The prevalence of Allergy is achieving epidemical proportions all over the world 1. In Cuba, it is estimated that about 20 % of the population is allergic. According to different sources from 5.7 % to 10 % of the total population suffer from asthma2,3, with a highest prevalence among teenagers, reaching 17.8 % 4. The increase of prevalence of allergic diseases is commonly associated with changes in lifestyle and environment. Particularly, allergic asthma is strongly associated with IgE sensitization to House Dust Mites 5. Previous studies have described 3 domestic mite species as the most relevant in Cuba regarding allergic sensitization: Dermatophagoides pteronyssinus (Dp), Dermatophagoides siboney (Ds) and Blomia tropicalis (Bt) 6.
Dermatophagoides species are well known all over the world as important inhalant allergen sources. D. siboney is a local endemic specie reported first in 1984 7 and is closely related to D. farinae, which is not found in Cuba. On the other hand, Blomia tropicalis is very common in tropical humid and hot climates. In general, tropical conditions with high temperatures and humidity all over the year favor the growth of domestic mites, and presumably, are decisive for the dominant role of mite allergens over the rest of inhalant allergens in our population 6.
Prevalence of sensitization to Dp and Ds in Cuban asthmatic population have been reported previously ranging from 70 to 85 % by a few authors 6,8,9. Considerably less data is known about sensitization to Bt, although prevalence rates of up to 79 % have been reported 6,10. Particularly, in this work we address the question about the degree of poly-sensitization and correlation of the skin response to these mite species, in a relatively large population of adult asthmatic patients, in the Havana urban region. Provided that, the cross-reactivity between Dermatophagoides and Blomia tropicalis is very limited, the study of the simultaneous sensitization, both in terms of prevalence and intensity, is of great importance for defining an etiological approach to manage the disease, based on prevention and allergen-specific immunotherapy.
MATERIALS AND METHODS
A multicenter, transversal, descriptive and analytical study was performed on 232 adult patients, who attended Allergy Services at two Havana University Hospitals: "Calixto Garcia" and "Joaquín Albarrán", from October, 2004 to March, 2005. Patient's mean age was 30.3 years (SD = 8.7); 92 were men and 140, women. The inclusion criteria were the presence of a clinical history of respiratory allergy to house dust (asthma with or without rhinitis) and a positive Skin-Prick-Test to at least one mite species. Written informed consent was given by every patient included in this study in agreement with Good Clinical Practice.
Skin Prick Test
Skin Prick Test (SPT) was performed on all patients following the method described by Dreborg 11. Briefly, a drop of the allergen solution was applied on to the forearm of the patient. The lancet (ALK, Denmark) was pressed slightly at 90° angle into the skin, during 1 second. After 15 min. the wheal contour was outlined using a pen and transferred to an adhesive tape. The wheal maximum (dm) and orthogonal (do) diameters were measured and mean diameter was calculated as d = (dm + do)/2. The test was considered valid when the skin reaction to the negative control (Buffer Solution) was smaller than 3 mm, and for the positive control (Histamine HCl, 10 mg/mL), greater than 3 mm. The test result was regarded positive if the wheal diameter was greater or equal to 3 mm. The wheal area was calculated according to the expression: A = π · d 2/4. The following standardized allergen extracts, manufactured by BIOCEN, Cuba, were used for prick testing:
VALERGEN-DP Dermatophagoides pteronyssinus
VALERGEN-DS Dermatophagoides siboney
VALERGEN-BT Blomia tropicalis
The extracts are stardandized in Biological Units (BU) acording to the definition of Nordic Guidelines for Registration of Allergen Products 12. The used concentration was 20 000 BU/mL, following manufacturer instructions.
Wheal area data was log-transformed and tested for normality using the Kolmogorov-Smirnov test (α = 0.05). Geometric means and 95 % Confidence Intervals were employed for comparing the reaction size between different allergens. The correlation of SPT size was calculated using the non-parametric Spearman rank correlation coefficient. Statistical processing was performed using STATISTICA v5.1 software package (StatSoft, USA).
The highest prevalence of positive test was observed to D. siboney (88.4 %) and D. pteronyssinus (87.1 %), followed by 68.1 % to B. tropicalis (table I). Most Dermatophagoides positive patients showed positive results to both species simultaneously (81.6 % of the total number of tested patients), whereas, up to 94.4 % were positive either to Dp or Ds or both; i.e. only 12.8 % (the difference) showed a specie specific response within Dermatophagoides genus.
The predominant role of sensitization to Dermatophagoides as compared to Blomia tropicalis is evidenced by the fact that 31.9 % of patients were exclusively positive to Dp or/and Ds whereas only 5.6 % were positive solely to Bt. Nevertheless, most patients (58.6 %) were sensitized against the three mite allergens, simultaneously.
The largest reactions were observed to D. siboney and D. pteronyssinus, followed by B. tropicalis, in full agreement with the prevalence data (table II). Nevertheless, the difference was not significant (p < 0.05). On the other hand, a highly significant Spearman correlation coefficient (r = 0.71, p = 5.3 x 1009) was found between SPT reaction size to Dp and Ds, whereas no significant correlation was reported between Bt and neither Dp nor Ds.
High prevalence of sensitization to D. siboney, D. pteronyssinus and Blomia tropicalis among Cuban asthmatic patients has been reported previously 6,8-10, and our results are in full agreement with that. All these species are commonly found in house settings in Havana geographical area 6. However, in our work, a slightly decreased prevalence of sensitization was found in the case of Blomia tropicalis as compared to Dermatophagoides. In other geographical areas with similar climatic conditions (for instance, Caribbean islands, Caribbean cost of Colombia, Central America, Brasil, Southeast Asia), sensitization to Bt in asthmatic patients is also very relevant, ranging from to 40 to 80 % 13-17. The observed reaction sizes were similar to the values reported previously by different authors 6,8,18.
According to our results, most patients (58.6 %) are poly-sensitized to Ds, Dp and Bt. Coincidence of positive SPT and significant correlation of reaction size between Dp and Ds, can be caused by the extensive IgE cross-reactivity described for these two species 19. In contrast, only limited IgE crossreactivity has been reported between Bt and Dermatophagoides species 19-21, suggesting that, in that case, poly-sensitization should be provoked by simultaneous exposure to different species, that possibly share a common environment; which is supported also by the lack of correlation concerning reaction size. Also, the genetic background of the population could also influence the intensity of the response to different allergens, leading to the lack of correlation regarding reaction size between Bt and Ds or Dp, even if the degree of exposure were the same. Other authors in the Caribbean geographical area have also reported lack of correlation between skin test to Dp and Bt 16.
Overall, the results of our study highlight the relevance of domestic mites as sensitized agents, strongly associated with asthma symptoms in our population, with a predominant role of D. siboney and D. pteronyssinus, but also, with a significant contribution of B. tropicalis. This work support the necessity of including allergens of these 3 mite species in routine skin test panels, for achieving higher diagnostic performance. In addition, the high degree of poly-sensitization should be taken into account for the effective administration of allergen-specific immunotherapy.
Raúl Lázaro Castro Almarales
National Center of Bioproducts (BIOCEN)
PO Box 6048, Havana 10600, Cuba