Malaria is a
deadly infectious disease caused by the protozoa Plasmodium. An
infectious disease is caused by a pathogen and is referred to as communicable
as it is transferred from an infected to uninfected person. Malaria has four
pathogens: Plasmodium falciparum, P. vivax, P. ovale, P. malariae, which are
all transferred by the by the vector
(a vector is an organism which carries a disease from one person to another or
from an animal to a human): the female Anopheles mosquito. Malaria
affects 500 million and kills more than 1 million people each year. It is
endemic in more than 100 third world countries in tropical and subtropical
regions such as Sub-Saharan Africa, Asia and Latin America. It is maintained
there by transmission between people.
NB: Endemic means native or confined to a
certain place or constantly present to a greater or lesser extent in a
particular locality.
Transmission of Malaria
There are three ways in which
malaria can be transmitted:
1. The
female anopheles mosquito
2. Blood
transfusion and when unsterile needles are re-used
3. Movement
from the placenta into the fetus
When a mosquito
bites a person, blood taken by mosquitoes provides protein and
iron needed for formation of eggs. If the person is a carrier for malaria or is
infected with Plasmodium, the mosquito takes some of the gametes from the blood
meal. These male and female gametes fuse in the mosquito’s gut to form
infective stages, which move to the mosquito’s salivary glands. When she feeds
again, she injects an anticoagulant from her salivary glands that prevents the
blood meal from clotting so blood can flow freely from the human to the mosquito.
The infective stages pass from the mosquito’s salivary glands into the human’s
blood flow including the anticoagulant and enter the red blood cells where they
multiply.
NB:
“anti”- against, opposite
“coagulant” causes coagulation which is to thicken or to change from liquid to
a solid state. Hence, they are chemical substances that prevent or reduce
coagulation of blood
Plasmodium is carried in the bloodstream to the
liver, where they invade liver cells and reproduce asexually. At about eight
days after infection, the liver cells burst, releasing millions of Plasmodium
into the bloodstream where they invade the red blood cells.
Plasmodium reproduces asexually inside the red
blood cells. Within the red cells the parasites grow in a membrane-bound digestive
vacuole, hydrolyzing haemoglobin through secreted enzymes. Some parasites
develop into sexual forms called Gametocytes, which infect the mosquito. Plasmodium
makes you ill about 10-15 days after infection, when they burst out of the red
blood cells and release a chemical that causes fever. The Plasmodium is then
free to invade more red blood cells. A person may have many bouts of malaria
fever.
At each stage, there is a huge
increase the number of parasites, increasing the chances of infecting another mosquito
or human.
Symptoms of Malaria
Typically, victims
who are bitten by malaria-carrying mosquitoes experience no symptoms until 10
to 28 days after infection. The first clinical signs may be any combination of chills, fever, headache, muscle ache, nausea, vomiting, diarrhea, and abdominal cramps. Chills
and fever occur in periodic attacks; these last 4 to 10 hours and consist first
of a stage of shaking and chills, then a stage of fever and severe headache,
and finally a stage of profuse sweating during which the temperature drops back
to normal. Persons with malaria commonly have anaemia (owing to the destruction of
red blood cells by the parasites), enlargement of the spleen (the organ responsible for
ridding the body of degenerate red blood cells), and general weakness and
debility.
Treatment of Malaria
This is through the use of
anti-malarial drugs including quinine and chloroquine. These can be used as
prophylactic (preventative) drugs, stopping an infection from occurring if the
person is bitten. This is taken before or after visiting an area where malaria
is endemic. Chloroquine inhibits protein synthesis and prevents the parasite from
spreading. Proguanil inhibits sexual reproduction of Plasmodium within the biting mosquito.
When these drugs are used
widely, Plasmodium has become resistant.
An example is chloroquine in parts of South Africa, Africa and New
Guinea, leading to the introduction of mefloquine, which is rather expensive
and has some unpleasant side effects such as restlessness, dizziness, vomiting
and disturbed sleep.
Prevention of Malaria
The way in which a pathogen passes from one host to another
is called a transmission cycle. Control methods try to break this cycle by
removing conditions that are favourable for the spread of the pathogen. Prevention
and control methods include:
§ Stocking
ponds, irrigation and drainage of ditches and other permanent bodies of water
with fish with which feed on mosquito larvae.
§ Spraying
a preparation containing bacteria that kills mosquito larvae.
§ Sleeping
under mosquito nets to avoid being bitten by mosquitoes.
§ Spraying
oil or paraffin on stagnant bodies of water.
§ Use of
drugs to kill the parasite
§ Killing
adult mosquitoes with insecticides such as DDT
Worldwide Concerns
Malaria prevention programs are unsuccessful as:
§ Plasmodium
became resistant to drugs used to control it.
§ Mosquitoes become resistant to DDT and other
insecticides.
The reason for worldwide concern over malaria stems from:
§ An
increase in drug-resistant forms of Plasmodium
§ An
increase in the proportion of cases caused by P. falciparum, the form that causes severe, often fatal malaria.
§ Difficulties
in developing vaccines against malaria
§ An
increase in the number of epidemics, caused by climatic and environmental
changes that favour the spread of mosquitoes.
§ Migration
of people from areas where malaria is endemic, for political and economic
reasons
Control methods now focus on working within health systems
to improve diagnosis, improve the supply of effective drugs and promote
appropriate methods to prevent transmission. Plasmodium genome has been
sequenced, which may lead to the development of effective vaccines. Drugs are
used in combinations to reduce the chances of drug resistance arising.
Three factors may lead to improvements in the control of
malaria:
§ Use of
modern techniques in gene sequencing and drug design
§ Development
of vaccines targeted against different stages of the parasites life cycle.
§ A
renewed international will to remove the burden of disease from the poorest
parts of the world, allied to generous donations from wealthy individuals and
foundations.
