 | Malaria | |
Malaria is caused by one of the four species of the protoctist Plamodium. The main features of malaria are summarised in the table below. Use the mouse rollover to reveal the hidden details that you should know!
| Feature | Description |
| Pathogen | > Plasmodium falciparum, P. vivax, P. ovale, P. malariae |
| Method of transmission | > Insect vector: female Anopheles mosquito |
| Global distribution | > Throughout the tropics and subtropics (91 countries) |
| Incubation period | > 1 week to 1 year |
| Site of action | > liver , red blood cells, brain |
| Clinical features | > fever, anaemia, nausea, headaches, muscle pain, shivering, sweating, enlarged spleen |
| Method of diagnosis | > microscopice analysis of blood |
| Worlwide annual incidence | > 300 million ( 90% in Africa) |
| Worldwide annual mortality | > 1.5 - 2.7 Million |
Distribution and Significance
The disease is well spread throughout the globe:
Malaria is of major worldwide significance, and is one of the biggest threats to health- 40% of the world's population live in areas where there is a risk of malaria. These risks are illustrated in the table below.
Spread
The disease is spread by an insect vector. Female Anopheles mosquitoes feed on human blood, and if the person is infected with Plasmodium, some of its gametes enter the mosquito where they fuse and develop in its gut beore multiplying. They reach an infective stage and move to the mosquito's salivary glands. When the mosquito feeds on a human, it injects an anticoagulant and the infective stage of the parasite enters with it. The parasites then enter red blood cells and liver cells where they multiply. For a diagrammatic representation of the infection cycle click here
Plasmodium may also be spread throught the use of unsterilised needles and through the placenta from mother to fetus.
Continually infected people may become immune, but only after around five years of exposure. Immunity only persits with continued exposure to the disease. This may explain why mortality is higher in areas not previously exposed to malaria.
- > reduce the number of mosquitoes
- > avoid being bitten
- > use drugs to prevent the parasite infecting people
The transmission cycle may be broken by killing the insect vector. Mosquitoes lay their eggs in water bodies, and the larvae breathe air at the surface. Spreading oil over the surface causes suffocation. Marshes may also be drained and the vegetation cleared. There wil always be suitable puddles left though.
- > stocking ponds and water bodies with fish that eat the mosquito larvae
- > spraying a preparation that contains the bacterium Bacillus thuringiensis which kills mosquito larvae but nothing else
The best protection is to > avoid being bitten.
Simple techniques include > sleeping beneath mosquito nets, using insecticide (and soaking your net in it), keeping skin covered at dusk. Check out your text book for a novel use of a pig in New Guineau!
Drugs and Chemical Control
Anti-malarial drugs can be used to treat people. Two examples are called > quinine and chloroquine. They can aslo be used as a preventive measure, the term for this is a > prophylactic. They have to be taken before, during and after a visit to an infected region.
Wide-use of drugs has lead to the evolution of drug-resistant strains of Plasmodium, and chloroquine resistance is common and widespread in many areas.
The WHO attempted a worldwide eradication program in the 1950's. It was generally unsuccessful. What are the main two reasons for this?
- > Plasmodium developed resitance to the drugs being used
- > Mosquitos became resistant to DDT nd other insecticides being used
People in areas where it was eradicated lost their resistance, and when it returned to the area many died as a consequence.
Reasons for Worldwide Concern There are 5 major reasons for the worldwide concern over the spread of malaria:
- > Increases in drug resiatant forms of Plasmodium
- > An increase in the incidence of cases caused by P falciparum- the form causing severe/fatal malaria
- > difficulties in developing a vaccine
- > An increase in the number of epidemics- possibly due to climatic and environmental changes that favour the spread of mosquito
- > Migration of people because of civil war and unrest
Solutions
Control methods focus on improving health systems to improve diagnosis, the supply of drugs and the promotion of methods to prevent transmission.New 'dip stick' diagnosis techniques speeds up the process, and doesn't require a lab. The Plasmodium genome is being sequenced and vaccines are under development. Treatment of the infected uses several drugs in combination to reduce the chances of drug resistance occurring.
