Essay: Dengue Virus

Viruses are tiny agents that can infect a variety of living organisms, including bacteria, plants, and animals. Like other viruses, the dengue virus has a microscopic structure that replicate inside a host organism (Halstead, 2008). Dengue viruses belong to the genus flavivirus within the Flaviviridae family. DENV-1–4 evolved in non-human primates from a common ancestor and each entered the urban cycle independently an estimated 500–1,000 years ago (Walts et al., 1987). The virion comprises a spherical particle, 40–50 nm in diameter, with a lipopolysaccharide envelope. The positive single-strand RNA genome, which is approximately 11 kb in length, has a single open reading frame that encodes three structural proteins — the capsid (C), membrane (M) and envelope (E) glycoprotein’s — and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5). Important biological properties of dengue viruses, including receptor binding, haemagglutination of erythrocytes and the induction of neutralizing antibodies and the protective immune response, are associated with the E glycoprotein (WHO, 2007). Each DENV shares around 65% of the genome, which is approximately the same degree of genetic relatedness as West Nile virus shares with Japanese encephalitis virus. Despite these differences, each serotype causes nearly identical syndromes in humans and circulates in the same ecological niche (Walts et al., 1987).
The mosquito vectors, principally Aedes aegypti, become infected when they feed on humans during the usual five-day period of viraemia. The virus passes from the mosquito intestinal tract to the salivary glands after an extrinsic incubation period, a process that takes approximately 10 days and is most rapid at high ambient temperatures (Schneider et al., 2004). Mosquito bites after the extrinsic incubation period result in infection, which might be promoted by mosquito salivary proteins (Cummings et al., 2005). In the skin, dengue viruses infect immature dendritic cells through the non-specific receptor dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN). Infected dendritic cells mature and migrate to local or regional lymph nodes where they present viral antigens to T cells, initiating the cellular and humoral immune responses (Lee et al., 2006). There is also evidence of abundant replication of DENVs in liver parenchymal cells and in macrophages in lymph nodes, liver and spleen, as well as in peripheral blood monocytes (Walts et al., 1987). Both in vitro and in vivo, macrophages and monocytes participate in antibody-dependent enhancement (ADE). ADE occurs when mononuclear phagocytes are infected through their Fc receptors by immune complexes that form between DENVs and non-neutralizing antibodies (Guzman et al., 2000). These non-neutralizing antibodies result from previous heterotypic dengue infections or from low concentrations of dengue antibodies of maternal origin in infant sera. The co-circulation of four DENV serotypes in a given population might be augmented by the ADE phenomenon (Goncalvez et al., 2007).
DENVs produce several syndromes that are conditioned by age and immunological status. During initial dengue infections, most children experience subclinical infection or mild undifferentiated febrile syndromes (WHO, 2007). During secondary dengue infections the pathophysiology of the disease changes dramatically, particularly sequential infections in which infection with DENV-1 is followed by infection with DENV-2 or DENV-3, or infection with DENV-3 is followed by infection with DENV-2 (Sierra et al., 2007). Such infections can result in an acute vascular permeability syndrome known as dengue shock syndrome (DSS). The severity of DSS is age-dependent, with vascular leakage being most severe in young children (WHO, 2007), a phenomenon that is thought to be related to the intrinsic integrity of the capillaries. In adults, primary infections with each of the four DENV serotypes, particularly with DENV-1 and -3, often results in DF. Some outbreaks of primary DENV-2 infections have been predominantly subclinical. Nonetheless, dengue infections in adults are often accompanied by a tendency for bleeding that can lead to severe haemorrhages (Fernandez et al., 2004).
Dengue infections can be life-threatening when they occur in individuals with asthma, diabetes and other chronic diseases (WHO, 2007). Host factors that increase the risk of severe dengue disease include female sex, several human leukocyte antigen (HLA) class I alleles, a promoter variant of the DC-SIGN receptor gene, a single-nucleotide polymorphism in the tumour necrosis factor (TNF) gene and AB blood group (Guzman et al., 2000). Host factors that reduce the risk of severe disease during a second dengue infection include race, second or third degree malnutrition, and polymorphisms in the Fcγ receptor and vitamin D receptor genes. Secondary dengue infections in adults can produce the classical DSS or severe disease complicated by haemorrhages group (Guzman et al., 2000). The severity of secondary dengue infections has been observed to increase from month-to-month during island outbreaks; the longer the interval between the first and second infection the more severe is the accompanying disease. Tertiary dengue infections can cause severe disease, but only rarely (Schneider et al., 2004).
In vitro studies demonstrate that the infection of human monocytes and mature dendritic cells results in increased virus replication as a result of the suppression of the interferon system. Type I interferon-associated genes are less abundantly activated in peripheral blood mononuclear cells taken from patients with severe dengue disease compared with milder disease (Halstead, 2008). Subsequently, the increased number of infected cells present targets for CD4+ and CD8+ T cells, resulting in large quantities of interleukin (IL)-10, IL-2, interferon (IFN)-γ and TNF that, singly or in combination, might contribute to endothelial damage and altered haemostasis. Virions released from infected cells might also directly damage endothelial cells and the uptake of the non-structural protein NS1 by hepatocytes might promote viral infection of the liver (Lee et al., 2006). During DHF, the complement cascade is also activated and the levels of the complement activation products C3a and C5a correlate with the severity of illness. Soluble and membrane-associated NS1 have been demonstrated to activate human complement. The levels of the terminal SC5b–9 complement complex and plasma NS1 correlated with disease severity, suggesting links between the virus, complement activation and the development of DHF/DSS. Alternative hypotheses of dengue pathogenesis include the suggestions that secondary T-cell responses are blunted because stimulation of T-cell memory results in the production of heterotypic CD4+ and CD8+ cells that have a diminished capacity to kill but nonetheless release inflammatory cytokines that contribute to disease severity; that severe disease is caused by DENVs of increased virulence; and the suggestion that cross-reactivity between NS1 and human platelets and endothelial cells raises antibodies that damage these cells (Guzman et al., 2000).
One working hypothesis of dengue pathogenesis that is consistent with the available evidence is that severe disease in infants with primary infections and in older individuals with secondary infections is the result of ADE of infection of mononuclear phagocytes. Infection by an antibody–virus complex suppresses innate immune responses, increasing intracellular infection and generating inflammatory cytokines and chemokines that, collectively, result in enhanced disease. Liver infection and a pathogenic role for NS1 add to the complexity. In patients with DF, IFN production and activated natural killer cells can limit disease severity.
1.1 History of Dengue Fever
Dengue fever was firstly recorded in a Chinese medical encyclopedia (Jin Dynasty) between the years 265- 420 AD which is referred to a ‘water poison’ been associated with flying insects (Gubler, 1998). A. aegypti, which is the primary vector of dengue spread out of Africa in the 15th to 19th centuries. Although, there have been descriptions of epidemics in the 17th century, but the most plausible early reports of dengue epidemics are from 1779 and 1780, when an epidemic swept across Asia, Africa and North America from that time until 1940, epidemics were infrequent (Gubler, 1998).
Dengue was widely spread during and after the Second World War, which was attributed to ecological disruption. This same trend led to the spread of different serotypes of the disease to new areas, and emergence of dengue hemorrhagic fever. In 1953, the severe case of this disease was reported in the Philippines, and a major cause of child mortality by the 1970’s, which had also emerged to the Pacific and Americas (Gubler, 1998). Dengue hemorrhagic fever (DHF) and Dengue Shock Syndrome (DSS- Complicated case of dengue) were first noted in South and Central America in 1981 as DENV-2 was contracted by people who had previously been infected with DENV-1 several years earlier (Gould et al., 2008).
A question that has been commonly asked about Dengue is why are there four distinct serotypes of the virus? The simplest answer could be due to geographic (allopatric) location, dengue virus got separated into distinct lineages or partition in different ecology, as a result, the four serotypes evolved independently (Barrett et al., 2009). Alternatively, the virus could have evolved in sympatry (within a single population) due to the four distinct antigen serotypes presence which facilitates viral transmission through the phenomenon of antibody-dependent enhancement (Barrett et al., 2009). Under this model, natural selection favours viruses with the degree of antigenic dissimilarity that maximizes immunological enhancement, thereby aiding their mutual transmission (Ferguson et al., 1999). At present, most evidence favours the independent evolution hypothesis. In particular, the initial lineage splits would have produced viruses most likely sufficiently antigenically similar for there to be virtually complete cross-protection between them. Furthermore, if antibody-dependent enhancement were the major force shaping dengue genetic diversity then it might be expected that the virus would be subject to continual (immune) selection pressure. The studies of natural selection in dengue virus undertaken to date show that this not the case (Twiddy et al., 2002a and Twiddy et al., 2002b). Consequently, rather than being a long-term evolutionary strategy to aid viral transmission, antibody-dependent enhancement is more likely the result of recent contact between four viruses that have evolved in isolation for an extended period of time and by chance have a level of antigenic dissimilarity that allows immune enhancement (Barrett et al., 2009). All analyses undertaken to date show that the four serotypes of dengue virus are phylogenetically distinct, and often to the same degree as different “species” of flaviviruses (Kuno et al., 1998)
1.1.1 Etymology of Dengue
The origin of Dengue fever is not clear, but a popular theory was that it was derived from dinga in the Swahili phrased Ka-dinga pepo, where the disease is said to be caused by an evil spirit. Slaves who contracted dengue were said to have the posture and gait of a dandy, and the disease was called “dandy fever” (Halstead, 2008).
The term “break-bone fever” was by a physician and United States Founding Father- Benjamin Rush, in a report of the 1780 epidemic in Philadelphia. He titled the report as “bilious remitting fever” (Barrett et al., 2009). The term dengue fever came into general use only after 1828 (Halstead, 2008). Other historical terms include “breakheart fever” and “la dengue” (Halstead, 2008). Terms for severe disease include “infectious thrombocytopenic purpura” and “Philippine”, “Thai”, or “Singapore hemorrhagic fever.
1.1.2Epidemiology of Dengue
40% of the world’s population or about 2.5 billion people live in dengue endemic areas. About 100 countries in Asia, America, Pacific, Caribbean and Africa are at risk of dengue transmission. The World Health Organization (WHO) estimates 50 to 100 million infections yearly, including 500,000 DHF cases and 22,000 deaths, mostly among children and the aged (WHO, 2013). Most people with dengue recover without any ongoing problems and less than 1% has access to adequate treatment (WHO, 2013).
In Asia, epidemic DHF has expanded geographically from Southeast to India, Sri Lanka, the Maldives, and Pakistan (Gubler, 1997). Several island countries of the South and Central Pacific (Niue, Palau, Yap, Cook Islands, Tahiti, New Caledonia, and Vanuatu) have experienced major or minor DHF epidemics (Gubler, 1997). Epidemiologic changes in the Americas, however, have been the most dramatic. About 12 countries in Southeast Asia were estimated to have about 3 million infections and 6,000 deaths annually (Shepard et al., 2013). Dengue fever, which was once confined to Southeast Asia, has now spread to Southern China, countries in the Pacific Ocean and America (Gubler, 2010) might pose a threat to Europe (Reiter, 2010).
About 22 countries in Africa have history of dengue infection; but is likely present in all of them with 20% of the population at risk (Amarasinghe et al., 2011). Dengue increased 30 fold between 1960 and 2010. This increase is believed to be due to a combination of urbanization, population growth, increased international travel (Whitehorn and Farrar, 2010). An infection with dengue is second only to malaria as a diagnosed cause of fever among travelers returning from the developing world (Chen and Wilson, 2010). There are just five (5) countries in Africa where there are no reported case of Aedes mosquitoes (the principal vector for dengue virus) which are: Western Sahara, Morocco, Algeria, Tunisia, and Libya. There are thirteen (13) countries with no reported cases of dengue virus in Africa. Dengue is endemic in Thirty- four (34) countries in Africa while others are reported cases in travelers (Amarasinghe et al., 2011).
Figure 1.1: Distribution of Dengue in Nigeria (Source: Brady et al., 2012)
1.1.3 Structure of Dengue Virus
Dengue viruses are circular in shape with a viral envelope. This envelope comes from the membrane of an infected human cell and is covered with two viral proteins: E and M. These proteins help the virus attach to and infect human skin cells. Inside of the envelope is the viral genome coated by C protein, which forms a nucleocapsid. The viral genome is a single piece of single-stranded RNA; this RNA is translated by the host cell to form a polypeptide, or long protein strand (Perera and Kuhn, 2008). The individual viral proteins are made by cutting up the polypeptide into smaller pieces. This process makes all ten of the viral proteins dengue virus needs to replicate, including the other nonstructural viral protein (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5).
The envelop protein on Dengue viral surface is used for attachment to the host cell. The virus is transmitted by a mosquito bite, some molecules interacts with the virus E protein like ICAM3-grabbing non-integrin, CD209, Rab 5, GRP 78, and the mannose receptor which are important factors mediating attachment and viral entry (Perera and Kuhn, 2008). The DENV prM (membrane) protein, which is important in the formation and maturation of the viral particle, consists of seven antiparallel β-strands stabilized by three disulfide bonds (Perera and Kuhn, 2008).
The glycoprotein shell of the mature DENV virion contains 180 copies of E protein and M protein. The immature virion begins with the E and prM proteins forming 90 heterodimers that give a spiky exterior to the viral particle. This immature viral particle buds into the endoplasmic reticulum and eventually travels via the secretory pathway to the Golgi apparatus. These homodimers lie flat against the viral surface giving the maturing virion a smooth appearance. During this maturation pr peptide is cleaved from the M peptide by the host protease, furin. The M protein then acts as a transmembrane protein under the E-protein shell of the mature virion.
The pr peptide stays associated with the E protein until the viral particle is released into the extracellular environment. This pr peptide acts like a cap, covering the hydrophobic fusion loop of the E protein until the viral particle has exited the cell (Perera and Kuhn, 2008).
Figure 1.3: Transmission cycle of dengue virus
1.1.4 Dengue Viral Replication
The principal mosquito vector- Aedes aegypti, gets infected when it feeds on human during the usual five-day period of viremia. The virus is passed from the mosquito intestinal tract to the salivary glands after an extrinsic incubation period, this process takes about 10 days (Watts et al., 1987). Mosquito bites after the extrinsic incubation period result in infection, which might be promoted by mosquito salivary proteins (Schneider et al., 2004).
On the skin, DENV is injected into the bloodstream, with spillover in the epidermis and dermis, which results in infection of immature Langerhans cells (epidermal dendritic cells [DC]) and keratinocytes (Limon et al., 2005). The infected cells migrate to lymph nodes, where monocytes and macrophages are recruited, which become targets of infection. Consequently, infection is amplified and virus is disseminated through the lymphatic system. Several cells of the mononuclear lineage, including myeloid DC, blood-derived monocytes , and liver macrophages becomes infected (Kou et al., 2008). It should be noted that during secondary infections with heterologous DENV, high concentrations of DENV-specific immunoglobulin G (IgG) will complex newly produced virus that adheres to and is taken up by mononuclear cells. Following infection, mononuclear cells predominantly die by apoptosis, while abortively infected or bystander DC are stimulated to produce the bulk of mediators that are involved in inflammatory and hemostatic responses of the host (Espina et al., 2003). Factors that influences the amount of target cells infected and the levels of viremia, may also determine the ratio of pro-inflammatory and anti-inflammatory cytokines, chemokines, and other mediators, as well as the way in which the inflammatory response affects the hemostatic system (Chao et al., 2008)
Figure 1.4: Dengue Viral Replication Source: (Hunsperger, 2009)
1.1.5 Antigen-Dependent Enhancement
This is a process by which certain strain of dengue virus complexes with non-neutralizing antibodies which can enter a greater portion of APCs, leading to higher yield of virus production. This occurs after a primary infection by a different strain and results in DHF or DSS (Halstead, 2002). Several scholars observations have led to a conclusion that subsequent infection of preimmune individuals with a different DENV serotype could exacerbate rather than mitigate disease, a phenomenon that was claimed to be caused by antibodies and termed antibody-dependent enhancement (ADE) of disease (Halstead, 1970). Subsequent epidemiological studies further support the role of preimmunity in the pathogenesis of DHF. ADE could lead to infection with a higher number of target cells, causing an increase in the viral load observed in many studies (Libraty et al., 2002).
Despite several clinical studies, evidence for the role of ADE in human disease, such as in DENV infections, remains circumstantial. Some studies have shown a correlation between enhancing activity of serum, high levels of viremia, and an increased risk for DHF/DSS (Chau et al., 2008), not all cases of severe disease are associated with ADE or preceded by infection with a heterologous serotype or by high viral loads. In some cases with DHF/DSS, the viral RNA became undetectable (Libraty et al., 2002). In general, however, a high viral load and the presence of virus on the day of defervescence are important risk factors for the development of severe disease. As stated above, it is not completely clear whether the absence of viremia always correlates with clearance of virus from infected tissues (Marchette and Halstead, 1973).
1.1.6 Laboratory diagnosis of dengue infection
1.1.6.1 Virus isolation
C6/36 cell line of the Aedes albopictus mosquito can be used in DENV isolation, although other mosquito (such as Aedes pseudoscutellaris AP61) and mammalian (including Vero cells, LLC-MK2 cells and BHK21 cells) cell lines can also be used (Singh and Paul, 1968). Sera collected from suspected dengue patient within the first 3–5 days of fever (the viraemic phase) can be used for virus isolation. After an incubation period permitting virus replication, viral identification is performed using dengue-specific monoclonal antibodies in immunofluorescence and PCR assays (Guzman and Kouri, 2004). Plasma, leukocytes, whole blood and tissues obtained at autopsy can also be used (Rosen et al., 1999).
1.1.6.2 Serological testing
Serological assays are commonly used for diagnosis of dengue infection as they are relatively inexpensive and easy to perform compared with culture or nucleic acid-based methods. Some of which include;
1.1.6.2.1 Hemagglutination-Inhibition (HI): This is easy to perform, requires minimal equipments, and is very reliable when properly done. HI antibodies persist for up to 48 years and probably longer (Halstead, 1974), the test is ideal for seroepidemiologic studies. HI antibody usually appear at detectable levels (titer of 10) by day 5 or 6 of illness, and antibody titers in convalescent-phase serum specimens are generally at or below 640 in primary infections, although there are exceptions (Barnes and Rosen, 1974). The major disadvantage of the HI test is its lack of specificity, which makes it unreliable for identifying the infecting virus serotype. However, some patients with primary infections show a relatively monotypic HI response that correlates with the virus isolated (Gubler and Sather, 1988).
1.1.6.2.2 MAC-ELISA: This has become the most widely used serologic test for dengue diagnosis in the past few years. It is a simple, rapid test that requires very little sophisticated equipment (Kapoor et al., 1995)). Anti-dengue IgM antibody develops a little faster than IgG antibody, MAC-ELISA shows a sensitivity and specificity of 90% and 98% in samples collected after 5 days of fever respectively (PAHO, 1994). In addition to serum, dengue-specific IgM can be detected in whole blood on filter paper (sensitivity 98.1% and specificity 98.5%) (Herrera et al., 2006) and in saliva (sensitivity 90.3% and specificity 92.0%), but not in urine. More than 50 commercial kits are available with variable sensitivity and specificity. False-positive results due to dengue-specific IgG and crossreactivity with other flaviviruses is a limitation of the MAC-ELISA in regions where multiple flaviviruses co-circulate. In areas where dengue is not endemic, it can be used in clinical surveillance for viral illness, population-based serosurveys, with the certainty that any positive results detected indicate recent infections (within the last 2 to 3 months). In areas where dengue is endemic, MAC-ELISA can be used as an inexpensive way to screen large numbers of serum specimens with relatively little effort. It is especially useful for hospitalized patients, who are generally admitted late in the illness after detectable IgM is present in the blood (Gubler and Sather 1988)
1.1.6.2.3 IgG ELISA: This is comparable to the HI test, which can also be used to differentiate primary and secondary dengue infections (Chungue et al., 1989). The test is simple and easy to perform and is thus useful for high-volume testing. It is nonspecific and exhibits the same broad cross-reactivity among flaviviruses as the HI test does. It can’t be used to identify the infecting dengue virus serotype. However, it has a slightly higher sensitivity than the HI test. As more data are accumulated on the IgG-ELISA, it is expected to replace the HI test as the most commonly used IgG test in dengue laboratories. The assay uses the same dengue antigens as MAC-ELISA and it correlates with results from the haemagglutination inhibition assay. It has been said that IgG response to the prM membrane glycoprotein is specific to individual flaviviruses as no cross reactivity was observed in sera collected from individuals infected with dengue or Japanese encephalitis virus (Cardosa et al., 2002).
1.1.6.2.4 Neutralization assays: The plaque reduction neutralization technique (PRNT) and the micro-neutralization assay are used to define the infecting serotypes following a primary infection. It is the most specific and sensitive serologic test for dengue viruses (Russell and Nisalak, 1967). The serum dilution plaque reduction NT protocols are mostly used in dengue laboratories. Neutralizing-antibody titers rise at about the same time or slightly more slowly than HI and ELISA antibody titers but more quickly than CF antibody titers and persist for at least 48 years. Neutralizing antibodies are present in the absence of detectable HI antibodies in some persons with past dengue infection. This assay is expensive, time consuming and it has high technical difficulty, as a result, they are used mainly for research and vaccine studies (Morens et al.,1985).
1.1.6.2.5 NS1 Antigen and Antibody Detection: NS1 is a glycoprotein produced by all flaviviruses, it is paramount in viral viability and replication. This protein is secreted into the bloodstream, many tests have been developed to diagnose DENV infections using NS1. These tests include antigen-capture ELISA, lateral flow antigen detection and measurement of NS1-specific IgM and IgG responses. As yet, these kits do not differentiate between the different DENV serotypes. (Young et al., 2000).
1.1.6.2.6 Reverse Transcriptase PCR (RT-PCR): Different dengue RT-PCR assays have been in use since the past years. These assays target different genes and use different amplification procedures. NAATs is mostly used based on a single RT-PCR assay, a nested RT-PCR assay or a one-step multiplex RT-PCR assay (Lanciotti et al., 1992). The nested PCR reaction involves an initial reverse transcription and amplification step using dengue primers that target a conserved region of the virus genome followed by a second amplification step that is serotype specific. The products of the reactions are separated by electrophoresis on an agarose gel, which allows the dengue serotypes to be differentiated on the basis of size. RT-PCR provides a rapid serotype-specific diagnosis. This method is easy to perform, reproducible, rapid, sensitive, and simple, when properly controlled. It can also be used to detect viral RNA in human clinical samples, autopsy tissues, or mosquitoes (Lanciotti et al., 1992). Although RT-PCR sensitivity is similar to virus isolation systems using C6/36 cell cultures, poor handling, poor storage, and the presence of antibody usually do not influence the outcome of PCR as they do virus isolation. A number of methods involving primers from different locations in the genome and different approaches to detect the RT-PCR products have been developed over the past several years (Guzman and Kouri, 1996).
1.1.6.2.7 Real-Time RT-PCR: The real-time RT-PCR assay is a one-step assay that allows virus titre to be quantified in approximately 1.5 hours. The detection of the amplified target by fluorescent probes replaces the need for post-amplification electrophoresis. Different real-time RT-PCR assays have been developed that are either ‘singleplex’, detecting one single serotype per reaction, or ‘multiplex’, identifying all four serotypes from a single sample (Johnson et al., 2005). One advantage of this assay is the ability to determine viral titre early in dengue illness, which is believed to be an important predictor of disease severity.
1.1.7 Prevention and Control of Dengue Virus
Prevention and control of dengue and DHF has become more urgent with the expanding geographic distribution and increased disease incidence in the past 20 years (Gubler and Clark, 1995). Unfortunately, tools available to prevent dengue infection are very limited. There is no vaccine currently available, and options for mosquito control are limited. Clearly, the emphasis must be on disease prevention if the trend of emergent disease is to be reversed. The risk of the disease can be reduced by preventing mosquito bites, use of mosquito nets and insect repellents, or with mosquito-control measures such as spraying insecticides and draining of stagnant water (Caraballo and King, 2014).
1.2 MALARIA PARASITE
Malaria is the most prevalent infectious disease in the tropical and subtropical regions of the world causing major morbidity in the tropics (Mishra et al., 2003). Malaria is a mosquito-borne infectious disease of humans and other animals caused by parasitic protozoan (a group of single-celled microorganism) belonging to the genus Plasmodium. Malaria symptoms include fever, fatigue, vomiting and headaches. In severe cases it can cause yellow skin, seizures, coma or death (Caraballo and King, 2014). The disease is transmitted by the biting of mosquitoes, and the symptoms usually begin ten to fifteen days after being bitten. In those who have recently survived an infection, re-infection typically causes milder symptoms. This partial resistance disappears over months to years if the person has no continuing exposure to malaria (Caraballo and King, 2014).
The disease is transmitted mostly by an infected female Anopheles mosquito. The mosquito bite introduces the parasites from the mosquito’s saliva into a person’s blood. The parasites travel to the liver where they mature and reproduce. There are five species of Plasmodium which can infect and spread malaria; they are P. falciparum,P. vivax, P. ovale, P. malariae and P. knowlesi which rarely causes disease in humans (Caraballo and King, 2014). Malaria is typically diagnosed by the microscopic examination of blood using blood films, or with antigen-based rapid diagnostic tests. Methods that use the polymerase chain reaction to detect the parasite’s DNA have been developed, but are not widely used in areas where malaria is common due to their cost and complexity (Nadjm and Behrens 2012).
Malaria is commonly associated with poverty and has a major negative effect on economic development (Worrall et al., 2005). In Africa it is estimated to result in losses of US$12 billion a year due to increased healthcare costs, lost ability to work, and effects on tourism (Greenwood, 2005). In the World Health Organization report, there were 198 million cases of malaria worldwide in 2013. This resulted in an estimated 584,000 to 855,000 deaths. According to the World Health Organization, malaria accounts for nearly 90% of deaths in Africa (Ogbodo et al., 2010), while records have shown that about 50% of the Nigerian population suffer from at least one episode of malaria annually with over 45% of all out-patient visits being associated with malaria. Approximately 0.25 million deaths of Nigerian children under the age of 5 have been associated with malaria yearly (UNICEF, 2009).
1.2.1 Origin of Malaria
About 10,000 years ago, malaria started having a major impact on human survival, coinciding with the start of agriculture in the Neolithic revolution. Consequences included natural selection for sickle-cell disease, thalassaemias, glucose-6-phosphate dehydrogenase deficiency, Southeast Asian ovalocytosis, elliptocytosis and loss of the Gerbich antigen (glycophorin C) and the Duffy antigen on the erythrocytes, because such blood disorders confer a selective advantage against malaria infection (balancing selection) (Canali, 2008). The three major types of inherited genetic resistance (sickle-cell disease, thalassaemias, and glucose-6-phosphate dehydrogenase deficiency) were present in the Mediterranean world by the time of the Roman Empire, about 2000 years ago (Sallares et al., 2004) . Human malaria likely originated in Africa and coevolved with its hosts, mosquitoes and non-human primates. Malaria protozoa are diversified into primate, rodent, bird, and reptile host lineages (Hayakawa et al., 2008). Humans may have originally caught Plasmodium falciparum from gorillas. P. vivax another malarial Plasmodium species among the six that infect humans, also likely originated in African gorillas and chimpanzees. Another malarial species recently discovered to be transmissible to humans, P. knowlesi, originated in Asian macaque monkeys (Lee et al., 2011). While P. malariae is highly host specific to humans, there is spotty evidence that low level non-symptomatic infection persists among wild chimpanzees. The name malaria derived from mal aria (‘bad air’ in Medieval Italian). This idea came from the Ancient Romans who thought that this disease came from the horrible fumes from the swamps. The word malaria has its roots in the miasma theory, as described by historian and chancellor of Florence Leonardo Bruni in his Historiarum Florentini populi libri XII, which was the first major example of Renaissance historical writing (Hempelmann and Krafts, 2013).
1.2.2 Etiology of Malaria
Malaria parasites belong to the genus Plasmodium (phylum Apicomplexa). In humans, malaria is caused by P. falciparum, P. malariae, P. ovale, P. vivax and P. knowlesi (Collins, 2012). In humans, P. falciparum is the most identified (~75%) causing high deaths rate, P. vivax (~20%) malaria is associated with potentially life-threatening conditions but it is more common outside Africa (Baird, 2013). There have been documented human infections with several species of Plasmodium from higher apes; however, except for P. knowlesi—a zoonotic species that causes malaria in macaques, these are mostly of limited public health importance (Parham et al., 2011).
1.2.3 Epidemiology of Malaria
Malaria is presently endemic in a broad band around the equator, in areas of the Americas, many parts of Asia, and much of Africa; in Sub-Saharan Africa, 85–90% of malaria fatalities occur. The WHO estimates that in 2010 there were 219 million cases of malaria resulting in 660,000 deaths (Nadjm and Behrens, 2012). The majority of cases (65%) occur in children under 15 years old, millions of pregnant women are at risk of infection each year. In Sub-Saharan Africa, maternal malaria is associated with up to 200,000 estimated infant deaths yearly (Hartman et al., 2010). In 2012, there were 207 million cases of malaria. That year, the disease is estimated to have killed between 473,000 and 789,000 people, many of whom were children in Africa. Efforts towards the reduction of the disease in Africa since the turn of millennium have been partially effective, with rates of the disease dropping by an estimated forty percent on the continent (Bhatt et al., 2015). The Malaria Atlas Project aims to map global endemic levels of malaria, providing a means to determine the global spatial limits of the disease and to assess disease burden (Hay et al., 2010). As of 2010, about 100 countries are endemic malaria. Every year, 125 million international travelers visit these countries, and more than 30,000 contract the disease. In Africa malaria is present in both rural and urban areas, with lower risk in the larger cities (Machault et al., 2011).
1.2.4 Life Cycle of Malaria Parasite
A female Anopheles mosquito (the definitive host) transmits a motile infective form (called the sporozoite) to a vertebrate host such as a human (the secondary host), thus acting as a transmission vector. A sporozoite travels through the blood vessels to liver cells (hepatocytes), where it reproduces asexually (tissue schizogony), producing thousands of merozoites. These infect new red blood cells and initiate a series of asexual multiplication cycles (blood schizogony) that produce 8 to 24 new infective merozoites, at which point the cells burst and the infective cycle begins anew. Other merozoites develop into immature gametocytes, which are the precursors of male and female gametes (Cowman et al., 2012). When a fertilised mosquito bites an infected person, gametocytes are taken up with the blood and mature in the mosquito gut. The male and female gametocytes fuse and form an ookinete—a fertilized, motile zygote. Ookinetes develop into new sporozoites that migrate to the insect’s salivary glands, ready to infect a new vertebrate host. The sporozoites are injected into the skin, in the saliva, when the mosquito takes a subsequent blood meal.
Only female mosquitoes feed on blood, male mosquitoes feed on plant nectar, and do not transmit the disease (Arrow et al., 2004). The females of the Anopheles genus of mosquito prefer to feed at night which begins at dusk. Malaria parasites can also be transmitted by blood transfusions, although this is rare (Owusu-Ofori et al., 2010).
Source: www.niaid.nih.gov/topics/Malaria/Pages/lifecycle.aspx
Figure 1.5: Life Cycle of Malaria Parasite
1.2.5 Diagnosis
Malaria is confirmed by microscopic examination of blood films or by the use of antigen-based rapid diagnostic tests (RDT) (Abba et al., 2011). Diagnosis by microscopy suffers from two main drawbacks: many settings (especially rural) are not equipped to perform the test, and the accuracy of the results depends on both the skill of the person examining the blood film and the levels of the parasite in the blood. The sensitivity of blood films ranges from 75–90% in optimum conditions, to as low as 50%. Commercially available RDTs are often more accurate than blood films at predicting the presence of malaria parasites, but they are widely variable in diagnostic sensitivity and specificity depending on manufacturer, and are unable to tell how many parasites are present (Wilson, 2012).
In areas that lacks access to laboratory diagnostic tests, it has become common to use only a history of fever as the indication to treat for malaria—thus the common teaching “fever eq
uals malaria unless proven otherwise”. A drawback of this practice is over diagnosis of malaria and mismanagement of non-malarial fever, which wastes limited resources, erodes confidence in the health care system, and contributes to drug resistance (Perkins and Bell, 2008) Although polymerase chain reaction-based tests have been developed, they are not widely used in areas where malaria is common as of 2012, due to their complexity (Nadjm and Behrens, 2012).
1.2.6 Treatments
Despite the effort to contain the Anopheles vectors of malaria in the 20th century, over a million deaths with more than 300 acute malaria cases are sill reported annually in the world, with greatest percentage occurring in Africa (WHO, 2005). Severe and complicated malaria are medical emergencies since mortality rates are high (10% to 50%) ((Pasvol, 2005). Cerebral malaria is the form of severe and complicated malaria with the worst neurological symptoms. Recommended treatment includes; the intravenous antimalarial drugs, parenteral artesunate which is superior to quinine in both children and adult (Sinclair et al., 2012), artemisinin derivatives (artemether and arteether) were as efficacious as quinine in the treatment of cerebral malaria in children (Kyu and Fernández, 2009). Treatment of severe malaria involves supportive measures that are best done in a critical care unit. This includes the management of high fevers and the seizures that may result from it. It also includes monitoring for poor breathing effort, low blood sugar, and low blood potassium (Sarkarv et al., 2009).
1.2.7 Prevention and Control
Methods used to prevent malaria include medications, mosquito elimination and the prevention of bites. There is no vaccine for malaria. The presence of malaria in an area requires a combination of high human population density, high anopheles mosquito population density and high rates of transmission from humans to mosquitoes and from mosquitoes to humans. If any of these is lowered sufficiently, the parasite will eventually disappear from that area, as happened in North America, Europe and parts of the Middle East (WHO, 1958). However, unless the parasite is eliminated from the whole world, it could become re-established if conditions revert to a combination that favours the parasite’s reproduction. Furthermore, the cost per person of eliminating anopheles mosquitoes rises with decreasing population density, making it economically unfeasible in some areas.
There is a wide cost difference between the costs of control (i.e. maintenance of low endemicity) and elimination programs in countries (Sabot et al., 2010). For example, in China—whose government in 2010 announced a strategy to pursue malaria elimination in the Chinese provinces—the required investment is a small proportion of public expenditure on health. In contrast, a similar program in Tanzania would cost an estimated one-fifth of the public health budget.