Malaria

Epidemiology of Infectious Diseases: Malaria

Causal agent
Malaria in humans is caused by four different species of the protozoan parasite Plasmodium: Plasmodium falciparum, P. vivax, P. ovale and P. malariae.

Common clinical features
Malaria typically produces a string of recurrent attacks, each of which has 3 stages - chills, followed by fever and then sweating.

• Plasmodium. falciparum is responsible for most malaria deaths worldwide. It is most prevalent in sub-Saharan Africa and in certain areas of South East Asia and the Western Pacific.
• P. falciparum may present with a varied clinical picture, including one or more of the following, fever, chills, sweats, anorexia, nausea, lassitude, headache, muscle and joint pain, cough and diarrhoea¹.  If treated inadequately the disease may progress to severe malaria, of which the most important manifestations are; acute encephalopathy (cerebral malaria), severe anemia, icterus, renal failure, hypoglycaemia and respiratory distress¹. 
• Plasmodium vivax, is the most geographically widespread of the species. Once found in temperate climates, P. vivax is now found mostly in the tropics, especially throughout Asia.
• P. vivax produces less severe symptoms including, a slowly rising fever of several days duration followed by a shaking chill and rapidly rising temperature, commonly accompanied by headache, nausea and profuse sweating.  Following a fever free period this cycle of symptoms may recur daily, every other day or every third day. An untreated primary attack may last from a week to a month. Relapses can occur at irregular intervals for up to 5 years¹. 
• Plasmodium malariae, infections produce typical malaria symptoms and can persist in the blood for very long periods (possibly for life) without producing symptoms.
• Plasmodium ovale, is less common and generally occurs in West Africa. It can cause relapses.
• Pregnant women are particularly vulnerable to malaria as pregnancy reduces a woman's immunity to malaria, making her more susceptible to malaria infection and increasing the risk of illness, severe anaemia and death. For the unborn child, maternal malaria increases the risk of spontaneous abortion, stillbirth, premature delivery and low birth weight - a leading cause of child mortality³.
• Persons who are partially immune or who have been taking prophylactic drugs may show an atypical clinical picture and a prolonged incubation period¹.

Epidemiology

• Malaria is one of the most important public health problems worldwide. It is the leading cause of morbidity and mortality in many developing countries, where young children and pregnant women are most affected.
• As of 2004, 107 countries and territories have reported malaria transmission. It is estimated that worldwide approximately 3.2 billion people are at risk of malaria infection⁴.
• WHO estimate that 350-500 million clinical cases and more than 1 million deaths from malaria occur worldwide each year⁴.
• Malaria transmission is widely distributed and occurs in countries of Africa, Asia, Central and South America, the Middle East and Oceania. However, significant geographical variation in  morbidity and mortality exists between countries and regions where malaria transmission exists.
• An estimated 200,000 newborn deaths occur each year as a result of malaria infection during pregnancy³.
   
• Africa  - has the greatest burden of malaria cases and deaths worldwide. An estimated 60% of  all cases and over 80% of malaria deaths occur in sub Saharan Africa⁴.
• In sub-Saharan Africa, malaria is responsible for 1 in 5 of all childhood deaths.
• Most malaria infections in sub-Saharan Africa are caused by P. falciparum (93% ) of which the principal vectors are Anopheles gambiae and Anopheles funestus³.  Both efficient vectors.
• WHO reports that during the 1980s and early 1990s, malaria mortality in rural Africa increased considerably, probably as a result of increasing resistance to chloroquine.
   
• Asia - an estimated 49% of individuals in Asia are at risk of infection with malaria. Asia contributes an estimated 38% of the global burden of clinical malaria cases and 10% of the global mortality burden.
• Multidrug resistant malaria is now prevalent in many parts of the world with the highest incidence of drug and insecticide resistance reported in South East Asia.
• In India, urban malaria has emerged as a serious health problem in several states.
   
• In the Americas malaria transmission occurs in 9 countries that share the Amazon rainforest in South America (Bolivia, Brazil, Columbia, Ecuador, French Guiana, Guyana, Peru, Suriname and Venezuela, and 8 countries in Central America (Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama and Mexico and in Haiti and the Dominican Republic.
• The Americas contribute approximately 3% of all malaria cases worldwide and <1% mortality. Infection with Plasmodium vivax is responsible for the majority of cases (72%) in this region.
   
• Between 1979 and 2002, the total number of imported cases of malaria in the UK has remained steady at around 2000 cases and around 8 deaths per year⁵.
• However, over the last 10 years an increasing proportion of imported malaria in the UK has been shown to be due to P. falciparum (a large proportion of which occurred in West Africa)⁵.
• Approximately half of all imported malaria infections occur in travellers visiting friends and relatives. Most imported malaria cases occur in non-white ethnic groups particularly among people of African origin⁵.
• A large proportion of imported cases occur in travellers who have taken no chemoprophylaxis. A particularly low level of chemoprophylaxis use is seen in ethnic minority travellers⁵.
• In 2005 at total of 1,754 (76% due to P. falciparum) were reported in the UK.

Reservoir
Humans.

Mode of transmission

• Transmitted by various species of  infective female Anopheles mosquitoes. Most species feed and night; some important vectors have biting peaks at dusk or in the early morning¹.
• Injection or transfusion of contaminated blood may also transmit malaria.
• Congenital transmission is rare.
• The mosquitoes that can transmit malaria are found not only in malaria endemic areas, but are also found in areas where malaria has been eliminated. The latter areas are thus constantly at risk of re-introduction of the disease.

Incubation period

Some strains of P. vivax, may have an incubation period of 8-10 months or longer¹.

Transfusion associated malaria (rare) has a shorter incubation period².

Period of Communicability
Humans may infect mosquitoes as long as infective gametocytes are present in the blood.

Anopheles mosquitoes remain infective for life.

Prevention and control
As outlined by Bradley et al. the A, B, C, D of malaria prevention, is essential to prevent the risk of malaria infection among UK travellers^6,7.

References

1. Heymann D L, editor, Control of Communicable Disease Manual. 18^th ed. American Public Health Association, 2004.
    
2. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
    
3. World Health Organization, Malaria in Pregnancy, Fact Sheet 2003. Available online at; http://www.who.int/features/2003/04b/en/
    
4. Korenromp E, Miller J, Nahlen B, Wardlaw T, Young M, World Health Organization (WHO) Roll Back Malaria (RBM) Department and the United Nations Children's Fund (UNICEF),  World Malaria Report 2005, Geneva, World Health Organization 2005. available online at; http://www.rbm.who.int/wmr2005/html/toc.htm
    
5. Health Protection Agency. Foreign travel-associated illness. England, Wales and Northern Ireland - Annual Repot 2005. London: Health Protection Agency Centre for Infections; 2005. Available at; http://www.hpa.org.uk/infections/topics_az/travel/pdf/Baseline/full_version.pdf
    
6. Bradley DJ, Bannister B, Health Protection Agency Advisory Committee on Malaria Prevention for UK Travellers, Guidelines for malaria prevention in travellers from the United Kingdom for 2003, Communicable Disease and Public Health, 2003; 6(3): 180-99.  Available online at: http://www.hpa.org.uk/cdph/issues/CDPHvol6/No3/6(3)p180-99.pdf
    
7. Bradley DJ, Bannister B. Update to the Guidelines for malaria prevention in travellers for the United Kingdom for 2003. Available online at: http://www.hpa.org.uk/infections/topics_az/malaria/pdf/further_update_guidelines.pdf

Further Resources

World Health Organization - Roll back malaria programme http://www.rbm.who.int/

© CM Kirwan 2006