How Environment Influences Malaria Transmission Patterns in India
Table of Contents
Introduction
Malaria remains one of the world’s most persistent and deadly diseases, especially in tropical countries like India. This excerpt from a Zoology thesis provides a detailed examination of how environmental factors—biological, physical, and socio-economic—play a critical role in shaping malaria transmission.
Understanding these dynamics is essential for public health planners, researchers, and communities working on malaria control.
The post discusses the mosquito vectors, climatic influence, human immunity, and housing conditions affecting the disease’s spread. If you’re studying parasitology, epidemiology, or tropical medicine, this deep dive offers rich insights into the environmental science of malaria.
Excerpt
The Biological Environment
“Plasmodium, often referred to as the ‘beast within’ the female Anopheles mosquito, is a single celled parasite that has four species: falciparum, vivax, ovale, and malariae. In India, vivax dominates the malaria cases at 65 percent and falciparum occurs in the remaining cases (Prakash et al, 2003). Ovale is not found in India and malariae occurs in small numbers in the foot hills of the state of Orissa.
All malaria mortality in India is due to P. falciparum, if it is left untreated, can cause cerebral malaria. P. vivax may cause relapsing malaria but seldom death (WHO 2005).
Nearly 3000 species of mosquitoes of 34 genera belonging to family: Culicidae, Order: Diptera, Class: Insecta and Phylum: Arthropoda are present all over the world. The Anopheles genus has 422 identified species of which only about 70 species are considered to be main vectors of malaria in the world (Subbarao, 1998).
In India 58 species have been reported, of which only six are major malaria vectors.
The most prominent vector is An. culicifacies, responsible for transmitting two-thirds of the malaria in India. The other two vectors of importance are An. stephensi, accounting for about 12 percent of the total malaria cases in the country, followed by An. fluviatilis.
Andhra Pradesh, the state selected for this study, is a host mainly to An. culicifacies in rural areas and An. stephensi in urban areas.
An. fluviatilis, usually active in hills and foothills up to an altitude of 2,500 metres, An. stephensi, An. sundaicus are found in the coastal regions. An. culicifacies and An. philippinensis are found in the plains.
The feeding habits can be zoophilic (feeding on animals) or anthropophilic (feeding on humans) as well as endophagic (feed indoors) or exophagic (feed outdoors). The resting habits can be endophilic (rest indoors after blood meals) or exophilic (rest outdoors after blood meals).
MRC 2003, Srivastava et al., 2001, WHO 2005, Sharma 2002 found that An. culicifacies is primarily endophagic, indoor resting, and mainly zoophilic, but readily feeds on humans.
This species shows varied biting behaviour but usually feeds from dusk until before midnight. Rural settings are more conducive to its breeding. An. stephensi, found in urban areas, is also zoophilic but has increasingly shown a preference for human blood meals and is primarily endophagic and feeds from late night until very early hours of the morning. Kam and Harada (2002) also found a high density of An.
stephensi in toilets in slums and squatter settlements of Mumbai. Adak et al., 2005, Ansari and Razdan 2004 observed that An. fluviatilis is primarily anthropophilic and breeds in fresh water spring.”
The Physical Environment
“Climate and topography influence the malaria transmission by supporting the habitat of the mosquitoes. The three main factors that affect malaria transmission are Temperature, precipitation and relative humidity (Pampana 1969).
Piyaratne et al., (2005) found that An. culicifacies occurrence was higher in the temperature range of 28-32 degrees Celsius as this provides the optimum egg, larval and pupal development conditions for this species. However, for most mosquito species the ideal breeding temperature is between 25 and 27 degrees Celsius and the maximum survival temperature for the vector as well as the parasite is 40 degrees Celsius. The minimum temperature for mosquito development is as low as 8-10 degrees Celsius and that for parasite development ranges between 14 and 19 degrees Celsius.
The transmission of P. vivax requires a minimum temperature of 15 degrees Celsius whereas that of P. falciparum needs at least 19 degrees Celsius. Furthermore, the temperatures need to prevail over a certain period of time in order to complete sporogony (Patz et al., 1998, Sharma 2002).
The malaria vector can be found at altitudes between 1,600 m and 3,300 m, but these altitudes are generally not very conducive to the survival of the Plasmodium parasite because of lower temperatures at these elevations. There is a negative relation between altitude and Plasmodium falciparum incidence in children in Kenya (Drakeley et al., 2005). Sharma 2002, Malakooti et al., 1998 found that microclimatic influences in the form of indoor heating can promote the spread of malaria even at high altitudes.
Humidity is essential for promoting the breeding of mosquitoes, but extreme conditions of rain can restrict mosquito proliferation. Singh and Sharma (2002) as well as Akhtar and McMichael (1996) did not find a clear relationship between rainfall and malaria incidence in Madhya Pradesh (central India) and west Rajasthan in western India, respectively.
In India, low malaria incidence can persist even in the dry months of the year when average rainfall is recorded (Bhattacharya et al., 2006).
Topography such as type of forest cover and terrain are also important in determining whether a location is likely to be exposed to malaria. For example, An. dirus species is found in the evergreen forests of north-east India and moist deciduous forests such as those found in some districts of Kerala, Karnataka, and Tamil Nadu (Srivastava et al., 2001).”
The Host Environment
“Malaria incidence is affected by socio-economic factors, living conditions as well as the level of immunity of the human host in the host environment.
In highly endemic areas individuals develop immunity over time and act as passive carriers of gametocytes, that is, they may not show symptoms of malaria but carry gametocytes in their blood that can infect a mosquito taking a blood meal (WHO 2004).
The sickle-cell trait disorder is developed over generations of being subjected to malaria. While it provides protection against malaria if the gene is inherited only from one parent, it is fatal for children who inherit it from both parents.
In India the sickle-cell trait is found among those belonging to the lowest castes and in tribal populations.
The Glucose-6-phosphate dehydrogenase (G6PD) deficiency is also a hereditary enzyme defect that tends to break down red blood cells (haemolysis) when a person is exposed to infection and has been found in tribal populations in India. Ethnic groups carrying this trait are partly protected against malaria.
Sachs and Malaney 2002, Pant et al., 1992 found that ethnicity, by being associated with such characteristics, can be an important factor influencing the incidence of malaria across populations.
Certain blood group types can also carry immunity to malaria. On the basis of data collected from the relevant literature Arthreya and Coriell (1967) found a correlation between blood group B and malaria endemicity.
They found that persons with blood group B develop immunity over time and have better chances of resisting the malaria parasite. Thus, individuals with A and AB groups would be more susceptible to this organism than those with blood groups O and B.
Socio-economic factors such as the proximity of the house to breeding sites, type of house (i.e., material used for construction and whether the house has caves that allow entry of mosquitoes or has crevices in which mosquitoes can hide) affect the malaria incidence.
Living conditions such as failure to clean or dry residual water after using it for domestic purposes, whether household members work in irrigated fields, and if people have poor knowledge and understanding about the disease also influence the malarial incidence.”
Source Citation
Researcher: Ramakrishna, S
Thesis Title: Studies on certain malaria transmission dynamics in Visakhapatnam and Vizianagaram districts of Andhra Pradesh
Guide: Sandeep B V
University: Andhra University
Completed Date: 2012
Exit Page Number: 7
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