Comparative Studies of Different Indices Related to Filarial Vector of a Rural and an Urban Area of West Bengal, India

Vector control programme in India is generally carried out by local bodies like municipalities or panchayats-linked with sanitation and solid waste disposal. Due to various reasons most of them unable to undertake effective vector control, which increased the vector and vector born disease problems like LF in the country largely. One of such reasons is incomplete information about vectors from many areas [3,4].


Introduction
Lymphatic Filariasis (LF) is one of the leading causes of disability worldwide. More than 1307 million people live in filariasis endemic territories including 553.7 million in India and are at risk of acquiring filarial infection [1]. 20 states and union territories of India are endemic for LF with estimated 28 million microfilaria carriers and 21 million clinical cases [2].
Vector control programme in India is generally carried out by local bodies like municipalities or panchayats-linked with sanitation and solid waste disposal. Due to various reasons most of them unable to undertake effective vector control, which increased the vector and vector born disease problems like LF in the country largely. One of such reasons is incomplete information about vectors from many areas [3,4].
Collection was done by one insect collector using hand collection method [11], between 0600 to 0800 hours during the year 2009-2011 (two years). So in 2 years, a total of 384 man-hours were employed (192 urban and 192 rural). The collected mosquitoes from each habitation were identified following Christophers [12] and Barraud [13], dissected to search for different developing stages of filarial larvae including microfilariae, and identified following Simonsen [14]. After staining with Leishman's stain, number of parasites detected in each infected mosquito was counted and noted separately for each human habitation.
For determination of age, ovaries were extracted by dissection of mosquitoes and then the ovarioles were isolated. After staining with Leishman's stain, the slides with ovarioles were examined under microscope for number of follicular dilatations, if any. The highest number of dilatations was noted for each mosquito. Average duration of gonotropic cycle was estimated in vitro by noting the time taken between artificial blood feeding and egg laying of mosquitoes [15]. Indices related to age grading were calculated following the methods of pioneer workers of the field [16][17][18][19]. One hundred vector mosquitoes per month were dissected for this study (i.e. 1200 each from rural and urban areas). Follicular dilatations, if any were also noted separately for each infected mosquito.
Available data were subjected to statistical analyses using standard normal deviate 'Z' and student's 't' test [20].
In urban area, overall MHD of Cx. quinquefasciatus was 27.56 which ranges from 22.88 (April) to 35.38 (August) in different months and in rural area, it ranges from 6.50 (May) to 14.88 (October) with overall value of 11.86 (Tables 2 and 3). Season wise calculation shows that, MHD was higher in Rainy season in both the area, but values were higher in urban area than in rural area in all the 3 seasons ( Figure 1).
10 or more vector mosquitoes were encountered in 4.27% of the searched shelters in urban study area (ranges from 0.0% to 7.50% in different months) whereas only in 1.56% of searched shelters in rural study area (ranges from 0.0% to 3.75% in different months). In the urban area, overall 8.85% (ranges 5.0% to 13.75% in different months)  in different months) and 0.31% (ranges 0.0% to 1.25% in different months) of the searched shelters was invaded by infected and infective Cx. quinquefasciatus, respectively (Tables 2 and 3).
Overall vector infection and infectivity rates among the mosquitoes collected from the human habitations of the urban study area were 3.49% and 0.34%, respectively, with highest numbers of infected and infective vectors encountered in October (5.00%) and September (0.80%) respectively. In rural area, overall vector infection and infectivity rates were 1.41% and 0.14%, respectively, with highest numbers of infected and infective vectors encountered in June (2.96% and 0.59% respectively) (Tables 2 and 3). Season wise calculation shows that, vector infection and infectivity rates were higher in Rainy season in both the areas, but values were much higher in urban area than in rural area in all the 3 seasons ( Figure 1).
In both the area, number of mosquitoes containing different developmental stages of W. bancrofti, total count of each stage larvae and average load of parasite in infected vectors shows a decrease from microfilaria (mf) to 1st stage to 2 nd stage and 3 rd stage. But the corresponding figures were much higher in urban area than in rural area (Tables 4 and 5).

Discussion
The urban study area of Kolkata has much dense human population and closely situated habitations in comparison to the rural study area of Tenya of Murshidabad, which increased the possibility of man-vector contact in urban area. First-hand information on different aspects  NP=nulliparous, P 1 =uniparous, P 2 =biparous, P 3 =triparous, P 4 =quadriparous, P 5 =pentaparous, P 6 =hexaparous, P 7 =heptaparous  Cx. quinquefasciatus was the dominant species among house frequenting mosquitoes in both urban and rural areas. MHD of vectors from both the study areas was lower than some other endemic areas of West Bengal but higher than some other area also [5,6,[8][9][10]. Comparison shows that, MHD was much higher in urban area than in rural area in all the months, seasons and overall (p<0.05). In both the areas MHD was higher in rainy season than those in other seasons (p>0.05).
Shelters with high density of vector mosquitoes and shelters with higher numbers of infected and infective vectors are of more epidemiological importance [21]. Present study shows that, percent of shelters with high density (10 or more) vectors, with infected and infective vectors were higher in urban area than in rural area (p<0.05), which increases the possibility of chance of parasitic transmission in urban area.
Vector infection and infectivity rates in both the area were lower than some other areas [9,10] but higher than some other endemic areas [8]. Seasonal data reveal that, in both the areas infection and infectivity rates were higher in rainy season closely followed by summer season (p>0.05). This tendency was also found in some other area [9] but unlike of some other area also, where higher rates were found in summer [10]. Vector infection and infectivity rates were significantly higher in urban area than in rural area under present study (p<0.05).
A strong fall in the average load of microfilaria to average load of infective stages in those vectors shows a natural bearing in control of filarial transmission in both urban and rural area, which was also reported in some other studies [9,10,22]. However, average load of parasites per vectors was higher in urban area than in rural area (p>0.05).
Analysis of the parity status of the infected Cx. quinquefasciatus mosquitoes in both the study areas revealed that a high proportion of the mosquitoes acquired infection during their first blood meal and most infective mosquitoes are triparous, quadriparous and pentaparous. So mosquitoes of higher parous were of more epidemiological importance [23]. Presumptive mortality between two successive age groups shows that after passing the initial age, the vectors tend to survive the age which generally harbours the infective parasites in both the study area. So the probability of filarial transmission was almost similar in urban and rural area in respect of this index. Moreover, rate of overall daily mortality of vectors and also in summer and rainy season were lower in urban area than in rural area (p>0.05).
From the present study it can be concluded that, in both the study areas of Kolkata (urban) and Tenya (rural) possibility of filarial transmission is higher in monsoon months followed by warm months. Study related to different indices shows that urban areas were more susceptible for transmission of filarial parasite. But vector related indices from rural study area indicate that a favourable situation for transmission of filarial parasite exists also in that area and there is a possibility to create a grave situation in near future, as some rural areas of West Bengal are already considerably become endemic [6,[24][25][26]. So, appropriate preventive measures including vector control, developing awareness among the peoples about the disease and concerned vector should be taken immediately to stop the situation to become worse.