Malaria
About 219 million people suffer from malaria in over 100 countries
(Figure 10.2).
In 2010 there were an estimated 660 000 malaria deaths according to
the World Health Organization.
The disease is caused by a protozoan parasite called Plasmodium which
is transmitted from person to person by the bites of infected
mosquitoes of the genus Anopheles.
The mosquito is said to be the vector of the disease.
When a mosquito ‘bites’ a human, it inserts its sharp, pointed
mouthparts through the skin till they reach a capillary (Figure 10.3).
The mosquito then injects saliva, which stops the blood from clotting.
If the mosquito is infected, it will also inject hundreds of malarial
parasites.
The parasites reach the liver via the circulation and burrow into the
liver cells where they reproduce.
A week or two later, the daughter cells break out of the liver cells and
invade the red blood cells.
Here they reproduce rapidly and then escape from the original red cells
to invade others (Figure 10.4).
The cycle of reproduction in the red cells takes 2 or 3 days (depending
on the species of Plasmodium).
Each time the daughter plasmodia are released simultaneously from
thousands of red cells the patient experiences the symptoms of malaria.
These are chills accompanied by violent shivering, followed by a fever
and profuse sweating.
With so many red cells being destroyed, the patient will also become
anaemic.
If a mosquito sucks blood from an infected person, it will take up the
parasites in the red cells.
The parasites reproduce in the mosquito and finally invade the salivary
glands, ready to infect the next human.
Control
There are drugs which kill the parasites in the bloodstream but they do
not reach those in the liver.
The parasites in the liver may emerge at any time and start the cycle
again.
If these drugs are taken by a healthy person before entering a malarious
country, they kill any parasites as soon as they are injected. This is a
protective or prophylactic use of the drug.
Unfortunately there are now many mutant forms of Plasmodium that
have developed resistance to these drugs.
A great deal of work has been devoted to finding an effective vaccine,
without much success. Trials are currently taking place of a vaccine that
may offer at least partial protection against the disease.
The most far-reaching form of malarial control is based on the
elimination of the mosquito.
It is known that mosquitoes lay their eggs in stagnant water and that the
larvae hatch, feed and grow in the water, but have to come to the
surface to breathe air.
Spraying stagnant water with oil and insecticides suffocates or poisons
the larvae and pupae.
Spraying must include not only lakes and ponds but any accumulation of
fresh water that mosquitoes can reach, e.g. drains, gutters, tanks, tin
cans and old car tyres.
By draining swamps and turning sluggish rivers into swifter streams, the
breeding grounds of the mosquito are destroyed.
Spraying the walls of dwellings with chemicals like DDT was once very
effective because the insecticide remained active for several months and
the mosquito picked up a lethal dose merely by settling on the wall.
However, in at least 60 countries, many species of Anopheles have
developed resistance to these insecticides and this method of control is
now far less effective.
The emphasis has changed back to the removal of the mosquito’s
breeding grounds or the destruction of the larvae and pupae.
