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INTRODUCTION

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This chapter summarizes the major features of selected arthropod-borne and rodent-borne viruses. Numerous viruses of this category are transmitted in nature among animals without ever infecting humans. Other viruses incidentally infect humans, but only a proportion of these viruses induce human disease. In addition, certain viral agents are regularly introduced into human populations or spread among humans by certain arthropods (specifically, insects and ticks) or by chronically infected rodents. These zoonotic viruses are taxonomically diverse and therefore differ fundamentally from one another in terms of virion morphology, replication strategies, genomic organization, and genome sequence. While a virus’s classification in a taxon is enlightening with regard to natural maintenance strategies, sensitivity to antiviral agents, and particular aspects of pathogenesis, the classification does not necessarily predict which clinical signs and symptoms (if any) the virus will cause in humans. Zoonotic viruses are evolving, and “new” zoonotic viruses are regularly discovered. The epizootiology and epidemiology of zoonotic viruses continue to change as a result of environmental alterations affecting vectors, reservoirs, wildlife, livestock, and humans. Zoonotic viruses are most numerous in the tropics but are also found in temperate and even frigid climates. The distribution and seasonal activity of a zoonotic virus may vary, and the rate at which it changes is likely to depend largely on ecologic conditions (e.g., rainfall and temperature), which can affect the density of virus vectors and reservoirs and the development of infection.

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Arthropod-borne viruses (arboviruses) infect their vectors after ingestion of a blood meal from a viremic, usually nonhuman vertebrate; some arthropods may also become infected by saliva-activated transmission. The arthropod vectors then develop chronic, systemic infection as the viruses penetrate the gut and spread throughout the body to the salivary glands; such virus dissemination, referred to as extrinsic incubation, typically lasts 1–3 weeks in mosquitoes. At this point, if the salivary glands become involved, the arthropod vector is competent to continue the chain of transmission by infecting a vertebrate during a subsequent blood meal. An alternative mechanism for virus maintenance in its arthropod vector is transovarial transmission. The arthropod generally is unharmed by the infection, and the natural vertebrate partner usually has only transient viremia with no overt disease.

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Rodent-borne viruses are maintained in nature by transmission between rodents, which become chronically infected. Usually a high degree of rodent–virus specificity is observed, and overt disease in the reservoir host is rare.

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ETIOLOGY

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Arthropod-borne and rodent-borne zoonotic viruses belong to at least seven families: Arenaviridae, Bunyaviridae, Flaviviridae, Orthomyxoviridae, Reoviridae, Rhabdoviridae, and Togaviridae (Table 106-1).

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TABLE 106-1ZOONOTIC ARTHROPOD- AND RODENT-BORNE VIRUSES THAT INFECT HUMANS

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