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On
This Page:
What Is Epidemiology? | Factors That Determine The Occurrence
of Malaria | Climate | Anopheles Mosquitoes | Humans | Parasites | Areas
Where Malaria Is No Longer Endemic
What
Is Epidemiology?
Epidemiology:
1: A branch of medical science that deals with the incidence, distribution,
and control of disease in a population; 2: The sum of the factors
controlling the presence or absence of a disease or pathogen.
(From Merriam-Webster's Collegiate Dictionary, Tenth Edition,
Merriam-Webster, Incorporated, 1999.)
Epidemiology:
The study of the distribution and determinants of health-related
states or events in specified populations, and the application of
this study to control of health problems. (From: Last, JM: A Dictionary
of Epidemiology, Fourth Edition, Oxford University Press, 2001.)
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Factors That Determine The Occurrence of
Malaria
Factors that
determine the occurrence of malaria
are those that influence the three components of the malaria
life cycle:
- Anopheles
mosquitoes must be present, which are in contact with humans, and in
which the parasites can complete the "invertebrate host" half
of their life cycle
- Humans
must be present, who are in contact with Anopheles mosquitoes,
and in whom the parasites can complete the "vertebrate host"
half of their life cycle
- Malaria
parasites must be present.
Notes:
- In
rare cases malaria parasites can be transmitted from one person to another
without requiring passage through a mosquito (from mother to child in
"congenital malaria", or through transfusion, organ transplantation
or shared needles)
- Some
species of monkeys and apes can be experimentally infected with some
of the four "human" malaria parasite species, and in nature
some apes are infected with parasites similar to those infecting humans.
But for all practical purposes the role of an animal reservoir is negligible.
Climate
Climate
can influence all three components of the life cycle. It is thus a key
determinant in the geographic distribution
and the seasonality of malaria.
Rainfall can create collections of water ("breeding sites")
where Anopheles eggs are deposited, and larvae and pupae develop
into adulthood, a process that takes approximately 9-12 days in tropical
areas. Such breeding sites may dry up prematurely in the absence of further
rainfall, or conversely they can be flushed and destroyed by excessive
rains.
Once
adult mosquitoes have emerged, the ambient temperature, humidity, and
rains will determine their chances of survival. To transmit malaria successfully,
female Anopheles must survive long enough after they have become
infected (through a blood meal on an infected human) to allow the parasites
they now harbor to complete their growth cycle ("extrinsic"
cycle). That cycle takes 9-21 days at 25°C or 77°F. Warmer ambient
temperatures shorten the duration of the extrinsic cycle, thus increasing
the chances of transmission. Conversely, below a minimum ambient temperature
(15°C or 59°F for Plasmodium vivax, 20°C or 68°F
for P. falciparum), the extrinsic cycle cannot be completed and
malaria cannot be transmitted. This explains in part why malaria transmission
is greater in warmer areas of the globe (tropical and semitropical areas
and lower altitudes), particularly for P. falciparum. It has been
speculated that current trends of global warming may increase the geographic
range of malaria and may be responsible for malaria epidemics.
Climate
also determines human behaviors that may increase contact with Anopheles
mosquitoes between dusk and dawn, when the Anopheles are most
active. Hot weather may encourage people to sleep outdoors or discourage
them from using bed nets. During harvest seasons, agricultural workers
might sleep in the fields or nearby locales, without protection against
mosquito bites.
Anopheles
Mosquitoes
What
types (species) of Anopheles are present in an area at a given
time will influence the intensity of malaria transmission. Not all Anopheles
are equally good "vectors" for transmitting malaria from one
person to another. Some species are biologically unable to carry human
malaria parasites, while others are readily infected and produce large
numbers of sporozoites (the parasite stage that is infective to humans).
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The
anthropophilic Anopheles gambiae is an extremely effective
vector and is one of the reasons why malaria is so prevalent in
Africa.
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Different
Anopheles species may differ in selected behavior traits, with
important consequences on their abilities as malaria vectors. In some
species the females prefer to get their blood meals from humans ("anthropophilic")
while in others they prefer animals ("zoophilic"). Some species
prefer to bite indoors ("endophagic"), and others prefer outdoor
biting ("exophagic"). All other factors being equal, the anthropophilic,
endophagic species will have more frequent contacts with humans and thus
will be more effective malaria vectors.
Some species prefer to rest inside the dwellings where they have just
obtained their blood meals ("endophilic") while others prefer
to rest outdoors ("exophilic"). The endophilic species will
be more likely to acquire lethal doses of insecticides sprayed on the
walls of the dwellings (a malaria control measure called "indoor
residual spraying") while this will not be the case for the exophilic
species.
An
important biologic factor is insecticide resistance. If the mosquitoes
are resistant to the insecticide(s) used locally for spraying or for
treating bed nets, these measures will be ineffective in curtailing
transmission.
See
also: Anopheles Mosquitoes
Humans
Biologic
characteristics (inborn and acquired) and behavioral traits can influence
an individual's malaria risk and, on a larger scale, the intensity of
transmission in a population.
more:
Human Factors And Epidemiology of Malaria
Parasites
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Plasmodium
falciparum predominates in Africa south of the Sahara, one reason
why malaria is so severe in that area.
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Characteristics
of the malaria parasite can influence the occurrence of malaria and its
impact on human populations:
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Areas where P. falciparum predominates (such as Africa south
of the Sahara) will suffer more disease and death than areas where other
species predominate
- P.
vivax and P. ovale have stages ("hypnozoites")
that can remain dormant in the liver cells for extended periods of time
(months to years) before reactivating and invading the blood. Such relapses
can result in resumption of transmission after apparently successful
control efforts, or can introduce malaria in an area that was malaria-free
- Various
strains of parasites can occur within the same species. Variant strains
of P. falciparum are considered one cause for repeated episodes
of malaria because immunity built against one strain is not necessarily
protective against another
- P.
falciparum (and to a lesser extent P. vivax) have developed
strains that are resistant to antimalarial drugs. Such strains are not
uniformly distributed. Constant monitoring of the susceptibility of
these two parasite species to drugs used locally is critical to ensure
effective treatment and successful control efforts. Travelers to malaria-risk
areas should use for prevention only those drugs that will be protective
in the areas to be visited.
Areas
Where Malaria Is No Longer Endemic
Malaria
transmission has been eliminated in many countries of the world, including
the United States and countries of western Europe. However, cases of malaria
still occur in these countries, mostly in returning travelers or immigrants
("imported malaria"). All patients must be diagnosed and treated
promptly for their own benefit but also to prevent the reintroduction
of malaria.
more:
Areas Where Malaria Is No Longer Endemic
Page last modified : April 30,
2004
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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