Resting and feeding preferences of Anopheles stephensi in an urban setting, perennial for malaria

Our plain-English summary of Resting and feeding preferences of Anopheles stephensi in an urban setting, perennial for malaria, by S. Thomas, S. Ravishankaran, N.A. Justin, et al (Malaria Journal, 2017).

The study

• Authors: S. Thomas, S. Ravishankaran, N.A. Justin, A. Asokan, M.T. Mathai, N. Valecha, J. Montgomery, M.B. Thomas, A. Eapen
• Journal: Malaria Journal
• Published: 2017;16(1):111
• Cited by: 60 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: The Indian city of Chennai is endemic for malaria and the known local malaria vector is Anopheles stephensi. Plasmodium vivax is the predominant malaria parasite species, though Plasmodium falciparum is present at low levels. The urban ecotype of malaria prevails in Chennai with perennial transmission despite vector surveillance by the Urban Malaria Scheme (UMS) of the National Vector Borne Disease Control Programme (NVBDCP). Understanding the feeding and resting preferences, together with the transmission potential of adult vectors in the area is essential in effective planning and execution of improved vector control measures.

Methods: A yearlong survey was carried out in cattle sheds and human dwellings to check the resting, feeding preferences and transmission potential of An. stephensi. The gonotrophic status, age structure, resting and host seeking preferences were studied. The infection rate in An. stephensi and Anopheles subpictus were analysed by circumsporozoite ELISA (CS-ELISA).

Results: Adult vectors were found more frequently and at higher densities in cattle sheds than human dwellings. The overall Human Blood Index (HBI) was 0.009 indicating the vectors to be strongly zoophilic. Among the vectors collected from human dwellings, 94.2% were from thatched structures and the remaining 5.8% from tiled and asbestos structures. 57.75% of the dissected vectors were nulliparous whereas, 35.83% were monoparous and the rest 6.42% biparous. Sporozoite positivity rate was 0.55% (4/720) and 1.92% (1/52) for An. stephensi collected from cattle sheds and human dwellings, respectively. One adult An. subpictus (1/155) was also found to be infected with P. falciparum.

Conclusions: Control of the adult vector populations can be successful only by understanding the resting and feeding preferences. The present study indicates that adult vectors predominantly feed on cattle and cattle sheds are the preferred resting place, possibly due to easy availability of blood meal source and lack of any insecticide or repellent pressure. Hence targeting these resting sites with cost effective, socially acceptable intervention tools, together with effective larval source management to reduce vector breeding, could provide an improved integrated vector management strategy to help drive down malaria transmission and assist in India's plan to eliminate malaria by 2030.

In plain English

This article is a malaria study, not a study of asbestos; asbestos appears only as one roof type on buildings where mosquitoes were collected. In 2014, the authors surveyed 17 cattle sheds and made 245 surveys of human dwellings in Besant Nagar, Chennai, India, for the malaria mosquito Anopheles stephensi. The mosquitoes were found more often in cattle sheds. Of 52 An. stephensi collected from human dwellings, 49 came from thatched structures, 2 from tiled structures and 1 from an asbestos-roofed house. Of 548 blood meals tested, 518 were from cattle and five from humans (Human Blood Index 0.009). Of 772 An. stephensi tested, five carried malaria parasites: four of 720 from cattle sheds and one of 52 from human dwellings. One Anopheles subpictus mosquito carried P. falciparum. The article reports the number of An. subpictus tested as 135 in its results but as 155 in its abstract. The authors concluded that the mosquitoes feed mainly on cattle and rest in cattle sheds. They noted that their density and infection estimates may be too low because other resting places were not studied.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Yearlong field survey of mosquitoes (entomological study), with laboratory testing of blood meals, age structure and malaria parasite infection
• Who was studied: Malaria mosquitoes (mainly Anopheles stephensi, also An. subpictus) collected from 17 cattle sheds and in 245 surveys of human dwellings in Besant Nagar, Chennai, India, from January to December 2014. No people were studied.
• Exposure: No asbestos exposure was studied. Asbestos was only one of the roof types of the houses and cattle sheds where mosquitoes were collected. The other roof types were tiled, thatched and concrete.
• Main results:
  • Of 882 Anopheles mosquitoes collected from cattle sheds, 743 (84.2%) were An. stephensi
  • Of the 52 An. stephensi collected from human dwellings, 49 (94.23%) were from thatched structures, 2 (3.85%) from tiled structures and 1 (1.92%) from an asbestos-roofed house
  • Of 548 blood meals analysed, 518 (94.5%) were bovine and five were human (four of the human ones came from human dwellings). The origin of 25 was unknown. The Human Blood Index was 0.009.
  • About 200 females were dissected. Of these, 57.75% were nulliparous, 35.83% monoparous and 6.42% biparous. None had oocysts or sporozoites on dissection.
  • Of 772 An. stephensi tested by ELISA (720 from cattle sheds, 52 from households), five were infected. Four carried P. vivax (three from cattle sheds, one from a human dwelling) and one carried P. falciparum (from a cattle shed).
  • One An. subpictus was infected with P. falciparum. The results section gives 135 An. subpictus tested; the article's abstract gives 1/155.
• The authors' conclusion: The authors concluded that adult An. stephensi feed mainly on cattle and rest in cattle sheds, and that human dwellings other than thatched structures are not preferred resting sites
• Limitations the authors state:
  • Only human dwellings and cattle sheds were sampled, so vector density and sporozoite rate may be underestimated
  • Sample sizes were too small to draw conclusions about seasonality or links to microclimate

Topics

Medical terms in this study, explained in our Lexicon: Endemic.

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