Chapter 15: Diseases Transmitted Primarily by Arthropod Vectors

VIRAL INFECTIONS

INTRODUCTION TO ARBOVIRUSES

Arboviral, or arthropod-borne diseases are among the most important emerging infections. Arthropods, of the phylum Arthropoda, are invertebrates with a chitinous exoskeleton and internal hemocoele for blood meals. Mosquitoes, ticks, sand flies (phlebotomus) and midges serve as the primary arboviral vectors. The transmission by these vectors is biologic, rather than mechanical, in that true arboviruses replicate in arthropod tissues and salivary glands prior to transmission, rather than being simply carried by vectors.¹

Arboviruses produce a wide range of clinical manifestations. Primary hosts generally develop an asymptomatic viremia exemplifying a balanced host-parasite relationship. In non-susceptible hosts an abortive infection can occur in which there are no clinical manifestations or viremia, but serum antibodies can be detected.² Because of the individual variability within host species, severe or medically recognizable disease is often dependent on age and occurs in a small fraction of the total number of individuals infected. The disease patterns of arboviruses in clinically susceptible hosts provide a basis of classification and include the following: (a) acute central nervous system disease, including aseptic meningitis, encephalitis, or encephalomyelitis; (b) undifferentiated febrile illness that can occur with or without a rash; (c) fever and arthritis with or without a rash; and (d) hemorrhagic fever, a febrile systemic illness with hemorrhagic manifestations, cardiovascular instability and varying degrees of hepatic and renal insufficiency. Arthropod-borne hemorrhagic fevers, most of which produce the hemorrhagic fever syndrome, will be described in another chapter that includes nonarthropodborne (often rodent-associated) zoonotic disease.

CLASSIFICATION OF ARBOVIRUSES

Arboviruses belong to a number of different taxonomic groups. The term arbovirus is a convenient, but imprecise, classification with different viral replication strategies and some viruses that are not transmitted by arthropods.¹ Of the more than 534 viruses listed in the International Catalogue of Arboviruses, only 134 have been shown to cause disease in humans; however, those that produce unusually severe disease or are of high incidence are much fewer.³ Most medically important arboviruses are included in the seven RNA virus families of Togoviridae, Flaviviridae, Bunyaviridae, Reoviridae, Arenaviridae, Filoviridae and Rhabdoviridae⁴ (Table 15-1).