The Bug
Malaria is an infection caused by protozoa of the genus
Plasmodium. Plasmodium parasites enter the
body through the saliva of female mosquitoes from the genus
Anopheles. These mosquitoes primarily inhabit the
tropical and subtropical parts of the world. Anopheles
mosquitoes have a life expectancy of 50 days. They bite before
laying eggs close to stagnant water, which they do every two
to seven days. During a bite, mosquito uses their saliva as
a local anaesthetic, so people do not sense a bite has occurred.
Mosquito saliva also has anti-coagulating properties to allow
the mosquito to feed for longer. If the mosquito is infected
with Plasmodium, thousands of these parasites will
be present in the saliva and enter the bloodstream of the
host.
The four known species of Plasmodium, which cause
malaria in humans, are P. falciparum, P. vivax,
P. ovale, and P. malariae. Malaria caused
by P. falciparum is referred to as malignant malaria,
since it usually results a more severe form of malaria. P.
vivax, P. ovale, and P. malariae cause
benign malaria, and while death from these parasites are not
as common, benign malaria still causes very serious symptoms.
The main transmission vector for malaria is the female mosquito
from the genus Anopheles. Thousands of Plasmodium
parasites in the saliva of infected mosquitoes are transferred
into the human bloodstream with a single bite. At this point,
the parasites are in the sporozoite stage, where they exist
as small elongated cells. Once in the bloodstream, the sporozoites
immediately enter the hepatic cells of the liver.
In the liver, sporozoites undergo multiple asexual fission
and transform into schizonts. These schizonts (containing
6 to 24 nuclei) enlarge and segment into smaller mononucleated
cells called merozoites. The infected hepatic cell eventually
lyses, releasing merozoites to either infect other liver cells
(continuing the preerythrocytic stage), or infect erythrocytes
(beginning the erythrocytic stage).
Once inside the erythrocyte, Plasmodium goes through
a process similar to the replication stages in the liver.
The merozoite enlarges into a uninucleate cell called a trophozoite.
The trophozoite’s nucleus then undergoes multiple asexual
fission to produce a schizont. The schizont divides and produces
mononucleated merozoites. Eventually, the erthrocyte lyses,
releasing merozoites into the bloodstream to infect other
erthrocytes.
The erythrocytic stage is cyclic and repeats itself every
36 to 72 hours, depending on the species of Plasmodium.
Sudden release of merozoites, toxins, and erythrocyte debris
into the bloodstream triggers the sudden onset of chills and
fever, which produces the cyclic paroxysms that are characteristic
of malaria. Many cases of P. falciparum will have
a continuous fever, instead of a periodic fever.
Sometimes merozoites will differentiate into gametocytes
which are not capable of rupturing the erythrocyte. When these
infected erythrocytes are ingested by mosquitoes during a
bite, they develop into female and male gametes. In the mosquito’s
gut, the infected erythrocytes lyse and the gametes fuse to
form diploid zygotes called ookinetes. The ookinete penetrates
the mosquito’s midgut wall and forms an oocyst on the
outer surface. The oocyst undergoes meiosis and forms sporozoites.
Once the oocyst ruptures, the sporozoites migrate to and penetrate
the salivary glands of the mosquito. At this point, the sporozoites
are present in the mosquito saliva, ready to enter the bloodstream
the next time the mosquito bites a human host.
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While mosquito bites are the most common mode of malaria
transmission, one can also get malaria from blood transfusions,
and contaminated needles. The risk of acquiring malaria from
blood transfusions is extremely low in countries where malaria
is not endemic, such as Canada and the United States. While
the risk is low, patients who develop a fever following blood
transfusions should be screened for malaria.
| Species |
Distribution |
| P. falciparum |
Africa, Haiti, New Guinea, Southeast
Asia, South America |
| P. vivax |
India, Pakistan, Bangladesh, Sri
Lanka, Central America |
| P. ovale |
Africa |
| P. malariae |
Sporadic distribution |
Environmental The
life cycles of Plasmodium as well at the Anopheles
mosquito are dependent on temperature. The optimal temperature
for Plasmodium replication within the mosquito is
27°C. Higher temperatures increase the number of times
female mosquitoes bite and lay eggs.
Precipitation and humidity are factors which affect the behaviour
of Anopheles mosquitoes. There must be a certain
level of precipitation in order to provide the stagnant water
pools for the female mosquito to lay her eggs. Too little
precipitation, and stagnant water can not be found. Too much
water, and the pools of water will be washed away, along with
the mosquito larvae. Anopheles mosquitoes are also
affected by humidity, as average mosquito life is shorten
at humidity levels below 60%.
Natural disasters, such as floods or earthquakes can change
the conditions of an area to promote malaria transmission.
Artificial factors such as the creation of war, dams, or irrigation
projects can change conditions to promote malaria transmission.
Malaria tends to be more frequent at elevations lower than
2000m, though global warming is increasing the altitudes where
malaria transmission can occur.
Travellers
Long-term travellers as well as frequent travellers to high
risk areas should consult their local heatlhcare provider
concerning the suggested use of chemoprophylaxis. Long-term
travellers should eventually seek the advice of a local healthcare
provider from that region who is familiar with malaria precautions
from that region.
When travelling, try to avoid outdoor activity from the hours
between sunset and sunrise. If possible, schedule your visits
to avoid the times of year when malaria transmission is highest.
When staying in high risk areas, seek accommodations that
are air-conditioned and well-screened to prevent mosquitoes
from entering.
Malaria and Pregnancy
Pregnant women are at risk of severe malaria and should
avoid visiting high risk areas if possible. During pregnancy,
the parasite will accumulate on the placenta and affect fetal
growth. Malaria infection during pregnancy may lead to maternal
anemia, abortion, stillbirth, prematurity, low birth weight,
and increased neonatal mortality.
Malaria and Children
Malaria infection in children may cause symptoms such as
fever, vomiting, headache, and flu-like symptoms. If left
untreated, malaria infection may eventually cause convulsions,
coma, permanent neurological damage, and/or death. Death can
occur quickly in children with malaria, due to lower red blood
cell mass.
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