INTRODUCTION
Malaria is a complex and deadly disease that puts billions of people at risk in several countries and territories around the world, contributing to the cycle of poverty and limiting economic development. Several countries lose billions per year in direct losses (e.g. illness, treatment, premature death) and many times more than that in lost economic growth. Malaria is endemic throughout most of the tropics. Malaria imposes great socioeconomic burden on humanity. 1,2
Malaria continues to be one of the important public health problems in India. As per World Health Organization report 2011 to 2012, South-East Asian region bears the second largest burden of malaria (13%), being next to African region (81%). Among South-East Asian regions, India shares two-thirds of the burden (66%).3,4
As the second most populous country in the world, with a population exceeding one billion people, India's public health system faces several challenges including implementation of surveillance programs to accurately estimate and control the national malaria burden.5
ETIOLOGY
Malaria is a vector borne disease caused by one of the protozoan species of the genus Plasmodium, transmitted to humans by female Anopheles mosquitoes. Malaria in humans is caused by one of the following species of Plasmodium: P. falciparum, P. malariae, P. ovale, P. vivax and P. knowlesi.3,6 Malaria is transmitted via the bite of an infective female Anopheles mosquito, which occurs mainly between dusk and dawn. Other comparatively rare mechanisms for transmission include: congenitally-acquired disease, blood transfusion, sharing of contaminated needles, and organ transplantation.4,6,72
The clinical features of malaria vary according to the species of parasite present, patient's state of immunity, the intensity of infection, presence of concomitant conditions like malnutrition and other diseases.8 Plasmodium falciparum is by far the deadliest of all the human malarial species.
EPIDEMIOLOGY
World Scenario
Malaria is a major international public health problem. About 3.4 billion people—half of the world's population—are at risk of malaria. In 2013, 97 countries had on-going malaria transmission. Despite the apparent progress in reducing the global prevalence of malaria, many areas still remain malaria endemic. Globally 207 million malaria cases have been reported in 2012 (uncertainty range: 135–287 million) and 627000 deaths have been reported globally attributed to malaria (uncertainty range: 473,000–789,000). Ninety percent of all malaria deaths occur in sub-Saharan Africa and 77 percent occur in children under five. 9,10 An estimated 2.5 billion people worldwide were at risk of P. falciparum malaria, in 201011 (Figs 1.1 to 1.3).
Malaria transmission occurs in large areas of Africa, Central and South America, parts of the Caribbean, Asia (including South-Asia, South-East Asia, and the Middle East), Eastern Europe, and the South Pacific (Figs 1.1 and 1.2).7
The malaria mortality rate was reduced in 2000 to 2012 globally by 45 percent. Fifty-two countries are on track to reduce their malaria case incidence rates by 75 percent, in line with World Health Assembly and Roll Back Malaria targets for 2015.
Fig. 1.1: Malaria endemic countries in eastern hemisphere (CDC) 7
Fig. 1.2: Malaria endemic countries in the Western Hemisphere (CDC) 7
However, these 52 countries only account for 4 percent of the total estimated malaria cases. Fifty-nine countries are on track to meet the Millennium Development Goal target of reversing the incidence of malaria (between 2000 and 2015).4
Figs 1.3A and B: The spatial distribution of Plasmodium falciparum malaria endemicity in 201011
Between 2000 and 2012, an estimated 3.3 million lives were saved as a result of a scale-up of malaria interventions. Ninety-percent of these lives saved are in the under-five age group, in Sub-Saharan Africa. 9,10
Indian Scenario
The malaria cases were brought down from 2,031,790 cases in 2000 to 1,816,569 cases in 2005 and further brought down to 1,067,824 cases in 2012. The annual incidence rate (cases of malaria/1000 population) of malaria has come down from 2.57 per thousand in 1990 to 1.10 per thousand in 2011, and to 0.88 cases per 1000 population in 2012. The malaria death rate in the country was0.09 deaths per lakh population in 2000 which has come down to 0.04 deaths per lakh population in 2012.12
Malaria was a major scourge in India contributing 75 million cases with about 0.8 million deaths annually, prior to the launching of the National Malaria Control Program (NMCP) in 1953. The implementation of NMCP resulted in a sharp decline in malaria cases in India and as a result the Government of India (GOI) converted the NMCP into the National Malaria Eradication Program (NMEP) in 1958. The NMEP was initially a great success with the malaria incidence dropping to a 0.1 million cases and no deaths due to malaria reported in 1965. 5The resurgence of malaria in the country resulted in escalation of incidence to 6.4 million cases in 1976. The resurgence was attributed to various operational, administrative and technical reasons, including emergence of drug resistance in the parasites and insecticide resistance in the vectors. In 1977, the Modified Plan of Operation (MPO) was implemented with the immediate objectives of preventing deaths due to malaria and reducing morbidity due to malaria. The national programme was also integrated with the primary health care delivery system. The malaria incidence declined to 1.66 million cases in 1987. By 1996, there was another malaria upsurge with 3.03 million cases and 2,803 deaths reported. Since the focus shifted from eradication to control, the programme was renamed as National Anti-Malaria Programme (NAMP) during year 1999.
With the convergence of responsibilities and strategies to prevent other vector borne diseases, NAMP was renamed as National Vector Borne Disease Control Programme (NVBDCP) in 2003. The NVBDCP is presently one of the most comprehensive and multi-faceted public health programmes in the country. The NVBDCP became an integral part of the NRHM launched in 2005. The special focus of the NVBDCP is on resource challenged settings and vulnerable groups. The incidence of malaria in the country started halting and sustaining reversal of cases for last one decade.
The total positive cases of malaria and deaths due to malaria have shown declining trend from 2011 and 2010 respectively. The indicators Annual Parasite Incidence (API) per 1000 population and deaths due to malaria are showing declining trend (Fig. 1.4) in the recent past and the challenge is to sustain that trend.12
As may be seen the annual incidence has been constantly declining, which reveals that the increasing trend of malaria incidence has already been halted and being reversed (Table 1.1).
At present, malaria control strategies are being implemented under the National Vector Borne Diseases Control Programme (NVBDCP). The strategies for control of malaria depend largely on the vector bionomics as well as the disease burden present in a particular area. NVBDCP has established different strategies based on different ecotypes of malaria. With the decline in the annual positive cases over a period of time, much progress has been made in the field of malaria. However, major challenges remain in the form of chloroquine-resistance, increasing proportion of Plasmodium falciparum cases in recent years and cases of fifth Plasmodium species (P. knowlesi) which need to be addressed at the earliest. As a result of these, major changes have been made in the strategic action plan for control of malaria. Future interventions in the form of malaria vaccine and development of genetically modified mosquito which is almost100 percent immune to malaria will be a boon for effective malaria control.36
Fig. 1.4: Declining trend of malaria in the countrySource: Ministry of Health and Family Welfare, India
Figs 1.5A and B: Parasite prevalence of Malaria and proportion of cases due to Plasmodium falciparum WHO Malaria Guidelines 20134
In India, the most common and deadliest species is P. falciparum (Figs 1.5A and B) contributing to 52 percent of the total malaria cases in 2010 which is closely followed by P. vivax. Other two species; P. malariae and P. ovale which contribute to <10 percent of the burden with very few cases being due to ovale species. Recently a fifth Plasmodium species, P. knowlesi which usually infects macaques has been identified and over the past few years hundreds of human cases have been reported in South and South-East Asian countries.3
Malaria distribution in India depends much upon vector bionomics. In India, Anopheles species have been described, six of which have been implicated to be main malaria vectors, namely: An. culicifacies, An. dirus, An. fluviatilis, An. minimus, An. sundaicus and An. stephensi. Besides, some are of local importance, viz. An. philippinensis-nivipes, An. varuna, An. annularis and An. jeyporiensis. The study of vector bionomics plays an important role in implementation of control strategies. As can be seen around 80 percent of malaria burden is confined to 20 percent of population residing in high-risk areas like Odisha, Jharkhand, Chhattisgarh, Madhya Pradesh, North-Eastern States except Sikkim, Maharashtra, Rajasthan and certain parts of Gujarat.38
Life Cycle of Malaria (Fig. 1.6)
The malaria parasite exhibits a complex life cycle. It undergoes 2 cycles of development—the human cycle (asexual cycle) and the mosquito cycle (sexual cycle). Man is the intermediate host and mosquito the definitive host.
Asexual cycle:
The infection is initiated when sporozoites are injected with the saliva of a feeding mosquito. Sporozoites are carried by the circulatory system to the liver and invade hepatocytes. The intracellular parasite undergoes an asexual replication known as exoerythrocytic schizogony producing shizonts within the hepatocyte. Exoerythrocytic schizogony culminates in the production of merozoites which are released into the bloodstream. A proportion of the liver-stage parasites from P. vivax and P. ovale go through a dormant period instead of immediately undergoing asexual replication. These hypnozoites will reactivate several weeks to months (or years) after the primary infection and are responsible for relapses. Once the merozoites are released into the blood stream, many of them are destroyed, but a significant number invade erythrocytes and form trophozoites and shizonts. Merozoites bud from the mature schizont and the merozoites are released following rupture of the infected erythrocyte.
Invasion of erythrocytes reinitiates another round of the blood-stage replicative cycle. The blood stage is responsible for the pathology associated with malaria. The intermittent fever paroxysms are due to the synchronous lysis of the infected erythrocytes. P. malariae exhibits a 72-hour periodicity, whereas the other three species exhibit 48 hours cycles. However, P. falciparum often exhibits a continuous fever rather than the periodic paroxysms. P. falciparum also is responsible for more morbidity and mortality than the other species. Some erythrocytic forms do not divide but become male and female gametocytes. These are the sexual forms of the parasite which are infective to mosquito.
Sexual cycle:
Sporogony begins when the gametocytes are ingested by the vector mosquito when feeding on an infected person. Microgametes, formed by a process known as exflagellation of the male gametocyte are flagellated forms which will fertilize the mature female gamete—the macrogamete leading to a zygote. The zygote develops into a motile ookinete which penetrates the gut epithelial cells and develops into an oocyst. The oocyst undergoes multiple rounds of replication resulting in the production of sporozoites within it. Rupture of the mature oocyst releases the sporozoites into the hemocoel (i.e. body cavity) of the mosquito. The sporozoites migrate to and invade the salivary glands, thus completing the life cycle and the mosquito now becomes infective to man (Fig. 1.6).6,8
REFERENCES
- Estimation of True Malaria Burden in India; A profile of National Institute of Malaria Research; www.mrcindia.org.
- Simon I Hay, Peter W Gething, Robert W Snow. India's Invisible Malaria Burden; Lancet. 2010;376(9754):1716–7.
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- WHO Malaria Guidelines 2013.
- Das A, et al. Malaria in India: The Center for the Study of Complex Malaria in India. Acta Tropica. 2012;121:267–73.
- Kliegman, Behrman, Jenson, Stanton. Nelson Text Book of Pediatrics, 18th edn., Chapter 285: Malaria (Plasmodium); Peter J. Krause.
- Infectious diseases related to travel; CDC; Chapter 3: Malaria; Paul Arguin, Kathrine Tan.
- Park K. Park's Textbook of Preventive and Social Medicine; 21st edn.; Chapter 5(111): Malaria.
- WHO Factsheet on the World Malaria Report 2013.
- WHO Roll Back Malaria Programme; www.rollbackmalaria.org.
- Gething P, et al. A new world malaria map: Plasmodium falciparum endemicity in 2010; Malaria Journal. 2011;10:378.
- Millenium Development Goals; India Country Report 2014.