Malaria

What is it?

Malaria is an infectious disease caused by Plasmodium parasites. The parasites are spread by the bite of infected female Anopheles mosquitos. There are four main types of Plasmodium (P) species which infect humans; the relapsing P. vivax and P. ovale, and the non-relapsing P. malariae and P. falciparum. Recently it has been recognised that a fifth species that normally infects macaques, P. knowlesi, can be naturally transmitted to humans. (Human Plasmodium knowlesi infections occur in young children in central Vietnam).

Rarely, transmission can also occur through congenital infection, blood transfusion and sharing of needles or syringes.

When a human is bitten by an infected mosquito the parasites enter the blood stream and travel to the liver. After infection there is usually an incubation period of 7-30 days, after which the parasites enter red blood cells and symptoms develop. Some people don’t develop symptoms for several months, particularly if they took inadequate doses of anti-malarial medication. P. vivax and P. ovale cause relapsing disease as the parasite can stay dormant in the liver, before re-entering the blood stream months, and even years, after the initial infection. Infection by P. falciparum can cause life-threatening disease, as can the newly recognised P knowlesi.

The World Health Organisation declared Australia malaria-free in 1981, however over 500 cases are diagnosed in Australia each year. These people contract the disease while travelling, most commonly in Papua New Guinea.

Most malaria infections and most malaria deaths occur in Africa. Malaria also exists in regions in Central and South America, parts of the Caribbean, Africa, Asia (including South Asia, Southeast Asia, and the Middle East), Eastern Europe and the South Pacific.

Worldwide, the World Health Organisation estimated there were 243 million cases of malaria and 863,000 deaths in 2008. The majority of deaths were caused by P. falciparum infections in African children.