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Biology (Year 12) - Specific Disease Studies

Ben Whitten

What biological agent causes the disease? Malaria is caused by the protozoan parasites, belonging to the genus Plasmodium, residing in some female mosquitoes of the genus Anopheles. Reservoirs; where is the organism usually found? Humans are the main reservoir of Plasmodium.

Transmission; how is it commonly contracted between hosts? A female Anopheles mosquito ingests gametocytes from an infected human. The parasite must undergo 8–35 days of development within the mosquito before infective sporozoites are formed. The sporozoites are transmitted to another person via the bite of an infected mosquito. In rare cases, malaria parasites can be transmitted blood-to-blood from one person to another without requiring passage through a mosquito (from mother to child in ‘congenital malaria’ or through transfusion, organ transplantation or shared needles).

Typical pathology (signs, symptoms and progress) of disease The most prominent feature of malaria is fever. The classic symptoms of fever with a regular recurring pattern every 2 to 3 days is not usually present when the disease begins. Irregular fever also may occur due to mixed infections, ineffective use of prophylactic drugs and partial treatment. Patients commonly feel well on the days when fever is absent. A presumptive diagnosis of malaria should be made for any person with a high fever who has been to a malarious area, particularly when that travel was recent. Early diagnosis with prompt appropriate treatment is essential, as malaria can be a fatal disease. If the initial blood film is negative for malarial parasites, the test should be repeated within 12 to 24 hours and preferably when the person’s temperature is rising. One negative test does not exclude the diagnosis, particularly if the patient has taken antimalarials, which may result in partial treatment of the infection. Rapidly rising temperature is commonly associated with rigors (shaking chills), muscle pains, back pain, nausea and headache. The episode frequently ends with profuse sweating. Other symptoms may include confusion or other neurological signs, diarrhoea, dark urine, jaundice, cough and respiratory distress. Describe the life cycle of the pathogen (include generation time). Once an infected female mosquito bites an individual, saliva from the mosquito is injected into the human. This saliva carries the plasmodium parasite. Once bitten, in a non-immune individual, symptoms usually appear 10–15 days after the infective mosquito bite. The parasite travels towards the liver to infect a liver cell, and once in the liver cell, it reproduces till it then leaves the liver and enters the blood stream. Once in the bloodstream, the parasite locates red blood cells to infect. It reproduces until it mutates the red blood cell. Then it lyses, increasing the concentration of the parasite in the blood. Some of these will go on to become gametes, and the rest will just remain infected cells. Then, when the next mosquito comes and bites the same individual it will then be infected with the parasite. Once in the mosquito, the parasite heads to its saliva glands to reinfect someone else.

Are vectors involved? Yes. Malaria is transmitted to humans by female mosquitoes of the genus Anopheles. Female mosquitoes take blood meals for egg production, and these blood meals are the link between the human and the mosquito hosts in the parasite life cycle.

Pathogenicity of pathogen (disease causing ability) Plasmodium parasites in the form of sporozoites are injected into the bloodstream. The sporozoites then travel to the liver, multiplying asexually for the next 7-10 days. During this period, there are no symptoms present in the affected individual.

Where is the disease prevalent (geographically) and why? Malaria typically is found in warmer regions of the world — in tropical and subtropical countries. Higher temperatures allow the Anopheles mosquito to thrive. Malaria parasites, which grow and develop inside the mosquito, need warmth to complete their growth before they are mature enough to be transmitted to humans. Malaria occurs in more than 100 countries and territories. About half of the world's population is at risk. Large areas of Africa and South Asia and parts of Central and South America, the Caribbean, Southeast Asia, the Middle East, and Oceania are considered areas where malaria transmission occurs.

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