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What is malaria?

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VOL: 98, ISSUE: 04, PAGE NO: 34

Carolyn Driver, MSc, RGN, RM, RHV, FPCert, is an independent travel health specialist nurse and chair of the British Travel Health Association

Although malaria has been around for almost as long as man, its cause and method of transmission were established just over 100 years ago. Before that various theories abounded.

The name malaria derives from an earlier notion that the disease was connected with bad air or ‘mal-aria’, because it usually occurs close to stagnant water as that is where mosquitoes lay their eggs. Ronald Ross, a Scot based in Calcutta, established the mosquito connection in 1897 (Schlagenhauf, 2001).

Since then, however, the mosquito has continually developed resistance to the insecticides used in eradication programmes, while the causative organism, plasmodium, has developed resistance to the drugs used to prevent or treat the disease.

About 300 million cases of malaria are reported every year, resulting in more than a million deaths, mainly in Africa and mostly among children aged under five.

An effective malaria vaccine has yet to be developed, although at the time of going to press a randomised trial on a new vaccine had shown promising results (Bojang et al, 2001). In a joint project between the UK and the Gambia, 306 men took part in the year-long trial. Not only was the new vaccine safe and well-tolerated, but also after four doses it was found to be 47% effective in preventing Plasmodium falciparum from entering the red blood cells.

Although a larger trial is planned and the researchers are hoping that the vaccine will prove more effective in children, it will be many years before one suitable for use by business travellers and tourists is available. The researchers also intend to investigate various ways to improve the effectiveness of the vaccine.

The spread of malaria

Malaria is thought to have originated in Africa. Human exploration allowed it to spread around the globe and the popularity of worldwide travel is exposing an increasing number of UK business travellers and tourists to the risk of contracting the disease.

An estimated 10 million trips to ‘exotic’ locations are made from the UK each year (Walker, 1998). As a consequence of the downturn in travel to the USA after last year’s September 11 attacks on Washington and New York, more people are expected to travel to malaria risk areas.

Malaria is present at many exotic destinations and tourists need to be aware of the risk and how to protect themselves. The disease occurs in more than 100 countries and territories, and over 40% of the world’s population lives in a malaria risk area (US Department of Health and Human Services, 2000). It is endemic in parts of Central and South America, Haiti and the Dominican Republic, Africa, the Indian subcontinent, South-East Asia, the Middle East and Oceania.

Until recently, the number of cases diagnosed in the UK had remained constant at about 2,000 a year. However, during the latter half of 2000, the malaria reference laboratory noted an increase of 34% over the previous year (Behrens, 2001).

Most of the ‘extra’ cases originated in the Gambia, which has become a popular winter destination for British tourists. The increase may have been due to unseasonal rainfall in the Gambia that year, but it also highlights the fact that when tourists travel to such destinations they are vulnerable to endemic diseases.

What is malaria?

Malaria is caused by plasmodium, a microscopic protozoa that requires both a mosquito and a human host to complete its life cycle. There are many species of plasmodium, but only four affect humans:

- Plasmodium falciparum;

- Plasmodium ovale;

- Plasmodium vivax;

- Plasmodium malariae.

Female anopheline mosquitoes transmit the malaria parasite while feeding on human blood. The mosquitoes require a blood meal to produce eggs and generally feed between dusk and dawn. When they bite they inject an inoculum containing an anticoagulant into the bloodstream, preventing the blood from clotting and enabling them to feed.

The inoculum of an infected mosquito may contain 20-200 plasmodium sporozoites. Once in the bloodstream these find their way to hepatocytes in the liver within 30 minutes. Schizogony (asexual reproduction in sporozoa) occurs here over the next five to seven days in P. falciparum and within 30 days in the other species. The sporozoites invade the cells, where they undergo repeated nuclear divisions, producing many thousands of merozoites.

Cyst-like parasitic host cells known as pre-erythrocytic schizonts are formed, each of which contains up to 40,000 merozoites. The schizonts eventually rupture, releasing the merozoites into the circulation where they invade the red blood cells, feeding on their contents and growing to become trophozoites.

The mature trophozoites then undergo further shizogony, dividing into separate merozoites again. The red blood cells burst, releasing the merozoites back into the circulation, where they invade fresh blood cells. Some of the merozoites will ultimately develop into gametocytes (the sexual stage of the life cycle).

When another fertilised female anopheline mosquito bites, the gametocytes are drawn into her stomach, where sexual reproduction (sporogony) occurs. Oocysts then form in the mosquito’s mid-gut and eventually become sporozoites. These accumulate in the salivary glands ready to be injected into a new host during the mosquito’s next meal, and so the cycle begins again (Fig. 1).

Signs, symptoms and diagnosis

Malaria is asymptomatic until it enters the blood phase. Once the destruction of red blood cells starts, the patient may experience fever, chills, sweats, jaundice, headaches, myalgia, nausea and vomiting, and occasionally a cough. The symptoms are similar to many viral infections so diagnosis is often missed or delayed.

The blood phase is not usually reached until six or seven days after infection, so malaria can usually be eliminated as the cause if the person becomes ill less than a week after entering an endemic area.

A diagnosis can only be made after examining thick and thin blood films. A thick film screens a larger volume of blood and shows whether malaria infection is present. A thin film allows detailed microscopic examination to identify the type of parasite and degree of parasitaemia.

P. falciparum can cause rapid illness resulting in multiple organ failure and death, sometimes within as little as 24 hours. Early diagnosis and treatment is vital to prevent fatalities. The incubation periods for P. ovale and P. vivax can be much longer, because during the liver phase they form hypnozoites - dormant cells which can become active months and sometimes years after infection.

Until recently, preventive medication focused on drugs that act on the disease’s blood phase, which meant that hypnozoites are not always destroyed and malaria could develop after chemoprophylaxis has stopped. Tourists should therefore be encouraged to mention their travel history if they have a severe febrile illness up to two years after a trip. It is equally important that health care professionals who treat people with febrile illnesses enquire about their travel history.n

- For further information on malaria see the Wellcome Trust website at: www.wellcome.ac.uk

- Part two in next week’s issue of NT looks at the prevention and treatment of malaria

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