FAUNAL DIVERSITY AND HABITAT PREFERENCE OF HOUSE DUST MITES IN WEST BENGAL IN RELATION TO NASOBRONCmAL ALLERGIC DISORDERS

Mites are ubiquitous in distribution and are reported to be present almost in all the habitats. An intricate association between mites and human beings are also noticed from long back. A large number of people are reported to suffer from several nasobronchial allergic complaints due to inhalation of mites present in the house dust. It is well documented that mites of the family Pyroglyphidae, particularly the genus Dermatophagoides are considered to be the most potent allergen in house dust responsible for asthma and other allergic manifestations (Voorhorst et al., 1964 and Pepys et al., 1968). Saha (1993, 1994, 1995 and 1997) and Saha et al., (1995 a,b, & c) confirmed that Dermatophagoides mites are the main source of allergen in house dust responsible for asthmatic problems in at least 80% of the sensitive patients of Calcutta.


INTRODUCTION
Mites are ubiquitous in distribution and are reported to be present almost in all the habitats.An intricate association between mites and human beings are also noticed from long back.A large number of people are reported to suffer from several nasobronchial allergic complaints due to inhalation of mites present in the house dust.It is well documented that mites of the family Pyroglyphidae, particularly the genus Dermatophagoides are considered to be the most potent allergen in house dust responsible for asthma and other allergic manifestations (Voorhorst et al., 1964 andPepys et al., 1968).Saha (1993Saha ( , 1994Saha ( , 1995Saha ( and 1997) ) and Saha et al., (1995 a,b, & c) confirmed that Dermatophagoides mites are the main source of allergen in house dust responsible for asthmatic problems in at least 80% of the sensitive patients of Calcutta.
Mites in house dust date back to seventeenth century (Leeuwenhoek, 1694).More than 130 species of mites have so far been isolated from the house dust samples all over the world since 1983 (Zheltikova et al., 1985).The first report on the occurrence of mites in Indian house dust was made by Krishna Rao et al., (1973).Since then studies on the house dust mite fauna have been conducted in different parts of the country from time to time and the presence of varying number of species of mites have been reported (Dixit and Mehta, 1973;Dar and Gupta, 1979;Krishna Rao etal., 1981;Maurya and Jamil, 1982;Tripathi and Parikh, 1983;Kumar etal., 1988;Kumud etal., 1988;Modak et al., 1991, 1995and Saha et al., 1995b).However, a comprehensive knowledge on the house dust mite fauna of West Bengal is still fragmentary.Earlier, Gupta and Datta Roy (1975) isolated 12 species of mites from mattress dust of patients with respiratory troubles from six different districts of the state.In recent past, Modak et al., (1991) in a comparative study reported the presence of 54 species from house dust samples collected from 24 Parganas, Burdwan and Calcutta and highest number of species were encountered from Calcutta metropolis.The present study has been designed to gain an insight into the faunal diversity of house dust mites in 14 different districts of West Bengal and to assess their habitat p'reference.

MA TERIALS AND METHODS
Dust samples from two different habitats namely bed and bedroom floor were collected manually from houses of 85 established cases of bronchial asthma patients residing in 14 different districts of West Bengal, namely, Calcutta, South 24 Parganas, North 24 Parganas, Howrah, Hooghly, Burdwan, Bankura, Birbhum, Purulia, Midnapur,'Murshidabad, MaIda, Coochbehar and Jalpaiguri.
Patients were selected from Asthma Clinic of Calcutta School of Tropical Medicine and District Health Centres of respective districts following the criteria mentioned earlier (Saha, 1993 and1994).
Dust samples were also collected from the houses 0' fifty healthy individuals to.serve as control.
Floor dust samples were 'collected by sweeping the floors while bed dust samples were obtained by brushing the mattresses, bed covers, pillows on clean sheets of n~wspapers, kept in separate polythene packets, labeled properly.Extraction of mites were done following the method of Channa Basavanna et al., (1985).Isolated mites were cleared and mounted temporarily.in lactic (\cid and identified following the classification of Hughes (1976).

OBSERVATIONS
A total of about '1,31,531 'mites were isolated from 270 samples of dust 'collected, from 14 different districts of West Be~gal.As many.as 68 species of m,ites b~longing to, 44 genera, 25 families and 3 orders namely Astigmata, rrostigmata and Mesos~igmata (Table 1) have been isolated.
A large number of Cryptostigmatids were also foul)d but they, were not identified.;,Among 68 species of mites, 26 specles of mites were reported for the firs~ time from Indian hou~e dust _samples, of which 6 species are new to science~ Among 14 di~ferent distric~s ~urveyed~ 9alcu~ta ~ ,h~lds.
maximum variety (4.1) of mites, followed by aurdwa~ (~O) and North 24-Parg~~,s (33) a~,d least number of ,species was isolated f~om Birbhum District.Dermatop~ag~ides pteronyssin14s, was the  (Voorhorst and Spieksma, 1973) were present in larget:l:umber in the state.Almost all the dust samples collected were positive for the presence of mites and'90% of the samples were positive for D. pteronyssinus.It was further noticed that the maximum number 141 of mites (13,750/gm of dust) was isolated from a bed dust sample collected from Calcutta during the month of November, while the minimum (2/gm of dust) was reported from one bed room floor dust sample of Bankura during the month of July.Among two different habitats examined, bed and bed room floor, apparently the former contained higher mite densities than the corresponding bed room floor dust samples, though the difference was not statistically significant.

DISCUSSION
Studies on the house dust mite fauna have been explored in other parts of the country from time to time and the presence of varying number of species have been reported by several workers.
To the best of our knowledge, other than the state of West Bengal, a maximum of 27 mite species distributed ov~r 21 genera and 10 families have been reported from Punjab and Himachal Pradesh by Kumar et al., (1988).Therefore, it is interesting to note that West Bengal is extremely rich in house dust mite fauna as was also observed in our earlier reports (Modak et al., 1991).This may presumably be attributed to the presence of ideal climatic conditions particularly the temperature and high humidity prevailing in this state as also suggested by Bronswljk and Sinha (1971) and Wharton (1976) in their studies.
Isolation of exceptionally large numbers of Dermatophagoides mites is an interesting finding which may be due to extremely favourable conditions to the survival of these mites in respect to temperature, RH and ample supply of food.It has been recognized that temperature and RH are the two most important limiting factors in controlling mite infestations.
Our study further reveals that members of the family Pyroglyphidae predominated in both bed and bedroom floor-dust samples~ Predominance of Pyroglyphid mites in house dust was also observed by Dixit andMehta (1973), Krishna Rao et al., (1981), Modak et al., (1991) and Kumud et al., (1988).Among different species of Pyroglyphidae the most predominant one is D. pteronyssinus (69%) followed by D.farinae (140/0) and Hirstia domicola (13%).Several authors: from different parts of the country also reported the dominance of D. pteronyssinus over D.~fari~e (Dixit and Mehta 1973;Nayar et al., 1974;Krishna Rao et al., 1981;Maurya et al., 198~ ,and Kumud et al., 1988).Similar observations have been reported from different parts of the globe and they constitute 42.32% to 92.61 % of the total mite population depending on the grographical location.In contrast, Dar et ale (1973), Jamil and Maurya (1981) and Tripathi and Parikh (1983) reported the predominance of D. farinae from other parts of the country.
The present study reveals that cent percent dust samples were positive for the presence of mites and the rate of occurrence of D. pteronyssinus was 90%, which confirms the earlier observation of Spieksma (1967) and Sesay and Dobson (1972).Dar and Gupta (1979) reported that 64.3%~ dust samples from Uttar Pradesh, 61 % from Punjab, 60% from Chandigarh, 54.9% from Delhi-and 45.5% from Haryana were positive for the presence of mites.They further opined that Dermatophagoides spp.were present in 100% mite positive dust samples while Tripathi and ParikQ (1983) reported 92% positivity among them from Mumbai.
Of the two different habitats examined to study the habitat preference of Dermatophagoides mites, it was observed that bed dust contained higher densities of mites than the corresponding bed-room floor dust, though, not statistically significant.Ho and Nadchatram (1984) also did not observe significant difference in mite densities between different niches.In contrast, Sesay and Dobson (1972) and Tripathi and Parikh (1983) observed significant difference between these two habitats.Saba et al., (1995b) also observed that of the two different habitats examined, the bed . .dust contained significantly higher (p < 0.01) mite density than the corresponding ~ed-room'floor dust sampies of patients.Higher mite densities in bed dust samples are attributed to 'the presence of ideal ecological conditions such as favourable temperature, required humidity and avrulable 'food.Mites usually feed on human skin scales (Spieksma, 1968 andWharton, 1976),-which is present' in abundance in the beds.Moreover, since the beds are occupied by human beings for about'I/3 of the day, the required optimum temperature and RH and sufficient amount of human skin scales are: automatically provided (Mulla and Medina, 1980).Secondly, the beds :provide .an' undistUrbed: habitat in comparison to the corresponding bed-room floors which ate' frequeritly cleaned and moped (Blythe, 1976 andMulla andMedina, 1980).
Observations regarding mite densities in house dust samples of individuals, hypersensitive to dust inhalation and control subjects residing in Calcutta metropolis and adjoining suburban' areas of West Bengal were made.The study indicates that in the suburbs, the difference in .the • total number of mites/gm of dust in allergic and control subjects were not significant.However,' the difference was significant in Calcutta population.Dixit and Mehta (1973), Nayar et-al~, (1974),  Murten and Madden (1977), Tripathi and Parikh (1983) 'and Korsgaard (1983) also, reported' that apparently the mean number of total mites as well as Dermatophagoides mites/gm of dust, was greater in houses of asthmatics, though such difference was not statistically significant:•: Saha et ale (1995a & b) was of the opinion that in Calcutta Dermatophagoides alone constituted more than 60% of the total mites isolated and patients' bed contained significantly higher (p < 0.02) mite population than the control subjects' bed dust.
It is worth mentioning that the maximum and minimum number of mites recorded during the course .ofpresent study were 13,750 and 2/gm of dust respectively.Observations in these resPfcts have also been made in other parts of the globe and the maximum and minimum number of the mites recorded so far were between 7000-8000/gm and 5-8/gm of dust respectively (Oshima 1970, Blythe et al., 1975and Artian et al., 1983).
In view of increasing trends in the incidence of nasobronchial allergic manifestations due to dust inhalation, the mite fauna of house dust has gained considerable attention throughout the country.Since the house dust mites particularly the genus Dermatophagoides present in the house dust are considered as the sole potent allergen responsible for nasobronchial allergic disorders, a detailed knowledge on the diversity and other ecological requirements of thIS mite species are of urgent need for better understanding of the disease pathogenesis.Saba (1994) further opined that the degree of house dust allergenecity is closely related to the density of mite populations prevalent in patients environment and thus it is reasonable to think that the physical appearance of these mites in higher concentration in the patients' environment may remind us to search for an allergic etiology towards these species of mites.

SUMMARY
House dust mite fauna in the state of West Bengal comprises of 68 species of mites belonging to 44 genera 25 families and 3 orders.Among those 6 species appeared to be new to science and another 20 species are reported for the first time from Indian house dust samples.Dermatophagoides pteronyssinus was the most predominating species comprising 69% of the total mite population followed by D.farinae (14%) and Hirstia domicola (13%).The relative density varies from 13,7501 g to 2/g of dust.Interestingly, all the well known allergen producing mites namely, D. pteronyssinus, D. farinae, Euroglyphus maynei, Tyrophagus putrescentiae, Acaru,,~ siro and Glycyphagus domesticus are present in the house dust collected from different districts of the state.Among two different habitats examined, the bed dust harbour comparatively higher mite population than the corresponding bedroom floor dust samples and their density being higher in dust samples collected from homes of asthmatic patients than non-sensitive control individuals.

Table 1 .
Mite fauna isolated from dust samples of different districts of West Bengal.Austroglycyphagus sp.nov.