Vector Ecology and behaviour

Behaviour of Aedes aegypti

Culex quinquefasciatus, the vector of bancroftian filariasis

Mansonioides, the vectors of Brugian filariasis

Vectors of Malaria

Behaviour of Aedes aegypti

Extensive surveys showed that Aedes aegypti, which normally breeds in containers was also found to breed in wells, coconut tree holes and septic tanks, indicating diversification of this species to newer habitats due to human pressure and the necessity for extending vector surveillance and change in control strategies.

Culex quinquefasciatus,the vector of bancroftian filariasis

Pondicherry agglomeration has been highly endemic for bancroftian filariasis. For planning effective vector control, information on bionomics and population dynamics of the vector Culex quinquefasciatus was lacking. Therefore, a detailed study was carried out on these aspects.

  • In the absence of extreme climate, environmental conditions such as breeding habitat and surface area were found to influence the population growth of the vector. Hence, it was recommended that an alteration in environmental conditions alone, like source reduction would cause considerable decrease in the population density. Larvicidal measure could be a supplementary.

  • Cx. quinquefasciatus was endophilic and highly anthropophagic with a human blood index of 0.98. Biting activity commenced after sunset and peaked between 2400 and 0100 hr.

These findings formed the basis for formulating an Integrated Vector Management approach to control bancroftian filariasis in Pondicherry.

Mansonioides, the vectors of Brugian filariasis

Ecological studies on Mansonioides were carried out in Chertala, Kerala, endemic for brugian filariasis to understand the vector adaptation to various host plants and develop bioenvironmental control strategy against the vectors.

  • Mansonia annulifera and Ma. uniformis were found to be the major vectors and Ma. indiana the secondary and seasonal vector.

  • Polluted ponds with Pistia stratiotes and mixed vegetation were found to be the major potential breeding habitats for the Mansonioides.

  • P. stratiotes was the most preferred plant by Ma. annulifera larvae whereas it was Eichhornia crassipes by Ma. uniformis and Ma. indiana. Survival rate was also maximum with these plants.

  • In addition to the most abundant and preferred host plants such as P. stratiotes, Salvinia molestes and E. crassipes, natural breeding of Mansonia species was observed with 16 other plants. Two indices viz., natural hospitability index (NHI) and Mansonia host plant index (MHI) were developed to rank host plants favouring mosquito breeding.

  • The larvae of Ma. annulifera showed frequent detachment and reattachment with the host plant. A periodicity phenomenon was observed in this mosquito species with more attachment during the early morning hours and more detachment during late night hours.

  • Vector density peaked during monsoon and post-monsoon period when extensive growth of host plants was noticed in the larval habitats.

  • It was recommended that effective control could be achieved if host plants are removed prior to the onset of monsoon. Since the host plants in the perennial habitats co-existed with no dominance over the other, control method should not be selective.

  • Daily emergence of the three mansonoides species was the maximum in October-November. Ma. annulifera and Ma. uniformis were recorded throughout the year while Ma. indiana was seasonal and in small numbers.

  • Mansonia annulifera was endophilic while Ma. uniformis was exophilic. Ma. indiana did not show a clear preference. Ma. annulifera and Ma. indiana were relatively more endophagic while Ma. uniformis was exophagic. All species showed a predilection towards cattle.

  • Mansonia annulifera bite throughout the night while Ma. uniformis bite predominantly during dusk hours.

Vectors of Malaria

The ecology of An. culicifacies was studied in the villages of Thenpennaiyar riverine tract, Tamil Nadu, where malaria transmission was persistent, despite vector control efforts by the Public Health Dept.

  • An. culicifacies, An. fluviatilis and An. varuna were found prevalent. Species A and B of An. culicifacies were incriminated as vectors in this area.

  • Outdoor resting behaviour of An. culicifacies was found to be one of the main reasons for the failure of the indoor residual spray.

  • Peri-domestic applications of ULV cold aerosol using technical malathion and antilarval measures were recommended to control malaria.

Extensive studies were also carried out on An. culicifacies, the vector in Rameswaram island, where malaria was endemic for decades. Indoor residual application of insecticides were not acceptable to the fishermen population in the island.

  • An. culicifacies was confirmed to be the only vector involved in transmission.

  • Perennial breeding habitats such as coconut and casuarina garden pits and wells, close proximity of breeding places and high preference of vector to human blood (HBI ; 0.63) due to scarcity of cattle were found to be important factors influencing continued transmission even when there was low vector density.

  • Water accumulation in low-lying areas during rains supported vector breeding during post-monsoon period that was responsible for increased incidence during this season. The island was stratified based on the distribution of breeding habitats and better targeting was suggested.

  • The vector was mainly endophilic and susceptible to malathion and it was suggested to continue ULV peridomestic application of malathion.

Malaria transmitted by An.stephensi was endemic in Salem town, Tamil Nadu and routine indoor residual spraying had no effect on transmission. Studies showed that

  • An.stephensi rest in wells and other inaccessible habitats.

  • The species bred mostly in wells and the population was found regulated by density dependent factors. The immature density peaked during March and April and it was suggested to carry out anti-larval measures in wells using temephos (50% EC, 0.01 ppm) at weekly intervals.

  • Since the vectorial capacity was higher during July- August due to high survival rate of adults, it was suggested to carry out pyrethrum space spraying for adult control during this period. Ultra low volume fogging with appropriate insecticide was also recommended as a supplement. These measures were effective in reducing malaria incidence.

  • Adults were found attracted more to violet and red lights than to white, blue, green and yellow. This information helped in improving the efficiency of traps designed to collect the adults.

The Jeypore hill tract of Koraput district is one of the present day hard core malarious areas in India, where involvement of multiple vectors in the transmission of malaria parasites was reported during the pre-DDT period. No information on bionomics and behaviour of the vectors was available for the post-DDT period to review or modify the ongoing malaria control programmes. Therefore, systematic studies on various aspects of bionomics and behaviour of the vectors were carried out in two of the different geoclimatic zones of Koraput district.

  • Two different behavioural population of An. fluviatilis were identified.

  • In one of the zones, Malkangiri, the vector was predominantly endophilic resting in human dwellings and endophagic.

  • In Jeypore zone, An. fluviatilis exhibited higher degree of exophilic and endophagic behaviour.

  • Host preference of An. fluviatilis differed in the two zones, the Human Blood Index being 0.82 in Malkangiri and 0.26 in Jeypore.

  • The vector rest indoors before and after feeding. Further, during cold season, the proportion of indoor resting population was relatively higher compared to the other two seasons. The residual spraying would be effective during this period. The indoor residual spray operation which is currently being undertaken can be continued with adequate coverage and the sprayed surfaces are retained without mud plastering.

  • Wall in the dwellings were the most preferred resting site followed by roof. The proportion resting on unsprayable surfaces was found to be much higher. Resting was also noticed on exterior of the walls during night. Since these populations may escape from the residual insecticides, alternate measures need to be carried out.

  • Biting occured throughout the night in all seasons and the maximum biting was seen between 22.00 and 02.00 h. However, in cold season the peak biting time was shifted to the early part of the night before the inhabitants retire to bed.

  • The modified version of the CDC light trap was shown to be an alternate sampling tool to hand catch in monitoring the relative density of An. fluviatilis.

  • Dispersal of An. fluviatilis was studied based on the distribution of adult females in outdoor habitats and immatures in terraced paddy fields at different distances. The maximum distance at which adults and immatures collected was 1500m and 1700m respectively. Information obtained in this study indicated the constraints in undertaking antilarval measures as well as outdoor adulticidal spray.

  • The phenotypic variations observed in 16 anophelines in hilly and forested areas in Koraput district indicated the process of microevolution underway in this area to overcome environmental stress brought about by deforestation and prolonged use of insecticides. In such situation, the morphological variations should be considered while identifying the type forms.

  • Further, during this season, majority of the residents sleeps near fire. Hence, use of insecticide impregnated bednets will have only limited impact in interrupting the transmission. In this situation, use of mosquito repellents may be advocated.

Last Updated on : 07/04/2014