Distribution of Malaria by Age and Sex
The incidence of malaria among different age groups varies depending on the levels of endemicity. In hyper/holoendemic areas malaria incidence is mostly confined to young children. The prevalence of the disease in more or less evenly distributed among different age groups during epidemics in the zone of epidemic malaria. As regards sex distribution it has been observed that there is some difference in malaria rates between the two sexes: this difference is marked in age-groups 15 and above where there was malaria preponderance among males than females. Some explanations have been offered for this variation.
Vector mosquitoes for Malaria
There are nearly 400 species of Anopheles mosquitoes of which some 60 are proVen vectors of human malaria. However. to each geographic area there arc usually not inure than three or four anopheline species that can be regarded as important vectors. To be an effective vector a species must be present in adequate numbers, should have marked preference for human blood rather than animal blood, and must live long enough in order to enable the malaria parasite to complete its sexual cycle in the mosquito.
Some 19 anopheline species arc known to be primary or secondary vectors. of malaria in the countries of the South-East Asia Region. Of these, some species (A. cuhicifacies. A. stephensi„A. aconitus, A. fluviatile) exhibit major operational implications for disease control due to their resistance to one or more insecticides and some other species (A. rntnimus, A. balabacensis. A. sundateus) owing to their refractory behavior. In Bangladesh, A. Philippines. A dims. A. sundaicus . A aconitus, and A. maculatus (group), have been incriminated as vectors of malaria.
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Socio-economic factors for Malaria
A demographic explosion has resulted in (a) pressure on the agricultural land. and (b) migrationOrTiopulation to other areas. These have created favorable conditions for the transmission of malaria. Increased irrigation, mechanization of agriculture and extensive use of pesticides are essential requirements for intensification of agriculture (green revolution): the direct relationship between irrigation and malaria has been known to malariologists for a long time. Surface irrigation by canal system and mismanagement of water resources provide extensive breeding grounds for malaria vectors and prolong their longevity due to the melanogenic potential of the area. The seepage from the dikes and irrigation dam reservoirs results in perennial water flow which provides ideal conditions for vector breeding.
In recent years the phenomenon of the population has assumed greater significance in the epidemiology of malaria. The importation of gametocyte carriers to a receptive area or non-immune to a highly endemic area have been responsible for many epidemic outbreaks.— localized or large scale. In order to determine the appropriate control strategy for a given area, it is important to collect necessary information relevant to the particular risks ‘and epidemiological types.
Diagnosis of Malaria
The diagnosis of malaria is based on (a) clinical features and (b) laboratory investigation.
Clinical features: The natural history of malaria is characterized by an incubation period, prepatent period, primary attack (composed of paroxysms), latent period (parasitic latency), and recurrences (long-term relapses). Incubation period: in mosquito-transmitted natural infection this varies due to (a) quantum of inoculation, and (b) immune response of the host. This is an interval between the infective mosquito bite and the first appearance of clinical manifestation (fever).
Prolonged /protracted incubation period (3-10, months) in P. vivax has been reported from some parts of the world. The incubation period may be prolonged by prophylaxis, which is inadequate to destroy completely developing parasites. Prepatera period: This is the time from the infective mosquito bite to the first appearance of parasites in1 the blood. Generally, this is shorter than incubation period by 1-2 days: it may coincide with the incubation period in P. vivax. Primary attack: Each group of paroxysms or an isolated one is called’ an attack, and the first attack, after the termination of the incubation period, is called primary attack. This is– generally more severe than relapses.
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Laboratory Malaria diagnosis
The presence of malaria parasite in the blood is a sign of infection. but not necessarily a cause of malaria disease. A person living in a highly malarious area may have fewer parasites but may not have malaria disease. A non- immune person with a parasite count of 1000,000/mm3 or a semi-immune person with a parasite count of 250,000/ mm3 deserves serious attention. Malaria parasites are found in the blood: (a) during an attack (in falciparum malaria only gametocytes are seen in peripheral blood); in the terminal stage, all forms arc seen): (h) before clinical manifestation (fever), i.e., during its period. but density is low; (e) between attacks (during clinical. latency). In P. falciparum infection. parasites are usually present in the blood for a few hours after an attack/paroxysm and then becomes negative, whereas P. vivax and P. material infections always yield positive blood films. The microscopic examination of blood slides is the only method of malaria diagnosis available at the present time. Thick blood films are ideal for scanty parasitemia, whereas thin films are ideal for species identification. For staining. Giemsa is better in mass staining than JSB. Duration of examination is important; 100 microscopic fields should be examined in 3-5 minutes; in case of doubt 200 fields should be examined. Serocliynosis: The detection of aetiological agents of infection is sometimes difficult, costly and time-consuming: and the absence of patent parasitemia can be misleading since patency is influenced by I .ise of drugs and immune status of the patient. Serodiagnosis gives present and past experience/prevalence provides information. about transmission serves as a useful tool in epidemiological surveillance. The tests commonly employed are II IA. IFA and ELISA. DNA probe: it is a new approach to malaria diagnosis. Presently it is in the developmental stage. and the probe is a P. falciparum specific. Antigen detection test (dipstick) for quick diagnosis by peripheral health workers is now available.
The provision of early diagnosis and prompt treatment (disease management) is the fundamental element of malaria control. The difficulty of diagnosing malaria with certainty if blood slides cannot be promptly examined by a skilled microscopist makes it necessary to develop practical guidelines for the management of the patient at different levels of the primary health care system. Referral services should be able to diagnose similar microscopically, preferably at the first referral level. This is needed to identify parasite species, confirm clinical diagnosis— especially severe and complicated malaria or treatment failure malaria and to guide or monitor the management of patients.