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38 C H A P T E R Arthropod-Borne and...

38 C H A P T E R Arthropod-Borne and Rodent-Borne Viral Diseases The arthropod-borne viruses (arboviruses) and rodent- borne viruses represent ecologic groupings of viruses with HUMAN ARBOVIRUS INFECTIONS complex transmission cycles involving arthropods or rodents. There are several hundred arboviruses, of which about 100 These viruses have diverse physical and chemical properties are known human pathogens. Those infecting humans are all and are classified in several virus families. believed to be zoonotic, with humans the accidental hosts who Arboviruses and rodent-borne viruses are classified play no important role in the maintenance or transmission cycle among the Arenaviridae, Bunyaviridae, Flaviviridae, of the virus. Exceptions are urban yellow fever, chikungunya, Reoviridae, and Togaviridae families. The African hem- Zika, and dengue. Some of the natural cycles are simple and orrhagic fever viruses are classified in the Filoviridae involve infection of a nonhuman vertebrate host (mammal or family (Table 38-1, Figure 38-1). A number of the diseases bird) transmitted by a species of mosquito or tick (eg, jungle yel- described here are considered emerging infectious diseases low fever, and Colorado tick fever). Others, however, are more (see Chapter 29). complex. For example, tick-borne encephalitis can occur after The arboviruses are transmitted by bloodsucking ingestion of raw milk from goats and cows infected by grazing arthropods from one vertebrate host to another. The vector in tick-infested pastures where a tick–rodent cycle is occurring. acquires a lifelong infection through the ingestion of blood Individual viruses were sometimes named after a dis- from a viremic vertebrate. The viruses multiply in the tis- ease (dengue, yellow fever) or after the geographic area where sues of the arthropod without evidence of disease or damage. the virus was first isolated (St. Louis encephalitis, West Nile Some arboviruses are maintained in nature by transovarian fever). Arboviruses are found in all temperate and tropical transmission in arthropods. zones, but they are most prevalent in the tropics with its The major arbovirus diseases worldwide are yellow abundance of animals and arthropods. fever, dengue, Japanese B encephalitis, St. Louis encephali- Diseases produced by arboviruses may be divided into tis, western equine encephalitis, eastern equine encephalitis, three clinical syndromes: (1) fevers of an undifferentiated type tick-borne encephalitis, West Nile fever, chikungunya fever, with or without a maculopapular rash and usually benign; (2) Zika virus disease, and sandfly fever. In the United States the encephalitis (inflammation of the brain), often with a high case- most important arboviral infections are La Crosse encepha- fatality rate; and (3) hemorrhagic fevers, also frequently severe litis, West Nile fever, St. Louis encephalitis, eastern equine and fatal. These categories are somewhat arbitrary, and many encephalitis, and western equine encephalitis. arboviruses may be associated with more than one syndrome. Rodent-borne viral diseases are maintained in nature by The degree of viral multiplication and its predominant direct intraspecies or interspecies transmission from rodent site of localization in tissues determine the clinical syndrome. to rodent without participation of arthropod vectors. Viral Thus, individual arboviruses can produce a minor febrile ill- infection is usually persistent. Transmission occurs by con- ness in some patients and encephalitis or a hemorrhagic diathe- tact with body fluids or excretions. sis in others. Notably, Zika virus can pass through the placenta Major rodent-borne viral diseases include hantavirus and infect fetal tissue, so it has become a major cause of birth infections, Lassa fever, and South American hemorrhagic defects in regions where transmission occurs (see below). fevers. In the United States, the most important rodent-borne Arbovirus infections occur in distinct geographic dis- diseases are hantavirus pulmonary syndrome (HPS) and tributions and vector patterns (Figure 38-2). Each continent Colorado tick fever. Also considered here are the African tends to have its own arbovirus pattern, and names are usu- hemorrhagic fevers—Marburg and Ebola. Their reservoir ally suggestive, including Venezuelan equine encephali- hosts are unknown but are suspected to be rodents or bats. tis, Japanese B encephalitis, and Murray Valley (Australia) 557 Riedel_CH38_p557-p580.indd 557 04/04/19 5:07 PM  558   SECTION IV  Virology TABLE 38-1 Classification and Properties of Some Arthropod-Borne and Rodent-Borne Viruses Important Arbovirus and Taxonomic Classification Rodent-Borne Virus Members Virus Properties Arenaviridae Genus Arenavirus New World: Chapare, Guanarito, Junin, Machupo, Sabia, Spherical, 50–300 nm in diameter (mean, 110–130 nm). and Whitewater Arroyo viruses. Old World: Lassa, Lujo, Genome: double-segmented, negative-sense and lymphocytic choriomeningitis viruses. Rodent and ambisense, single-stranded RNA, 10–14 kb borne in overall size. Virion contains a transcriptase. Four major polypeptides. Envelope. Replication: cytoplasm. Assembly: incorporate ribosomes and bud from plasma membrane Bunyaviridae Genus Orthobunyavirus Anopheles A and B, Bunyamwera, California encephalitis, Spherical, 80–120 nm in diameter. Genome: triple- Guama, La Crosse, Oropouche, and Turlock viruses. segmented, negative-sense or ambisense, Arthropod borne (mosquitoes) single-stranded RNA, 11–19 kb in total size. Virion contains a transcriptase. Four major polypeptides. Genus Hantavirus Hantaan virus (Korean hemorrhagic fever), Seoul virus Envelope. Replication: cytoplasm. Assembly: (hemorrhagic fever with renal syndrome), Sin Nombre budding into the golgi virus (hantavirus pulmonary syndrome). Rodent borne Genus Nairovirus Crimean-Congo hemorrhagic fever, Nairobi sheep disease, and Sakhalin viruses. Arthropod borne (ticks) Genus Phlebovirus Heartland virus, Lone Star virus, Rift Valley fever virus, sandfly (Phlebotomus) fever viruses, severe fever with thrombocytopenia syndrome virus (SFTSV), and Uukuniemi viruses. Arthropod borne (mosquitoes, sandflies, ticks) Filoviridae Genus Marburgvirus Marburg viruses Long filaments, 80 nm in diameter × varying length (>10,000 nm), although most average ~1000 nm. Genus Ebolavirus Ebola viruses Genome: negative-sense, nonsegmented, single- stranded RNA, 19 kb in size. Seven polypeptides. Envelope. Replication: cytoplasm. Assembly: budding from plasma membrane Flaviviridae Genus Flavivirus Brazilian encephalitis (Rocio virus), dengue, Japanese Spherical, 40–60 nm in diameter. Genome: positive- B encephalitis, Kyasanur Forest disease, louping ill, sense, single-stranded RNA, 11 kb in size. Genome Murray Valley encephalitis, Omsk hemorrhagic fever, RNA infectious. Envelope. Three structural Powassan virus, St. Louis encephalitis, West Nile polypeptides, two glycosylated. Replication: fever, yellow fever, and Zika viruses. Arthropod borne cytoplasm. Assembly: within endoplasmic (mosquitoes, ticks) reticulum. All viruses serologically related Reoviridae Genus Coltivirus Colorado tick fever virus. Arthropod borne (ticks, Spherical, 60–80 nm in diameter. Genome: mosquitoes) 10–12 segments of linear, double-stranded RNA, 16–27 kbp total size. No envelope. Ten to Genus Orbivirus African horse sickness and bluetongue viruses. 12 structural polypeptides. Replication and Arthropod borne (mosquitoes) assembly: cytoplasm (see Chapter 37) Togaviridae Genus Alphavirus Chikungunya, Eastern, Western, and Venezuelan equine Spherical, 70 nm in diameter, nucleocapsid has encephalitis viruses, Mayaro, O’Nyong-nyong, Ross 42 capsomeres. Genome: positive-sense, single- River, Semliki Forest, and Sindbis viruses. Arthropod stranded RNA, 11–12 kb in size. Envelope. Three borne (mosquitoes) or four major structural polypeptides, two glycosylated. Replication: cytoplasm. Assembly: budding through host cell membranes. All viruses serologically related encephalitis. Many encephalitides are alphavirus and flavi- with conditions permissive for further transmission, as evi- virus infections spread by mosquitoes, although the group of denced by the introduction of West Nile, chikungunya, and California encephalitis diseases is caused by bunyaviruses. Zika viruses from the Old to New World continents. On a given continent, there may be a shifting distribution Several arboviruses cause significant human infections depending on viral hosts and vectors in a given year. There in the United States (Table 38-2). The numbers of cases vary can also be major shifts in viral distribution to new areas widely from year to year. Riedel_CH38_p557-p580.indd 558 04/04/19 5:07 PM  CHAPTER 38 Arthropod-Borne and Rodent-Borne Viral Diseases    559 A B C D FIGURE 38-1 Electron micrographs of typical arboviruses and rodent-borne viruses. A: An alphavirus, Semliki Forest virus (Togaviridae). B: A representative member of Bunyaviridae, Uukuniemi virus. C: An arenavirus, Tacaribe virus (Arenaviridae). D: Ebola virus (Filoviridae). (Courtesy of FA Murphy and EL Palmer.) TOGAVIRUS AND FLAVIVIRUS infections in mosquitoes and are transmitted between ver- tebrates by mosquitoes or other blood-feeding arthropods. ENCEPHALITIS They have a worldwide distribution. All alphaviruses are Classification and Properties of antigenically related. The viruses are inactivated by acid pH, heat, lipid solvents, detergents, bleach, phenol, 70% alcohol, Togaviruses and Flaviviruses and formaldehyde. Most possess hemagglutinating ability. In the Togaviridae family, the Alphavirus genus consists of Rubella virus, classified in a separate genus in the Togaviridae about 30 viruses 70 nm in diameter that possess a single- family, has no arthropod vector and is not an arbovirus (see stranded, positive-sense RNA genome (see Table 38-1). The Chapter 40). envelope surrounding the particle contains two glycopro- The Flaviviridae family consists of about 70 viruses teins (see Figure 38-1). Alphaviruses often establish persistent 40–60 nm in diameter that have a single-stranded, Riedel_CH38_p557-p580.indd 559 04/04/19 5:07 PM  560   SECTION IV  Virology A B C D E F G FIGURE 38-2 Known distributions of selected flaviviruses causing human disease. A: Yellow fever virus. B: Dengue virus. C: St. Louis encephalitis virus. D: Japanese B encephalitis virus. E: Murray Valley encephalitis virus. F: Tick-borne encephalitis virus. G: West Nile virus. (Reproduced with permission from Monath TP, Tsai TF: Flaviviruses. In Richman DD, Whitley RJ, Hayden FG [editors]. Clinical Virology, 2nd ed. Washington DC: ASM Press, 2002. ©2002 American Society for Microbiology. No further reproduction or distribution is permitted without the prior written permission of American Society for Microbiology.) positive-sense RNA genome. Initially, the flaviviruses were alphaviruses, and many also exhibit hemagglutinating abil- included in the togavirus family as “group B arboviruses” ity. Hepatitis C virus, classified in a separate genus in the but were moved to a separate family because of differences Flaviviridae family, has no arthropod vector and is not an in viral genome organization. The viral envelope contains arbovirus (see Chapter 35). two glycoproteins. Some flaviviruses are transmitted among vertebrates by mosquitoes and ticks, but others are transmit- ted among rodents or bats without any known insect vec- Replication of Togaviruses and Flaviviruses tors. Many have worldwide distribution. All flaviviruses are The alphavirus RNA genome is positive sense (Figure 38-3). antigenically related. Flaviviruses are inactivated similarly to Genomic-length and subgenomic (26S) mRNAs are produced Riedel_CH38_p557-p580.indd 560 04/04/19 5:07 PM  CHAPTER 38 Arthropod-Borne and Rodent-Borne Viral Diseases    561 TABLE 38-2 Summary of Major Human Arbovirus and Rodent-Borne Virus Infections That Occur in the United States Infection:Case Ratio (Age Mortality Rate Diseasea Exposure Distribution Major Vectors Incidence) Sequelaeb (%) Eastern equine Rural Atlantic, southern Aedes, Culex 10:1 (infants) + 30–70 encephalitis coastal 50:1 (middle-aged) (Alphavirus) 20:1 (elderly) Western equine Rural Pacific, Mountain, Culex tarsalis, 50:1 (15 years) (Alphavirus) Venezuelan equine Rural South (also South Aedes, 25:1 (15 years) (Alphavirus) America) Culex St. Louis encephalitis Urban–rural Widespread Culex 800:1 (60 years) West Nile fever Urban–rural Widespread Culex, Aedes, 5:1 (fever) ± 3–15 (Flavivirus) Anopheles 150:1 (encephalitis) California encephalitis Rural North central, A triseriatus Unknown ratio Rare ~1 (La Crosse) Atlantic, South (most cases (Orthobunyavirus)