Malaria

Malaria is a complex tropical disease caused by infection with one of several types of malaria parasites, known as Plasmodia.  Some of the types cause more serious illness than others.  Although these parasites can be found in many different animals, four types infect humans causing mild to severe illness.  They are called falciparum, vivax, ovale and malariae. A fifth type called knowlesi that is known to infect monkeys and was previously confused with the malariae type is now recognized as a serious infection in humans. The most dangerous of the strains is falciparum. 

The incubation period depends on the type of parasite.  For the falciparum strain, the incubation period is generally 7-14 days; for the vivax and ovale strains, 8-14 days; for the malariae strain, 7-30 days; for the knowlesi strain, 10-12 days.  Because of the somewhat lengthy incubation period for all strains, many travellers do not develop clinical illness until they return to their home country. Because the early manifestation of the infection is marked by spiking fevers, it is often difficult to distinguish malaria from other infectious diseases.  Where malaria is not commonly seen, physicians are not well versed in the diagnosis and treatment of this illness. 

Malaria parasites are found in many tropical and subtropical areas.  Malaria is no longer a problem in countries with temperate climates.  

Many of the parasites have developed resistance to commonly used anti-malaria medications, especially in Southeast Asia and sub-Saharan Africa. Recently, high levels of resistance to the most effective drug combination (artemisinin-mefloquine) for treating malaria has been found along the Thai-Myanmar border.

Humans are universally susceptible to malaria infection.  Persons who live in malarious regions and who are infected repeatedly, develop some degree of immunity and tolerance to the clinical effects of the disease.

It is often difficult to distinguish the initial symptoms of malaria from any other infectious disease that begins with fever.  Laboratory tests are essential to identify the presence of the parasites in the blood.

Falciparum malaria can cause a varied clinical picture, but in general, the patient develops sudden onset of fever, chills, sweats, cough, diarrhoea, difficulty breathing and headaches.  The severe form of the illness may include jaundice, shock, liver and kidney failure, acute inflammation of the brain, swelling of the brain, coma and death.  Prompt treatment is essential, even for mild cases, since the disease may progress suddenly and rapidly to severe consequences and death.

The vivax and ovale strains usually do not cause life threatening illness.  The illness is characterized by fatigue and a rising fever, followed by shaking chills, headache and nausea.  The fever ends with severe sweating.  After an interval of 1, 2 or 3 days, the fever cycle is repeated for a week or a month or longer.  Relapses occur over a period as long as 5 years.  Gradual development of immunity reduces the frequency of attacks. and a low grade life-long infection may occur without symptoms.

The knowlesi strain causes peak fevers every 24 hours. Although infection with this strain can be fatal, the outcome may also be quite variable, including self-cure or chronic illness.

Treatment depends on the type of malaria parasite.  Prompt treatment with appropriate anti-malarial medication is essential since delays in diagnosis and treatment are associated with increased mortality, especially for the falciparum strain.  Treatment has become more complicated for this strain due to the development of serious drug resistance.  There are, however, effective anti-malarial medications that should be taken under the care of a physician who is knowledgeable about malaria therapy.  

Malaria is most prevalent in the world's tropical areas where year-round transmission can occur. In other regions, such as sub-tropical and temperate areas, transmission may only occur during seasons that provide optimal climate to support the growth and development of the mosquito. Malaria is endemic (i.e., constantly occurring) in most of sub-Saharan Africa and New Guinea; in large areas of South Asia, Southeast Asia, Oceania, Haiti, Central and South America; and in parts of Mexico, the Dominican Republic, North Africa and the Middle East.

The knowlesi strain is found in the forested areas of many countries in Southeast Asia, especially where among farmers, hunters and loggers, or wherever humans, mosquitos and monkeys interact.

Local transmission can vary significantly depending on factors such as rainfall, temperature and altitude that affect the numbers of mosquitoes in the local area.