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Bionomics of malaria vectors in two physiographically different areas of the epidemic-prone Thar Desert, north-western Rajasthan (India)
Journal of Arid Environments (2001) 47: 161–172 doi:10.1006/jare.2000.0698, available online at http://www.idealibrary.com on
Bionomics of malaria vectors in two physiographically different areas of the epidemic-prone Thar Desert, north-western Rajasthan (India)
B. K. Tyagi & S. P. Yadav Desert Medicine Research Centre, Post Box 122, New Pali Road, Jodhpur 342 005, India (Received 15 May 1998, accepted 15 August 2000) Entomological and parasitological investigations were carried out on malaria vectors and disease prevalence in two sets of villages, the highly irrigated Indira Gandhi Nahar Pariyojana (IGNP) command-area villages (Madassar and Awai), and the truly desertic non-command (unirrigated) area villages (Kanasar and Khetusar), located in different ecological conditions in the Thar Desert, north-western Rajasthan (India). Malaria prevalence, as determined through sustained fever surveys, was higher in the IGNP villages with a slide positivity rate (32%) marginally more than that of the unirrigated villages (25)5%), but with a high proportion of Plasmodium falciparum (76)6%) in the former villages as compared to the latter (16)6%). Anopheles stephensi, A. culicifacies and A. subpictus were amongst the eight anopheline species collected from all the four villages which were found positive for malarial parasites. Anopheles stephensi was the predominant species in the unirrigated villages ('95%), although in the irrigated villages A. culicifacies was also found. The major ecological changes associated with irrigation in the Thar Desert are understood to be playing an important role in accentuating the transmission of malaria by improving vector breeding conditions and survival in an otherwise hostile arid environment. 2001 Academic Press Keywords: malaria; vectors; bionomics; Thar Desert
Introduction Recently, malaria has re-emerged and/or formed new foci in several parts of India (Tyagi, 1997). The resulting epidemics are quite often associated with a high Plasmodium falciparum prevalence associated with high morbidity and mortality (Sharma, 1996). Due to its aridity, the Thar Desert, in north-western India, nearly 90% of which is within the Rajasthan state, has been traditionally regarded as an area only hypoendemic for malaria with variable endemicities and potential for sporadic outbreaks, especially in the flood-prone lower reaches of the Luni river basin (Sharma, 1986; Tyagi, 1994a). During the past decade, however, disease distribution has changed a great deal in the Thar Desert and this is attributed, amongst several other factors, largely to the major ecological and physical changes brought about by an extensive canalized irrigation scheme, called the Indira Gandhi Nahar Pariyojana (IGNP). Along with the other two main canal systems in the Thar, the Gang canal and Bhakra-Sirhind 0140-1963/01/020161#12 $35.00/0
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feeder canal, the irrigated area covers about 2 million ha with a total of 10,000 km of canals, which is one of the world’s largest canal irrigation systems in a desert environment (Anonymous, 1991, 1992, 1994). The continued supply of canal water to the desert over the past few decades has raised the water table, increased the water-retention characteristic of the soil, allowed cultivation of rice and water-demanding crops which were previously absent in the desert, and unintentionally converted vast areas into marshes along the main canals. All these changes in the environment have attracted mosquitoes and created preferred sites for breeding on a perennial basis. As a consequence, the Thar Desert has been experiencing malaria outbreaks since 1983 (Mathur et al., 1992; Tyagi, 1995). Anopheles stephensi is considered to be the original malaria vector in the Thar region (Tyagi, 1994b), but is numerous throughout the year, surviving high temperatures and low relative humidity. Anopheles stephensi is also well-adapted to breeding in underground water reservoirs (tanka and beri), commonly a household’s sole water supply (Tyagi & Yadav, 1996). With the availability of irrigation water and waste water collection from seepage through canals, however, several other species, including the efficient vector A. culicifacies, have also been attracted to the interior of the Thar region (Tyagi & Verma, 1991; Tyagi & Yadav, 1994). An outbreak in late 1992 warranted a detailed investigation into the factors responsible for repeated epidemics in the desert.
Transformation in Thar Desert climate During the last few years the Thar Desert has undergone a major change in part of its topography and has largely converted to verdure, greatly affecting the epidemiology of a disease like malaria. The best way to demonstrate a shift in the area’s climate is by plotting changes in rainfall, air temperature and relative humidity.
Rainfall Pant & Hingane (1988) studied trends in rainfall and temperature during 1901}1982, covering the north-western Rajasthan, and showed an increasing trend in mean annual rainfall (141)3 mm per 100 years) and a decreasing trend in air temperature (!0)523C for 100 years), which contradicts the conclusions of Winstanely (1973a, 1973b). One of the most reliable studies on the meteorological parameters of a desert district, Sri Ganganagar, was made by Ramakrishna & Rao (1991) who found an increase in the mean decadal rainfall in the district, which had been subjected to canal-based irrigation for a considerable period. The Gang canal and IG canal projects aimed to utilize 7)59 MAF of water from Ravi-Beas water to irrigate an 1)143 mill-ha area. In the Ganganagar district, the Bikaner district and the Jaisalmer district, 708,775 ha, 37,022 ha and 84 ha of land, respespectively, has been put under irrigation (Roy, 1983). The impact of irrigation in Rajasthan is seen from the vegetational and micro-climatic changes that have occurred in the region. Ganganagar receives an annual rainfall of 243 mm. The annual rainfall of Ganganagar showed an increasing trend, at the rate of 1029 mm year\, during the period 1926}1993. The decade-wise mean annual rainfall shows that the increase in rainfall was high during the last four decades, the period of intense irrigation in the region. Even in periods of drought faced by western Rajasthan, Ganganagar has received adequate rainfall for crop production, which is attributed to the increased moisture through irrigation imposed for long periods in the region. On average, climatic droughts
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prevailed in the region once every 5 years (22% chance) during 1926}1993. The mean annual rainfall of Bikaner is 286 mm with a coefficient of variation of 48%. The region experienced droughts once every 3 years (38% chance). The rainfall (annual) at Bikaner showed an overall decreasing trend at a rate of 0)147 mm year\ during 1926}1993, which is forcibly attributable to consecutive droughts which occurred after 1961. Jaisalmer receives a mean annual rainfall of 185 mm with a coefficient of variation of 64%. The rainfall is very erratic and poorly distributed with droughts occurring once every 4 years (24% chance). However, the overall trend in annual rainfall at Jaisalmer remained the same during 1901}1993.
Air temperature In a recent study, Rao (1996) established a general decreasing trend in air temperature of 0)0393C year\ a Ganganagar, 0)0233C year\ at Bikaner, and 0)0093C year\ at Jaisalmer.
Relative humidity Rao (1996) also recorded 5}10% higher humidity measurements in cropped surfaces at 1-m height in the irrigated villages compared to the unirrigated cropped surfaces. This highlights the fact that the amount of canal water available in the desert region can influence atmospheric humidity. In conclusion, these observations provide sufficient evidence for a visible shift in both the biotic and abiotic factors in the IGNP-command area.
Material and methods Study site Four villages, Madassar and Awai, located near the main Indira Gandhi canal in the Jaisalmer district under the IGNP, and Kanasar and Khetusar, about 30}40 km away from the Indira Gandhi canal in the unirrigated area, were selected for various entomological and parasitological investigations (Fig. 1). All the houses in these villages were numbered, the population enumerated and family details recorded on schedules pretested for epidemiological research (Black, 1968).
Entomological investigations Sampling of resting vector mosquitoes Adult mosquitoes were sampled indoors and outdoors at six households and cattlesheds in each village between 0600 h and 1000h using mouth-operated aspirators and flash lights. The collections were transported to a temporary laboratory established at the Guest House of IGNP at Bhikampur about 20 km from Madassar village. Identification of the samples was carried out following standard keys (Christophers, 1933; Harrison & Scanlon, 1975), and they were then classified for their abdominal condition and follicular development.
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Figure 1. Sketch showing sites of villages investigated in the highly irrigated Indira Gandhi Nahar Pariyojana command area (1"Madassar village, 2"Awai village), and in the truly desertic non-command area (3"Kanasar village, 4"Khetusar village) in Jaisalmer and Jodhpur districts, respectively, in the Thar Desert, north-western Rajasthan (India).
Sampling of biting mosquitoes from cattle and humans Collections of biting mosquitoes were simultaneously sampled from a volunteer who lay on a cot and from a buffalo calf tethered about 10 feet away, between 1900h and 2100h, to determine the feeding preference of the vector species. To determine the peak periods of vector feeding, collections of biting mosquitoes were also made overnight (1800}0600h) from two sleeping volunteers. Larval sampling Larvae were sampled from the limited water sources available, such as ‘tanka’, ‘beri’, pitchers, cement tanks, pond and seepage water, within a radius of 2 km from the study villages. At the smaller sites, 10 dips with a long-handled dipper were used for sampling.
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However, in the case of ‘tanka’ and ‘beri’ (underground manmade reservoirs in the precincts of houses or villages or in the bed of seasonal ponds), the larvae were sampled with a 5 l bucket (Tyagi, 1998). Parasitological investigations Vector incrimination Females of various anopheline species from the collections were dissected in normal saline for the examination of guts and the salivary glands for parasitization with Plasmodium species. Prevalence of malaria parasites Monthly fever surveys of all febriles were conducted in all villages to determine prevalence of each malarial parasite species. The slides with both thick and thin blood-films were stained either in Geimsa or JSB. All fever cases were provided with presumptive treatment at the time of blood slide collection and later, after confirmation of positive malarial parasitization, the patient was also given radical treatment according to the policy of the National Malaria Eradication Programme (NMEP, recently renamed as the National Anti-Malaria Programme or NAMP), through either the Auxilary Nurse Midwife (ANM) or the Multi-Purpose Worker (MPW) present in the village. Observations and results Parasitological and epidemiological evaluation Following a rapid mass blood survey immediately after the epidemic in the late autumn of 1992 in the two IGNP command villages (P.v. 11)1%, P.f. 88)7%) and two unirrigated villages (P.v. 17)1%, P.f. 82)7%), fever surveys conducted for one full year between April 1993 and March 1994 showed a slightly higher slide positivity rate in the IGNP villages (32)3%) than in the unirrigated villages (25)5%) (Table 1). The most important feature was, however, the higher proportion of P. falciparum in the IGNP villages (76)6%) compared to that in the unirrigated villages (16)6%). Influenced by the high percentage of P. falciparum in the study villages, the slide positivity rate maintained a high profile throughout. Plasmodium falciparum exhibited two distinct peaks; first, during the summer months of March}April and, second, during December}January (Fig. 2). Mixed infections occurred in all villages, although more occurred in the unirrigated villages (11)5%) than the other two villages (3)2%). Interestingly, more gametocyte carriers were present in the IGNP-command villages (16, 17)8%) compared to the unirrigated villages (2, 8)3%). Cumulatively, 73)7% of all the positive cases were found in children of less than 14 years of age. The male population (68)4%) suffered more than the females (31)5%). Questions arise pertaining to (1) the higher percentage of P. vivax compared to P. falciparum in the unirrigated villages, and (2) the higher number of slides collected in the irrigated villages. Despite the fact that the Thar Desert is hypoendemic for malaria, it was P. vivax which naturally occurred in the region (e.g. the xeric and unirrigated villages in the present study) until the more virulent P. falciparum could invade the region with A. culicifacies and other vectors, in the highly irrigated part of the desert (e.g. the IGNP-command villages). The Thar Desert has a unique agrarian system with only the main villages conveniently located near basic amenities such as roads, hospitals and water sources, and the agricultural field being several kilometers away. Consequently,
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Table 1. Results of fever surveys in the four study villages from April 1993 to March 1994 Village
BSE/BSC Positive Pf (%) (%)
177
Awai
101
Kanasar
65
Khetusar
29 372
8 (15)3) 10 (26)6) 13 (72)2) 5 (83)3)
Mix (%)
52 (29)3) 38 (37)6) 18 (27)6) 6 (20)6)
42 (80)0) 27 (71)0) 4 (22)2) 0 (0)0)
114 (30)6)
73 36 5 (64)0) (31)5) (4)3)
Gametocyte (%)
2 12 (3)8) (23)0) 1 4 (2)6) (10)5) 1 2 (5)5) (11)6) 1 0 (16)6) (0)0) 18 (15)8)
Prevalence of positives by age group
M (%)
F (%)
ABER
API
7)1
20)8
8)8
32)9
4)4
12)2
2)4
5)0
5)9
18)0
(11 12}23 2}4 m m yr
5}9 yr
10}14 yr
2 (3)8) 3 (3)8) 7 (38)8) 0 (0)0)
7 (13)4) 11 (28)9) 6 (33)3) 12 (16)6)
16 (30)7) 6 (15)7) 2 (11)1) 1 (16)6)
16 (30)7) 10 (26)3) 2 (11)1) 2 (33)3)
36 (69)2) 23 (60)5) 14 (77)7) 5 (83)3)
16 (30)7) 15 (39)4) 4 (22)2) 1 (16)6)
14 25 (12)2) (21)9)
25 (21)9)
30 (26)3)
8 (68)4)
36 (31)5)
3 (5)7) 3 (3)8) 0 (0)0) 2 (33)3)
12 8 (10)5) (7)5)
8 (15)3) 5 (13)1) 1 (5)5) 0 (0)0)
'15 yr
BSC/BSE"Blood slide collected/examined; Pf"Plasmodium falciparum; Pv"Plasmodium vivax; M"male; F"female; ABER"annual blood examination rate, i.e. slides examined as % of population; API"annual parasite incidence, i.e. slide positive cases per 1000 population.
B. K. TYAGI & S. P. YADAV
Madassar
Pv (%)
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Figure 2. Graph showing the relative number of Plasmodium vivax and P. falciparum cases in both irrigated and unirrigated villages (note the high prevalence of P. falciparum in the irrigated villages), during 1993}1994; P. falciparum: irrigated villages, unirrigated villages; P. vivax: irrigated villages, unirrigated villages.
when the epidemic occurred in 1992}1993, most of the families shifted to far-flung hamlets (called ‘Dhani’ in local dialect) and were difficult to access during the investigation. Contrary to this, the irrigated villages in the IGNP-command area were more compact and the families were easily accessible, hence more blood slides were collected from the irrigated villages. The reason for more males suffering with infection may be due to their different sleeping and clothing habits, as they prefer to sleep outdoors at night, with fewer clothes covering their skin, as compared to females who essentially sleep indoors while more or less completely covered with the traditional clothes they wear. This difference in sleeping and clothing habits determines the extent and magnitude of exposure of a person to the biting vectors during sleep. Vector composition and prevalence A total of eight anopheline species were sampled from all the types of collection (Table 2), of which A. stephensi (38)8%) was the most numerous. Anopheles nigerrimus (7)1%) was prominent during dusk collections in the vicinity of the forest along the Indira Gandhi main canal, as they were attracted towards the light of torches and vehicle headlights. The relative density, expressed as the number of females collected by one man during a 1 h period, is graphically presented in Fig. 3. The average density per man-hour of A. stephensi (17)1; range 0)0}47)2) was the highest, with maximum density in April and minimum in January. Another important vector, A. culicifacies, (average pmh 3)3; range 0)0}14)1) showed maximum density during the monsoon months of July}September. Anopheles subpictus rose in density with the onset of the monsoon in late June due to the existence of rainwater-fed surface breeding habitats, and remained abundant until December (average pmh 11)6; range 9)7}33)1). Anopheles stephensi is conspicuous by its constant presence in unirrigated villages despite extremes of ambient temperature (average pmh 7)0; range 0)0}18)7). In these villages A. subpictus also occurred in low densities between August and December (average pmh 1)1; range 0)0}7)0), but A. culicifacies was characteristically absent throughout.
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Table 2. Females of anopheline species collected in the irrigated and unirrigated villages
Species
Irrigated villages Indoor collection HD CS
A. stephensi A. culicifacies A. subpictus A. annularis A. nigerrimus Other species
635 316 479 257 29 99
Unirrigated villages PMH Density
Total (%)
588 1223(29)6) 342 658(15)9) 606 1085(26)2) 236 493(11)9) 23 52(1)2) 47 146(3)5)
Indoor collection
PMH Density
HD CS Total (%) 23)8 13)5 22)6 10)2 1)0 2)8
230 200 430(10)4) 0 0 0 24 31 55(1)3) 0 0 0 0 0 0 0 0 0
8)1 0)0 1)1 0)0 0)0 0)0
HD"Human dwelling; CS"cattleshed; PMH"per-man hour density; Other species"Anopheles vagus, A. splendidus and A. d’thali.
Sporozoite rates A total of 2625 females belonging to all eight species were dissected during 1993}1995, but only A. stephensi, A. culicifacies and A. subpictus samples during dawn and dusk were found infected with the malaria parasite (for salivary glands, p-test or s"1)768; for Gut, s"0)36) (Table 3).
Larval collection Larval instars for as many as four species, A. culicifacies, A. stephensi, A. subpictus and A. annularis, were sampled from a variety of breeding habitats including irrigation canals, ponds, ‘khadin’, ‘nadi’, ‘tanka’ and ‘beri’. Only A. stephensi bred in the ‘tanka’ and
Figure 3. Comparison of the relative density (per man-hour density) amongst Anopheles stephensi, A. culicifacies and A. subpictus in the irrigated and unirrigated villages (1993}1994); A. stephensi; A. culicifacies; A. subpictus; A. stephensi; A. culicifacies; A. subpictus.
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Table 3. Anopheline species naturally incriminated with malaria parasite
Species
Irrigated villages
Unirrigated villages
No. SG#ve G#ve Total dissected
No. SG#ve G#ve Total dissected
Anopheles stephensi Anopheles culicifacies Anopheles subpictus Anopheles nigerrimus
850
3
0
3
102
1
1
2
938
1
6
7
0
0
0
0
451
1
1
2
284
0
1
1
40
0
0
0
0
0
0
0
SG#ve"Salivary gland positive for sporozoite; G#ve"gut positive for oocyst.
‘beri’, while A. culicifacies was dominant in irrigation channels, stagnat water along the canals, and the swamp. Anopheles subpictus was found breeding in the post-monsoon open-ground pools and the cement tanks filled with water for cattle drinking. Feeding behaviour During the simultaneous collections at dusk from a human volunteer and a calf, A. stephensi was collected about equally from each (Table 4). Surprisingly, A. nigerrimus showed the strongest preference for orientation on the human, but only in the presence of flash lights. It did not, however, feed preferably on human blood. Anopheles culicifacies proved to be zoophilic, like A. annularis and A. subpictus. Night biting behaviour was studied for four species, A. stephensi, A. culicifacies, A. subpictus and A. nigerrimus. Both A. stephensi and A. culicifacies showed two feeding peaks, one early in the night and the other just before the dawn (Fig. 4). Anopheles culicifacies fed earlier than A. stephensi and stayed in higher densities until just before drawn. Anopheles nigerrimus was collected in large numbers in the early hours of the Table 4. Anopheline species collected simultaneously from man and cattle to show anthropophily/zoophily of five of the eight species
Species
Anopheles stephensi Anopheles culicifacies Anopheles annularis Anopheles subpictus Anopheles nigerrimus
Number of females collected Man (%)
Cattle (%)
151 (52)2) 78 (26)8) 2 (3)7) 11 (4)8) 71 (98)6)
138 (47)8) 213 (73)2) 51 (96)3) 215 (95)2) 1 (1)4)
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Figure 4. Graph showing orientation of four important anopheline species for anthropophily during all-night human-bait collection (note characteristic twin peaks for orientation of the vector Anopheles stephensi and A. culicifacies, in particular); A. stephensi; A. culicifacies; A. A. nigerrimus. subpictus;
night only and was apparently attracted to the flash lights. Blood meal identification indicated to the greater preference of A. stephensi for human blood (58)2%) compared to that of either A. culicifacies (23)4%) or A. subpictus (13)4%). Discussion Epidemiologically, the Thar Desert was never regarded as an important region for malaria outbreaks until the series of epidemics which began in the late 1980s and had exceptionally high morbidity and mortality due to P. falciparum (Tyagi et al., 1994; Tyagi & Chaudhary, 1997). In fact, the earliest report of malaria cases from any part of Rajasthan is from 1909 when 134 hospital admissions were registered, of which 10% were P. falciparum (Green, 1911). The first major outbreak in the Sri Ganganagar district in the northern Thar Desert during 1983 and 1984 nearly went unreported, but in 1990, following heavy rains (average 450 mm; maximum 960 mm), a malaria epidemic broke out in the lower reaches of the Thar, including the Barmer, Jodhpur and Pali districts (Tyagi et al., 1995). The canal system in this part of the Thar Desert is so extensive and favourable to vector mosquito breeding that the severe epidemic which occured during late 1992 in the Jodhpur, Bikaner and Jaisalmer districts in the IGNPcommand area is likely to recur (Tyagi, 1995). The present investigation in two IGNP-command villages was carried out near the boundaries of the above three districts where the ambient temperature and relative humidity in the vicinity of canal are about 223C and 50}60%, respectively. In comparison, two other villages were studied in typical desert conditions with ambient temperatures and the relative humidity in summer averaging around 35}403C and 25}30%, respectively. Mass blood survey results showing more than 80% P. falciparum cases in both irrigated and unirrigated areas suggest that there must be high malaria morbidity in the region, particularly the IGNP-command area (Tyagi et al., 1994). This was supported by the fever survey results showing 76% of malaria infections was P. falciparum in the IGNP-command villages compared to about 16% P. falciparum in the unirrigated villages. Furthermore, there was a relatively high percentage of P. falciparum gametocyte carriers in the irrigated villages (17)8%). Most of the malaria occurred in children (73)3%).
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Anopheles stephensi seems to be reproductively best-adapted and chronologically the original vector species in the Thar Desert (Zahar, 1990; Tyagi, 1994a). Although A. culicifacies, A. subpictus and A. annularis were reported earlier from the urban limits of Bikaner (MacDonald, 1931), none were parasitologically proved then to be a vector in the desert. For the first time, therefore, A. culicifacies has been shown to be a vector in the highly canal-irrigated Sri Ganganagar district (Tyagi & Verma, 1991), while A. stephensi was proved to be the sole vector in the truly desert environment of the Jodhpur district (Tyagi & Yadav, 1994). Anopheles subpictus was found to be malaria-infected both in irrigated and unirrigated villages, while the infected individuals could be found only in the IGNP-command area in the north-eastern Jaisalmer district (Tyagi, 1996). This situation calls for a strong effort to annihilate vector populations before the situation worsens. Lack of sustained vector control efforts through indoor residual insecticide spraying has been a major drawback, coupled with a moderate to high degree of tolerance/resistance by vector species to DDT, the only insecticide used extensively over the past three decades in the Thar Desert. Taking account of the fact that the Thar Desert is a very different ecosystem from the rest of the country, the problem of malaria control has to be tackled with an improvised vector containment methodology which must essentially suit the local conditions, including human behaviour, but must also effectively dissipate vector populations and the disease burden. The authors are grateful to various technical staff, especially Shri Santosh Dhawal and Rajaneesh, for their help in laboratory and field investigations.
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Sharma, V.P. (1996). Re-emergence of malaria in India. Indian Journal of Medical Research, 103: 26}45. Tyagi, B.K. (1994a). Impact of extensive canalization on the distributions of malaria vectors in the north-western Rajasthan. Abstract Paper. Indian Science Congress Association, Jaipur, pp. 39}40. Tyagi, B.K. (1994b). Chronological and spatial distribution of anopheline vectors of malaria in the Thar Desert. In: Ghosh, A.K., Baqri, Q.H. & Prakash, I. (Eds), Faunal Diversity in the Thar Desert, pp. 227}240. Jodhpur: Scientific Publishers. Tyagi, B.K. (1994c). Thanatosis as an adaptive defence mechanism in Anopheles stephensi larvae breeding in ‘tanka’ and ‘beri’ in the desert villages of Rajasthan. Geobios New Reports, 13: 132}135. Tyagi, B.K. (1995). Malaria in the Thar Desert: A critical Review. ICMR Bulletin, 25: 85}91. Tyagi, B.K. (1996). Phenology of Anopheles subpictus Grassi, an unusual vector of malaria in the Thar Desert, India. Abstract Paper. XX International Congress of Entomology, Firenze. Tyagi, B.K. (1997). Emerging and re-emerging vector-borne diseases in the Great Indian Thar Desert. WHO-SEARO Regional Health Forum, 2: 9}15. Tyagi, B.K. (1998). Malaria and its vectors in the Thar Desert. In: Goel, S.C. (Ed.), Advances in Medical Entomology and Human Welfare, pp 1}10. Muzaffar Nagar: UPZS. Tyagi, B.K. & Chaudhary, R.C. (1997). Outbreak of falciparum malaria in the Thar Desert (India), with particular emphasis on physiographic changes brought about by extensive canalization and their impact on vector density and dissemination. Journal of Arid Environments, 36: 541}555. Tyagi, B.K. & Verma, K.V. (1991). Anopheline mosquitoes of Sri Gangangar District (Rajasthan), transmitting malaria parasite. Journal of Applied Zoological Researches, 2: 85}91. Tyagi, B.K. & Yadav, S.P. (1994). Impact of agricultural patterns and water management on the distribution of disease transmitting mosquitoes. Abstract Paper, Meeting of Experts on the Gaps in Researches on Faunal Diversity in the Thar Desert, Jodhpur. 36 pp. Tyagi, B.K. & Yadav, S.P. (1996). Malariological and sociological significance of ‘tanka’ and ‘beri’ in the Thar desert, Western Rajasthan, India. Journal of Arid Environments, 33: 497}501. Tyagi, B.K., Singh, K.V., Bansal, S.K. & Yadav, S.P. (1994). Malaria epidemic in some villages of north-western desert Rajasthan. Journal of Applied Zoological Researches, 5: 152}155. Tyagi, B.K., Chaudhary, R.C. & Yadav, S.P. (1995). Epidemic malaria in Thar Desert, India. The Lancet, 346: 634}635. Winstanley, D. (1973a). Recent rainfall trends in Africa. The Middle-East & India. Nature, 243: 464}466. Winstanley, D. (1973b). Rainfall patterns and general atmospheric circulation. Nature, 245: 190}194. Zahar, A.R. (1990). Vector bionomics in the epidemiology and control of malaria. II. The WHO European Region and the WHO Eastern Mediterranean Region. Vol. II. Applied Field Studies. WHO/VBC/90.1, Mal/90.1, p. 1}90.
Bionomics of malaria vectors in two physiographically different areas of the epidemic-prone Thar Desert, north-western Rajasthan (India)
B. K. Tyagi & S. P. Yadav Desert Medicine Research Centre, Post Box 122, New Pali Road, Jodhpur 342 005, India (Received 15 May 1998, accepted 15 August 2000) Entomological and parasitological investigations were carried out on malaria vectors and disease prevalence in two sets of villages, the highly irrigated Indira Gandhi Nahar Pariyojana (IGNP) command-area villages (Madassar and Awai), and the truly desertic non-command (unirrigated) area villages (Kanasar and Khetusar), located in different ecological conditions in the Thar Desert, north-western Rajasthan (India). Malaria prevalence, as determined through sustained fever surveys, was higher in the IGNP villages with a slide positivity rate (32%) marginally more than that of the unirrigated villages (25)5%), but with a high proportion of Plasmodium falciparum (76)6%) in the former villages as compared to the latter (16)6%). Anopheles stephensi, A. culicifacies and A. subpictus were amongst the eight anopheline species collected from all the four villages which were found positive for malarial parasites. Anopheles stephensi was the predominant species in the unirrigated villages ('95%), although in the irrigated villages A. culicifacies was also found. The major ecological changes associated with irrigation in the Thar Desert are understood to be playing an important role in accentuating the transmission of malaria by improving vector breeding conditions and survival in an otherwise hostile arid environment. 2001 Academic Press Keywords: malaria; vectors; bionomics; Thar Desert
Introduction Recently, malaria has re-emerged and/or formed new foci in several parts of India (Tyagi, 1997). The resulting epidemics are quite often associated with a high Plasmodium falciparum prevalence associated with high morbidity and mortality (Sharma, 1996). Due to its aridity, the Thar Desert, in north-western India, nearly 90% of which is within the Rajasthan state, has been traditionally regarded as an area only hypoendemic for malaria with variable endemicities and potential for sporadic outbreaks, especially in the flood-prone lower reaches of the Luni river basin (Sharma, 1986; Tyagi, 1994a). During the past decade, however, disease distribution has changed a great deal in the Thar Desert and this is attributed, amongst several other factors, largely to the major ecological and physical changes brought about by an extensive canalized irrigation scheme, called the Indira Gandhi Nahar Pariyojana (IGNP). Along with the other two main canal systems in the Thar, the Gang canal and Bhakra-Sirhind 0140-1963/01/020161#12 $35.00/0
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feeder canal, the irrigated area covers about 2 million ha with a total of 10,000 km of canals, which is one of the world’s largest canal irrigation systems in a desert environment (Anonymous, 1991, 1992, 1994). The continued supply of canal water to the desert over the past few decades has raised the water table, increased the water-retention characteristic of the soil, allowed cultivation of rice and water-demanding crops which were previously absent in the desert, and unintentionally converted vast areas into marshes along the main canals. All these changes in the environment have attracted mosquitoes and created preferred sites for breeding on a perennial basis. As a consequence, the Thar Desert has been experiencing malaria outbreaks since 1983 (Mathur et al., 1992; Tyagi, 1995). Anopheles stephensi is considered to be the original malaria vector in the Thar region (Tyagi, 1994b), but is numerous throughout the year, surviving high temperatures and low relative humidity. Anopheles stephensi is also well-adapted to breeding in underground water reservoirs (tanka and beri), commonly a household’s sole water supply (Tyagi & Yadav, 1996). With the availability of irrigation water and waste water collection from seepage through canals, however, several other species, including the efficient vector A. culicifacies, have also been attracted to the interior of the Thar region (Tyagi & Verma, 1991; Tyagi & Yadav, 1994). An outbreak in late 1992 warranted a detailed investigation into the factors responsible for repeated epidemics in the desert.
Transformation in Thar Desert climate During the last few years the Thar Desert has undergone a major change in part of its topography and has largely converted to verdure, greatly affecting the epidemiology of a disease like malaria. The best way to demonstrate a shift in the area’s climate is by plotting changes in rainfall, air temperature and relative humidity.
Rainfall Pant & Hingane (1988) studied trends in rainfall and temperature during 1901}1982, covering the north-western Rajasthan, and showed an increasing trend in mean annual rainfall (141)3 mm per 100 years) and a decreasing trend in air temperature (!0)523C for 100 years), which contradicts the conclusions of Winstanely (1973a, 1973b). One of the most reliable studies on the meteorological parameters of a desert district, Sri Ganganagar, was made by Ramakrishna & Rao (1991) who found an increase in the mean decadal rainfall in the district, which had been subjected to canal-based irrigation for a considerable period. The Gang canal and IG canal projects aimed to utilize 7)59 MAF of water from Ravi-Beas water to irrigate an 1)143 mill-ha area. In the Ganganagar district, the Bikaner district and the Jaisalmer district, 708,775 ha, 37,022 ha and 84 ha of land, respespectively, has been put under irrigation (Roy, 1983). The impact of irrigation in Rajasthan is seen from the vegetational and micro-climatic changes that have occurred in the region. Ganganagar receives an annual rainfall of 243 mm. The annual rainfall of Ganganagar showed an increasing trend, at the rate of 1029 mm year\, during the period 1926}1993. The decade-wise mean annual rainfall shows that the increase in rainfall was high during the last four decades, the period of intense irrigation in the region. Even in periods of drought faced by western Rajasthan, Ganganagar has received adequate rainfall for crop production, which is attributed to the increased moisture through irrigation imposed for long periods in the region. On average, climatic droughts
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prevailed in the region once every 5 years (22% chance) during 1926}1993. The mean annual rainfall of Bikaner is 286 mm with a coefficient of variation of 48%. The region experienced droughts once every 3 years (38% chance). The rainfall (annual) at Bikaner showed an overall decreasing trend at a rate of 0)147 mm year\ during 1926}1993, which is forcibly attributable to consecutive droughts which occurred after 1961. Jaisalmer receives a mean annual rainfall of 185 mm with a coefficient of variation of 64%. The rainfall is very erratic and poorly distributed with droughts occurring once every 4 years (24% chance). However, the overall trend in annual rainfall at Jaisalmer remained the same during 1901}1993.
Air temperature In a recent study, Rao (1996) established a general decreasing trend in air temperature of 0)0393C year\ a Ganganagar, 0)0233C year\ at Bikaner, and 0)0093C year\ at Jaisalmer.
Relative humidity Rao (1996) also recorded 5}10% higher humidity measurements in cropped surfaces at 1-m height in the irrigated villages compared to the unirrigated cropped surfaces. This highlights the fact that the amount of canal water available in the desert region can influence atmospheric humidity. In conclusion, these observations provide sufficient evidence for a visible shift in both the biotic and abiotic factors in the IGNP-command area.
Material and methods Study site Four villages, Madassar and Awai, located near the main Indira Gandhi canal in the Jaisalmer district under the IGNP, and Kanasar and Khetusar, about 30}40 km away from the Indira Gandhi canal in the unirrigated area, were selected for various entomological and parasitological investigations (Fig. 1). All the houses in these villages were numbered, the population enumerated and family details recorded on schedules pretested for epidemiological research (Black, 1968).
Entomological investigations Sampling of resting vector mosquitoes Adult mosquitoes were sampled indoors and outdoors at six households and cattlesheds in each village between 0600 h and 1000h using mouth-operated aspirators and flash lights. The collections were transported to a temporary laboratory established at the Guest House of IGNP at Bhikampur about 20 km from Madassar village. Identification of the samples was carried out following standard keys (Christophers, 1933; Harrison & Scanlon, 1975), and they were then classified for their abdominal condition and follicular development.
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Figure 1. Sketch showing sites of villages investigated in the highly irrigated Indira Gandhi Nahar Pariyojana command area (1"Madassar village, 2"Awai village), and in the truly desertic non-command area (3"Kanasar village, 4"Khetusar village) in Jaisalmer and Jodhpur districts, respectively, in the Thar Desert, north-western Rajasthan (India).
Sampling of biting mosquitoes from cattle and humans Collections of biting mosquitoes were simultaneously sampled from a volunteer who lay on a cot and from a buffalo calf tethered about 10 feet away, between 1900h and 2100h, to determine the feeding preference of the vector species. To determine the peak periods of vector feeding, collections of biting mosquitoes were also made overnight (1800}0600h) from two sleeping volunteers. Larval sampling Larvae were sampled from the limited water sources available, such as ‘tanka’, ‘beri’, pitchers, cement tanks, pond and seepage water, within a radius of 2 km from the study villages. At the smaller sites, 10 dips with a long-handled dipper were used for sampling.
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However, in the case of ‘tanka’ and ‘beri’ (underground manmade reservoirs in the precincts of houses or villages or in the bed of seasonal ponds), the larvae were sampled with a 5 l bucket (Tyagi, 1998). Parasitological investigations Vector incrimination Females of various anopheline species from the collections were dissected in normal saline for the examination of guts and the salivary glands for parasitization with Plasmodium species. Prevalence of malaria parasites Monthly fever surveys of all febriles were conducted in all villages to determine prevalence of each malarial parasite species. The slides with both thick and thin blood-films were stained either in Geimsa or JSB. All fever cases were provided with presumptive treatment at the time of blood slide collection and later, after confirmation of positive malarial parasitization, the patient was also given radical treatment according to the policy of the National Malaria Eradication Programme (NMEP, recently renamed as the National Anti-Malaria Programme or NAMP), through either the Auxilary Nurse Midwife (ANM) or the Multi-Purpose Worker (MPW) present in the village. Observations and results Parasitological and epidemiological evaluation Following a rapid mass blood survey immediately after the epidemic in the late autumn of 1992 in the two IGNP command villages (P.v. 11)1%, P.f. 88)7%) and two unirrigated villages (P.v. 17)1%, P.f. 82)7%), fever surveys conducted for one full year between April 1993 and March 1994 showed a slightly higher slide positivity rate in the IGNP villages (32)3%) than in the unirrigated villages (25)5%) (Table 1). The most important feature was, however, the higher proportion of P. falciparum in the IGNP villages (76)6%) compared to that in the unirrigated villages (16)6%). Influenced by the high percentage of P. falciparum in the study villages, the slide positivity rate maintained a high profile throughout. Plasmodium falciparum exhibited two distinct peaks; first, during the summer months of March}April and, second, during December}January (Fig. 2). Mixed infections occurred in all villages, although more occurred in the unirrigated villages (11)5%) than the other two villages (3)2%). Interestingly, more gametocyte carriers were present in the IGNP-command villages (16, 17)8%) compared to the unirrigated villages (2, 8)3%). Cumulatively, 73)7% of all the positive cases were found in children of less than 14 years of age. The male population (68)4%) suffered more than the females (31)5%). Questions arise pertaining to (1) the higher percentage of P. vivax compared to P. falciparum in the unirrigated villages, and (2) the higher number of slides collected in the irrigated villages. Despite the fact that the Thar Desert is hypoendemic for malaria, it was P. vivax which naturally occurred in the region (e.g. the xeric and unirrigated villages in the present study) until the more virulent P. falciparum could invade the region with A. culicifacies and other vectors, in the highly irrigated part of the desert (e.g. the IGNP-command villages). The Thar Desert has a unique agrarian system with only the main villages conveniently located near basic amenities such as roads, hospitals and water sources, and the agricultural field being several kilometers away. Consequently,
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Table 1. Results of fever surveys in the four study villages from April 1993 to March 1994 Village
BSE/BSC Positive Pf (%) (%)
177
Awai
101
Kanasar
65
Khetusar
29 372
8 (15)3) 10 (26)6) 13 (72)2) 5 (83)3)
Mix (%)
52 (29)3) 38 (37)6) 18 (27)6) 6 (20)6)
42 (80)0) 27 (71)0) 4 (22)2) 0 (0)0)
114 (30)6)
73 36 5 (64)0) (31)5) (4)3)
Gametocyte (%)
2 12 (3)8) (23)0) 1 4 (2)6) (10)5) 1 2 (5)5) (11)6) 1 0 (16)6) (0)0) 18 (15)8)
Prevalence of positives by age group
M (%)
F (%)
ABER
API
7)1
20)8
8)8
32)9
4)4
12)2
2)4
5)0
5)9
18)0
(11 12}23 2}4 m m yr
5}9 yr
10}14 yr
2 (3)8) 3 (3)8) 7 (38)8) 0 (0)0)
7 (13)4) 11 (28)9) 6 (33)3) 12 (16)6)
16 (30)7) 6 (15)7) 2 (11)1) 1 (16)6)
16 (30)7) 10 (26)3) 2 (11)1) 2 (33)3)
36 (69)2) 23 (60)5) 14 (77)7) 5 (83)3)
16 (30)7) 15 (39)4) 4 (22)2) 1 (16)6)
14 25 (12)2) (21)9)
25 (21)9)
30 (26)3)
8 (68)4)
36 (31)5)
3 (5)7) 3 (3)8) 0 (0)0) 2 (33)3)
12 8 (10)5) (7)5)
8 (15)3) 5 (13)1) 1 (5)5) 0 (0)0)
'15 yr
BSC/BSE"Blood slide collected/examined; Pf"Plasmodium falciparum; Pv"Plasmodium vivax; M"male; F"female; ABER"annual blood examination rate, i.e. slides examined as % of population; API"annual parasite incidence, i.e. slide positive cases per 1000 population.
B. K. TYAGI & S. P. YADAV
Madassar
Pv (%)
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Figure 2. Graph showing the relative number of Plasmodium vivax and P. falciparum cases in both irrigated and unirrigated villages (note the high prevalence of P. falciparum in the irrigated villages), during 1993}1994; P. falciparum: irrigated villages, unirrigated villages; P. vivax: irrigated villages, unirrigated villages.
when the epidemic occurred in 1992}1993, most of the families shifted to far-flung hamlets (called ‘Dhani’ in local dialect) and were difficult to access during the investigation. Contrary to this, the irrigated villages in the IGNP-command area were more compact and the families were easily accessible, hence more blood slides were collected from the irrigated villages. The reason for more males suffering with infection may be due to their different sleeping and clothing habits, as they prefer to sleep outdoors at night, with fewer clothes covering their skin, as compared to females who essentially sleep indoors while more or less completely covered with the traditional clothes they wear. This difference in sleeping and clothing habits determines the extent and magnitude of exposure of a person to the biting vectors during sleep. Vector composition and prevalence A total of eight anopheline species were sampled from all the types of collection (Table 2), of which A. stephensi (38)8%) was the most numerous. Anopheles nigerrimus (7)1%) was prominent during dusk collections in the vicinity of the forest along the Indira Gandhi main canal, as they were attracted towards the light of torches and vehicle headlights. The relative density, expressed as the number of females collected by one man during a 1 h period, is graphically presented in Fig. 3. The average density per man-hour of A. stephensi (17)1; range 0)0}47)2) was the highest, with maximum density in April and minimum in January. Another important vector, A. culicifacies, (average pmh 3)3; range 0)0}14)1) showed maximum density during the monsoon months of July}September. Anopheles subpictus rose in density with the onset of the monsoon in late June due to the existence of rainwater-fed surface breeding habitats, and remained abundant until December (average pmh 11)6; range 9)7}33)1). Anopheles stephensi is conspicuous by its constant presence in unirrigated villages despite extremes of ambient temperature (average pmh 7)0; range 0)0}18)7). In these villages A. subpictus also occurred in low densities between August and December (average pmh 1)1; range 0)0}7)0), but A. culicifacies was characteristically absent throughout.
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Table 2. Females of anopheline species collected in the irrigated and unirrigated villages
Species
Irrigated villages Indoor collection HD CS
A. stephensi A. culicifacies A. subpictus A. annularis A. nigerrimus Other species
635 316 479 257 29 99
Unirrigated villages PMH Density
Total (%)
588 1223(29)6) 342 658(15)9) 606 1085(26)2) 236 493(11)9) 23 52(1)2) 47 146(3)5)
Indoor collection
PMH Density
HD CS Total (%) 23)8 13)5 22)6 10)2 1)0 2)8
230 200 430(10)4) 0 0 0 24 31 55(1)3) 0 0 0 0 0 0 0 0 0
8)1 0)0 1)1 0)0 0)0 0)0
HD"Human dwelling; CS"cattleshed; PMH"per-man hour density; Other species"Anopheles vagus, A. splendidus and A. d’thali.
Sporozoite rates A total of 2625 females belonging to all eight species were dissected during 1993}1995, but only A. stephensi, A. culicifacies and A. subpictus samples during dawn and dusk were found infected with the malaria parasite (for salivary glands, p-test or s"1)768; for Gut, s"0)36) (Table 3).
Larval collection Larval instars for as many as four species, A. culicifacies, A. stephensi, A. subpictus and A. annularis, were sampled from a variety of breeding habitats including irrigation canals, ponds, ‘khadin’, ‘nadi’, ‘tanka’ and ‘beri’. Only A. stephensi bred in the ‘tanka’ and
Figure 3. Comparison of the relative density (per man-hour density) amongst Anopheles stephensi, A. culicifacies and A. subpictus in the irrigated and unirrigated villages (1993}1994); A. stephensi; A. culicifacies; A. subpictus; A. stephensi; A. culicifacies; A. subpictus.
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Table 3. Anopheline species naturally incriminated with malaria parasite
Species
Irrigated villages
Unirrigated villages
No. SG#ve G#ve Total dissected
No. SG#ve G#ve Total dissected
Anopheles stephensi Anopheles culicifacies Anopheles subpictus Anopheles nigerrimus
850
3
0
3
102
1
1
2
938
1
6
7
0
0
0
0
451
1
1
2
284
0
1
1
40
0
0
0
0
0
0
0
SG#ve"Salivary gland positive for sporozoite; G#ve"gut positive for oocyst.
‘beri’, while A. culicifacies was dominant in irrigation channels, stagnat water along the canals, and the swamp. Anopheles subpictus was found breeding in the post-monsoon open-ground pools and the cement tanks filled with water for cattle drinking. Feeding behaviour During the simultaneous collections at dusk from a human volunteer and a calf, A. stephensi was collected about equally from each (Table 4). Surprisingly, A. nigerrimus showed the strongest preference for orientation on the human, but only in the presence of flash lights. It did not, however, feed preferably on human blood. Anopheles culicifacies proved to be zoophilic, like A. annularis and A. subpictus. Night biting behaviour was studied for four species, A. stephensi, A. culicifacies, A. subpictus and A. nigerrimus. Both A. stephensi and A. culicifacies showed two feeding peaks, one early in the night and the other just before the dawn (Fig. 4). Anopheles culicifacies fed earlier than A. stephensi and stayed in higher densities until just before drawn. Anopheles nigerrimus was collected in large numbers in the early hours of the Table 4. Anopheline species collected simultaneously from man and cattle to show anthropophily/zoophily of five of the eight species
Species
Anopheles stephensi Anopheles culicifacies Anopheles annularis Anopheles subpictus Anopheles nigerrimus
Number of females collected Man (%)
Cattle (%)
151 (52)2) 78 (26)8) 2 (3)7) 11 (4)8) 71 (98)6)
138 (47)8) 213 (73)2) 51 (96)3) 215 (95)2) 1 (1)4)
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Figure 4. Graph showing orientation of four important anopheline species for anthropophily during all-night human-bait collection (note characteristic twin peaks for orientation of the vector Anopheles stephensi and A. culicifacies, in particular); A. stephensi; A. culicifacies; A. A. nigerrimus. subpictus;
night only and was apparently attracted to the flash lights. Blood meal identification indicated to the greater preference of A. stephensi for human blood (58)2%) compared to that of either A. culicifacies (23)4%) or A. subpictus (13)4%). Discussion Epidemiologically, the Thar Desert was never regarded as an important region for malaria outbreaks until the series of epidemics which began in the late 1980s and had exceptionally high morbidity and mortality due to P. falciparum (Tyagi et al., 1994; Tyagi & Chaudhary, 1997). In fact, the earliest report of malaria cases from any part of Rajasthan is from 1909 when 134 hospital admissions were registered, of which 10% were P. falciparum (Green, 1911). The first major outbreak in the Sri Ganganagar district in the northern Thar Desert during 1983 and 1984 nearly went unreported, but in 1990, following heavy rains (average 450 mm; maximum 960 mm), a malaria epidemic broke out in the lower reaches of the Thar, including the Barmer, Jodhpur and Pali districts (Tyagi et al., 1995). The canal system in this part of the Thar Desert is so extensive and favourable to vector mosquito breeding that the severe epidemic which occured during late 1992 in the Jodhpur, Bikaner and Jaisalmer districts in the IGNPcommand area is likely to recur (Tyagi, 1995). The present investigation in two IGNP-command villages was carried out near the boundaries of the above three districts where the ambient temperature and relative humidity in the vicinity of canal are about 223C and 50}60%, respectively. In comparison, two other villages were studied in typical desert conditions with ambient temperatures and the relative humidity in summer averaging around 35}403C and 25}30%, respectively. Mass blood survey results showing more than 80% P. falciparum cases in both irrigated and unirrigated areas suggest that there must be high malaria morbidity in the region, particularly the IGNP-command area (Tyagi et al., 1994). This was supported by the fever survey results showing 76% of malaria infections was P. falciparum in the IGNP-command villages compared to about 16% P. falciparum in the unirrigated villages. Furthermore, there was a relatively high percentage of P. falciparum gametocyte carriers in the irrigated villages (17)8%). Most of the malaria occurred in children (73)3%).
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Anopheles stephensi seems to be reproductively best-adapted and chronologically the original vector species in the Thar Desert (Zahar, 1990; Tyagi, 1994a). Although A. culicifacies, A. subpictus and A. annularis were reported earlier from the urban limits of Bikaner (MacDonald, 1931), none were parasitologically proved then to be a vector in the desert. For the first time, therefore, A. culicifacies has been shown to be a vector in the highly canal-irrigated Sri Ganganagar district (Tyagi & Verma, 1991), while A. stephensi was proved to be the sole vector in the truly desert environment of the Jodhpur district (Tyagi & Yadav, 1994). Anopheles subpictus was found to be malaria-infected both in irrigated and unirrigated villages, while the infected individuals could be found only in the IGNP-command area in the north-eastern Jaisalmer district (Tyagi, 1996). This situation calls for a strong effort to annihilate vector populations before the situation worsens. Lack of sustained vector control efforts through indoor residual insecticide spraying has been a major drawback, coupled with a moderate to high degree of tolerance/resistance by vector species to DDT, the only insecticide used extensively over the past three decades in the Thar Desert. Taking account of the fact that the Thar Desert is a very different ecosystem from the rest of the country, the problem of malaria control has to be tackled with an improvised vector containment methodology which must essentially suit the local conditions, including human behaviour, but must also effectively dissipate vector populations and the disease burden. The authors are grateful to various technical staff, especially Shri Santosh Dhawal and Rajaneesh, for their help in laboratory and field investigations.
References Anonymous (1991). Prospects of Indira Gandhi Canal Project. New Delhi, Indian Council of Agricultural Research. Anonymous (1992). Profile of Indira Gandhi Nahar Project. Jaipur, Government of Rajasthan, Indira Gandhi Nahar Board. 38 pp. Anonymous (1994). Resource Atlas of Rajasthan. Jaipur, Department of Science and Technology, Government of Rajasthan. 231 pp. Black, R.H. (1968). Manual of Epidemiology and Epidemiological Services in Malaria Programmes. Geneva: World Health Organization. Christophers, S.R. (1933). The Fauna of British India, including Ceylon and Burma. Diptera IV. Family Culicidae: Tribe Anophelini. London: Taylor & Francis. 271 pp. Green J.S. (1911). Malaria in Nasirabad. Journal of Royal Army Medical Corps, 26: 44. Harrison, B.A. & Scanlon, J.E. (1975). Medical Entomology Series—XIII. The myzomia series of Anopheles (Cellia) in Thailand, with emphasis on intra-interspecific variations (Diptera: Culicidae). Contributions of the American Entomological Institute, 17: 1}195. MacDonald, G. (1931). Report on a malaria survey in Bikaner State. Records of the Malaria Survey of India, 2: 603}617. Mathur, K.K., Harpalani, G., Kalra, N.L., Murthy, G.G.K. & Narsimham, M.V.V.L. (1992). Epidemic of malaria in Barmer (Thar desert) Rajasthan during 1990. Indian Journal of Malariology, 29: 1}10. Pant, G.B. & Hingane, L.S. (1988). Climatic changes in and around the Rajasthan desert during the 20th Century. Journal of Climatology, 8: 391}401. Ramakrishana, Y.S. & Rao, A.S. (1991). Climatic features of the Indira Gandhi Canal region. In: Abrol, I.P. & Venkateshvaralu, J. (Eds), Prospects of Indira Gandhi Canal Project, pp. 1}10. New Delhi: ICAR. Rao, A.S. (1996). Climatic changes in the irrigated tracts of Indira Gandhi Canal region of arid western Rajasthan, India. Annals of Arid Zone, 35: 111}116. Roy, T.K. (1983). Impact of Rajasthan Canal Project on Social, Economic and Environmental Conditions. New Delhi: NCEAR. 179 pp. Sharma, G.K. (1986). Malaria and its control in India. Part III. Directorate of National Malaria Eradication Programme, Delhi, pp. 1}618.
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