sickle cell anaemia

52

This article can be downloaded from http://www.ijlbpr.com/currentissue.php

Int. J. LifeSc. Bt & Pharm. Res. 2014 Otoikhian C S O et al., 2014

MALARIA RESISTANCE

AND SICKLE CELL TRAIT: A REVIEW

Otoikhian C S O1*, Osakwe A A2, Utieyin M C1 and Igue U B2

Review Article

*Corresponding Author:Otoikhian C S O � [email protected]

ISSN 2250-3137 www.ijlbpr.com

Vol. 3, No. 3, July 2014

© 2014 IJLBPR. All Rights Reserved

Int. J. LifeSc. Bt & Pharm. Res. 2014

1 Department of Biological Sciences, Novena University Ogume, Delta State Nigeria.

2 Department of Chemical Sciences, Novena University Ogume, Delta State Nigeria.

INTRODUCTION

Malaria is caused by an infection with protozoa

of the genus plasmodium. The name malaria,

from the Italian male aria, meaning “bad air”,

comes from the linkage suggested by Giovanni

Maria Lancisi (1717) of malaria with the poisonous

vapors of swamps. This species name comes

from the Latin falx, meaning “sickle”, and parere

meaning “to give birth”. The organism itself was

first seen by Laveran on November 6, 1880 at a

military hospital in Constantine, Algeria, when he

discovered a microgametocyte exflagellating.

Patrick Manson (1894) hypothesized that

mosquitoes could transmit malaria. This

hypothesis was experimentally confirmed

independently by Giovanni Battista Grassi and

Ronald Ross in 1898. Grassi (1900) proposed

an exerythrocytic stage in the life cycle, later

confirmed by short, Garnham et al. (1948), who

found plasmodium vivax in the human liver.

Around the world, malaria is the most

significant parasitic disease of humans, and

claims the lives of more children worldwide than

any other infections disease. Since 1900, the area

of the world exposed to malaria has been halved,

yet two billion more people are presently exposed.

Morbidity, as well as mortality, is substantial.

Infection rates in children in endemic areas are

of the order of 50%. Chronic infection has been

shown to reduce school scores by up to 15%.

Reduction in the incidence of malaria coincides

with increased economic output (Perkins et al.,

Sickle cell disease or sickle cell anaemia is an autosomal recessive disease caused byhaemoglobin S, an oxygen-carrying protein in blood cells. A single point mutation in the nucleobasesequence of chromosome 11 (Eleven) causes the sixth amino acid in the haemoglobin protein,glutamine acid, to be replaced by valine, changing standard haemoglobin beta into haemoglobinS. Translocation of sickle cell erythrocyte MicroRNAs into plasmodium falciparum inhibits parasitetranslation and contributes to malaria resistance. Elucidation of this mechanism can lead to abetter understanding of sickle cell trait protection against plasmodium falciparum infection.

Keywords: Malaria resistance, Sickle cell anaemia, Sixth amino acid

53



2011). While there are no effective vaccines for

any of the six or more species that cause human

malaria, drugs have been employed for centuries.

In 1640, Huan del Vego first employed the tincture

of the cinchona bark for treating malaria; the native

Indians of Peru and Ecuador had been using it

even earlier for treating fevers.

Thompson (1650) introduced this “Jesuits’

bark” to England. It is first recorded use there was

by Dr. John Metford of Northampton in 1656.

Mortan (1696) presented the first detailed

description of the clinical picture of malaria and

of its treatment with cinchona. Gize (1816) studied

the extraction of crystalline quinine from the

cinchona bark, and Pelletier and Caventou (1820)

in France extracted pure quinine alkaloids, which

they named quinine and Cinchonine (Rich et al.,

2009). Sickle cell disease, or sickle cell anaemia

is an autosomal recessive disease caused by

haemoglobin S, an oxygen-carrying protein in

blood cells. A single point mutation in the

nucleobase sequence of chromosome 11

(eleven) causes the sixth amino acid in the

haemoglobin protein, glutamic acid, to be replaced

by valine, changing standard haemoglobin beta

into haemoglobin S (National Institute of Health,

2009). The sickle shape is caused by

haemoglobins resulting structural change, and it

is from this structure that the disease gets its

name (Genetic Home Reference, 2012). People

with either HbAS (heterozygous with haemoglobin

A and haemoglobin S) or HbSS (homozygous with

haemoglobin S) are considered to have sickle cell

trait and can show symptoms of sickle cell

disease. However, when an individual is

homozygous recessive for haemoglobin S,

severe symptoms occur and the most infamous

being anemia, which can, in turn, cause shortness

of breath, jaundice, and hypertension. Individuals

heterozygous for the trait are not prone to the

same severity of symptoms as homozygous

recessive individual (Genetic Home Reference,

2012). Despite the disease’s lethal symptoms, it

protects the carrier from malaria, which is why

sickle cell alleles are not common in people of

African descent (about 7% of people of African

descent carry an allele), and other areas where

malaria is prevalent (Ashley et al., 2000). Sickle

cell trait is hypothesized to have evolved because

of the vital protection from malaria it provides

(Taylor et al., 2013). Furthermore, individuals with

HbAS tend to survive better than individuals with

HbSS as they are not exposed to the same

severity of risks yet still receive protection form

malaria (Ashley et al., 2000).

SICKLE CELL ANAEMIA

The first report of sickle cell anaemia may have

been in a report in 1846 where the autopsy report

of an executed runaway slave was discussed.

(Lebby, 1846). The author noted the curious

absence of a spleen in this case. In 1910 a

Chicago physician, James B Herrick, reported the

presence of sickle cells in the blood of an anemic

dental student, Walter Clement Noel. (Herrick,

1910). An association with pigmented gall stones

was noted in 1911 by Washborn. A genetic basis

for this disease was proposed in 1915 by Cook

and Meyer. The disease was named sickle cell

anaemia in 1922 by Verne Mason after several

additional cases were reported. All the known

cases had been reported in blacks and he

concluded that this disease was confined to those

of black African descent. The heterozygous

condition was independently recognized in 1923

by Huck and Syndestrickler. Syndestrickler also

was the first to note the splenic atrophy that occurs

in the condition it was recognized as a Mendelian

54



autosomal characteristic by Taliaffero and Huck

also in 1923. (Taliaffero et al., 1923). A

predisposition to pneumonia was noted in 1924

by Graham. The concept of progressive splenic

atrophy was proposed by Hahn and Gilespie in

1927. Pneumococcal meningitis in this condition

was first reported in 1928 by Wollstein and Kriedel,

but it was not until 1966 that the association

between splenic atrophy and infection was made

by Robinson and Watson.

In 1927 Vernon Hahn and Elizabeth Biermann

Gillespie showed that sickling of the red cells was

related to low oxygen. (Hahn et al., 1927). In some

individuals that change occurs at partial pressures

of O2 prevalent in the body, and produces anemia

and other disorders, termed sickle-cell disease.

In other persons sickling occurs only at very low

O2 partial pressures; these are asymptomatic

sickle-cell trait carriers. The association with

kidney and lung infarction was noted in 1931 by

Yater and Mollari and Baird in 1934, respectively.

The term sickle cell trait was coined by Samuel

Diggs in Memphis in 1933 to distinguish

heterozygotes from those with sickle cell

anaemia. Digs also reported the association with

splenic fibrosis in 1935. The pathological

mechanism of vaso-occlusion was proposed by

Ham and Castle in 1940. In 1946, EA Beet, a

British medical officer stationed in Southern

Rhodesia (Zimbabwe), observed that blood from

malaria patients who had sickle cell trait had

fewer malarial parasites than blood from patients

without the trait and suggested that this might be

a protective feature. In 1947 Beet published that

the incidence of enlarged spleens in sickle cell

patients was much lower than in non sickle cell

and suggested that this was due to recurrent

thromboses which resulted in fibrosis and

shrinkage of the spleen. In 1949 Lehmann and

Raper published a map of Uganda and showed

that the presence of sickle cell anaemia

correlated with the presence of malaria.

(Lechmann et al., 1946). In 1950 Singer et al.

noted the abrupt cessation of marrow activity that

may occur and coined the term aplastic crisis.

The role of parvovirus in aetiology of this condition

was not recognized until 1981. Brain also while

working in Northern Rhodesia confirmed the lower

incidence of splenomegaly and suggested that

while homozytotes for the sickle cell gene

suffered from several problems heterozygotes

might be protected against malaria. (Brain, 1952).

The modern phase of research on this disorder

was initiated by the famous chemist Linus Pauling

in 1949. Pauling postulated that the hemoglobin

(Hb) in sickle-cell disease is abnormal; when

deoxygenated it polymerizes into long, thin, helical

rods that distort the red cell into a sickle shape.

In his laboratory, electrophoretic studies showed

that sickle-cell Hb (S) is indeed abnormal, having

at physiological pH a lower negative charge than

normal adult human Hb (A) (Pauling et al., 1949).

In sickle-cell trait carriers there is a nearly equal

amount of HbA and HbS, whereas in persons with

sickle-cell disease nearly all the Hb is of the S type,

apart from a small amount of fetal Hb. These

observations showed that most patients with

sickle-cell disease are homozygous for the gene

encoding HbS, while trait carriers are heterozygous

for this gene. Persons inheriting a sickle-cell gene

and another mutant at the same locus, e.g., a

thalassemia gene, can also have a variant form of

sickle-cell disease. Pauling also introduced the term

“molecular disease”, which together with molecular

medicine, has become widely used.

55



The next major advance was the discovery by

Vernon Ingram in 1959 that HbS differs form HbA

by only a single amino-acid substitution in the β-

polypeptide chain (β6Glu Val). (Ingram, 1959). It

was later established that this results from a

substitution of thymine for adenine in the DNA

codon (GAG GTG). This was the first example in

any species of the effects of a mutation on a

protein. Sickle Hb induces the expression of heme

oxygenase-1 in hemoatopoietic cells. Carbon

monoside, a byproduct of heme catabolism by

heme oxygenase-1, prevents an accumulation of

circulating free heme after Plasmodium infection,

suppressing the pathogenesis of experimental

cerebral malaria (Ferreira et al., 2011). Other

mechanisms, such as enhanced tolerance to

disease mediated by HO-1 and reduced parasitic

growth due to translocation of host micro-RNA

into the parasite, have been described (Gong et

al., 2013).

BIOCHEMISTRY OF SICKLE

CELL TRAIT

Sickle haemoglobin (HbS) is the result of

substitution of valine for the normally occurring

glutamic acid residue at the 6th position of the β-

chain and so, is designated as: Hbβ6(A3) Glu-Val.

This substitution creates a sticky hydrophobic

contact point which is on the outer surface of the

molecule. These sticky spots cause

deoxygenated haemoglobin (Hb) molecules to

associated abnormally with each other to form

the long fibrous aggregates responsible for

sickling of the red cells. Deoxygenated HbS then

polymerizes and the polymers are in the form of

long needle-like structures that distort the cell.

This reduces the flexibility of the red cell making

it more mechanically fragile and as it passes

through capillaries, the cell sickles, decreasing

its life span and leading to anaemia.

DISTRIBUTION OF THE

SICKLE-CELL GENE

Since sickle-cell homozygotes are at a strong

selective disadvantage, while protection against

malaria favors the heterozygotes, it would be

expected that high frequencies of the HbS gene

would be found only in populations living in regions

where malaria transmission is intense, or was

so until the disease was eradicated. In a second

study conducted in 1953. Allison showed that this

was true in East Africa (Allison, 1954).

Frequencies of sickle-cell heterozygotes were 20-

40% in malarious areas, whereas they were very

low or zero in the highlands of Kenya, Uganda,

and Tanzania/ later studies by many investigators

filled in the picture (Allison, 2009). High frequencies

of the HbS gene are confined to a broad belt

across Central Africa, but excluding most of

Ethiopia and the East African highlands; this

corresponds closely to areas of malaria

transmission. Sickle-cell heterozygote

frequencies up to 20% also occur in pockets of

India and Grecce that were formerly highly

malarious. Tens of thousands of individuals have

been studied, and high frequencies of abnormal

hemolobins have not been found in any population

that was malaria free.

INDEPENDENT ORIGINS OF

THE SICKLE-CELL GENE

Two mutations found in nontranscribed

sequences of DNA adjacent to the β-globin

gene are so close to each other that the

likelihood of crossover is very small. Restriction

endonuclease digests of the β-globin gene cluster

56



have shown five different patterns associated with

the sickle-cell (GAG GTG) mutation. Four are

observed in Africa, the Bantu, Benin, Senegal and

Cameroon types, (Dunda et al., 1992), and a fifth

type is found in the India subcontinent and Arabia

(Labie et al., 1989). The cited authors report that

haplotype analysis in the β-globin region shows

strong linkage disequilibrium over the distance

indicated, which is evidence that the HbS mutation

occurred independently at least five times. The

high levels of AS in parts of Africa and India

resulted from independent selections occurring

in different populations living in malarious

environments. In summary, the demonstration that

sickle-cell heterozygotes have some degree of

protection against falciparum malaria was the first

example of genetically controlled innate

resistance to human malaria, as recognized by

experts on inherited factors affecting human

infectious diseases (Able et al., 2009). It was also

the first demonstration of Darwinian selection in

humans, as recognized by evolutionary biologists

(Carroll et al., 2009).

GLUCOSE-6-PHOSPHATE

DEHYDROGENASE

DEFICIENCY

Glucose-6-phosphate dehydrogenase (G6PD) is

an important enzyme in red cells, metabolizing

glucose through the pentose phosphate pathway

and maintaining a reducing environment. G6PD

is present in all human cells but is particularly

important to red blood cells. Since mature red

blood cells lack nuclei and cytoplasmic RNA, they

cannot synthesize new enzyme molecules to

replace genetically abnormal or ageing ones. All

proteins, including enzymes, have to last for the

entire lifetime of the red blood cell, which is

normally 120 days. In 1956 Alving and colleagues

showed that in some African Americans the

antimalarial drug primaquine induces hemolytic

anemia, and that those individuals have an

inherited deficiency of G6PD in erythrocytes.

(Alving et al., 1956). G6PD deficiency is sex

linked, and common in Mediterranean, African and

other populations. In Mediterranean countries

such individuals can develop a hemolytic diathesis

(favism) after consuming fava beans. G6PD

deficient persons are also sensitive to several

drugs in addition to primaquine. G6PD deficient

is the most common enzyme deficiency in

humans, estimated to affect some 400 million

people (Beutler, 2008). There are many mutations

at this locus, two of which attain frequencies of

20% or greater in African and Mediterranean

populations; these are termed the A- and Med

mutations (Verelli, 2004), mutant varieties of

G6PD can be more unstable than the naturally

occurring enzyme, so that their activity declines

more rapidly as red cells age.

MALARIA IN G6PD-

DEFICIENT SUBJECT

This question has been studied in isolated

populations where antimalarial drugs were not

used in Tanzania, East Africa (Allison et al., 1961).

And in the Republic of the Gambia, West Africa,

following children during the period when they are

most susceptible of falciparum malaria.

(Ruwende et al., 1995). In both cases parasite

counts were significantly lower in G6PD-deficient

persons than in those with normal red cell

enzymes. The association has also been studied

in individuals, which is possible because the

enzyme deficiency is sex-linked and female

heterozygotes are mosaics due to lionization,

where random inactivation of an X-chromosome

in certain cells creates a population of G6PD

57



deficient red blood cells coexisting with normal

red cell than in enzyme-deficient cells. An

evolutionary genetic analysis of malarial selection

on G6PD deficiency genes has been published

by Tishkoff and Verelli (Verelli, 2004). The enzyme

deficiency is common in many countries that are,

or were formerly, malarious, but not elsewhere.

DUFFY ANTIGEN RECEPTOR

The malaria parasite Plasmodium vivax is

estimated to infect 75 million people annually. P.

vivax has a wide distribution in tropical countries,

but is absent or rare in a large region in West and

Central Africa, as recently confirmed by PCR

species typing. (Mita et al., 2008). This gap in

distribution has been attributed to the lack of

expression of the Duffy Antigen Receptor for

Chemokines (DARC) on the red cells of many

sub-Saharan Africans. Duffy negative individuals

are homozygous for a DARC allele, carrying a

single nucleotide mutation (DARC 46T C), which

impairs promoter activity by disrupting a binding

site for the hGATA1 crythroid lineage transcription

factor (Collin et al., 1995). In widely cited in vitro

and in vivo studies, Miller et al reported that the

Duffy blood group is the receptor for P. vivax and

that the absence of the Duffy blood group on red

cells is the resistance factor to P. vivax in persons

of African descent. (Miller et al., 1976). This has

become a well-known example of innate

resistance to an infectious agent because of the

absence of a receptor for the agent on target cells.

However, observations have accumulated

showing that the original report needs

qualification. P. vivax can be transmitted in

Squirrel monkeys (Saimiri boliviensis and S.

sciureus), and Barnwell et al. (Barmwell et al.,

1989). Have obtained evidence that P. has an

alternative pathway for invading these cells. The

Duffy binding protein, the one and only invasion

ligand DARC, does not bind to Saimiri

erythrocytes although these cells express DARC

and obviously become infected with P. vivax

(Barmwell et al., 1989). The question is whether

these observations are relevant to naturally

occurring human transmission of P. vivax. Ryan

et al., presented evidence for the transmission of

P. vivax infections in Duffy antigen-negative

among a Duffy-negative population in Western

Kenya. (Amon et al., 2006). Independently,

Cavasini et al., have reported P. vivax infections

in Duffy antigen-negative individuals from the

Brazilian Amazon region (Cavasini et al., 2007).

P. vivax and Duffy antigen expression were

identified by genotypic and other methods. A

subsequent investigation in Madagascar has

extended these observations. (Mendes et al.,

2010). The Malagasy people in this island have

an admixture of Duffy-positive and Duffy-negative

people of diverse ethnic backgrounds. At eight

sentinel sites covering different parts of the island

72% of the population were Duffy-negative, as

shown by genotyping of flow cytometry. P. vivax

positivity was found in 8.8% of 476 asymptomatic

Duffy-negative people, and clinical P. vivax

malaria was found in 17 such persons.

Genotyping of polymorphic and microsatellite

markers suggested that multiple P. vivax strains

were invading the red cells of Duffy-negative

people. The authors suggest that among

Malagasy populations there are enough Duffy-

positive people to maintain mosquito transmission

and liver infection. From this internal source P.

vivax variants can develop, using receptors other

than Duffy to enter red cells and multiply. More

recently, Duffy negative individuals infected with

two different strains (VK247 and classic stains)

of P. vivax were found in Angola and Equatorial

58



Guinea; further, P. vavax infection were found

both in humans and mosquitoes, which means

that active transmission is occurring. This finding

reinforces the idea that this parasite is able to

use receptors other than Duffy to invade

erythroucytes, which may have an enormous

impact in P. vivax current distribution (Mendes et

al., 2011). Because of these several report from

different parts of the world it is clear that some

variants of P. vivax are being transmitted to

humans who are not expressing DARC on their

red cells. The frequency of such transmission is

still unknown. Identification of the parasite and

host molecules that allow Duffy-independent

invasion of human erythrocytes is an important

task for the future, because it may facilitate

vaccine development.

PLASMODIUM FALCIPARUM

Plasmodium falciparum is a protozoan parasite,

one of the species of Plasmodium that cause

malaria in humans. It is transmitted by the female

Anophetes mosquito. Malaria caused by this

species (also called malignant (Rich et al., 2009)

or falciparum (Perkins et al., 2011) malaria) is the

most dangerous form of malaria, (Perlmann et al.,

2000) with the highest rates of complication and

mortality. As of 2006, there were estimated 247

million human malarial infections (98% in Africa,

70% being 5 years or younger). (World malaria

Report, 2008). It is much more prevalent in sub-

Saharan Africa than in many other regions of the

world; in most African countries, over 75% of cases

were due to P. falciparum, whereas in most other

countries with malaria transmission, other less

virulent plasmodial species predominate. Almost

every malarial death is caused by P. falciparum

(World Malaria Report, 2008).

PLASMODIUM LIFE CYCLE

The life cycle of all Plasmodium species is

complex. Infection in humans begins with the bite

of an infected female Anopheles mosquito enter

the bloodstream during feeding quickly invading

liver cells (hepatocytes). Sporozoites are cleared

from the circulation within 30 min. During the next

14 days in the case of P. falciparum, the liver-

stage parasites differentiate and undergo asexual

multiplication, resulting in tens of thousands of

merozoites that burst from the hepatocyte.

Individuals merozoites invade red blood cells

(erythrocytes) and undergo an additional round

of multiplication, producing 12-16 merozoties

within a schizont. The length of this crythroucytic

stage of the parasite lifecycle depends on the

parasite species: irregular cycle for P. falciparum,

48 h for P. vivax and P. ovale, and 72 h for P.

malariae (Sylvie et al., 2008). The clinical

manifestations of malaria, fever, and chills are

associated with the synchronous rupture of the

infected crythrocytes. The released mcrozoites

go on to invade additional erythrocytes. Not all of

the mcrozoites divide into schizonts; some

differentiate into sexual forms, male and female

gametocytes. These gametocytes are taken up

by a female Anopheles mosquito during a blood

meal. Within the mosquito midgut, the male

gametocyte undergoes a rapid nuclear division,

producing eight flagellated microgametes that

fertilize the female macrogamete. The resulting

ookinete traverses the mosquito gut wall and

encysts on the exterior of the gut wall as an

oocyst. Soon, the oocyst ruptures, releasing

hundreds of sporozoites into the mosquito body

cavity, where they eventually migrate to the

mosquito salivary glands.

59



The life cycle of malaria parasite. A mosquito

causes infection by taking a blood meal. First,

sporozoites enter the bloodstream, and migrate

to the liver. They infect liver cells, where they

multiply into merozoites, rupture the liver cells,

and return to the bloodstream. Then, the

merozoites infect red blood cells, where they

develop into ring forms, trophozoites and

schizonts that in turn produce further merozoites.

Sexual forms are also produced, which, if taken

up by a mosquito, will infect the insect and

continue the life cycle.

PATHOGENESIS OF


Plasmodiuim falciparum causes severe malaria

via a distinctive property not shared by any other

human malaria, that of scquestration. Within the

48 h asexual blood stage cycle, the mature forms

change the surface properties of infected red

blood cells, causing them to stick to blood vessels

(a process called cytoadherence). This leads to

obstruction of the microcirculation and results in

dysfunction of multiple organs, typically the brain

in cerebral malaria (Dandorp et al., 2004).

THE PLASMODIUM

FALCIPARUM GENOME

In 1995, a consortium, the Malaria Genome

Project (MGP), was set up to sequence the

genome of P. flaciparum. The genome of its

mitochondrion was reported in 1995, that of the

nonphotosynthetic plastid known as the

apicoplast in 1996 (Wilson et al., 1996) and the

sequence of the first nuclear chromosome

(chromosome 2) in 1998. The sequence of

chromosome 3 was reported in 1999, and the

entire genome on October 3, 2002 (Gardner et

al., 2002). Annotated genome data can now be

fully analyzed at several database resources

including the Malaria Genome Browser,

PlasmoDB and GeneDB. The 24 megabase

genome is extremely AT-rich (80%) and is

organized into 14 chromosomes: Just over 5,300

genes were described.

P. falciparum and Sickle cell Anemia

Individuals with sickle-cell anemia or sickle-cell

trait do have reduced parasitermia when

compared to wild-type individuals for the

hemoglobin protein in red blood cells. Studies have

shown these genetic deviations of hemoglobin

from normal states provide protection against the

deadly parasite that causes malaria (Allison, b of

the six malarial parasites, P. falciparum causes

the most fatal and medically severe form. Malaria

is prevent in tropical countries with an incidence

of 300 million per year and a mortality rate of 1 to

2 million per year. Roughly 50% of all malarial

infections are caused by P. falciparum (Roberts

and Janovy Jr., 2005). Upon infection via a bite

from an infected Anopheles mosquito,

sporozoites devastate the human body by first

infecting the liver. While in the liver, sporozoites

undergo asexual development and merozoites

60



are released into the bloodstream. The

trophozoites further develop and reproduce by

invading red blood cells. During as reproduction

cycle, Plasmodium falciparum produces up to

40,000 merozoites in one day. Other blood

sporozoans, such as Plasmodium vivax,

Plasmodium ovale, and Plasmodium malaria,

that infect humans and cause malaria do not have

such a productive cycle for invasion. The process

of bursting red blood cells does not have any

symptoms, however destruction of the cells does

cause anemia, since the bone marrow cannot

compensate for the damage. When red blood

cells rupture, hemozoin wastes cause cytokine

release, chills, and then fever (Roberts and

Janovy Jr., 2005).

Plasmodiuim falciparum trophozoties develop

sticky knobs in red blood cells, which then adhere

to endothelial cells in blood vessels, thus evading

clearance in the spleen. The acquired adhesive

nature of the red blood cells may cause cerebral

malaria when sequestered cells prevent

oxygenation of the brain, symptoms of cerebral

malaria include impaired consciousness,

convulsions, neurological disorder, and coma

(Brown University). Additional complications from

Plasmodium falciparum induced malaria include

advanced immunosuppression (Roberts and

Janovy Jr., 2005).

Individuals with sickle-cell trait and sickle-cell

anemia are privileged because they have altered

sticky knobs. Research by Cholera et al. (2007)

has shown that parsitemia (the ability of a parasite

of infect) because merozoites of each parasite

species that cause malaria invade the red blood

cell in three stages: contact, attachment, and

endocytosis. Individuals suffering from sickle-cell

anemia have deformed red blood cells that

interfere with the attachment phase, and

Plasmodium falciparum and the other forms of

malaria have trouble with endocytosis.

These individuals have reduced attachment

when compared to red blood cells with the

normally functioning hemoglobin because of

differing protein interactions. In normal

circumstances, merozoites enter red blood cells

through two PfEMP-1 protein-dependent

interactions. These interactions promote the

malaria inflammatory response associated with

symptoms of chills and fever. When these

proteins are impaired, as in sickle-cell cases,

parasites cannot undergo cytoadhereance

interactions and cannot infect the cells; therefore,

sickle-cell anemic individuals and individuals

carrying the sickle-cell trait have lower parasite

loads and shorter time for symptoms than

individuals expressing normal red blood cells

(Mockenhaupt, 2004). Individuals with sickle-cell

anemia may also experience greatly reduced

symptoms of malaria because Plasmodium

falciparum trophozites cannot bind to hemoglobin

in order to form sticky knobs. Without knob-binding

complexes, which is an exclusive feature of

Plasmodium falciparum, red blood cells do not

stick to endothelial walls of blood vessels, and

infected individuals do not experience symptoms

such as cerebral malaria (Cholera et al., 2007).

Many may wonder why natural selection has not

phased out sickle-cell anemia. The answer lies

within answers generated by Cholera et al. (2007).

Individuals with sickle-cell trait are greatly desired

in areas where malarial infections are endemic.

Malaria kills between 1 and 2 million people per

year. It is the leading cause of death among

children in tropical regions. Individuals with sickle-

cell deformities are able to fight Plasmodium

61



parasite infections and do not become victims of

malarial demise. Therefore, individuals

expressing the genes and individual carrying

genes are selected to remain within the population

(Allison, 1964). It is no surprise that the incidence

of sickle-cell anemia malarial infections.

MECHANISMS OF MALARIAL

PROTECTION BY SICKLE

CELL GENES

During the peripheral blood stage of replication,

malaria parasites have a high rate of oxygen

consumption and ingest large amounts of

haemoglobin A (normal haemoglobin gene). Unlike

haemoglobin A, haemoglobin S (HbS) in endocytic

vesicles of the parasite is deoxygenated,

polymerizes an is poorly digested. In the digestive

process, the toxic portion of the haemoglobin

called haemin (ferriprotoporphyrin IX) is converted

to the non-toxic substance called haemozoin by

the parasite enzyme call malarial haem

polymerase enabling the parasite to survive.

However, HbS is poorly digested by the parasite

and as a result haemin accumulates thereby

inhibiting the replication and survival of the

parasite in HbS containing red blood cell (Vaidya

et al., 2009).

1. In red cells containing abnormal haemoglobin

(HbS) or which are glucose-6-phosphate

dehydrogenase (G6PD) deficient, oxygen

radicals are produced and malaria parasites

induce additional oxidative stress and this result

in changes in red cells membranes, including

translocation of phosphatidylserine to their

surface, followed by macrophage recognition

and ingestion. This mechanism occurs earlier

in abnormal (HbS) than in normal red cells

(HbA), thereby restricting the multiplication of

malaria parasite. (Elliott et al., 2008).

2. In addition, binding of parasitized sickle cells

to endothelial cells is significantly decreased

because of an altered display of plasmodium

falicparum erythrocyfe membrane protein – 1

Normal Heamoglobin 1A

Abnormal Process in M.P.

Normal Heamoglobin 2A

62



(PfMP-1). This protein is the parasite’s main

cytoadherence ligand and virulence factor on

the cell surface. During the late stages of

parasite replication red cells are adherent to

venous endothelium, an inhibiting this

attachment suppresses replication of the

parasite in HbS containing red blood cell (Foller

et al., 2009).

CURRENT RESEARCH

ISSUES ON MALARIA AND

SICKLE CELL GENES

Translocation of sickle cell Erythrocyte MicroRNAs

into plasmodium falciparum inhibits parasite

translation and contributes to malaria Resistance.

The study finds that individuals have three

microRNAs (miR-223, miR-451, let-7i) that are

effective in reducing Plasmodium falciparum

growth and replication, and the latter two are

increased in HbAS and HbSS individuals when

compared to HbAA individuals. The microRNAs

are transformed into the parasite and are inserted

Abnormal Heamoglobin 2B

PfMP–1 = Plasmodium FaciparumErythrocyte Membrace Protein – I

Abnormal Heamoglobin 3B

Percentage Showing the Resistanceof Malaria

63



into its mRNA in a method similar to trans-leader

trans-splicing, inhibiting translation and reducing

plasmodium falciparum growth. Therefore, HbAS

and HbSS individuals have a genetic advantage

over HbAA individuals

Known Vectors

• Anopheles gambiae (Principal vector) (Mbogo

et al., 2003)

these two species originated from a parasite of

birds (Rathore et al., 2001). More recent analyses

do not support this, however, instead suggesting

that the ability to parasitize mammals evolved only

once within the genus Plasmodium (Yotoko et al.,

2006).

New evidence based on analysis of more than

1,100 mitochondrial, apicoplastic, and nuclear

DNA sequences has suggested that Plasmodium

falciparum may in fact have speciated from a

lineage present in gorillas. (Liv et al., 2010; Dvva

et al., 2010; Rayner et al., 2011). According to

this theory, P. falciparum and P. falciparum and

P. reichenowi may both represent host switches

from an ancestral line that infected primarily

gorillas; P. falciparum went on to infect primarily

humans, while P. reichenowi specialized in

chimpanzees. The ongoing debate over the

evolutionary origin of Plasmodium falciparum will

likely be the focus of continuing genetic study

(Prugnolle et al., 2011). A third species that

appears to related to these two has been

discovered: Plasmodiium gaboni. This putative

species is currently (2009) known only from two

DNA sequences and awaits a full species

description before it can be regarded as valid.

Molecular clock analyses suggest that P.

falciparum is as old as the human line; the two

species diverged at the same time as humans

and chimpanzees. (Escalante et al., 1995).

However, low levels of polymorphism within the

P. falciparum genome suggest a much more

recent origin (Hartl et al, 2004). It may be that this

discrepancy exists because P. falciparum is old,

but its population recently underwent a great

expansion. (Joy et al., 2003). Some evidence still

indicates that P. reichenowi was the ancestor of

P. falciparum (Rich et al., 2009). The timing of

Current Research

• Anopheles albimanus

• Anopheles freeborni

• Anopheles maculates

• Anopheles stephensi

ORIGIN AND EVOLUTION OF


The closest relative of Plasmodium falciparum

is Plasmodium reichenowi, a parasite of

chimpanzees. P. falciparum and P. reichenowi are

not closely related to the other Plasmodium

species that parasite humans, or indeed

mammals in general. It has been argued that

64



this event is unclear at present but it has been

proposed that it may have occurred about 10,000

years ago.

More recently, P. falciparum has evolved in

response to human interventions. Most strains

of malaria can be treated with chloroquine, but P.

falciparum has developed resistance to this

treatment. A combination of quinine and

tetracycline has also been used, but there are

strains of P. falciparum that have grown resistant

to this treatment as well. Different stains of P.

falciparum have grown resistant to different

treatment. Often, the resistant of the strain

depends on where it was contracted. Many cases

of malaria that come from parts of the Caribbean

and west of the Panama Canal as well as the

Middle East and Egypt can often be treated with

chloroquine, since they have not yet developed

resistance. Nearly all cases contracted in Africa,

India and Southeast Asia have grown resistant to

this medication, and there have been cases in

Thailand and Cambodia in which the strain has

been resistant to nearly all treatment. Often the

strain grows resistant to the treatment in areas

where the use is not as tightly regulated. Like most

apicomplexans, malaria parasites harbor a

plastid, an apicoplast, similar to plant

chlorophalsts, which they probably acquired by

engulfing (or being invaded by)a cukaryotic alga,

and retaining the algal plastid as a distinctive

organelle encased within four membranes – (sce

endosymbiotic theory). The apicoplast, is an

essential organelle, thought to be involved into

the synthesis of lipids and several other

compounds, and it provides an attractive target

for antimalarial drug development, in particular in

light of the emergence of parasites resistant to

chloroquine and other existing antimalarial agents.

TREATMENT OF

FALCIPARUM MALARIA

Treatment of Uncomplicated FalciparumMalaria

According to WHO guidelines 2010 (WHO, 2010)

artemisinin-based combination therapies (ACTs)

are the recommended first line antimalarial

treatments for uncomplicated malaria caused by

P. falciparum. (WHO, 2010). The following ACTs

are recommended By the WHO (WHO, 2010).

• Artemether plus lumefantrine

• Artesunate plus amodiaquine

• Artesunate plus mefloquine

• Artesunate plus sulfadoxine-pyrimethamine

• Dihydroartemisinin plus piperarquine

The choice of ACT in a country or region will

be based on the level of resistance to the

combination. (WHO, 2010). Arteminsinin and its

derivatives should not be used as monotherapy

in uncomplicated Falciparum malaria. (WHO,

2010). As second-line antimalaria treatment, wheninitial treatment doesn’t work or stops working, itis recommended to use an alternative ACT knownto be effective in the region, such as:

• Artesunate plus tetracycline or doxycycline orclindamycin (WHO, 2010).

• Quinine plus tetracycline or doxycyline orclindamycin (WHO, 2010).

Any of these combination are to be given for 7days (WHO, 2010).

For pregnant women, the recommended first-line treatment during the first trimester is quinineplus clindamycin for 7 days (WHO, 2010).Artesunate plus clindamycin for 7 days is

65



indicated if this treatment fails (WHO, 2010). Still,

an ACT is indicated only if this is the only

treatment immediately available, or if treatment

with 7 days quinine plus clindamcin fails or if there

is uncertainty of compliance with a 7 day

treatment (WHO, 2010). In second and third

trimesters, the recommended treatment is an

ACT known to be effective in the country/region

or artesunate plus clindamycin for 7 days, or

quinine plus clindamycin for 7 days (WHO, 2010).

Lactating women should receive standard

antimalarial treatment (including ACTs) except for

dapsone, primaquine and tetracyclines. In infants

and young children, the recommended first-line

treatment is ACTs, with attention to accurate

dosing and ensuring the administered dose is

retained. (WHO, 2010). For travelers returning to

non-endemic countries, any of the following is

recommended (WHO, 2010).

• Atovaquone-proguanil

• Artemether-lumefantrine;

• Quinine plus doxycycline or clindamycin.

Treatment of Severe Falciparum Malaria

In severe falciparum malaria, it is recommended

that the rapid clinical assessment and

confirmation of the diagnosis be made, followed

by administration of full doses of parenteral

antimalarial treatment without delay with

whichever antimalarial is first available. (WHO,

2010). For adults, intravenous (IV) or

intramuscular (IM) artesunate is recommended

(WHO, 2010). Quinine is an acceptable

alternative if parenteral artesunate is not available

(WHO, 2010). For children, especially in the

malaria-endemic areas of Africa, any the following

antimalarial medicines is recommended (WHO,

2010).

• Artesunate IV or IM.

• Quinine (IV infusion or divided IM injection).

• Artemether IM. It should be used only if none

of the alternative are available, as its absorption

may be erratic (WHO, 2010).

Parenteral antimalarials should be

administered for a minimum of 24 h in the

treatment of severe malaria, irrespective of the

patient’s ability to tolerate oral medication earlier.

(WHO, 2010). Thereafter, it is recommended to

complete treatment by giving a complete course

of nay of the following. (WHO, 2010).

• An ACT

• Artesunate plus clindamycin or doxycycline

• Qunine plus clindamycin or doxycycline

If complete treatment of severe malaria is not

possible, it is recommended that patients be

given pre-referral treatment and referred

immediately to an appropriate facility treatment.

The following are options for pre-referral

treatment. (WHO, 2010).

• Rectal artesunate

• Quinine IM

• Artesunate IM

• Artemether IM

History of Falciparum Treatment

Attempts to make synthetic antimalarials began

in 1891. Altabrine, developed in 1933, was used

widely throughout the Pacific in World War II but

was deeply unpopular because of the yellowing

of the skin it caused. In the late 1930s, the

Germans developed chloroquine, which went into

use in the North African campaigns. Mao Zedong

encouraged Chinese scientists to find new

66



antimalarials after seeing the causalities in the

Vietnam War. Artemisinin was discovered in the

1980s based on a medicine described in China

in the year 340. This new drug became known to

Western scientists in the late 1980s and early

1990s and is now a standard treatment. In 1976,

P. falciparum was successfully cultured in vitro

for the first time, which facilitated the development

of new drugs substantially. (Trager et al., 1976).

A 2008 study published in the New England

Journal of Medicine highlighted the emergence

of artemisinin-resistant strains of P. falciparum

in Cambodia.

Vaccination

Although an antimalarial vaccine is urgently

needed, infected individuals never develop a

sterilizing (complete) immunity, making the

prospects for such a vaccine dim. The parasites

live inside cells, where they are largely hidden

from the immune response. Infection has a

profound effect on the immune system including

immune suppression. Dendritic cells suffer a

maturation defect following interaction with infect

erythrocytes directly adhere to and activate

peripheral blood B cells from nonimmune donors.

The var gene products, a group of highly

expressed surface antigens, bind in Fab and Fe

fragments of human immunoglobulins in a

fashion similar to protein A to Staphylococcus

aureus, which may offer some protection to the

parasite from the human immune system. Despite

the poor prospects for a fully protective vaccine,

it may be possible to develop a vaccine that would

that would reduce the severity of malaria for

children living in endemic areas. (Noedl et al.,

2008).

CONCLUSION

People with either HbAS or HbSS are considered

to have sickle cell trait and can show symptoms

of sickle cell disease. Recent findings have

showed that translocation of sickle cell erythrocyte

MicroRNAs into plasmodium falicparum inhibits

parasite translation and contributes to malaria

resistance, and that this mechanism is higher in

HbAS and HbSS individuals when compared to

HbAA individuals, thus, people with sickle cell trait

have malaria resistance than those without sickle

cell trait (HbAA).

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