Non-Climatic Factors and Malaria

            Although there have been few studies done on the relationship between climatic variables and malaria rates in India, many studies have addressed the ways that other factors have affected malaria rates.  Because these other factors often confound the effect of meteorological variables on malaria, it is important to understand them.

Urbanization

            India has been becoming more industrialized, and with industrialization comes urbanization.  Urban areas did not experience much malaria in India in the past and so no effort was put into controlling malaria in those areas.  In the 1990s, however, due to an increase in industrial growth, many forest areas where malaria is endemic have been cleared and those areas developed.  These areas become more easily accessed.  Migration of non-immune people creates an environment for epidemics.  An increasing urban population creates a large number of peri-urban areas on the outer limits of the city which now account for 25-40% of the Indian population (Sharma, 1996a).  These areas are unplanned and poor people live there in unsanitary conditions.  This creates the right environment for epidemics caused by increases in An. culicifacies which breeds in clean water on the ground due to rain and An. stephensi which breeds in wells and stored water.  An. stephensi has extended its distribution in India over the past four decades by entering more towns and peri-urban areas.  This spread in An. stephensi has been found to be related to the spread of piped water systems throughout the country (Sharma, 1996b).  Peri-urban malaria is a new malaria paradigm because migrants often have chronic malaria and the poor environmental conditions in their temporary settlements foster mosquito breeding and malaria transmission (Sethi et al., 1990).

Irrigation

            Irrigation has also increased tremendously in India from 26.8 million hectares in 1950 to more than 90 million hectares in 1995.  Irrigation was introduced by the green revolution in some areas in order to increase agricultural productivity to feed the growing population.  This irrigation was created by building a large number of dams and canals, which often caused seepage from canals and a rise in the water table, thus creating a source of still water in which malaria vectors could breed (Sharma, 1996b).  Irrigation can increase the transmission season of malaria, and in some parts of India, irrigation has changed areas from epidemic to endemic malarious regions (Sharma et al., 1994).  

            Until the 1980's, there were no malaria epidemics in the Thar desert despite significant rains at different times in the year since 1901.  A study of malaria in the Thar desert by Tyagi and Chaudhary (1997) found that increases in malaria were due to the "mismanagement of the widespread developmental activities of canal-based irrigation."  Starting in 1928, many different canal projects were carried out in this area in order to stimulate agricultural production through increased irrigation.  With more irrigation, more water-intensive crops were planted.  Tyagi and Chaudhary found that, due to seepage from the canals, 8600 hectares were permanently inundated, 1000 hectares were converted to marshy land, there was a rise in the water table, and hydroponic weeds, the preferred breeding grounds of An. culicifacies, increased in number.  The area used to be dominated by An. stephensi, which is a desert species, but now An. culicifacies has entered and dominates in the areas around the canals.  Both of these vectors increased their tolerance of DDT due to the incorrect use of insecticides.  This study also found that the slide positivity rate (SPR) and the slide falciparum rate (SfR) in the study area increased.  The location of both the canal project and the outbreaks of malaria were correlated.

            Another example of the relationship between irrigation and changes in malaria transmission comes from an area in Uttar Pradesh.  An. culicifacies took over for An. fluviatilis when irrigation was implemented in the region creating more problems since An. culicifacies is resistant to DDT and HCH in India (Sharma, 1996b).  Also, the Sardar Sarovar Project, an irrigation project on the Namada, which is intended to irrigate 1.8 million hectares in drought prone areas of Gujarat and Rajasthan, caused the invasion of An. culicifacies and An. fluviatilis, thus extending the malaria season and changing the area into an endemic malaria region and causing a ten to fifteen fold increase in malaria (Sharma, 1996b).

  Agricultural Practices

            In addition to increased irrigation, some agricultural practices, such as rice farming, also created large areas of stagnant water that are suitable breeding grounds for malaria vectors.  Rice fields in India provide breeding habitats for twenty Anopheline species (Sharma et al., 1994).  However, there are differing opinions about whether increases in rice cultivation area correlate with increases in malaria (Tyagi and Chaudhary, 1997).  A study of rice cultivation and annual parasite index (API) by Sharma et al. (1994) found that in some areas there was a statistically significant negative correlation, and in only three states (Punjab, Nagaland, and Meghalaya) was any positive statistically significant correlation between malaria rates and rice cultivation found.  The study also found low API rates in many places with large areas under rice cultivation, and in only one state was there high API and a large amount of area used for rice cultivation.  Sharma et al. (1994) found that there was not necessarily a correlation between an increase in rice cultivation area and an increase in malaria transmission, while at the same time admitting that more research is needed in order to better understand the relationship between these two factors because other studies have found such a relationship.

Deforestation

            India has experienced a large amount of deforestation because of increased needs from the land by a large and growing population.  In 1950, India had 40.48 million hectares of forest and in 1991, only 22.3 million hectares were left (Sharma, 1996b).  Forests are a reservoir of high levels of malaria in India where the majority of the people are from tribal groups that have lived there for a long time.  Currently malaria in the forests of India is stable with high transmission, and accounts for 30% of all malaria cases in India (Singh et al., 1996).  Most of these cases are caused by P. falciparum which is growing increasingly more resistant to chloroquine and, in certain locations, to other anti-malarial drugs (Pattanayak et al., 1994).  Mostly tribal groups live in the forest and have developed herd immunity to malaria.  Most of these populations are immune to malaria and therefore, even if a few people became infected, a large epidemic would not likely occur.  Deforestation of the area allows new vectors to invade the forest fringes (Sharma, 1996b), producing epidemics, especially in the non-tribal non-immune people who have moved to these areas because of the development projects that have caused the deforestation.  Some forest areas in India also experience moderate levels of chloroquine resistant P. falciparum (Pattanayak et al., 1994).

            A study comparing malaria in forested and deforested areas in the Mandla District in the state of Madhya Pradesh in central India, found that the patterns of malaria in the two areas were very different (Singh et al., 1996).  This supports the idea that changes in the ecology of an area can lead to changes in the malaria dynamics.  The study compared four forest villages with two villages along the road in the forest-fringe area.  The villages in the forests suffered more from P. falciparum infections, while the majority of infections in the villages outside the forest were caused by P. vivax.  The villages outside the forest also had a lower API than the villages in the forest and the pattern of malaria transmission throughout the year was different because of the density and seasonality of different vectors.  This study noted that there were many possible reasons for the differences in malaria transmission such as climatic factors related to the ecology of the forest that were more favorable to vectors in the forests, easier access to hospitals by people in the villages outside of the forest, or lack of schools or public health centers in the forests (Singh et al., 1996).

            Deforestation of jungle areas in Uttar Pradesh has also changed malaria in that region by changing relative parasite survival.  Clearing of the forest caused a reduction in humidity which shortened the longevity of the Anopheles vectors until they did not live long enough to successfully transmit P. malariae, which needs a longer time within the mosquito before it can be transmitted to the human host.  P. vivax and P. falciparum have increased and P. malariae has not returned to the area since (Sharma, 1996b).

            In the Malnad foothill areas in Karnataka, many different ecological changes have affected the malaria situation.  Economic pressure caused deforestation in order to create coffee plantations.  This caused increased malaria breeding sites because with less forest litter, water did not percolate through the soil as easily and therefore ran down the hills.  In addition, dams were built in the valleys of these hills, which trapped water.  The results of deforestation and the creation of dams eliminated the streams in which An. fluviatilis, the main vector in that area, had bred.  Therefore, An. fluviatilis and consequently malaria were eliminated from the area.  Although this ecological change eliminated malaria from the area, different zoonotic diseases entered because it became more arid and rodents entered, leading to the spread of other diseases such as Kysanore Forest Disease, which is spread by ticks (Sharma, 1996b).