Malaria and Man seem to have evolved together. It is believed that
most, if not all, of today's populations of human malaria may have had their
origin in West Africa (P. falciparum) and West and Central Africa (P.
vivax) on the basis of the presence of homozygous alleles for hemoglobin C
and RBC Duffy negativity that confer protection against P.
falciparum and P. vivax respectively. Recent molecular studies have found evidence that
human malaria parasites probably jumped onto humans from the great apes,
probably through the bites of vector mosquitoes.
The ancestors of the malaria parasites have probably existed at
least half a billion years ago. Molecular genetic evidence strongly suggests
that the pre-parasitic ancestor for malaria parasite was a
choroplast-containing, free-living protozoan which became adapted to live in
the gut of a group of aquatic invertebrates. This single-celled organism
probably had obligate sexual reproduction, within the midgut lumen of a host
species. At some relatively early stage in their evolution, these
"premalaria parasites" acquired an asexual, intracellular form of
reproduction called schizogony and with this, the parasites greatly increased
their proliferative potential. (This schizogony in the RBCs of humans causes
the clinical manifestations of malaria). Among the invertebrates to which the
ancestors of the malaria parasites became adapted were probably aquatic insect
larvae, including those of early Dipterans, the taxonomic order to which
mosquitoes and other blood-sucking flies belong. These insects first appeared around
150 million to 200 million years ago. During or following this period, certain
lines of the ancestral malaria parasites achieved two-host life cycles which
were adapted to the blood-feeding habits of the insect hosts. In the 150
million years since the appearance of the early Diptera, many different lines
of malaria and malaria-like parasites evolved and radiated. The malaria
parasites of humans evolved on this line with alternate cycles between human
and the blood-feeding female Anopheles mosquito hosts. Fossil mosquitoes have been found in geological
strata 30 million years old.[1]
From Great Apes to Man: P. falciparum is found
to be very closely related to a malaria parasites of chimpanzees, P. reichenowi andthese
two are more closely related to the malaria parasites of birds than to those of
other mammals. The lineage of these parasites possibly occurred around
130 million years ago, nearly about the same time as the origin of the two-host
life cycle involving blood-feeding Dipterans and land vertebrates. The
separation of the lines that led to P.
falciparum and P. reichenowi probably
occurred only 4 million to 10 million years ago, overlapping the period in
which the human line diverged from that of the African great apes. Recent
phylogenetic analysis indicates that all extant P.
falciparum populations
originated from P.
reichenowi, likely by a single host transfer, occurring as early as 2–3
million years ago, or as recently as 10,000 years ago.[1,2] The modern, lethal
strains of P.
falciparum probably
emerged within the last 5,000– 10,000 years, after agriculture took roots in
Africa.[1]
P. falciparum probably
jumped from Gorillas: Different
studies have suggested that P.
falciparum malaria
probably jumped from great apes to man, probably by a single host transfer by
vector mosquitoes. While
earlier reports suggested the origin from chimpanzees[2] or bonobos[3], a new
study from central Africa points to Gorillas. A single-genome amplification
strategy to identify and characterize Plasmodium spp., DNA sequences in nearly 3,000 faecal samples from wild-living
apes from field sites throughout central Africa, found Plasmodium infection in
chimpanzees (Pan troglodytes) and western gorillas (Gorilla gorilla),
but not in eastern gorillas (Gorilla beringei) or bonobos (Pan
paniscus). Ape plasmodial infections were highly prevalent, widely
distributed and almost always made up of mixed parasite species. Analysis of
more than 1,100 mitochondrial, apicoplast and nuclear gene sequences from
chimpanzees and gorillas revealed that 99% grouped within one of six
host-specific lineages representing distinct Plasmodium species within the subgenus Laverania. One of
these from western gorillas comprised parasites that were nearly identical to P. falciparum. In phylogenetic analyses of full-length
mitochondrial sequences, humanP. falciparum formed a monophyletic lineage within the gorilla parasite
radiation. These findings indicate that P.
falciparum is of
gorilla origin and not of chimpanzee, bonobo or ancient human origin.[4-8]
P. malariae, P. ovale, and P. vivax diverged
over 100 million years ago along the lineage of the mammalian malaria
parasites. P. ovale is the the sole known surviving representative of its line and
causes infection only in humans. P.
malariae was a
parasite of the ancestor of both humans and African great apes and had the
ability to parasitize and cross-infect both host lineages as they diverged
around five million years ago. P.
malariae is found as a natural parasite of chimpanzees in West Africa
and P. brazilianumthat
infects New World monkeys in Central and South America is morphologically
indistinguishable from P.
malariae. P.
malariae, like P. ovale, is the
only confirmed and extant representative of its line. P. vivax, closely
related to P. shwetzi, a parasite of African great apes, belongs to a group of
malaria parasites like P. cynomolgi, that infect monkeys. The time of divergence of P. vivax from P. cynomolgi is put at
2-3 million years ago.[1] Several cases of P.
knowlesi infection,
zoonotic from macaque monkeys, have been recently reported from Southeast Asia,
including Malaysia, Thailand, Viet Nam, Myanmar and Phillippines.[9-13]
Mosquitoes adapt: End of the
last glacial period and warmer global climate heralded the beginnings of agriculture
about 10000 years ago. It is argued that the entry of agricultural practice
into Africa was pivotal to the subsequent evolution and history of human
malaria. The Neolithic agrarian revolution, which is believed to have begun
about 8,000 years ago in the "Fertile Crescent," southern Turkey and
northeastern Iraq, reached the western and Central Africa around 4,000 to 5,000
years ago. This led to the adaptations in the Anopheles vectors of human malaria. The human populations in sub-Saharan
Africa changed from a low-density and mobile hunting and gathering life-style
to communal living in settlements cleared in the tropical forest. This new,
man-made environment led to an increase in the numbers and densities of humans
on the one hand and generated numerous small water collections close to the
human habitations on the other. This led to an increase in the mosquito
population and the mosquitoes in turn had large, stable, and accessible sources
of blood in the human population, leading to very high anthropophily and great
efficiency of the vectors of African malaria. Even though the practice of
agriculture had developed throughout the tropics and subtropics of Asia and the
Middle East up to several thousand years before those in
Africa, simultaneous animal domestication in Asia probably prevented the
mosquitoes from developing exclusive anthropophilic habits. In most parts of
the world, the anthropophilic index (the probability of a blood meal being on a
human) of the vectors of malaria is much less than 50% and often less than 10
to 20%, but in sub-Saharan Africa, it is 80 to almost 100%. This is probably
the most important single factor responsible for the stability and intensity of
malaria transmission in tropical Africa today.[1]
|
|
Spread of Malaria Disease
|
|
From its origin in the West and Central Africa, malaria spread all
across the globe to become the worst killer disease ever suffered by mankind.
The parasites spread to other areas through the journey of man, following the
human migrations to the Mediterranean, Mesopotamia, the Indian peninsula and
South-East Asia.[1] Although P. vivax and P.
malariaehad achieved the widest global distribution, today P. malariae has lost
its predominance and P. vivax and P.
falciparum are the
most commonly encountered malaria parasites. Almost 85% of the nearly 500
million annual malaria cases occur in sub-Saharan Africa and about 85% of cases
in Africa are caused by P.
falciparum, the remaining cases being caused by the other three strains. P. vivax is now the
most geographically widespread of the human malarias, estimated to account for
100-300 million clinical cases across much of Asia, Central and South America,
the Middle East, where 70–90% of the malaria burden is of this species and the
rest due to P.
falciparum.[1,14] P.
malariae causes
sporadic infections in Africa, parts of India, western Pacific and South
America, whereas P. ovale is restricted to tropical Africa, New Guinea, and the
Philippines.[1]
Malaria seems to have been known in China for almost 5,000 years.
(Men from ancient China, who traveled to malarious areas were advised to
arrange for their wives to be remarried). Sumerian and Egyptian texts dating
from 3,500 to 4,000 years ago mention about fevers and splenomegaly suggestive
of malaria. (The enlarged spleens of Egyptian mummies are believed to have been
caused by malaria). It appears that P.
falciparum had
reached India by around 3,000 years ago. It is believed that malaria reached
the shores of the Mediterranean Sea between 2,500 and 2,000 years ago and
northern Europe probably mainly between 1,000 and 500 years ago. The waves of
invasions that swept across the continents helped the cause of malaria parasite
as well. By the Middle Ages, Kings and feudal lords had the best wetlands under
their control, but in turn had to fear marshes as breeding grounds of plagues
and incurable fevers (The term ‘paludismo' comes from the Latin ‘Palus' for
lagoon). A royal decree was passed in 11th-century Valencia sentencing any
farmer to death who planted rice too close to villages and towns and the
conflict between rice growers and the authorities continued for centuries. The
disease continued spread and decimated local populations with the increase in
rice farming.
By the beginning of the Christian era, malaria was widespread
around the shores of the Mediterranean, in southern Europe, across the Arabian
peninsula and in Central, South, and Southeast Asia, China, Manchuria, Korea,
and Japan. Malaria probably began to spread into northern Europe in the Dark
and Middle Ages via France and Britain. The growth in international trade in
the sixteenth century contributed to the spread of disease, as international
traders introduced new sources of infection. Europeans and West Africans
introduced malaria in the New World at the end of 15th century AD. P. vivax and P. malariae were
possibly brought to the New World from South-East Asia by early trans-Pacific
voyages. P.
falciparum probably
reached the Americas through the African slaves brought by the Spanish
colonisers of Central America. At first the Caribbean and parts of Central and
South America were affected and from the mid-18th century, it spread across the
North American continent. Over the next 100 years, malaria spread across the
United States of America and Canada and by around 1850 A.D., it prevailed
through the length and breadth of the two American continents. At this time,
malaria was common in Italy, Greece, London, Versailles, Paris, Washington
D.C., and even New York City.
Thus by 19th century, malaria reached its global limits with over
one-half of the world's population at significant risk and 1 in 10 affected expected
to die from it. From the time of the voyages of Columbus until the mid-19th
century, European trade and colonization in the tropics were marked by enormous
losses of life from malaria. On the coasts of West Africa, mortality rates
often exceeding 50% of a company per year of contact were the norm. From the
mid-19th century onward, with the use of the Cinchona bark, mortality rates
fell rapidly to less than one-quarter of this. Up to early 20th century,
repeated untreated infections of P. vivaxand prolonged
infections of P.
malariae also
contributed significantly to the mortality along with the lethal P. falciparum. Poor living conditions, poverty and famine
probably contributed to the high mortality. During the past 100 years, nearly
150 million to 300 million people would have died from the effects of malaria,
accounting for 2-5% of all deaths. In the early part of the century, malaria
probably accounted for 10% of global deaths to malaria and in India it probably
accounted for over half.
By mid 20th century, the mortality started dropping, mainly as a
result of the spontaneous decline in contact between human and vector
populations as a result of improved living conditions as well as by the vector
control measures. By the early 1950s, malaria almost disappeared from North
America and from almost all of Europe. However, from the tropics where it is
endemic, it can spread across continents through the vectors (mosquitoes) and
the hosts (men) carried on the boats, trawlers, ships, jets and surface transport.
