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07 Feb 2008


Mass murderers of the insect world

Roddy Smith

here is unquestionably one creature in Africa that is in a league of its own when it comes to causing human deaths. The Anopheles mosquito is Hitler, Stalin and Mao all tied up in one on its own. This is of course a major concern to anyone who likes the bush, but as with every other potentially deadly animal there are a lot of misconceptions. So let’s look at this little mass murderer and its modus operandi.

Not every mosquito that bites is dangerous. The tiny ones that make you itch round the feet and ankles around sunset are probably Aedes. The bigger ones that drive you frantic in the small hours of the night by sounding like a Stuka diving into your ear (and which also give the itchiest bites) are probably Culex. Neither carries malaria.

The only carrier, and only under certain circumstances, is the female Anopheles mosquito (the male is a harmless vegetarian), a silent assassin and rather finicky in its habits. It only bites between dusk and dawn. It apparently doesn’t like coming indoors, it likes shallow puddles and is not usually found in deep, dirty or running water or swamps.

There are four species of malaria — Plasmodium falciparum is the only one that can kill (contrary to popular belief, “cerebral malaria” is not a different type) and unfortunately is also the most common.

If a non-immune person (that’s us) is bitten by a carrier mosquito, about 15-20 sporozoites are injected into his or her blood. These end up in the liver where they divide into thousands of little parasites. After five days the infected liver cells burst and these are washed out into the blood, where they look for a host red blood cell where they grow.

After two days (we’re now on day seven) these red cells become sticky and stick in the capillaries in deep organs while the parasites divide again and after a few hours the red cell bursts and another wave is released. This is repeated every two days, each time releasing about 10 times as many parasites as well as toxins from their excreta, which is what causes the fever, nausea and other malaria symptoms. This is why the symptoms come every two days with increasing severity.

Initially you may have a mild fever for a few hours. Two days later you will feel sick, have a fever and will experience vomiting but malaria may still not show up in tests.

In two more days you will have become very sick and now tests will show a clear positive. Another couple of days without treatment (12-14 days from the bite) and you could be in real trouble.

Once five percent of the red cells are infected they start sticking together and blocking capillaries. This can cause deep organ failure affecting the kidneys, lungs and brain resulting in death (this is what is commonly known as “cerebral malaria” — not a separate strain but normal falciparum which has gone untreated).

This time scale after five days applies to a non-immune adult from one infected bite — with more bites, or in a baby, it could happen more quickly.

People who are repeatedly infected and survive — usually because of treatment — can develop immunity. (If you were considering not taking the prophylatics so you can do this, don’t even think about it. You have to get sick several times a year for years on end.) In particular breast-fed children of immune mothers start with some immunity, which increases with each infection. Unfortunately, these people are the source of the disease. The mosquito can’t be infected by biting a non-immune person with malaria, only by biting an infected but immune person.

One of the big problems with malaria is identifying it, because there are no specific symptoms. Fever, nausea, headaches and joint pains can be due to other causes and many people are sometimes wrongly diagnosed with malaria. The problem with this is you are treated for malaria and don’t get better because you haven’t got it, so the doctor mutters about resistant strains and puts you on quinine which can have nasty side effects. In the mean time your gastro enteritis clears up and you are pronounced cured. So how do you know if you’ve got malaria?

Firstly, think about how long ago it was when you first arrived in a malarial area? If it was less than eight days, then it’s not malaria.

Secondly, are you taking a recommended prophylactic? If so, it’s possible but very unlikely.

Finally, take a test and take another one a day or two later to be sure. There are excellent self-administered rapid tests for falciparum on the market that are sensitive to parasite counts well below the danger level. But be warned: the test kits go out of date, they don’t like excessive heat and they show positive for some time after you’re cured (which at least confirms that you did have malaria).

The best treatment for malaria is Act — arthemether combination treatments. The arthemether kills most of the parasites in about four hours but itself only remains in the system a very short time. The longer-acting combination drug looks after any relapse due to parasites that are released later. What confuses people is that you feel a lot worse after taking the Act.

Remember that the symptoms result from the toxins released when the red cells burst and this is going to happen as the parasites die, which means that the drug is working.

Much of this information was gleaned from a series of articles in The Eye magazine by Dr Dick Stockley of Uganda.

• Roddy Smith works as a guide and conservationist in the lower Zambezi valley.

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