was higher in spring, and crab and shrimp sensitization in
summer may be influenced by seasonal differences in UVB exposure29. There were differences in the distributions of
allergen sensitizations in different seasons, which can be used for
prospective prevention and treatment of allergic diseases.