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.