Togaviruses and Flaviviruses 511.E1

C H A P T E R

TOGAVIRUSES AND 52 FLAVIVIRUSES

A 5-year-old Indonesian girl died of hemorrhagic shock. The presence of dengue virus serotype 3 in her blood was conﬁrmed by reverse transcriptase polymerase chain reaction (RT-PCR). 1. How was the child infected with dengue virus? 2. What are the diseases caused by dengue virus? 3. What types of immune responses are protective? Potentially harmful? 4. Where is dengue prevalent? Why? Answers to these questions are available on StudentConsult.com.

• SUMMARIES CLINICALLY SIGNIFICANT ORGANISMS

Togaviruses • Rubella: • Virus spreads in blood to neurons and brain Aerosol spread, only infects humans • Prodrome of fulike symptoms due to Trigger Words Unvaccinated individuals at risk interferon and cytokine response Arboviruses: mosquito, encephalitis Fetus at high risk • Arboviruses Rubella: German measles, congenital disease, • Encephalitis viruses: St. Louis, West Nile, rash, vaccine Diagnosis Japanese encephalitis viruses • Hemorrhagic disease: • RT-PCR, ELISA Biology, Virulence, and Disease Yellow fever: jaundice, black vomit Dengue: hemorrhagic fever, breakbone • Small size, envelope surrounds icosahedral Treatment, Prevention, and Control nucleocapsid, (+) RNA genome fever, dengue shock syndrome • Encodes RNA-dependent RNA polymerase, • Arboviruses: mosquito control replicates in cytoplasm • Live attenuated rubella vaccine at 1 year of Epidemiology age in MMR; booster at 4-6 years • Early and late mRNA and proteins produced • Endemic to habitat of mosquito • Antibody can block disease • Arboviruses: Flaviviruses • Virus spreads in blood to neurons and brain Zoonosis • Prodrome of fulike symptoms due to Trigger Words Reservoir in birds interferon and cytokine response Arboviruses: mosquito, encephalitis Vectors are Aedes or Culex mosquitoes • Arboviruses: equine encephalitis viruses Hepatitis C virus: see Chapter 55, Hepatitis (WEE, EEE, VEE) Viruses Diagnosis • Rubella: benign childhood rash, swollen glands. Adult complications: arthritis, • RT-PCR, ELISA Biology, Virulence, and Disease encephalitis. Congenital infection: teratogenic, cataracts, deafness, • Small size, envelope surrounds icosahedral Treatment, Prevention, and Control microcephaly, etc. nucleocapsid, (+) RNA genome • Arboviruses: mosquito control • Encodes RNA-dependent RNA polymerase, • Yellow fever virus: attenuated live vaccine Epidemiology replicates in cytoplasm • Neutralizing antibody can block disease • Arboviruses: • Nonneutralizing antibody promotes dengue Zoonosis virus infection Reservoir in birds Vectors are Aedes and Culex mosquitoes

511 CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 511.E1

Answers 1. Dengue is a mosquito-borne virus. 2. Dengue hemorrhagic fever and dengue shock syndrome. 3. Neutralizing antibody is protective, but a nonneutralizing antibody can facilitate uptake into macrophages, where the virus replicates and travels throughout the body. In addition, immune responses are more intense and exacerbate infammatory responses. 4. Dengue is prevalent where the Aedes mosquito vector is prevalent, in tropical regions of the world, including regions of the United States. 512 MEDICAL MICROBIOLOGY

he members of the Togaviridae and Flaviviridae families • Alphaviruses and Flaviviruses Tare enveloped, positive-sense, single-stranded ribonucleic acid (RNA) viruses (Box 52-1). Te Alphavirus genus Alphaviruses and faviviruses are classifed as arboviruses of togaviruses and Flavivirus are discussed together because because they are usually spread by arthropod vectors. Tese of similarities in the diseases they cause and in their epide- viruses have a very broad host range, including vertebrates miology. Most are transmitted by arthropods and are there- (e.g., mammals, birds, amphibians, reptiles) and inverte- fore arboviruses (arthropod-borne viruses). Tey difer in brates (e.g., mosquitoes, ticks). Diseases spread by animals size, morphology, gene sequence, and replication. or with an animal reservoir are called zoonoses. Examples Te Togaviridae (togaviruses) can be classifed into the of pathogenic alphaviruses and faviviruses are listed in following major genera (Table 52-1): Alphavirus, Rubivirus, Table 52-2. and Arterivirus. No known arteriviruses cause disease in humans, so this genus is not discussed further. Rubella virus Structure and Replication of Alphaviruses is the only member of the Rubivirus group; it is discussed Te alphaviruses have an icosahedral capsid and a positive- separately because its disease manifestation (German sense, single-strand RNA genome that resembles messenger measles) and its means of spread difer from those of the RNA (mRNA). Tey are slightly larger than picornaviruses alphaviruses. Te Flaviviridae include the faviviruses, pesti- (45 to 75 nm in diameter) and are surrounded by an viruses, and hepaciviruses (hepatitis C and G viruses). Hepa- titis C and G are discussed in Chapter 55. Table 52-1 Togaviruses and Flaviviruses Virus Group Human Pathogens Box 52-1 Unique Features of Togaviruses and Flaviviruses Togaviruses Viruses have enveloped, single-stranded, positive-sense RNA. Alphavirus Arboviruses Togavirus replication includes early (nonstructural) and late (structural) Rubivirus Rubella virus protein synthesis. Arterivirus None Togaviruses replicate in the cytoplasm and bud at plasma membranes. Flaviviruses Arboviruses Flaviviruses replicate in the cytoplasm and bud at intracellular Hepaciviridae Hepatitis C virus membranes. Pestivirus None

Table 52-2 Arboviruses Virus Vector Host Distribution Disease Alphaviruses Sindbis* Aedes and other mosquitoes Birds Africa, Australia, India Subclinical Semliki Forest* Aedes and other mosquitoes Birds East and West Africa Subclinical Venezuelan equine Aedes, Culex Rodents, horses North, South, and Central America Mild systemic; severe encephalitis encephalitis Eastern equine encephalitis Aedes, Culiseta Birds North and South America, Caribbean Mild systemic; encephalitis Western equine encephalitis Culex, Culiseta Birds North and South America Mild systemic; encephalitis Chikungunya Aedes Humans, monkeys Africa, Asia Fever, arthralgia, arthritis Flaviviruses Dengue* Aedes Humans, monkeys Worldwide, especially tropics Mild systemic; breakbone fever, dengue hemorrhagic fever, and dengue shock syndrome Yellow fever* Aedes Humans, monkeys Africa, South America Hepatitis, hemorrhagic fever Japanese encephalitis Culex Pigs, birds Asia Encephalitis West Nile encephalitis Culex Birds Africa, Europe, Central Asia, North Fever, encephalitis, hepatitis America St. Louis encephalitis Culex Birds North America Encephalitis Russian spring-summer lxodes and Dermacentor Birds Russia Encephalitis encephalitis ticks Powassan encephalitis lxodes ticks Small mammals North America Encephalitis *Prototypical viruses. CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 513 envelope (Latin toga, “cloak”). Te togavirus genome translated in early and late phases. Te initial two thirds of encodes early and late proteins. the alphavirus RNA is translated into a polyprotein that is Alphaviruses have two or three glycoproteins that associ- subsequently cleaved into four nonstructural early proteins ate to form a single spike. Te carboxy (COOH) terminus of (NSPs 1 through 4). Te protease is part of the polyprotein the glycoproteins is anchored in the capsid, forcing the enve- and precedes the site of cleavage. Each of these proteins is a lope to wrap tightly (“shrink-wrap”) and take on the shape portion of the RNA-dependent RNA polymerase. Te of the capsid (Figure 52-1). Te capsid proteins of all the enzymes for replication of the genome assemble on a mem- alphaviruses are similar in structure and are antigenically brane scafold, a full-length 42S negative-sense RNA is syn- cross-reactive. Te viruses can be grouped (complexes) and thesized as a template for replication of the genome, and also distinguished by diferent antigenic determinants on more 42S positive-sense mRNA is produced. In addition, a their envelope glycoproteins. 26S late mRNA, corresponding to one third of the genome, Te alphaviruses attach to specifc receptors expressed on is transcribed from the template. Te 26S RNA encodes the many diferent cell types from many diferent species (Figure capsid (C) and envelope (E1 through E3) proteins. Late in 52-2). Te host range for these viruses includes vertebrates the replication cycle, viral mRNA can account for as much (e.g., humans, monkeys, horses, birds, reptiles, amphibians) as 90% of the mRNA in the infected cell. Te abundance of and invertebrates (e.g., mosquitoes, ticks). However, the late mRNAs allows production of a large amount of the individual viruses have diferent tissue tropisms, accounting structural proteins required for packaging the virus. somewhat for the diferent disease presentations. Te structural proteins are produced by protease cleavage Te virus enters the cell by means of receptor-mediated of the late polyprotein that was produced from the 26S endocytosis (see Figure 52-2). Te viral envelope then fuses mRNA. Te C protein is translated frst and is cleaved from with the membrane of the endosome on acidifcation of the polyprotein. A signal sequence is then made that associ- the vesicle to deliver the capsid and genome into the ates the nascent polypeptide with the endoplasmic reticu- cytoplasm. lum. Tereafer, envelope glycoproteins are translated, Once released into the cytoplasm, the alphavirus genomes glycosylated, and cleaved from the remaining portion of the bind to ribosomes as mRNA. Te alphavirus genome is polyprotein to produce the E1, E2, and E3 glycoprotein

E1, E2 glycoproteins Envelope

E1, E2 glycoproteins

(+) RNA

B Togavirus C Flavivirus

FIGURE 52-1 Alphavirus morphology. A, Morphology of the alphavirus virion obtained from cryoelectron microscopy and image process- ing of the micrographs to show that the envelope is held tightly and conforms to the icosahedral shape and symmetry of the capsid. B, Cross section of alpha-togavirus. Te envelope is tightly associated with the capsid. C, Cross section of favivirus. Te envelope protein surrounds the membrane envelope, which encloses an icosahedral nucleocapsid. RNA, Ribonucleic acid. (A, From Fuller SD: Te T = 4 envelope of Sindbis virus is organized by interactions with a complementary T = 3 capsid, Cell 48:923–934, 1987.) 514 MEDICAL MICROBIOLOGY

Togavirus genome (Sindbis virus)

Nucleus 5؅ 42S genomic RNA 3؅ CAP 1 2 NSP-1 NSP-2 NSP-3 NSP-4 PolyA

C Pe2 6K E1 RNA genome CAP Genome 5 26S late transcript 5′ – + 3 + 7 6 5′ – E1 E3 E2 26s mRNA Nonstructural protein genes Structural protein genes AAA E2 E3 Polyprotein NC Flavivirus genome (yellow fever virus) p230 4 8 9 p270 CAP C PrM E NS-1 NS2a NS3 NS4a NS4b NS5 NS2b

Pr M FIGURE 52-2 Replication of a togavirus. 1, Togaviruses bind to cell Structural protein Nonstructural protein genes genes receptors and are internalized in a coated vesicle. 2, On acidifcation of the endosome, the viral envelope fuses with the endosomal FIGURE 52-3 Comparison of the togavirus (alphavirus) and favi- membrane to release the nucleocapsid into the cytoplasm. 3, Ribo- virus genomes. Alphavirus: Te enzymatic activities are translated somes bind to the positive-sense ribonucleic acid (RNA) genome, from the 5′ end of the input genome, promoting their early rapid and the p230 or p270 (full-length) early polyproteins are made. 4, translation. Te structural proteins are translated later from a Te polyproteins are cleaved to produce nonstructural proteins 1 smaller messenger ribonucleic acid (mRNA) transcribed from the to 4 (NSP1 to NSP4), which include a polymerase to transcribe the genomic template. Flavivirus: Te genes for the structural proteins genome into a negative-sense RNA template. 5, Te replication of the faviviruses are at the 5′ end of the genome/mRNA, and only enzymes assemble onto cellular membrane scafolds and the tem- one species of polyprotein is made, which represents the entire plate is used to produce a full-length 42S positive-sense mRNA genome. PolyA, Polyadenylate. genome and a late 26S mRNA for the structural proteins. 6, Te capsid (C) protein is translated frst and cleaved. A signal peptide is exposed, the peptide associates with the endoplasmic reticulum Structure and Replication of Flaviviruses 7, where the E glycoproteins are synthesized and glycosylated. Tey are transferred to the Golgi apparatus and then the plasma mem- Te faviviruses also have a positive-strand RNA genome, an brane. 8, Te capsid proteins assemble on the 42S genomic RNA icosahedral capsid, and an envelope but are slightly smaller and then associate with regions of cytoplasmic and plasma mem- than an alphavirus (40 to 65 nm in diameter). Te E viral branes containing the E1, E2, and E3 spike proteins. 9, Budding glycoprotein folds over, pairs up with another E glycoprotein, from the plasma membrane releases the virus. AAA, Polyadenylate; and lies fat across the surface of the virion to form an outer mRNA, messenger ribonucleic acid. protein layer (see Figure 52-1). Most of the faviviruses are antigenically related, and antibodies to one virus may neutralize another virus. Attachment and penetration of the faviviruses occur in the same way as described for the alphaviruses. Antibody spikes. Te E3 is released from most alphavirus glycoprotein can enhance infectivity and promote viral uptake into mac- spikes. Te glycoproteins are processed by the normal cel- rophages, monocytes, and other cells that have Fc receptors lular machinery in the endoplasmic reticulum and Golgi when the virus is coated with antibody. Te major difer- apparatus and are also acetylated and acylated with long- ences between alphaviruses and faviviruses are in the orga- chain fatty acids. Alphavirus glycoproteins are then trans- nization of their genomes and their mechanisms of protein ferred efciently to the plasma membrane. synthesis. Te entire favivirus genome is translated into a Te C proteins associate with the genomic RNA soon single polyprotein in a manner more similar to the process afer their synthesis and form an icosahedral capsid. Once for picornaviruses than for alphaviruses (Figure 52-3). As a this step is completed, the capsid associates with portions of result, there is no temporal distinction in the translation of the membrane expressing the viral glycoproteins. Te alpha- the diferent viral proteins. Te polyprotein produced from virus capsid has binding sites for the C-terminus of the gly- the yellow fever genome contains fve nonstructural proteins, coprotein spike, which pulls the envelope tightly around including a protease and components of the RNA-dependent itself in a manner like shrink-wrapping (see Figures 52-1 and RNA polymerase, plus the capsid and envelope structural 52-2). Alphaviruses are released on budding from the plasma proteins. membrane. Unlike in the alphavirus genome, the structural genes are Of interest, the western equine encephalitis virus (WEEV) at the 5′-end of the favivirus genome. As a result, the por- was created by recombination of two alphaviruses, the tions of the polyprotein containing the structural (not the eastern equine encephalitis virus (EEEV) and the Sindbis catalytic) proteins are synthesized frst and with the greatest virus. Te beginning of the WEEV genome is almost identi- efciency. Tis arrangement may allow production of more cal to EEEV, with similar glycoproteins and virulence genes, structural proteins, but it decreases the efciency of non- whereas the end of the genome resembles Sindbis. structural protein synthesis and the initiation of viral CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 515 replication. Tis feature of faviviruses may contribute to the On biting a host, the female mosquito regurgitates virus- lag before detection of their replication. containing saliva into the victim’s bloodstream. Te virus Te entire favivirus polyprotein associates with the endo- then circulates freely in the host’s plasma and comes into plasmic reticulum membrane and then is cleaved into its contact with susceptible target cells, such as the endothelial components. Unlike the togaviruses, the faviviruses acquire cells of the capillaries, monocytes, dendritic cells, and their envelope by budding into the endoplasmic reticulum macrophages. rather than at the cell surface. Te virus is then released by Te ultimate nature of alphavirus and favivirus disease is exocytosis or cell lysis mechanisms. Tis route is less ef- determined by (1) the specifc tissue tropisms of the indi- cient, and the virus may remain cell associated. vidual virus type, (2) the concentration of infecting virus, and (3) individual responses to the infection. Tese viruses Pathogenesis and Immunity are associated with mild systemic disease, encephalitis, Because the arboviruses are acquired from the bite of an arthrogenic disease, or hemorrhagic disease. arthropod such as a mosquito, knowledge of the course of Te initial viremia produces systemic symptoms such as infection in both the vertebrate host and the invertebrate fever, chills, headaches, backaches, and other fulike symp- vector is important for an understanding of the diseases. toms within 3 to 7 days of infection. Most of these symptoms Tese viruses can cause lytic or persistent infections of both can be attributed to the efects of the interferon and other vertebrate and invertebrate hosts (Box 52-2). Infections of cytokines produced in response to the viremia and infection invertebrates are usually persistent, with continued virus of host cells. Most viral infections do not progress beyond production. the mild systemic disease associated with viremia. A second- Te death of an infected cell results from a combination ary viremia can produce sufcient virus to infect target of virus-induced insults. Te large amount of viral RNA organs (e.g., brain, liver, skin, vasculature), depending on the produced on the replication and transcription of the genome tissue tropism of the virus (Figure 52-4). Te virus gains blocks cellular mRNA from binding to ribosomes. Increased access to the brain by infecting the endothelial cells lining permeability of the target cell membrane and changes in ion the small vessels of the brain or the choroid plexus. Hemor- concentrations can alter enzyme activities and favor the rhagic disease and shock, as for dengue virus, results from translation of viral mRNA over cellular mRNA. Te dis- viral and immune-induced cytolysis of infected vascular placement of cellular mRNA from the protein synthesis endothelial cells exacerbated by extensive cytokine produc- machinery prevents rebuilding and maintenance of the cell tion (cytokine storm), which induces vascular leakage. and is a major cause of the death of the virus-infected cell. Te primary target cells of the f aviviruses are of the Female mosquitoes acquire the alphaviruses and favivi- monocyte-macrophage lineage. Although these cells are ruses by taking a blood meal from a viremic vertebrate host. found throughout the body and may have diferent charac- A sufcient viremia must be maintained in the vertebrate host teristics, they express Fc receptors for antibody and release to allow acquisition of the virus by the mosquito. Te virus cytokines on challenge. Flavivirus infection is enhanced 200- then infects the epithelial cells of the midgut of the mosquito, to 1000-fold by nonneutralizing antiviral antibody that pro- spreads through the basal lamina of the midgut to the circu- motes binding of the virus to the Fc receptors and its uptake lation, and infects the salivary glands. Te virus sets up a into the cell. persistent infection and replicates to high titers in these cells. Te salivary glands can then release virus into the saliva. Not Immune Response all arthropod species support this type of infection, however. Replication of the alphaviruses and faviviruses produces a For example, the normal vector for WEEV is the Culex tar- double-stranded RNA replicative intermediate that is a good salis mosquito, but certain strains of virus are limited to the inducer of interferon (IFN)-α and IFN-β. Te interferon midgut of this mosquito, cannot infect its salivary glands, limits replication of the virus and is also released into the and therefore cannot be transmitted to humans. bloodstream to stimulate innate and immune responses.

Box 52-2 Disease Mechanisms of Togaviruses and Flaviviruses

Viruses are cytolytic, except for rubella and hepatitis C. Viruses establish viremia and systemic infection. Viruses are good inducers of interferon and cytokines, which can account for the fulike symptoms during prodrome. Viruses, except rubella and hepatitis C, are arboviruses. Flaviviruses can infect cells of the monocyte-macrophage lineage. Nonneutralizing antibody can enhance favivirus infection via Fc receptors on cells. Flulike Syndrome Encephalitis Hepatitis Hemorrhage Shock Dengue + + + + Yellow fever + + + + St. Louis encephalitis + + West Nile encephalitis + + Venezuelan encephalitis + + Western equine encephalitis + + Eastern equine encephalitis + + Japanese encephalitis + + 516 MEDICAL MICROBIOLOGY

Box 52-3 Epidemiology of Alphavirus and Flavivirus Infection

Disease/Viral Factors Enveloped virus must stay wet and can be inactivated by drying, soap, and Mild or asymptomatic detergents. 2-3 presentation Virus can infect mammals, birds, reptiles, and insects. days Asymptomatic or nonspecifc (fulike fever or chills), encephalitis, hemorrhagic fever, or arthritis Transmission Prodrome Specifc arthropods characteristic of each virus (zoonosis: arbovirus) Viremia Who Is at Risk? People who enter ecologic niche of arthropods infected by arboviruses

Mild systemic Geography/Season disease, fever, Endemic regions for each arbovirus are determined by habitat of mosquito aches, chills 3-7 or other vector. days Aedes mosquito, which carries dengue and yellow fever, is found in urban areas and in pools of water. Vascular Macrophage Liver Spleen, Culex mosquito, which carries St. Louis encephalitis and West Nile endothelium lymph nodes encephalitis viruses, is found in forest and urban areas. Disease is more common in summer. Modes of Control Viremia Mosquito breeding sites and mosquitoes should be eliminated. Severe or life-threatening Live attenuated yellow fever virus and inactivated Japanese encephalitis presentation virus vaccines Encephalitis

Hemorrhagic fever response can destroy tissues and signifcantly contribute to the pathogenesis of encephalitis. Hypersensitivity reactions Yellow fever, (initiated by formation of immune complexes with virions hepatitis and viral antigens) and the activation of complement can DHF/DSS cause arthritides and contribute to hemorrhagic symptoms. Severe systemic disease An antibody to another favivirus that does not neutralize the virus can enhance the uptake of faviviruses into macro- FIGURE 52-4 Disease syndromes of the alphaviruses and favivi- phages and other cells that express Fc receptors. Immune ruses. Primary viremia may be associated with mild systemic responses to a related strain of dengue virus that do not disease. Most infections are limited to this. If sufcient virus is prevent infection can exacerbate immunopathogenesis, produced during the secondary viremia to escape innate and leading to dengue hemorrhagic fever or dengue shock immune protection and to reach critical target tissues, severe sys- syndrome. temic disease or encephalitis may result. If antibody is present (X), viremia is blocked. For dengue virus, rechallenge with another Epidemiology strain can result in severe dengue hemorrhagic fever (DHF), which Alphaviruses and most faviviruses are prototypical arbovi- can cause dengue shock syndrome (DSS) because of the loss of ruses (Box 52-3). To be an arbovirus, the virus must be able fuids from the vasculature. to (1) infect both vertebrates and invertebrates, (2) initiate a sufcient viremia in a vertebrate host for a sufcient time to Interferon and other cytokines are produced following infec- allow acquisition of the virus by the invertebrate vector, and tion of plasmacytoid dendritic and other cells in blood, (3) initiate a persistent productive infection of the salivary causing rapid onset of the fulike symptoms characteristic of gland of the invertebrate to provide virus for the infection of mild systemic disease. other host animals. Humans are usually “dead-end” hosts Circulating immunoglobulin (Ig)M is produced within 6 in that they cannot spread the virus back to the vector days of infection, followed by production of IgG. Antibody because they do not maintain a persistent viremia. If the virus to the viral attachment protein blocks viremic spread of the is not in the blood, the mosquito cannot acquire it. A full cycle virus and subsequent infection of other tissues. Trough of infection occurs when the virus is transmitted by the recognition of the type-common antigens expressed on all arthropod vector and amplifed in a susceptible, immuno- viruses in the family, immunity to one favivirus can provide logically naïve host (reservoir), allowing reinfection of other some protection against infection with other faviviruses. arthropods (Figure 52-5). Te vectors, natural hosts, and Cell-mediated immunity is also important in controlling the geographic distribution of representative alphaviruses and primary infection. faviviruses are listed in Table 52-2. Immunity to these viruses is a double-edged sword. Tese viruses are usually restricted to a specifc arthropod Infammation resulting from the cell-mediated immune vector, its vertebrate host, and their ecologic niche. Te most CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 517

Natural host Vector Host through blood transfusions. Documentation of two such cases has led to screening blood donors for WNV and reject- ing donors who have fever and headache during the week of WEEV blood donation. EEEV Arbovirus diseases occur during the summer months and rainy seasons, when the arthropods breed, and the arbovi- (Culex) ruses are cycled among a host reservoir (birds), an arthropod (e.g., mosquitoes), and human hosts. Tis cycle maintains and increases the amount of virus in the environment. In the winter, the vector is not present to maintain the virus. Te VEEV virus may either (1) persist in arthropod larvae or eggs or in reptiles or amphibians that remain in the locale or (2) migrate (Culex, Aedes) with the birds and then return during the summer. When humans travel into the ecologic niche of the mosquito vector, they risk being infected by the virus. Pools of St. Louis, standing water, drainage ditches, and trash dumps in cities West Nile can also provide breeding grounds for mosquitoes such as encephalitis Aedes aegypti, the vector for yellow fever, dengue, and chi- (Culex) kungunya viruses. An increase in the population of these Dengue mosquitoes, as has occurred in the United States, puts the Yellow fever human population at risk for infection. Health departments Jungle Urban in many areas monitor birds and mosquitoes caught in traps (Aedes) for arboviruses and initiate control measures such as insec- ticide spraying when necessary. Russian Urban outbreaks of arbovirus infections occur when the spring- summer reservoirs for the virus are humans or urban animals. encephalitis Humans can be reservoir hosts for yellow fever, dengue, and (Ixodes) chikungunya viruses (see Figure 52-5). Tese viruses are maintained by Aedes mosquitoes in a sylvatic or jungle Milk cycle, in which monkeys are the natural host, and also in an urban cycle, in which humans are the host. A. aegypti, a FIGURE 52-5 Patterns of alphavirus and favivirus transmission. vector for each of these viruses, is a household mosquito. It Birds and small mammals are the hosts that maintain and amplify breeds in pools of water, open sewers, and other accumula- an arbovirus, which is spread by the insect vector upon a blood tions of water in cities. St. Louis encephalitis and WNV are meal. A double arrow indicates a cycle of replication in both host maintained in an urban environment because their vectors, (including man) and vector. “Dead-end” infections with no trans- Culex mosquitoes, breed in stagnant water, including puddles mission of the virus back to the vector are indicated by the single and sewage, and the reservoir group includes common city arrow. EEEV, Eastern equine encephalitis virus; VEEV, Venezuelan birds (e.g., crows). A large number of inapparent infections equine encephalitis virus; WEEV, western equine encephalitis virus. accompanies the incidence or an outbreak of arbovirus encephalitis. common vector is the mosquito, but ticks and sandfies spread some arboviruses. Even in a tropical region overrun Clinical Syndromes with mosquitoes, spread of these viruses is still restricted to More humans are infected with alphaviruses and faviviruses a specifc genus of mosquitoes. Not all arthropods can act as than show signifcant characteristic symptoms. Te inci- good vectors for each virus. For example, Culex quinquefas- dence of arbovirus disease is sporadic. Alphavirus infections ciatus is resistant to infection by WEEV (alphavirus) but are usually asymptomatic or cause low-grade disease such as is an excellent vector for St. Louis encephalitis virus fulike symptoms (chills, fever, rash, aches) that correlate (favivirus). with systemic infection during the initial viremia. EEEV, Birds and small mammals are the usual reservoir hosts WEEV, and Venezuelan equine encephalitis virus (VEEV) for the alphaviruses and faviviruses, but reptiles and amphib- infections can progress to encephalitis in humans. Te ians can also act as hosts. A large population of viremic equine encephalitis viruses are usually more of a problem to animals can develop in these species to continue the infec- livestock than to humans. An afected human may experi- tion cycle of the virus. For example, West Nile encephalitis ence fever, headache, and decreased consciousness 3 to 10 virus (WNV) was frst noted in 1999 as an outbreak in New days afer infection. Unlike herpes simplex virus encephali- York by the unusual deaths of captive birds at the Bronx Zoo. tis, the disease generally resolves without signifcant sequelae, Reverse transcriptase polymerase chain reaction (RT-PCR) but there is the possibility of paralysis, mental disability, analysis identifed the virus as WNV. Te virus is transmitted seizures, and death. Te name chikungunya (Swahili for by Culex pipiens mosquitoes, and crows, blue jays, and other “that which bends up”) refers to the crippling arthritis associ- wild birds are the reservoir. Te virus spread throughout the ated with serious disease caused by infection with these United States, and by 2006, the virus and human disease had viruses. Like dengue virus, chikungunya virus is spread by been noted in almost every state. WNV establishes a suf- A. aegypti, but a recently developed mutant virus can be cient viremia in humans to be a risk factor for transmission spread by A. albopictus (the Asian tiger mosquito). Te 518 MEDICAL MICROBIOLOGY incidence of chikungunya has greatly increased since 2000. Te virus and its vector are present in central and northern Tis disease is prevalent from western Africa across southern South America, and cases have occurred in Puerto Rico, Asia to the Philippines and in South America and has spread Texas, and Florida. Te incidence of the more serious DHF to the Caribbean Islands and United States because of the has quadrupled since 1985. Dengue fever is also known as return of the A. aegypti mosquito, its vector. breakbone fever; the symptoms and signs consist of high Most favivirus infections are relatively benign, but serious fever, headache, rash, and back and bone pain that last 6 to aseptic meningitis and encephalitic or hemorrhagic 7 days. On rechallenge with another of the four related disease can occur. Te encephalitis viruses include St. Louis, strains, dengue can also cause DHF and dengue shock syn- West Nile, Japanese, Murray Valley, and Russian spring- drome (DSS). Nonneutralizing antibody promotes uptake of summer viruses. Symptoms and outcomes are similar to the virus into macrophages, which causes memory T cells to those of the togavirus encephalitides. Hundreds to thou- become activated, release cytokines, and initiate infamma- sands of cases of St. Louis encephalitis virus disease are noted tory reactions. Tese reactions and the virus result in weak- in the United States annually. Approximately 20% of indi- ening and rupture of the vasculature, internal bleeding, and viduals infected with WNV will develop West Nile fever, loss of plasma, leading to shock symptoms and internal characterized by fever, headache, tiredness, and body aches, bleeding. In 1981 in Cuba, dengue-2 virus infected a popula- occasionally with a rash on the trunk of the body and swollen tion previously exposed to dengue-1 virus between 1977 and lymph glands usually lasting only a few days (Clinical Case 1980, leading to an epidemic of more than 100,000 cases of 52-1). Encephalitis, meningitis, or meningoencephalitis DHF/DSS and 168 deaths. occurs in approximately 1% of WNV-infected individuals. Yellow fever infections are characterized by severe sys- Individuals older than 50 years and the immunocompro- temic disease, with degeneration of the liver, kidney, and mised are at higher risk for serious disease. heart, as well as hemorrhage. Liver involvement causes the Te hemorrhagic viruses are dengue and yellow fever jaundice from which the disease gets its name, but massive viruses. Dengue virus is a major worldwide problem, with gastrointestinal hemorrhages (“black vomit”) may also occur. at least 100 million cases of dengue fever and 300,000 cases Te mortality rate associated with yellow fever during epi- of dengue hemorrhagic fever (DHF) occurring per year. demics is as high as 50%. Laboratory Diagnosis Detection and characterization of the alphaviruses and fa- Clinical Case 52-1 West Nile Encephalitis Virus (WNV) viviruses is now performed by RT-PCR testing of viral Hirsch and Warner (N Engl J Med 348:2239–2247, 2003) described the mRNA in blood or other samples. Monoclonal antibodies to case of a 38-year-old Massachusetts woman who presented with a pro- the individual viruses have become a useful tool for distin- gressively worsening headache with photophobia and fever. Because it guishing the individual species and strains of viruses. Te was August, she was on summer vacation and 10 days earlier (−10) had alphaviruses and faviviruses can be grown in both vertebrate traveled to St. Louis and stayed for 8 days. While there, she walked in the and mosquito cell lines, but most are difcult to isolate. A woods and visited the zoo. A day before the onset of these symptoms (−1), variety of serologic methods can be used to diagnose infec- she vacationed along the Atlantic shore and noted that she had been bitten tions, but the serologic cross-reactivity among viruses limits by mosquitos and removed ticks from her dog. Four days later (+4), she distinction of the actual viral species in many cases. was admitted with fever (40° C), chills, rapid heartbeat, confusion, light- Treatment, Prevention, and Control headedness, and lethargy. Although appearing alert, oriented, and only slightly ill, her neck was rigid and Kernig sign was present. The signs of No treatments exist for arbovirus diseases, other than sup- meningitis prompted testing of cerebrospinal fuid, which contained immu- portive care. Te easiest means of preventing the spread of any noglobulin (Ig)M to WNV and low titers to St. Louis encephalitis (SLE) virus. arbovirus is elimination of its vector and breeding grounds. Patient antibody neutralized WNV but not SLE virus infection of tissue Afer 1900, when Walter Reed and his colleagues discovered culture cells, suggesting that the activity to SLE was due to cross-reactivity that yellow fever was spread by A. aegypti, the number of between faviviruses. Tests for other organisms were negative. She was cases was reduced from 1400 to none within 2 years, purely treated empirically for meningitis and for herpes simplex virus (HSV) through control of the mosquito population. Many public (acyclovir). Antibacterial and anti-HSV treatment for meningitis and health departments monitor bird and mosquito populations encephalitis were necessary until the laboratory results were available. On in a region for arboviruses and periodically spray to reduce day 5 post onset, she became more lethargic and had diffculty answering the mosquito population. Avoidance of the breeding grounds questions. Magnetic resonance imaging (MRI) indicated subtle changes in of a mosquito vector is also a good preventive measure. the brain. On day 6, she could not distinguish her right from her left hand, A live vaccine against yellow fever virus and killed vac- but her headache lessened, and she could respond to commands. On day cines against EEEV, WEEV, Japanese encephalitis virus, and 7, she had a tremor in her right arm, but her mental status was improving, Russian spring-summer encephalitis virus are available. A and by day 8, she was alert and lucid. On day 9, a cranial MRI was normal; live Japanese encephalitis virus vaccine is used in China. on day 10, she was recovered; and on day 11, she was released from the Tese vaccines are meant for people working with the virus hospital. The season of the year, exposure to insects, and travel by this or at risk for contact. A live vaccine against VEEV is available woman were suggestive of several different arboviral encephalitis diseases but only for use in domestic animals. Vaccines consisting of in addition to WNV. Viruses in the differential diagnosis included eastern all four strains of dengue virus are being developed to ensure equine encephalitis, SLE, Powassan virus (tick-borne favivirus), HSV, and that immune enhancement of the disease on subsequent WNV. Unlike HSV encephalitis, favivirus meningoencephalitis usually challenge does not occur. An interesting approach to the resolves with limited sequelae. dengue virus vaccine consists of chimeric viruses in which the glycoprotein and other genes for each of the other dengue CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 519 virus strains is inserted into either an attenuated dengue 2 virus or the 17D yellow fever virus. Te yellow fever vaccine is prepared from the 17D strain isolated from a patient in 1927 and grown for long periods in monkeys, mosquitoes, embryonic tissue culture, and embryonated eggs. Te vaccine is administered intrader- mally and elicits lifelong immunity to yellow fever and pos- sibly other cross-reacting faviviruses.

• Rubella Virus

Rubella virus has the same structural properties and mode of replication as the other togaviruses. However, unlike the other togaviruses, rubella is a respiratory virus and does not Lymph Liver cause readily detectable cytopathologic efects. nodes Rubella is one of the fve classic childhood exanthems, along with measles, roseola, ffh disease, and chickenpox. Rubella, meaning “little red” in Latin, was frst distinguished Spleen from measles and other exanthems by German physicians; thus the common name for the disease, German measles. In Macrophage 1941, an astute Australian ophthalmologist, Norman McAli- ster Gregg, recognized that maternal rubella infection was Primary the cause of congenital cataracts. Maternal rubella infection Reticuloendothelial viremia has since been correlated with several other severe congeni- system tal defects. Tis fnding prompted the development of a unique program to vaccinate children to prevent infection of Tissue and pregnant women and neonates. skin Pathogenesis and Immunity Rubella virus is not cytolytic, but the replication of rubella Secondary prevents (in a process known as heterologous interference) viremia the replication of superinfecting picornaviruses. Tis prop- erty allowed the frst isolations of rubella virus in 1962. Placenta, Rubella infects the upper respiratory tract and then fetus spreads to local lymph nodes, which coincides with a period of lymphadenopathy (Figure 52-6). Tis stage is followed by Congenital establishment of viremia, which spreads the virus through- infection out the body. Infection of other tissues and the characteristic mild rash occur. Te prodromal period lasts approximately 2 weeks (Figure 52-7). Te infected person can shed virus in respiratory droplets during the prodromal period and for as long as 2 weeks afer the onset of the rash. Immune Response FIGURE 52-6 Spread of rubella virus within the host. Rubella Antibody is generated afer the viremia, and its appearance enters and infects the nasopharynx and lung and then spreads to correlates with the appearance of the rash. Te antibody the lymph nodes and monocyte-macrophage system. Te resulting limits viremic spread, but cell-mediated immunity plays an viremia spreads the virus to other tissues and the skin. Circulating important role in resolving the infection. Only one serotype antibody can block the transfer of virus at the indicated points (X). of rubella exists, and natural infection produces lifelong pro- In an immunologically defcient pregnant woman, the virus can tective immunity. Most important, serum antibody in a preg- infect the placenta and spread to the fetus. nant woman prevents spread of the virus to the fetus. Immune complexes most likely cause the rash and arthralgia associated with rubella infection. chromosomal structure of the cells of the fetus can be altered Congenital Infection by the infection. Te alterations can lead to improper devel- Rubella infection in a pregnant woman can result in serious opment of the fetus, small size of the infected baby, and the congenital abnormalities in the child. If the mother does not teratogenic efects associated with congenital rubella infec- have antibody, the virus can replicate in the placenta and tion. Te nature of the disorder is determined by the (1) spread to the fetal blood supply and throughout the fetus. tissue afected and (2) stage of development disrupted. Since Rubella can replicate in most tissues of the fetus. Te virus the vaccine era, cytomegalovirus has replaced rubella as the may not be cytolytic, but the normal growth, mitosis, and most common cause of congenital defects. 520 MEDICAL MICROBIOLOGY

100 Table 52-3 Estimated Morbidity Associated with the Virus in 1964-1965 U.S. Rubella Epidemic pharynx 80 Clinical Events Number Affected Viremia Rubella cases 12,500,000 60 Arthritis-arthralgia 159,375 Rash Encephalitis 2084 40 Lymphadenopathy

Percent positive Deaths Fever Excess neonatal deaths 2100 20 Other deaths 60 TOTAL DEATHS 2160 0 Excess fetal wastage 6250 ؊9 ؊6 ؊3 0 ؉3 ؉6 ؊12 ؉7 ؉11 ؉15 Congenital rubella syndrome to to to to ؊10 ؉10 ؉14 ؉19 Deaf children 8055 Prodrome Posteruption Deaf/blind children 3580 Onset of rash Mentally retarded children 1790 Other congenital rubella syndrome symptoms 6575 FIGURE 52-7 Time course of rubella disease. Rubella production TOTAL CONGENITAL RUBELLA SYNDROME 20,000 in the pharynx precedes the appearance of symptoms and continues Therapeutic abortions 5000 throughout the course of the disease. Te onset of lymphadenopathy coincides with the viremia. Fever and rash occur later. Te From National Communicable Disease Center: Rubella surveillance, Report No. 1, person is infectious as long as the virus is produced in the pharynx. Washington, DC, June 1969, U.S. Department of Health, Education, and Welfare. (Modifed from Plotkin SA, Orenstein WA, Oft PA: Vaccines, ed 5, Philadelphia, 2008, Saunders.)

Box 52-4 Epidemiology of Rubella Virus

Disease/Viral Factors Rubella infects only humans. Virus can cause asymptomatic disease. There is one serotype. Transmission Respiratory route Who Is at Risk? Children: mild exanthematous disease Adults: more severe disease with arthritis or arthralgia FIGURE 52-8 Fetus < 20 weeks: congenital defects Close-up of the rubella rash. Small erythematous macules are visible. (From Hart CA, Broadwell RL: A color atlas of Modes of Control pediatric infectious disease, London, 1992, Wolfe.) Live attenuated vaccine administered as part of the measles-mumps- rubella vaccine Before the development and use of the rubella vaccine, cases of rubella in schoolchildren would be reported every Te virus may persist in tissues such as the lens of the eye spring, and major epidemics of rubella occurred at regular for 3 to 4 years and may be shed up to a year afer birth. 6- to 9-year intervals. Te severity of the 1964-1965 epidemic Presence of the virus during the development of the baby’s in the United States is shown in Table 52-3. Congenital immune response may even have a tolerogenic efect on the rubella occurred in as many as 1% of all the children born system, preventing efective clearance of the virus afer birth. in cities such as Philadelphia during this epidemic. Te immunization program has succeeded in eliminating Epidemiology endemic rubella virus infection in the United States. Humans are the only host for rubella (Box 52-4). Te virus is spread in respiratory secretions and is generally acquired Clinical Syndromes during childhood. Spread of virus, before or in the absence Rubella disease is normally benign in children. Afer a 14- to of symptoms, and crowded conditions (e.g., day-care centers) 21-day incubation period, the symptoms in children consist promote contagion. of a 3-day maculopapular or macular rash and swollen Approximately 20% of women of childbearing age escape glands (Figure 52-8). Infection in adults, however, can be infection during childhood and are susceptible to infection more severe and include problems such as bone and joint unless vaccinated. Programs in many U.S. states test expect- pain (arthralgia and arthritis) and (rarely) thrombocytope- ant mothers for antibodies to rubella. nia or postinfectious encephalopathy. Immunopathologic CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 521

Box 52-5 Prominent Clinical Findings in Congenital Rubella Rubella cases per CRS cases per Syndrome 100,000 population 100,000 live births 30 Cataracts and other ocular defects 14 Heart defects 26 Rubella, total Rubella, Ն15-year-old Deafness Vaccine 12 CRS Intrauterine growth retardation 22 licensed Failure to thrive 10 Mortality within the frst year 18 Microcephaly 8 Mental retardation 14 6 10 4 6 2 Box 52-6 Clinical Summaries 2 West Nile encephalitis: During August, a 70-year-old man from a 0 swampy area of Louisiana develops fever, headache, muscle weak- 1966 68 70 72 74 76 78 80 82 84 ness, nausea, and vomiting. He has diffculty answering questions. He Year progresses into a coma. Magnetic resonance imaging results show no FIGURE 52-9 Efect of rubella virus vaccination on the incidence specifc localization of lesions (unlike in herpes simplex virus encepha- of rubella and congenital rubella syndrome (CRS). (Modifed from litis). His disease progresses to respiratory failure and death. His Williams MN, Preblud SR: Current trends: rubella and congenital 25-year-old niece, living next door, complains of sudden onset of fever rubella—United States, 1983, MMWR Morb Mortal Wkly Rep (39° C [102.2° F]), headache, and myalgias, with nausea and vomiting 33:237–247, 1984.) lasting 4 days. (See website: www.postgradmed.com/issues/2003/ 07_03/gelfand.shtml.) Yellow fever: A 42-year-old man had fever (103° F), headache, vomiting, and backache that started 3 days after returning from a trip to Central Treatment, Prevention, and Control America. He appeared normal for a short time, but then his gums No treatment is available for rubella. Te best means of pre- started to bleed, he had bloody urine and vomited blood, and he venting rubella is vaccination with the live cold-adapted developed petechiae, jaundice, and a slower and weakened pulse. He RA27/3 vaccine strain of virus (Figure 52-9). Te live rubella started to improve 10 days after the onset of disease. vaccine is usually administered with the measles and mumps Rubella: A 6-year-old girl from Romania developed a faint rash on her vaccines (MMR vaccine) afer 12 months of age. Te triple face, accompanied by mild fever and lymphadenopathy. Over the next vaccine is included routinely in well-baby care. Vaccination 3 days, the rash progressed to other parts of the body. She had no promotes both humoral and cellular immunity. history of rubella immunization. Te primary reason for the rubella vaccination program is to prevent congenital infection by decreasing the number of susceptible people in the population, especially children. As a result, there are fewer seronegative mothers and a efects resulting from cell-mediated immunity and hypersen- smaller chance they will be exposed to the virus from contact sitivity reactions are a major cause of the more severe forms with infectious children. Because only one serotype for of rubella in adults. rubella exists and humans are the only reservoir, vaccination Congenital disease is the most serious outcome of rubella of a large proportion of the population can signifcantly infection. Te fetus is at major risk until the 20th week of reduce the likelihood of exposure to the virus. pregnancy. Maternal immunity to the virus resulting from prior exposure or vaccination prevents spread of the virus to the fetus. Te most common manifestations of congenital Bibliography rubella infection are cataracts, mental retardation, cardiac abnormalities, and deafness (Boxes 52-5 and 52-6; see Table Chambers TJ, Hahn CS, Galler R, et al: Flavivirus genome organization, expression, and replication, Annu Rev Microbiol 44:649–688, 1990. 52-3). Te mortality in utero and within the frst year afer Chambers TJ, Monath TP: Te faviviruses: detection, diagnosis, and birth is high for afected babies. vaccine development, vol 61; Te faviviruses: pathogenesis and immunity, vol 60. In Adv Virus Res, San Diego, 2003, Elsevier Academic. Laboratory Diagnosis Cohen J, Powderly WG: Infectious diseases, ed 2, St Louis, 2004, Mosby. Isolation of the rubella virus is difcult and rarely attempted. Fernandez-Garcia MD, Mazzon M, Jacobs M, et al: Pathogenesis of favivirus infections: using and abusing the host cell, Cell Host Microbe 5:318– When isolation of the virus is necessary, the virus is usually 328, 2009. obtained from urine. Presence of the virus can be detected Flint SJ, Enquist LW, Racaniello VR, et al: Principles of virology: molecular by RT-PCR detection of viral RNA. Te diagnosis is usually biology, pathogenesis and control of animal viruses, ed 3, Washington, confrmed by the presence of antirubella-specifc IgM. Anti- DC, 2009, American Society for Microbiology Press. Gelfand MS: West Nile virus infection. What you need to know about this bodies to rubella are assayed early in pregnancy to determine emerging threat, Postgrad Med 114:31–38, 2003. the immune status of the woman; this test is required in Gorbach SL, Bartlett JG, Blacklow NR: Infectious diseases, ed 3, Philadelphia, many states. 2004, Saunders. 522 MEDICAL MICROBIOLOGY

Gould EA, Solomon T: Pathogenic faviviruses, Lancet 371:500–509, 2008. Nash D, Mostashari F, Fine A, et al: Te outbreak of West Nile virus infection Guabiraba R, Ryfel B: Dengue virus infection: current concepts in immune in the New York City area in 1999, N Engl J Med 344:1807–1814, 2001. mechanisms and lessons from murine models, Immunology 141:143– Plotkin SA, Reef S: Rubella vaccine. In Plotkin SA, Orenstein WA, editors: 156, 2014. Vaccines, ed 4, Philadelphia, 2004, Saunders. Johnson RT: Viral infections of the nervous system, Philadelphia, 1998, Richman DD, Whitley RJ, Hayden FG: Clinical virology, ed 3, Washington, Lippincott-Raven. DC, 2009, American Society for Microbiology Press. Knipe DM, Howley PM: Fields virology, ed 6, Philadelphia, 2013, Lippincott Strauss JM, Strauss EG: Viruses and human disease, ed 2, San Diego, 2007, Williams & Wilkins. Academic. Koblet H: Te “merry-go-round”: alphaviruses between vertebrate and Tsai TF: Arboviral infections in the United States, Infect Dis Clin North Am invertebrate cells, Adv Virus Res 38:343–403, 1990. 5:73–102, 1991. Kuhn RJ, Zhang W, Rossmann MG, et al: Structure of dengue virus: implica- tions for favivirus organization, maturation, and fusion, Cell 108:717– Websites 725, 2002. Centers for Disease Control and Prevention: Arboviruses. www.cdc.gov/ Mackenzie JS, Barrett ADT, Deubel V: Japanese encephalitis and West Nile ncezid/dvbd/. Accessed May 13, 2015. viruses, Curr Top Microbiol Immunol (vol 267), Berlin, 2002, Centers for Disease Control and Prevention: Dengue. www.cdc.gov/ Springer-Verlag. dengue/. Accessed April 22, 2015. Monath TP: Yellow fever vaccine. In Plotkin SA, Orenstein WA, editors: Centers for Disease Control and Prevention: West Nile virus. www.cdc.gov/ Vaccines, ed 4, Philadelphia, 2004, Saunders. ncidod/dvbid/westnile/index.htm. Accessed April 22, 2015. Mukhopadhyay S, Kim BS, Chipman PR, et al: Structure of West Nile virus, Dengue map. www.healthmap.org/dengue/index.php. Accessed April 22, Science 302:248, 2003. 2015. CHAPTER 52 TOGAVIRUSES AND FLAVIVIRUSES 522.E1

Case Studies and Questions 9. All unimmunized individuals are at risk for this infec- A 27-year-old businessman experienced a high fever, serious tion. However, the most serious outcomes occur to the retroorbital headache, and severe joint and back pain 5 days fetus of women who are infected before the 20th week afer he and his family returned from a trip to Malaysia. Te of pregnancy. Rubella causes severe congenital defects. symptoms lasted for 4 days, and then a rash appeared on his 10. Immunization of the populace (especially children) for palms and soles, which lasted for 2 days. At the same time, rubella prevents congenital defects in babies. the man’s 5-year-old son experienced mild fulike symptoms and then collapsed afer 2 to 5 days. Te boy’s hands were cold and clammy, his face was fushed, and his body was warm. Tere were petechiae on his forehead and ecchymoses elsewhere. He bruised very easily. He was breathing rapidly and had a weak, rapid pulse. He then rapidly recovered afer 24 hours. 1. What features of these cases pointed to the diagnosis of dengue virus infection? 2. Of what signifcance was the trip to Malaysia? 3. What was the source of infection in the father and son? 4. What were the signifcance of and the pathogenic basis for the petechiae and ecchymoses in the child? Two weeks afer returning from a trip to Pakistan, a 25-year-old man had arthralgia (joint aches) and a mild rash that started on his face and spread to his body. He recalled that he had felt as if he had the fu a few days before the onset of the rash. Te rash disappeared in 4 days. 5. What features of this case pointed to the diagnosis of rubella infection? 6. Why is it signifcant that the symptoms started afer a trip outside the United States? 7. What precaution could the man have taken to prevent this infection? 8. How was this infection transmitted? 9. Who was at risk for a serious outcome of this infection? 10. If this disease is normally mild in children, why is their immunization so important?

Answers 1. Te diagnosis of dengue virus infection is indicated by the disease signs of high fever, severe headache, and joint and back pain. His trip to Malaysia would have increased his risk of exposure to Aedes mosquitoes car- rying the virus. 2. Te Aedes mosquito is endemic in Malaysia and is a carrier of dengue virus, which is prevalent in Malaysia. 3. Te virus was transmitted independently by diferent mosquitoes to the father and son. 4. Petechiae and ecchymoses are indicators of hemorrhagic disease. 5. Te diagnosis of rubella infection is suggested by the arthralgia and especially the mild rash. Tese immune- mediated responses occur afer viral replication and viremic spread, which induces interferon, causing the fulike syndrome. 6. Exposure to rubella in the United States is unlikely because of the efective vaccine program there. 7. If the man had been immunized with the measles- mumps-rubella vaccine and received his booster immunization at 15 years of age, he should have been protected against rubella disease. 8. Rubella is the only togavirus that is transmitted by aero- sols as a respiratory virus.