Dengue in India

Epidemiology of dengue

The epidemiology of dengue fevers in the Indian subcontinent has been very complex and has substantially changed over almost past six decades in terms of prevalent strains, affected geographical locations and severity of disease. The very first report of existence of dengue fevers in India was way back in 194615. Thereafter, for the next 18 years, there was no significant dengue activity reported anywhere in the country. In 1963-1964, an initial epidemic of dengue fever was reported on the Eastern Coast of India716–20, it spread northwards and reached Delhi in 196721 and Kanpur in 19682223. Simultaneously it also involved the southern part of the country2425 and gradually the whole country was involved with wide spread epidemics followed by endemic/hyperendemic prevalence of all the four serotypes of DV. The epidemiology of dengue virus and its prevalent serotypes has been ever changing. The epidemic at Kanpur during 1968 was due to DV-422 and during 1969 epidemic, both DV-2 and DV-4 were isolated26. It was completely replaced by DV-2 during 1970 epidemic in the adjoining city of Hardoi27. Myers et al2428 had reported the presence of DV-3 in patients and Ae. aegypti at Vellore during the epidemic of 1966 while during the epidemic of 1968, all the four types of DV were isolated from patients and mosquitoes29. In another study Myers & Varkey30 reported an instance of a third attack of DV in one individual. DV-2 was isolated during the epidemics of dengue in urban and rural areas of Gujarat State during 1988 and 198931. Outbreaks of dengue occurred in Rajasthan by DV- 1 and DV-332, DV-333, Madhya Pradesh by DV-334, Gujarat by DV-231 and in Haryana by DV-235. DV-2 was the predominant serotype circulating in northern India, including Delhi, Lucknow and Gwalior121336 while DV-1 was isolated during the 1997 epidemic at Delhi37. The phylogenetic analysis by the Molecular Evolutionary Genetics Analysis programme suggests that the 1996 Delhi isolates of DV-2 were genotype IV. The 1967 isolate was similar to a 1957 isolate of DV-2, from India, and was classified as genotype V. This study indicates that earlier DV-2 strains of genotype V have been replaced by genotype IV38. The Gwalior DV-2 viruses were classified into genotype-IV, and were most closely related to Delhi 1996 DV-2 viruses and FJ 10/11 strains prevalent in the Fujian State of China. However, two earlier Indian isolates of DV-2 were classified into genotype-V. Genotype V of DV-2 has been replaced by genotype IV during the past decade, which continues to circulate silently in north India, and has the potential to re-emerge and cause major epidemics of DF and DHF39. DV-2 has also been reported from southern India - in Kerala alongwith DV-340.

DV-3 has been isolated during the epidemics at Vellore in 19662428, at Calcutta in 198341 and in 199010, at Jalore city, Rajasthan in 198533 at Gwalior in 2003 and 20044243 and at Tirupur, Tamil Nadu in 201044. Phylogenetic analysis showed that the Madurai isolates were closely related to Gwalior and Delhi isolates. The emergence of DV-4 has been reported in Andhra Pradesh45 and Pune, Maharashtra46, which was also implicated in increased severity of disease. At Delhi, till 2003, the predominant serotype was DV-2 (genotype IV) but in 2003 for the first time all four dengue virus subtypes were found to co-circulate in Delhi thus changing it to a hyperendemic state47 followed by complete predominance of DV serotype 3 in 200548. During the 2004 epidemic of DHF/DSS in northern India a sudden shift and dominance of the DV serotype-3 (subtype III) occurred replacing the earlier circulating serotype-2 (subtype IV)43. Co-circulation of DV serotypes in Delhi in 2003-2004 has also been reported43, which may have implications for increased DHF/DSS. Emergence of a distinct lineage of DV-1, having similarity with the Comoros/Singapore 1993 and Delhi 1982 strains, but quite different from the Delhi 2005 lineage and microevolution of the pre-circulating DV-3 has been reported49. Co-circulation of several serotypes of dengue viruses has resulted in concurrent infection in some patients with multiple serotypes of DV50. Further, replacement of DV-2 and 3 with DV-1 as the predominant serotype in Delhi over a period of three years (2007-2009) has been reported51. Concurrent infection by Chikungunya and DV-2 was reported from Vellore52 and Delhi53 (Table I).

Table I Epidemiological studies where dengue virus was identified

Dengue virus and its serotypes

DV-1 was isolated in 1956 at Vellore. All the Indian DV-1 isolates belong to the American African (AMAF) genotype. The Indian DV-1 isolates are distributed into four lineages, India I, II, III and the Africa lineage. Of these, India III is the oldest and extinct lineage; the Afro-India is a transient lineage while India I is imported from Singapore and India II, evolving in situ, are the circulating lineages75. The American genotype of DV-2 which circulated predominantly in India during the pre-1971 period, was subsequently replaced by the Cosmopolitan genotype. Post-1971 Indian isolates formed a separate subclade within the Cosmopolitan genotype. DV-2 strains were isolated in India over a time span of more than 50 years (1956-2011). The re-emergence of an epidemic strain of DV type-3 in Delhi in 2003 and its persistence in subsequent years marked a changing trend in DV circulation in this part of India49. Occasional reports of circulation of DV-4 are also seen, though it is not the predominant type in India7647.

Clinical presentation

The classical clinical presentation of dengue virus infection has been observed in the country, however, several atypical clinical presentations have also been reported (Table II).

Table II A typical clinical presentations of dengue virus infection

DV-specific follicular helper T cells (ThF)

DV induces generation of helper T cells (ThF) in mouse spleen which enhance clonal expansion of DV-specific B cells111 (Table III). DV-specific ThF secrete a cytokine, the helper factor (HF)112 which is composed of two chains, one has antigen and the other has I-A determinants; both chains are essential for helper activity113. The helper signal is transmitted only by a close physical contact of the plasma membranes of B cells and ThF or HF-adsorbed macrophage. ThF and HF help in production of DV-specific antibodies115116.

Table III T cell functions in dengue virus infection in mice

T cells producing cytotoxic factor (TCF)

Cytotoxic factor (CF), a unique cytokine, is produced by CD4+ T cells in DV infected mice and man. CF has no homology with any of the known proteins in their amino-terminal sequence134. Most of the patients with dengue virus infection have CF in their sera, with peak amounts in the most severe cases of DHF136139. CD4⁺ T cells and H-2A^- macrophages are killed by CF while it induces H-2A⁺ macrophage to produce another cytokine, the macrophage cytotoxin (CF2) which amplifies the effect of CF, thus producing immunosuppression to heterologous antigens106. CF appears in the serum before the clinical illness and is present in 100 per cent patients with DF/DHF up to day 4 of illness, detectable up to day 20 of illness139. CF is produced in ex vivo cultures of CD4+ T cells obtained from peripheral blood of the patients with severe dengue disease137. Human peripheral blood leucocyte cultures inoculated with DV, produce CF139140. CF is DV-specific, therefore, can be used for developing a diagnostic kit.

Suppressor T cells

For the first time microbe-induced suppressor cells or T cells cascade was shown in DV-infected mice117118120123 which was subsequently confirmed in a large number of viruses138. DV-specific suppressor T cell (TS) cascade has three sequential subpopulations of TS1, TS2, TS3 cells (Table III) and their secretary soluble suppressor cytokines (SF1, SF2, SF3). DV-infected macrophage transmits the signal to recruit TS1 cells, which secrete a suppressor cytokine, SF1. SF1 is composed of two polypeptide chains141, the alpha-chain binds to the beta-chain of the SF receptor (SF-R) present on macrophage142 while the beta-chain of SF1 binds to H-2A determinants on macrophage143144. SF1 is internalized by live syngeneic macrophage, processed and binds to H-2K antigen and is transported to a site other than SF-R on macrophage membrane for recruitment of TS2 cells145 (Table IV). TS2 produce a prostaglandin-like suppressor cytokine (SF2). Live syngeneic macrophage transmits the SF2 signal to recruit a third subpopulation of TS3, which suppresses humoral immune response in an antigen-specific and genetically restricted manner118–120123. DV-induced suppressor pathway suppresses antigen-specific antibody production (immunosuppression to homologous antigen). Thus, suppression of neutralizing antibody would delay elimination of DV from the body causing pathological lesions123125. In another study also suppressor T cell activity in dengue type 3 virus infected mice has been shown124.

Table IV Macrophage functions in dengue virus infection in mice

Macrophage & macrophage-like cells

Macrophages are the primary component of the host innate immune system and provide first line of defence against viral infections. But in dengue viral infection, the macrophages play multiple paradoxical roles (Table IV), sometimes these help in eradicating the virus, while sometimes these actually increase its replication within the host158. On one hand, macrophages are the main cells which replicate dengue virus in man, mouse and monkey128 and the presence of macrophages is obligatory for the transmission of DV-specific suppressor signal from the first subpopulation of suppressor T cells (TSl) to the second subpopulation, TS2123. On the other hand, macrophages are responsible for processing and presentation of DV antigen to B lymphocytes leading to their clonal expansion and immune response107108. Further, it has been observed that DV induces generation of follicular helper T cells (ThF) in mice which secrete a helper cytokine (HF) which enhances clonal expansion of B lymphocytes in an antigen-specific and H-2 restricted manner112149. The DV- induced CF kills H2-A negative macrophages111 by causing calcium influx, whereas it induces H2-A positive macrophages to produce a cytotoxin - CF2 which acts synergistically with CF150. CF-2 is a biologically active protein and causes various immunopathological effects including increased vascular permeability and damage to the blood brain barrier132152159160. It has also been demonstrated that CF-2 induces production of NO2^- in the spleen cells of mice thus mediating its cytotoxic effect on target cells155. This might be also one possible important trigger for switch from DF to DHF/DSS. In addition, in response to variety of stimuli, including viral infections, macrophages release migration inhibitory factor (MIF), which is a hormone released by different cells in many tissues in response to a variety of stimuli161. Macrophages also secrete a number of cytokines in viral infections, including DV infection158.

Cerebral oedema/encephalopathy during DV infection

Earliest reference to involvement of brain in dengue disease was encephalopathy or cerebral oedema, which was rare162. Therefore, the mechanism of cerebral oedema was studied in mouse model. A breakdown of the blood-brain barrier occurs in mice inoculated intracerebrally or intraperitoneally with DV 2 resulting in leakage of protein-bound Evans blue dye and51 Cr-labelled erythrocytes into the brain tissue. Similar breakdown of the blood-brain barrier also occurred in mice inoculated intravenously with CF and CF2; the damage is dose-dependent and the vascular integrity is restored during the 3 h period after inoculation. Treatment of mice with antihistamine drugs, blocking H1 or H2 receptors, decreases the DV2-induced protein leakage. Pretreatment with CF-specific or DV2-specific antiserum inhibits protein leakage. Thus CF/CF2-mediated breakdown of the blood-brain barrier leads to cerebral oedema during DV infection112132136163.

Capillary leakage in DV infection

One of the cardinal features of severe dengue is capillary leakage resulting into accumulation of fluids in various body cavities. Therefore, experiments were conducted to find out the mechanism of this phenomenon. It was observed that intraperitoneal inoculation of CF or CF2 in mice results in increased capillary permeability in a dose-dependent manner, as shown by leakage of intravenously injected radiolabelled iodine or Evans blue dye. Peak leakage occurs 30 min after inoculation of CF and the vascular integrity is restored by 2 h. The increase in capillary permeability is abrogated by pretreatment of mice with anti-CF antibodies, avil (H1 receptor blocker) or ranitidine (H2 receptor blocker)132152. CF purified from the pooled sera of the DHF patients on intravenous inoculation into mice increased capillary permeability and damaged the blood-brain barrier136.

CF and CF2 appear to be pathogenesis-related proteins, that can produce DHF-like pathological lesions in mice, such as capillary leakage, cerebral oedema, and blood leukocyte changes112132137152163. Pretreatment of mice with the anti-CF antibodies prevents pathological lesions produced by CF/CF2. Immunization of mice with CF protects them against subsequent challenge with CF, while challenge of such mice with a lethal intracerebral dose of DV prevents only the clinical symptoms not the death135. With the availability of endothelial cell monolayer models, extensive work has been done in recent times to understand the pathophysiology of vascular endothelium during dengue virus infection leading to plasma leakage as seen in severe dengue164–167.

Cardiac damage during DV infection

During the 1968 epidemic of DF at Kanpur, a few cases were suspected to have myocarditis2223. Therefore, an effort was made to develop a mouse model to study it. Infant mice inoculated with DV show minimal histological cardiac injury in the form of cytoplasmic vacuolation of myocardium and foci of infiltration by mononuclear cells27. Subsequently, cardiac involvement in dengue disease was reported during 1996 epidemic of DHF at Delhi (Table I).

Effect of DV infection on megakaryocytes and platelets

DV-2 inhibits in vitro megakaryopoiesis and induces apoptotic cell death in a subpopulation of early megakaryocytic progenitors which may contribute to thrombocytopoenia in dengue disease166. In another study it was shown that DV-2 may directly interact with and activate platelets and thus may be responsible for thrombocytopenia168. Significant ultrastructural changes in DV infected cells specially endomembrane re-organization and formation of autophagososmes have been shown using whole mount transmission electron microscopy169. These changes, taken together with a later study, that showed marked elongation of endothelial cell processes after transfection with the DV-E protein, provided early insights that the replication biology of the virus is coupled closely with the host cell physiology167.

Pathogenesis of DF/DHF

Understanding the factors that are involved in the pathogenesis of DHF continues to be one of the most active areas of dengue research. It has been established that DHF is caused by a “Cytokine Tsunami” but despite extensive studies for over four decades, its genesis is still not fully understood. The mechanisms that have been considered to cause DHF include antibody-dependent enhancement (ADE)170, T cell response122123171, and a shift from Th-1 to Th-2 response172. The combined effect of all of these is cytokine tsunami125 resulting in movement of body fluids in extravascular space. Various cytokines have been implicated in the immuno-pathogenesis of DF/DHF as summarized in Table V. It has been suggested that in dengue a Th1 response is linked to recovery from infection while a Th2 type response leads to severe pathology and exacerbation of the disease172182. The role of Th17 cells in dengue pathogenesis has been examined and warrants serious consideration by researchers183. CF/CF2 induces macrophage to produce free radicals, nitrite, reactive oxygen and peroxynitrite153154158182184185. The free radicals, besides killing the target cells by apoptosis also directly upregulate production of pro-inflammatory cytokines; interleukin (IL-1), tumour necrosis factor (TNF)-alpha, IL-8, and hydrogen peroxide in macrophage. Oxidative stress develops from the onset of dengue infection. Plasma protein carbonylation, protein carbonylation to protein-bound sulphydryl group ratio are reported to predict DHF/DSS9394. The change in relative levels of IL-12 and transforming growth factor (TGF)-beta shifts a Th1-dominant response to a Th2 biased response resulting in an exacerbation of dengue disease. The vascular permeability is increased due to combined effect cytokine tsunami, release of histamine, free radicals and the products of the complement pathway, etc. Thus the key player appears to be CF/CF2, but the activity is regulated by CF-autoantibodies generated in patients with dengue disease186.

Table V Serum cytokine levels in the pathogenesis of dengue

The accompanying factors that have been discussed from time to time are dengue non-structural protein of virus type 1 (NS1)-antibodies cross-reacting with vascular endothelium (a type of autoimmune phenomenon), immune complex disease, complement and its products, memory T cells, various soluble mediators including cytokines selection of virulent strains and virus virulence, etc.125157172182187. Further, DV has been shown to evade the innate immune mechanisms of the host by inhibiting both type I interferon (IFN) production and signaling in susceptible human cells, including dendritic cells (DCs). DV also encodes proteins that antagonize type I IFN signaling, including NS2A, NS4A, NS4B and NS5 by targeting different components of this signaling pathway, such as STATs. This contributes to the pathogenesis and host tropism of this virus188. Further, a critical role for invariant natural killer (iNK) T cells in mice189; altered plasma concentrations of vitamin D and mannose binding lectin190; shift from Th1 cytokine to Th2 cytokine expression; role of saliva of Ae. aegypti191; and intracellular changes in host proteins192 have been reported. Two loci on chromosomes 6 and 10 have been identified that are associated with susceptibility to DSS193. Classical and non-classical HLA alleles have been attributed to be related with disease severity in the host158194195. Other mechanisms that have been suggested are that DV utilizes calcium modulating cyclophilin-binding ligand to subvert the apoptotic process which in turn favoured efficient virus production196. A correlation of elevated lipopolysaccharide levels with disease severity has also been reported197.

Still the exact cascade of mechanisms involved in dengue disease pathogenesis remains unexplained and lot more needs to be done.

Establishment of mosquito cell line

A forerunner of mosquito cell line C6/C36 was established at Pune198 for the isolation of dengue viruses. This was the first time when mosquito cells were used as cell culture.

Diagnosis of dengue virus infection

Diagnosis of DV infection is routinely done by demonstration of anti DV IgM antibodies or by NS-1 antigen in patients’ serum depending upon day of illness using ELISA kits (prepared by National Institute of Virology, Pune) and commercial kits199. Molecular methods (reverse transcriptase PCR) are being increasingly used in diagnosis of DV infection. A single tube nested PCR for detection and serotyping of DV was developed and used for detection of co-infection by two viruses200. DV isolation in tissue culture cells and its sequencing is also being done175.

Treatment of dengue virus infection

The management of dengue virus infection is essentially supportive and symptomatic. No specific treatment is available. However, there are Indian studies which have contributed in terms of better management of DHF/DSS. A rapid response to platelet and fresh frozen plasma (FFP) transfusion is reported in a study201. Anti-D has been used in children with DHF and severe refractory thrombocytopenia202. In experimental study pre-feeding mice with trivalent chromium picolinate (CrP) in drinking water could abolish the adverse effects of DV infection on most of the haematological parameters203. Hippophae rhamnoides (Seabuckthorn, SBT) leaf extract has been shown to have a significant anti-dengue activity204.

Vaccine for dengue virus

Dengue vaccines have been under development since the 1940s, but a tetravalent vaccine which simultaneously provides long-term protection against all DV serotypes is round the corner205.

A tetravalent antigen was designed by splicing the EDIIIs of DV-1, DV-2, DV-3 and DV-4 using flexible pentaglycyl linkers. A synthetic gene encoding this tetravalent antigen was expressed in Pichia pastoris and purified to near homogeneity. This tetravalent antigen when injected into inbred BALB/c mice, elicited neutralizing antibodies specific to each of the four DVs in plaque reduction neutralization tests206. Efforts are underway to present the tetravalent antigen on a chimeric VLP platform. Some promising dengue antigens have been developed using different systems (Table VI).

Table VI* Dengue antigens developed with potential for vaccine purposes

Recombinant dengue virus antigens

Several studies have contributed in terms of developing new reagents or technology for diagnostic purposes (Table VI). A recombinant DV3 envelope domain III (rDen 3 EDIII) protein has been produced in Escherichia coli for potential use in diagnosis210226. A biotinylated chimeric dengue antigen to exploit the high affinity of biotin-streptavidin interaction to detect anti-dengue antibodies has been developed which incorporates the envelope domain III of all four DV serotypes216217. Immunosensor has been established for label free and real time assay for the serological diagnosis of DV infection. Scope for development of biosensors for diagnosis was demonstrated227. The recombinant dengue multiepitope (rDME-M) protein specific to IgM in E. coli was produced in a 5-L fermentor for use in diagnostic purpose228.

Vector control

Aedes aegypti is the commonest vector of DV in India, followed by Ae. albopictus. Larval indices indicate that Ae. aegypti is well established in peri-urban areas and is beginning to displace Ae. albopictus. Water-holding containers, viz. plastic, metal drums and cement tanks facilitate breeding of Ae. aegypti229230. Expansion in the risk area of diseases borne by it in the context of urbanization, transport development and changing habitats is a major concern231.

Vector control is known to be a good method for prevention of vector borne diseases. There are several reports from India which have demonstrated resistance of mosquito vector with anti larval substances like DDT and dieldrin but susceptibility to malathione is reported232. Temephos is relatively more effective in controlling Ae. aegypti, followed by fenthion, malathion and DDT233. Peridomestic thermal fogging reduced the resting and biting for the 3 days after treatment, whereas indoor fogging suppressed adult populations for 5 days234.

Plant based repellent against mosquito borne diseases235 have also been described. Flavonoid compounds derived from Poncirus trifoliata compounds have various activities against different life stages of Ae. aegypti239. Larvicidal and ovicidal activities of benzene, hexane, ethyl acetate, methanol and chloroform leaf extract of Eclipta alba have shown potential for controlling Ae. aegypti mosquito237. Hydrophobic nanosilica at 112.5 ppm is effective against mosquito species238.

Assessment of public awareness on dengue virus infection

Dengue is one of the major public health problems which can be controlled with active participation of the community. Need is to organize health education programmes about dengue disease to increase community knowledge and sensitize the community to participate in integrated vector control programmes229239.

Conclusions

Dengue disease continues to involve newer areas, newer populations and is increasing in magnitude, epidemic after epidemic. Every aspect of dengue viral infection continues to be a challenge; the pathogenesis of severe dengue disease is not known, no vaccine is yet available for protection and the vector control measures are inadequate. Dengue virus was isolated in India in 1944, but the scientific studies addressing various problems of dengue disease have been carried out at limited number of centres. Though clinical studies have reported on dengue disease in India, but these are largely based on diagnosis made by kits of doubtful specificity and sensitivity. A lot more remains to be achieved for creating an impact.