Blomia tropicalis, a mite whose time has come

The paper by Puccio et al. (1) describes a high prevalence of sensitization in Caracas, Venezuela to the mite Blomia tropicalis (Fig. 1). The authors determined prick skin test reactivity and specific immunoglobulin E (IgE) by a multiple blot antigen assay (MBAA) against B. tropicalis and Dermatophagoides pteronyssinus in a group of 115 patients with persistent allergic respiratory symptoms. Cross-reactivity between these two mites was also evaluated. A total of 106 patients (92.2%) were skin prick test-positive to at least one of the mite extracts; 81 (70.4%) were positive to both, 12 (10.4%) patients reacted only to D. pteronyssinus, and 13 (11.3%) only to B. tropicalis. In vitro results were similar to the in vivo findings. Specific IgE to either or both mite extracts was detected in 107 patients (93%); 80 patients (69.6%) showed specific IgE to both mite extracts, 13 sera (11.3%) reacted only to D. pteronyssinus, and 14 different sera (12.2%) reacted only to B. tropicalis. The MBAA inhibition was used to evaluate allergenic cross-reactivity between D. pteronyssinus and B. tropicalis. Sera of 12 allergic patients were used; five serum samples reacted to both mite extracts, four reacted only to D. pteronyssinus, and three reacted only to B. tropicalis. In patients who had specific IgE only to D. pteronyssinus, soluble B. tropicalis did not inhibit the binding to D. pteronyssinus and in patients who reacted only to B. tropicalis, the soluble form of D. pteronyssinus did not inhibit the binding to B. tropicalis. In contrast, two of the five sera that reacted to both mite extracts, soluble D. pteronyssinus inhibited the binding to B. tropicalis and B. tropicalis also inhibited the binding to D. pteronyssinus. These results suggest that there is a moderate degree of cross-reactivity between these two mites. Frontal view of Blomia tropicalis enlarged 200 times. Mite sensitivity in asthmatic patients is high in Latin American countries. The prevalence of positive prick skin tests to the mite species D. pteronyssinus, D. farinae, B. tropicalis, Chortoglyphus arcuatus, Lepidoglyphus destructor and Aleuroglyphus ovatus was determined in 297 asthmatic adults and children living in seven cities of five Latin American countries in a collaborative international study on mite allergy (2). The prevalence of sensitization to D. pteronyssinus varied from 61% in Cartagena, Colombia to 91% in São Paolo, Brazil, and to B. tropicalis from 47% in Mexico City to 93.7% in São Paulo. The mean age of the patients in São Paolo (n = 80) was 9 ± 2.9 years. Overall, the highest degrees of sensitization were present in São Paulo and Caracas (n = 36, mean age 12.6 years). The prevalence rates of sensitization in Caracas for D. pteronyssinus and B. tropicalis were 75.5% and 77.8%, respectively. Similar, but higher results were obtained 10 years later in another study by Sánchez Borges et al. (3), who evaluated 176 patients (mean age: 26.2 ± 16 years) who attended the allergy clinics of two medical institutions. Patients were prick skin tested with standardized extracts of D. pteronyssinus and B. tropicalis and six cockroach species. Sensitization to D. pteronyssinus was present in 97.2% and sensitivity to B. tropicalis in 91.6% of the patients. These results are very similar to those described in the present study by Puccio et al. and suggest that sensitization to D. pteronyssinus and B. tropicalis is very common. These results are more meaningful, since sensitivity to B. tropicalis has been described in many countries of the world and is not only limited just to tropical and subtropical environments. Different species of the genus Blomia, including B. tropicalis, have been described in countries with a temperate climate. These species include B. tjibodas, B. kulagini, and B. thori. There are a few studies showing sensitization to Blomia spp. in temperate countries. A study in Germany demonstrated a high rate of specific IgE to B. tjibodas and B. tropicalis in allergic city dwellers and farmers (4) and suggested that B. tjibodas and B. tropicalis share a high degree of cross-reactivity. Blomia tjibodas was originally isolated from a barn dust sample and cultured in the laboratory. Accordingly, agricultural workers showed a significantly higher frequency of sensitization with B. tjibodas than city dwellers. A clinical implication of this finding may be that a patient sensitized to B. tjibodas in Continental Europe or the USA could experience symptoms when exposed to other Blomia species in geographical regions such as the Caribbean, Central America, or in other tropical areas, such as Singapore, where this mite is common. More than a billion people may be exposed regularly to B. tropicalis allergens in the tropics and therefore, the use of B. tropicalis standardized extracts (in the tropics and subtropical regions of the world) is critical for diagnosis and treatment of allergic respiratory diseases. In vitro cross-reactivity studies between whole extracts of B. tropicalis and other mite species demonstrate that these mites share common, as well as species-specific allergens (5-7). Puerta et al. (5) demonstrated a greater degree of cross-reactivity between B. tropicalis and Lepidoglyphus destructor than between B. tropicalis and Dermatophagoides spp. Arlian et al. (6) using immunoelectrophoresis, demonstrated that the majority of the allergens present in B. tropicalis are species-specific. Only three allergens seem to be common with D. farinae body and faeces extracts, two and one with body and faeces extracts of D. pteronyssinus, respectively. Morgan et al. (7) demonstrated corresponding IgE-binding proteins/bands in extracts of Euroglyphus maynei and B. tropicalis. However, the majority of IgE-binding proteins did not show corresponding bands in both extracts. The authors concluded that E. maynei and B. tropicalis are the source of both species-specific and cross-reactive allergens and that most allergens in each extract were species-specific. Several allergens of B. tropicalis have been cloned and sequenced (8-16). Some of them show sequence homology with purified allergens of D. pteronyssinus, such as Blo t 5, a homologue of Der p 5; Blo t 13, a fatty acid-binding protein; Blo t 11, a homologue to paramyosin; Blo t 10, a homologue to tropomyosin and Der p 10; Blo t 3, a trypsin-like protease; and Blo t 1, homologous to Der p 1, a cysteine protease. Recently, it has been shown that Blo t 1, with an estimated molecular weight of 26 kDa, is an important allergen. Recombinant Blo t 1 reacted positively with IgE in 90% and 65% of sera from asthmatic children and adults from Singapore, respectively. Furthermore, there is a low correlation between IgE reactivity to Blo t 1 and Der p 1 (17). Several studies have focused on the in vitro cross-reactivity of purified Blo t 5 and Der p 5 (7, 18, 19) and Blo t 10 and Der p 10 (tropomyosin). Most group five studies demonstrate low-to-moderate cross-reactivity at the molecular level. There is less information about group 10 allergens. Yi et al. (14) concluded in their study that Blo t 10 and Der p 10 are highly conserved molecules, sharing 95% amino acids identity, and that they are cross-reactive allergens, although unique IgE epitopes do exist. In vivo studies using inhalation tests to investigate the clinical significance of sensitivity to B. tropicalis and its cross-reactivity with D. pteronyssinus are scarce. Stanaland et al. (20) demonstrated that 83% of B. tropicalis-sensitive patients in Florida had a positive nasal challenge with a B. tropicalis extract. Therefore, a positive skin test to B. tropicalis is a good indicator of possible allergic symptoms after inhalation of B. tropicalis allergens. Other studies in Brazil and Singapore have demonstrated the allergenicity of B. tropicalisin vivo using nasal challenges (21, 22). In Brazil, a group of D. pteronyssinus- and B. tropicalis-sensitive patients was evaluated; 90% of the patients had a positive nasal challenge to D. pteronyssinus and 60% to B. tropicalis. The study conducted in Singapore included 20 adults with persistent allergic rhinitis, five of whom had a history of asthma. Significant increases in subjective and objective nasal symptoms, together with a significant increase of tryptase and LTC4 concentrations in nasal secretion, were found in all patients after each challenge with B. tropicalis. This study also demonstrated that a nasal challenge with B. tropicalis might also provoke a concomitant asthmatic response during the late-phase reaction. A study conducted by García Robaina et al. (23) in the subtropical Canary Islands demonstrated that patients who are sensitized to two mite species (D. pteronyssinus and B. tropicalis) may only react to one of them. This had been previously suggested for even more closely related species such as D. pteronyssinus and D. farinae (24). In some cases, it may therefore be necessary to conduct challenge tests in order to better define the relevant allergen from a clinical point of view and to start the appropriate specific immunotherapy treatment. The study by García Robaina et al. (23) confirms previous in vivo and in vitro cross-reactivity observations using whole extracts as well as purified allergens. It suggests that, although there is some in vitro and in vivo allergenic cross-reactivity between B. tropicalis and D. pteronyssinus, clinical symptoms induced by the inhalation of B. tropicalis and D. pteronyssinus allergens seem to be species-specific, although some patients may react to common allergens. To the best of our knowledge, standardized extracts of B. tropicalis are available in some European and Asian countries, in Cuba and through licensing in some South American and African countries, such as Brazil, Chile, Colombia, Morocco, and South Africa. However, these extracts are only available under experimental conditions in the USA and many other countries throughout the world. There is a definitive need to use standardized extracts of B. tropicalis in countries with tropical, subtropical, and temperate climates of the world where Blomia species are endemic. Because of the wide distribution of B. tropicalis, the high rate of sensitization detected in many countries and the specificity of the allergic reactions, the inclusion of B. tropicalis for the diagnosis and treatment of specific IgE-mediated allergic reactions to this mite species is warranted.