Malaria is a disease, transmitted to humans through the bite of Anopheles mosquitoes, which causes infection of the liver and red blood cells by any one of four protozoan parasites from the genus Plasmodium. It is arguably one of the three most persistent, prevalent and devastating diseases to afflict humans (along with tuberculosis and AIDS) and is considered to be a risk for almost half of the world’s population. In has been estimated that at least 110 million people will develop the disease each year, with a further 280 million also carrying the parasite. Most recently in 2013, 198 million cases of infection were reported, resulting in approximately 554,000 deaths1 o of which more than 80 percent were children under five years of age.
Malaria occurs in over 100 countries but is typically restricted to the poorer tropical areas of Africa, Asia and Latin America. An intensive international effort aimed at both prevention and control of the foremost disease-carrying mosquitoes has resulted in a 54 percent reduction in mortality rates in Africa (and 47 percent globally). However, as the figures above demonstrate, there remains considerable need for ongoing preventative efforts.
Malaria is classed as either uncomplicated (classic) or severe (complicated); the symptoms experienced may vary according to the species of parasite and presence of comorbidities in the affected person. Incubation time of the parasite can also vary from five to 30 days and may also be prolonged if the person has been (or is) taking anti-malarial prophylactics.
Most people infected with uncomplicated malaria will develop mild to severe flu-like symptoms (fever, sweating, nausea, headaches and vomiting etc.) that last for six to 10 hours and progress in stages and symptoms over the course of several days.
Severe malaria occurs when infections are complicated by serious organ failures or abnormalities in the patient's blood or metabolism. Symptoms that manifest can be seizures, coma, severe anemia, acute respiratory distress syndrome (ARDS), acute kidney failure and ultimately death.
The Centres for Disease Control and Prevention recommend that severe malaria be treated as a medical emergency2.
Transmission of the malaria parasite is fundamentally simple. From mosquito to human, the parasite is transmitted via saliva injected as anticoagulant and anesthetic; from human to mosquito via presence in the host’s blood. However, the life cycle of the parasite is otherwise quite complex and involves several stages within both human and insect host in order to achieve such a degree of competence.
Briefly, a spore-like stage of the parasite is first injected into a human host in the saliva of an infected Anopheles mosquito. It then enters the liver where it invades cells and rapidly multiplies over the course of five to 16 days. Eventually, the parasite enters red blood cells and continues to grow in numbers. The onset of malaria symptoms normally corresponds to when the parasite begins leaving red blood cells. If a mosquito feeds on an infected person at this stage, the parasites are then carried into the insect’s gut where they mate and pass through the mid-gut of the mosquito. From this point, the spore-like stage will emerge between nine to 30 days later and subsequently invade the mosquito’s salivary glands, thereby restarting the cycle of transmission3 to a new host.
Once infected, a mosquito will remain infective for up to 12 weeks (effectively the term of its life) and thus is a capable vector via human-human transmission in regions where the parasite is endemic. Close to 70 different species of Anopheles mosquito have been associated with malaria transmission, with this number expected to increase as molecular analysis of the species identifies genetically distinct (but morphologically identical) ‘sibling’ species.
Outbreaks of malaria are extremely hard to control and require an integrated approach towards all aspects of the disease (both parasite and vector) through engagement with the affected community.
Prevention is best achieved through both short-term personal and community-wide chemoprophylaxis (anti-malarial drugs chosen according to the prevailing resistance profiles of the local parasites) and use of personal protective clothing, barriers and repellents. Targeted insecticide application around interiors and the use of mosquito bed nets will limit the risk of being bitten during the night, although mosquitoes have developed resistance to many modern insecticides and evidence is building of behavioral adaptations to insecticidal residues such as those used in nets.
Breeding holes and water sources may be chemically treated by commercial, government or non-governmental organizations, or by engagement with the local community, to physically fill or modify drainage to limit mosquito larval survival. Widespread application (e.g. aerial spraying, ground fogging) of insecticides is rarely successful in rural areas although it may have limited success in urban and peri-urban areas if appropriately planned and monitored.
REFERENCES AND FURTHER INFORMATIONhttp://www.niaid.nih.gov/topics/malaria/pages/lifecycle.aspx