Smarter Decisions,
Better Care

UpToDate synthesizes the most recent medical information into evidence-based practical recommendations clinicians trust to make the right point-of-care decisions.

  • Rigorous editorial process: Evidence-based treatment recommendations
  • World-Renowned physician authors: over 5,100 physician authors and editors around the globe
  • Innovative technology: integrates into the workflow; access from EMRs

Choose from the list below to learn more about subscriptions for a:


Subscribers log in here


Pathogenesis of dengue virus infection

INTRODUCTION

Substantial gaps remain in the basic understanding of the pathogenesis of dengue infection. In large part this limitation is related to the lack of a suitable animal model [1]. Rhesus monkeys develop viremia similar in pattern to humans after dengue virus challenge but do not develop clinical disease. Careful epidemiologic and experimental challenge studies in humans have provided valuable information on dengue virus infection, but detailed data on virus distribution in vivo are available only from small numbers of patients with more severe disease, unusual manifestations, or the later stages of infection. Little pathogenetic information is available concerning milder infections, which constitute the vast majority of cases.

THE DENGUE VIRAL REPLICATION CYCLE

Dengue viruses are members of the family Flaviviridae genus Flavivirus. They are small, enveloped viruses containing a single-strand RNA genome of positive polarity [2]. Dengue viruses infect a wide range of human and nonhuman cell types in vitro. Viral replication involves the following steps:

Attachment to the cell surface

Entry into the cytoplasm

Translation of viral proteins

                  

Subscribers log in here

To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information or to purchase a personal subscription, click below on the option that best describes you:
Literature review current through: Nov 2014. | This topic last updated: Feb 19, 2014.
The content on the UpToDate website is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding any medical questions or conditions. The use of this website is governed by the UpToDate Terms of Use ©2014 UpToDate, Inc.
References
Top
  1. Rico-Hesse R. Dengue virus evolution and virulence models. Clin Infect Dis 2007; 44:1462.
  2. Henchal EA, Putnak JR. The dengue viruses. Clin Microbiol Rev 1990; 3:376.
  3. Anderson R, King AD, Innis BL. Correlation of E protein binding with cell susceptibility to dengue 4 virus infection. J Gen Virol 1992; 73 ( Pt 8):2155.
  4. Modis Y, Ogata S, Clements D, Harrison SC. A ligand-binding pocket in the dengue virus envelope glycoprotein. Proc Natl Acad Sci U S A 2003; 100:6986.
  5. Modis Y, Ogata S, Clements D, Harrison SC. Structure of the dengue virus envelope protein after membrane fusion. Nature 2004; 427:313.
  6. Mukhopadhyay S, Kuhn RJ, Rossmann MG. A structural perspective of the flavivirus life cycle. Nat Rev Microbiol 2005; 3:13.
  7. Chen Y, Maguire T, Hileman RE, et al. Dengue virus infectivity depends on envelope protein binding to target cell heparan sulfate. Nat Med 1997; 3:866.
  8. Tassaneetrithep B, Burgess TH, Granelli-Piperno A, et al. DC-SIGN (CD209) mediates dengue virus infection of human dendritic cells. J Exp Med 2003; 197:823.
  9. Morens DM. Antibody-dependent enhancement of infection and the pathogenesis of viral disease. Clin Infect Dis 1994; 19:500.
  10. Stadler K, Allison SL, Schalich J, Heinz FX. Proteolytic activation of tick-borne encephalitis virus by furin. J Virol 1997; 71:8475.
  11. Marchette NJ, Halstead SB, Falkler WA Jr, et al. Studies on the pathogenesis of dengue infection in monkeys. 3. Sequential distribution of virus in primary and heterologous infections. J Infect Dis 1973; 128:23.
  12. Wu SJ, Grouard-Vogel G, Sun W, et al. Human skin Langerhans cells are targets of dengue virus infection. Nat Med 2000; 6:816.
  13. Kyle JL, Beatty PR, Harris E. Dengue virus infects macrophages and dendritic cells in a mouse model of infection. J Infect Dis 2007; 195:1808.
  14. SABIN AB. Research on dengue during World War II. Am J Trop Med Hyg 1952; 1:30.
  15. Vaughn DW, Green S, Kalayanarooj S, et al. Dengue in the early febrile phase: viremia and antibody responses. J Infect Dis 1997; 176:322.
  16. Scott RM, Nisalak A, Cheamudon U, et al. Isolation of dengue viruses from peripheral blood leukocytes of patients with hemorrhagic fever. J Infect Dis 1980; 141:1.
  17. King AD, Nisalak A, Kalayanrooj S, et al. B cells are the principal circulating mononuclear cells infected by dengue virus. Southeast Asian J Trop Med Public Health 1999; 30:718.
  18. Durbin AP, Vargas MJ, Wanionek K, et al. Phenotyping of peripheral blood mononuclear cells during acute dengue illness demonstrates infection and increased activation of monocytes in severe cases compared to classic dengue fever. Virology 2008; 376:429.
  19. Rosen L, Khin MM, U T. Recovery of virus from the liver of children with fatal dengue: reflections on the pathogenesis of the disease and its possible analogy with that of yellow fever. Res Virol 1989; 140:351.
  20. Boonpucknavig S, Boonpucknavig V, Bhamarapravati N, Nimmannitya S. Immunofluorescence study of skin rash in patients with dengue hemorrhagic fever. Arch Pathol Lab Med 1979; 103:463.
  21. Hall WC, Crowell TP, Watts DM, et al. Demonstration of yellow fever and dengue antigens in formalin-fixed paraffin-embedded human liver by immunohistochemical analysis. Am J Trop Med Hyg 1991; 45:408.
  22. Jessie K, Fong MY, Devi S, et al. Localization of dengue virus in naturally infected human tissues, by immunohistochemistry and in situ hybridization. J Infect Dis 2004; 189:1411.
  23. Fresh JW, Reyes V, Clarke EJ, Uylangco CV. Philippine hemorrhagic fever: a clinical, laboratory, and necropsy study. J Lab Clin Med 1969; 73:451.
  24. Schmidt AC. Response to dengue fever--the good, the bad, and the ugly? N Engl J Med 2010; 363:484.
  25. Kurane I, Janus J, Ennis FA. Dengue virus infection of human skin fibroblasts in vitro production of IFN-beta, IL-6 and GM-CSF. Arch Virol 1992; 124:21.
  26. Kurane I, Ennis FA. Production of interferon alpha by dengue virus-infected human monocytes. J Gen Virol 1988; 69 ( Pt 2):445.
  27. Libraty DH, Endy TP, Houng HS, et al. Differing influences of virus burden and immune activation on disease severity in secondary dengue-3 virus infections. J Infect Dis 2002; 185:1213.
  28. Kurane I, Hebblewaite D, Brandt WE, Ennis FA. Lysis of dengue virus-infected cells by natural cell-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity. J Virol 1984; 52:223.
  29. Muñoz-Jordan JL, Sánchez-Burgos GG, Laurent-Rolle M, García-Sastre A. Inhibition of interferon signaling by dengue virus. Proc Natl Acad Sci U S A 2003; 100:14333.
  30. Jones M, Davidson A, Hibbert L, et al. Dengue virus inhibits alpha interferon signaling by reducing STAT2 expression. J Virol 2005; 79:5414.
  31. Simmons CP, Popper S, Dolocek C, et al. Patterns of host genome-wide gene transcript abundance in the peripheral blood of patients with acute dengue hemorrhagic fever. J Infect Dis 2007; 195:1097.
  32. Schlesinger JJ, Brandriss MW, Walsh EE. Protection of mice against dengue 2 virus encephalitis by immunization with the dengue 2 virus non-structural glycoprotein NS1. J Gen Virol 1987; 68 ( Pt 3):853.
  33. He RT, Innis BL, Nisalak A, et al. Antibodies that block virus attachment to Vero cells are a major component of the human neutralizing antibody response against dengue virus type 2. J Med Virol 1995; 45:451.
  34. Kaufman BM, Summers PL, Dubois DR, Eckels KH. Monoclonal antibodies against dengue 2 virus E-glycoprotein protect mice against lethal dengue infection. Am J Trop Med Hyg 1987; 36:427.
  35. Kaufman BM, Summers PL, Dubois DR, et al. Monoclonal antibodies for dengue virus prM glycoprotein protect mice against lethal dengue infection. Am J Trop Med Hyg 1989; 41:576.
  36. Pierson TC, Diamond MS. Molecular mechanisms of antibody-mediated neutralisation of flavivirus infection. Expert Rev Mol Med 2008; 10:e12.
  37. Halstead SB. In vivo enhancement of dengue virus infection in rhesus monkeys by passively transferred antibody. J Infect Dis 1979; 140:527.
  38. Goncalvez AP, Engle RE, St Claire M, et al. Monoclonal antibody-mediated enhancement of dengue virus infection in vitro and in vivo and strategies for prevention. Proc Natl Acad Sci U S A 2007; 104:9422.
  39. Balsitis SJ, Williams KL, Lachica R, et al. Lethal antibody enhancement of dengue disease in mice is prevented by Fc modification. PLoS Pathog 2010; 6:e1000790.
  40. Dejnirattisai W, Jumnainsong A, Onsirisakul N, et al. Cross-reacting antibodies enhance dengue virus infection in humans. Science 2010; 328:745.
  41. Gagnon SJ, Ennis FA, Rothman AL. Bystander target cell lysis and cytokine production by dengue virus-specific human CD4(+) cytotoxic T-lymphocyte clones. J Virol 1999; 73:3623.
  42. Mathew A, Kurane I, Rothman AL, et al. Dominant recognition by human CD8+ cytotoxic T lymphocytes of dengue virus nonstructural proteins NS3 and NS1.2a. J Clin Invest 1996; 98:1684.
  43. Kontny U, Kurane I, Ennis FA. Gamma interferon augments Fc gamma receptor-mediated dengue virus infection of human monocytic cells. J Virol 1988; 62:3928.
  44. Anderson KB, Gibbons RV, Cummings DA, et al. A shorter time interval between first and second dengue infections is associated with protection from clinical illness in a school-based cohort in Thailand. J Infect Dis 2014; 209:360.
  45. Montoya M, Gresh L, Mercado JC, et al. Symptomatic versus inapparent outcome in repeat dengue virus infections is influenced by the time interval between infections and study year. PLoS Negl Trop Dis 2013; 7:e2357.
  46. Halstead SB, Shotwell H, Casals J. Studies on the pathogenesis of dengue infection in monkeys. II. Clinical laboratory responses to heterologous infection. J Infect Dis 1973; 128:15.
  47. Vaughn DW, Green S, Kalayanarooj S, et al. Dengue viremia titer, antibody response pattern, and virus serotype correlate with disease severity. J Infect Dis 2000; 181:2.
  48. Murgue B, Roche C, Chungue E, Deparis X. Prospective study of the duration and magnitude of viraemia in children hospitalised during the 1996-1997 dengue-2 outbreak in French Polynesia. J Med Virol 2000; 60:432.
  49. Wang WK, Chen HL, Yang CF, et al. Slower rates of clearance of viral load and virus-containing immune complexes in patients with dengue hemorrhagic fever. Clin Infect Dis 2006; 43:1023.
  50. Kuberski T, Rosen L, Reed D, Mataika J. Clinical and laboratory observations on patients with primary and secondary dengue type 1 infections with hemorrhagic manifestations in Fiji. Am J Trop Med Hyg 1977; 26:775.
  51. Sudiro TM, Zivny J, Ishiko H, et al. Analysis of plasma viral RNA levels during acute dengue virus infection using quantitative competitor reverse transcription-polymerase chain reaction. J Med Virol 2001; 63:29.
  52. Srikiatkhachorn A, Wichit S, Gibbons RV, et al. Dengue viral RNA levels in peripheral blood mononuclear cells are associated with disease severity and preexisting dengue immune status. PLoS One 2012; 7:e51335.
  53. Green S, Pichyangkul S, Vaughn DW, et al. Early CD69 expression on peripheral blood lymphocytes from children with dengue hemorrhagic fever. J Infect Dis 1999; 180:1429.
  54. Mongkolsapaya J, Dejnirattisai W, Xu XN, et al. Original antigenic sin and apoptosis in the pathogenesis of dengue hemorrhagic fever. Nat Med 2003; 9:921.
  55. Zivna I, Green S, Vaughn DW, et al. T cell responses to an HLA-B*07-restricted epitope on the dengue NS3 protein correlate with disease severity. J Immunol 2002; 168:5959.
  56. Simmons CP, Dong T, Chau NV, et al. Early T-cell responses to dengue virus epitopes in Vietnamese adults with secondary dengue virus infections. J Virol 2005; 79:5665.
  57. Friberg H, Bashyam H, Toyosaki-Maeda T, et al. Cross-reactivity and expansion of dengue-specific T cells during acute primary and secondary infections in humans. Sci Rep 2011; 1:51.
  58. Dung NT, Duyen HT, Thuy NT, et al. Timing of CD8+ T cell responses in relation to commencement of capillary leakage in children with dengue. J Immunol 2010; 184:7281.
  59. Zivny J, DeFronzo M, Jarry W, et al. Partial agonist effect influences the CTL response to a heterologous dengue virus serotype. J Immunol 1999; 163:2754.
  60. Mangada MM, Endy TP, Nisalak A, et al. Dengue-specific T cell responses in peripheral blood mononuclear cells obtained prior to secondary dengue virus infections in Thai schoolchildren. J Infect Dis 2002; 185:1697.
  61. Hatch S, Endy TP, Thomas S, et al. Intracellular cytokine production by dengue virus-specific T cells correlates with subclinical secondary infection. J Infect Dis 2011; 203:1282.
  62. Burke DS, Nisalak A, Johnson DE, Scott RM. A prospective study of dengue infections in Bangkok. Am J Trop Med Hyg 1988; 38:172.
  63. Sangkawibha N, Rojanasuphot S, Ahandrik S, et al. Risk factors in dengue shock syndrome: a prospective epidemiologic study in Rayong, Thailand. I. The 1980 outbreak. Am J Epidemiol 1984; 120:653.
  64. Martina BE, Koraka P, Osterhaus AD. Dengue virus pathogenesis: an integrated view. Clin Microbiol Rev 2009; 22:564.
  65. Rico-Hesse R, Harrison LM, Salas RA, et al. Origins of dengue type 2 viruses associated with increased pathogenicity in the Americas. Virology 1997; 230:244.
  66. Watts DM, Porter KR, Putvatana P, et al. Failure of secondary infection with American genotype dengue 2 to cause dengue haemorrhagic fever. Lancet 1999; 354:1431.
  67. Pryor MJ, Carr JM, Hocking H, et al. Replication of dengue virus type 2 in human monocyte-derived macrophages: comparisons of isolates and recombinant viruses with substitutions at amino acid 390 in the envelope glycoprotein. Am J Trop Med Hyg 2001; 65:427.
  68. Cologna R, Rico-Hesse R. American genotype structures decrease dengue virus output from human monocytes and dendritic cells. J Virol 2003; 77:3929.
  69. Guzmán MG, Kourí G, Martínez E, et al. Clinical and serologic study of Cuban children with dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). Bull Pan Am Health Organ 1987; 21:270.
  70. Díaz A, Kourí G, Guzmán MG, et al. Description of the clinical picture of dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) in adults. Bull Pan Am Health Organ 1988; 22:133.
  71. Thein S, Aung MM, Shwe TN, et al. Risk factors in dengue shock syndrome. Am J Trop Med Hyg 1997; 56:566.
  72. Kourí G, Guzmán MG, Bravo J. Hemorrhagic dengue in Cuba: history of an epidemic. Bull Pan Am Health Organ 1986; 20:24.
  73. Guzmán MG, Kouri GP, Bravo J, et al. Dengue hemorrhagic fever in Cuba, 1981: a retrospective seroepidemiologic study. Am J Trop Med Hyg 1990; 42:179.
  74. Kliks SC, Nimmanitya S, Nisalak A, Burke DS. Evidence that maternal dengue antibodies are important in the development of dengue hemorrhagic fever in infants. Am J Trop Med Hyg 1988; 38:411.
  75. Simmons CP, Chau TN, Thuy TT, et al. Maternal antibody and viral factors in the pathogenesis of dengue virus in infants. J Infect Dis 2007; 196:416.
  76. Libraty DH, Acosta LP, Tallo V, et al. A prospective nested case-control study of Dengue in infants: rethinking and refining the antibody-dependent enhancement dengue hemorrhagic fever model. PLoS Med 2009; 6:e1000171.
  77. Thisyakorn U, Nimmannitya S. Nutritional status of children with dengue hemorrhagic fever. Clin Infect Dis 1993; 16:295.
  78. Halstead SB, Streit TG, Lafontant JG, et al. Haiti: absence of dengue hemorrhagic fever despite hyperendemic dengue virus transmission. Am J Trop Med Hyg 2001; 65:180.
  79. de la C Sierra B, Kourí G, Guzmán MG. Race: a risk factor for dengue hemorrhagic fever. Arch Virol 2007; 152:533.
  80. Chiewsilp P, Scott RM, Bhamarapravati N. Histocompatibility antigens and dengue hemorrhagic fever. Am J Trop Med Hyg 1981; 30:1100.
  81. Stephens HA, Klaythong R, Sirikong M, et al. HLA-A and -B allele associations with secondary dengue virus infections correlate with disease severity and the infecting viral serotype in ethnic Thais. Tissue Antigens 2002; 60:309.
  82. Paradoa Pérez ML, Trujillo Y, Basanta P. Association of dengue hemorrhagic fever with the HLA system. Haematologia (Budap) 1987; 20:83.
  83. Loke H, Bethell DB, Phuong CX, et al. Strong HLA class I--restricted T cell responses in dengue hemorrhagic fever: a double-edged sword? J Infect Dis 2001; 184:1369.
  84. Fernández-Mestre MT, Gendzekhadze K, Rivas-Vetencourt P, Layrisse Z. TNF-alpha-308A allele, a possible severity risk factor of hemorrhagic manifestation in dengue fever patients. Tissue Antigens 2004; 64:469.
  85. Loke H, Bethell D, Phuong CX, et al. Susceptibility to dengue hemorrhagic fever in vietnam: evidence of an association with variation in the vitamin d receptor and Fc gamma receptor IIa genes. Am J Trop Med Hyg 2002; 67:102.
  86. Sakuntabhai A, Turbpaiboon C, Casadémont I, et al. A variant in the CD209 promoter is associated with severity of dengue disease. Nat Genet 2005; 37:507.
  87. Kalayanarooj S, Gibbons RV, Vaughn D, et al. Blood group AB is associated with increased risk for severe dengue disease in secondary infections. J Infect Dis 2007; 195:1014.
  88. Sahaphong S, Riengrojpitak S, Bhamarapravati N, Chirachariyavej T. Electron microscopic study of the vascular endothelial cell in dengue hemorrhagic fever. Southeast Asian J Trop Med Public Health 1980; 11:194.
  89. Srikiatkhachorn A, Ajariyakhajorn C, Endy TP, et al. Virus-induced decline in soluble vascular endothelial growth receptor 2 is associated with plasma leakage in dengue hemorrhagic Fever. J Virol 2007; 81:1592.
  90. Avirutnan P, Punyadee N, Noisakran S, et al. Vascular leakage in severe dengue virus infections: a potential role for the nonstructural viral protein NS1 and complement. J Infect Dis 2006; 193:1078.
  91. Bhamarapravati N, Tuchinda P, Boonyapaknavik V. Pathology of Thailand haemorrhagic fever: a study of 100 autopsy cases. Ann Trop Med Parasitol 1967; 61:500.
  92. Anderson R, Wang S, Osiowy C, Issekutz AC. Activation of endothelial cells via antibody-enhanced dengue virus infection of peripheral blood monocytes. J Virol 1997; 71:4226.
  93. Bosch I, Xhaja K, Estevez L, et al. Increased production of interleukin-8 in primary human monocytes and in human epithelial and endothelial cell lines after dengue virus challenge. J Virol 2002; 76:5588.
  94. Talavera D, Castillo AM, Dominguez MC, et al. IL8 release, tight junction and cytoskeleton dynamic reorganization conducive to permeability increase are induced by dengue virus infection of microvascular endothelial monolayers. J Gen Virol 2004; 85:1801.
  95. Hober D, Poli L, Roblin B, et al. Serum levels of tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and interleukin-1 beta (IL-1 beta) in dengue-infected patients. Am J Trop Med Hyg 1993; 48:324.
  96. Bethell DB, Flobbe K, Cao XT, et al. Pathophysiologic and prognostic role of cytokines in dengue hemorrhagic fever. J Infect Dis 1998; 177:778.
  97. Juffrie M, van Der Meer GM, Hack CE, et al. Inflammatory mediators in dengue virus infection in children: interleukin-8 and its relationship to neutrophil degranulation. Infect Immun 2000; 68:702.
  98. Kurane I, Innis BL, Nimmannitya S, et al. Activation of T lymphocytes in dengue virus infections. High levels of soluble interleukin 2 receptor, soluble CD4, soluble CD8, interleukin 2, and interferon-gamma in sera of children with dengue. J Clin Invest 1991; 88:1473.
  99. Green S, Vaughn DW, Kalayanarooj S, et al. Early immune activation in acute dengue illness is related to development of plasma leakage and disease severity. J Infect Dis 1999; 179:755.
  100. Bokisch VA, Top FH Jr, Russell PK, et al. The potential pathogenic role of complement in dengue hemorrhagic shock syndrome. N Engl J Med 1973; 289:996.
  101. Malasit P. Complement and dengue haemorrhagic fever/shock syndrome. Southeast Asian J Trop Med Public Health 1987; 18:316.
  102. Hober D, Delannoy AS, Benyoucef S, et al. High levels of sTNFR p75 and TNF alpha in dengue-infected patients. Microbiol Immunol 1996; 40:569.
  103. Kalayanarooj S, Vaughn DW, Nimmannitya S, et al. Early clinical and laboratory indicators of acute dengue illness. J Infect Dis 1997; 176:313.
  104. BIERMAN HR, NELSON ER. HEMATODEPRESSIVE VIRUS DISEASES OF THAILAND. Ann Intern Med 1965; 62:867.
  105. Rothwell SW, Putnak R, La Russa VF. Dengue-2 virus infection of human bone marrow: characterization of dengue-2 antigen-positive stromal cells. Am J Trop Med Hyg 1996; 54:503.
  106. Nakao S, Lai CJ, Young NS. Dengue virus, a flavivirus, propagates in human bone marrow progenitors and hematopoietic cell lines. Blood 1989; 74:1235.
  107. Murgue B, Cassar O, Guigon M, Chungue E. Dengue virus inhibits human hematopoietic progenitor growth in vitro. J Infect Dis 1997; 175:1497.
  108. Mitrakul C, Poshyachinda M, Futrakul P, et al. Hemostatic and platelet kinetic studies in dengue hemorrhagic fever. Am J Trop Med Hyg 1977; 26:975.
  109. Cardier JE, Rivas B, Romano E, et al. Evidence of vascular damage in dengue disease: demonstration of high levels of soluble cell adhesion molecules and circulating endothelial cells. Endothelium 2006; 13:335.
  110. Sosothikul D, Seksarn P, Pongsewalak S, et al. Activation of endothelial cells, coagulation and fibrinolysis in children with Dengue virus infection. Thromb Haemost 2007; 97:627.
  111. Srichaikul T, Nimmanitaya S, Artchararit N, et al. Fibrinogen metabolism and disseminated intravascular coagulation in dengue hemorrhagic fever. Am J Trop Med Hyg 1977; 26:525.
  112. Chungue E, Poli L, Roche C, et al. Correlation between detection of plasminogen cross-reactive antibodies and hemorrhage in dengue virus infection. J Infect Dis 1994; 170:1304.
  113. Falconar AK. The dengue virus nonstructural-1 protein (NS1) generates antibodies to common epitopes on human blood clotting, integrin/adhesin proteins and binds to human endothelial cells: potential implications in haemorrhagic fever pathogenesis. Arch Virol 1997; 142:897.
  114. Marianneau P, Cardona A, Edelman L, et al. Dengue virus replication in human hepatoma cells activates NF-kappaB which in turn induces apoptotic cell death. J Virol 1997; 71:3244.
  115. Solomon T, Dung NM, Vaughn DW, et al. Neurological manifestations of dengue infection. Lancet 2000; 355:1053.
  116. Ramos C, Sánchez G, Pando RH, et al. Dengue virus in the brain of a fatal case of hemorrhagic dengue fever. J Neurovirol 1998; 4:465.