Classification, epidemiology and control of arthropod-borne viruses

R SWANEPOEL

Introduction

As recently as the late 1960s the term arboviruses, a partial acronym derived from arthropod-borne viruses, was used in a taxonomic sense to denote the existence of an inferred underlying relationship between viruses with similar modes of transmission. After morphological and physicochemical characterization, it became evident that the viruses actually belonged to a range of widely disparate families, and since then there has been a tendency to stress the diversity of the viruses, at the cost of overlooking shared epidemiological features which dictate a common approach to the investigation of arbovirus diseases. Indeed, the study of the arthropod-borne viruses has always been a specialized branch of virology in which there is, nevertheless, a need for co-operation between such varied investigators as virologists, medical and veterinary clinicians, entomologists, zoologists, ecologists and climatologists,amongothers. Except where otherwise indicated, the following outline of the subject is drawn from a number of general accounts of arboviruses.^{22, 109, 228, 240}

For centuries it was suspected that certain diseases were transmitted by arthropods, but it was not until 1878 that Patrick Manson demonstrated in China that mosquitoes serve as vectors for the filarial worm Wuchereria bancrofti. In 1893 Smith and Kilborne established that ticks transmit the babesia, which causes Texas redwater fever of cattle, and between 1895 and 1898 Ronald Ross performed investigations in India that proved that mosquitoes transmit the parasite of malaria. Shortly thereafter, in 1900, Walter Reed and his associates in Cuba demonstrated human to human transmission of the virus of yellow fever by mosquitoes, as had been postulated earlier by Carlos Finlay. Not only did this constitute the first proof of transmission of a virus by arthropods, but it was also the first occasion on which evidence was produced to indicate that an agent causing disease in humans, as opposed to lower animals, was caused by a virus. However, it was not until 1927 that Stokes and his co-workers in Nigeria succeeded in maintaining yellow fever virus by serial passage in monkeys. By then, several arboviruses of veterinary significance had been isolated, the first two being African horse sickness and bluetongue viruses in South Africa. Although the exact dates and authorship of isolation of these two viruses are debatable, they had been maintained by subinoculation in horses and sheep respectively, long before they were shown to be filterable agents or were adapted to laboratory hosts.^{4, 65, 164, 176, 216, 217, 239}

By 1939, 16 arboviruses had been isolated (Table 7.1), although this figure is somewhat misleading as it does not take into account that African horse sickness and bluetongue viruses are each made up of multiple serotypes. Of the 19 arboviruses isolated between 1940 to 1949 (Table 7.2) eight were human pathogens from various parts of the world, but 11 were viruses of unknown significance that had been isolated from mosquitoes and a sentinel monkey in the course of yellow fever investigations in which the Rockefeller Foundation Virus Laboratories (RFVL) of New York played a leading role. During the Second World War, RFVL concentrated on the investigation of yellow fever, but during the 1950s it seconded staff to laboratories in many parts of the world, including what is now known as the National Institute for Communicable Diseases in Johannesburg, to establish and foster arbovirus units which were intended to continue operating as regional centres with locally trained staff.

The newly established arbovirus unit in South Africa, initially under the leadership of Smithburn of RFVL, conducted surveys for antibodies to a range of arboviruses on sera collected from humans and domestic animals in South Africa, ^{62, 124, 127, 148, 155, 213} Mozambique,¹²³ Angola,¹¹⁶ Namibia and Botswana.¹²⁹ The highest prevalence of antibodies was found in sera from Tongaland, a subtropical area occupying the coastal plain of the northern part of KwaZulu-Natal Province in South Africa, and consequent efforts to isolate new viruses were concentrated mainly on this area. In the decade from 1950 to 1959, members of the Onderstepoort Veterinary Institute isolated Wesselsbron virus from a lamb in the Wesselsbron district of the Free State Province, while personnel of the arbovirus unit in Johannesburg isolated 10 new viruses from Tongaland and four from elsewhere in southern African (Table 7.3).

Table 7.1 Arthropod-borne viruses isolated throughout the world prior to 1940. Information adapted from Karabatsos (1985)¹⁰⁹

* See text for...