Arthropods and Disease: The Evolution of Knowledge about Vector-Borne Disease

Prior to the late nineteenth century, medical understanding of "contagion" was based on a direct two-stage person to person model of transmission, formalised in 1584 by Fracastorius (1484-1553). The agents of such disease were demonstrated to be pathogenic microorganisms by Pasteur in his germ Theory of Disease on 1870-1880. A new concept - that a living creature, itself unaffected, might be an immediate carrier of infectious disease, a "vector" came late to medical science. This paradigm shift in understanding, that there might be three or even multi-stage links in the chain of transmissibility, was to unlock the secret of prevention in what had hitherto been the killing epidemic diseases of humankind - plague, typhus, malaria and yellow fever. The intermediate causes of these diseases were shown to be arthropods - fleas, flies, lice and mosquitoes. Their discovery as "intermediate agents", over a brief three decades in the late nineteenth century, brought enlightenment to the long-known associations of disease with poor personal and civic hygiene, with swamps and with the miasmata associated with them. Linnaeus (1707-1778) was the first to suggest that insects might be carriers of disease, a theoretical idea further reinforced by Ehrenberg in 1838. The Scottish physician, Sir Henry Holland, (1788-1873), in his Medical Notes: On the Hypothesis of Insect Life as Cause of Disease, developed this theoretical model further. In July 1878, Pasteur showed that earthworms could carry anthrax ("charbon") spores in the earth casts of their alimentary canals and so be external agents for disease transmission. However, it was the meticulous research of six doctors - Dr Joseph Bancroft (1876) in Brisbane; Dr Patrick Manson (1879) in China; Dr David Bruce (1894) in Africa; Sir Ronald Ross (1897) in India; Dr Thomas Lane Bancroft (1899) in Burpengary near Brisbane in Australia; and Dr Charles-Jules-Henri Nicolle in Tunis in 1909 - who established the science of an extra "intermediate stage" in disease transmission, such links being essential to the lifecycles and hence transmission of the pathogenic microorganisms involved. The primary research linking insects as potential vectors of human disease was undertaken by Dr Joseph Bancroft (1836-1894) who identified the adult filarial worm (later named Wucheria bancrofti) in a lymphatic abscess on the arm of a butcher's apprentice in Brisbane, Australia on December 21, 1876. In 1879, Dr Patrick Manson demonstrated that microfilariae underwent development in the mosquito, thus confirming that mosquitoes could serve as intermediate hosts of parasites which infected vertebrates. A crucial discovery was that of Sir David Bruce, who in 1894 showed that trypanosomiasis (sleeping sickness) could be passed experimentally from big-game animals to cattle by blood transmission of Trypanosomoa brucei, following the bite of the tsetse fly, Glossina morsitans. This work was followed by the work of Sir Ronald Ross of the Indian Medical Service, who in 1897 demonstrated Plasmodia parasites in the stomach wall of Anopheles mosquitoes which had been fed upon the blood of malarial patients. Bancroft himself believed that mosquitoes sucked up the microfilarial worms and then conveyed them to water by which further victims were infected by drinking. Bancroft's son, Dr Thomas Lane Bancroft (1860-1933) demonstrated in a meticulous series of experiments (1893-1901) that the mosquito was the intermediate host ad transmitting agent of Wucheria bancrofti and thus the cause of human clinical filariasis. Although it had been long known that the body louse, Pediculus, was associated with typhus, it was not until 1909 that Dr Charles Nicolle (1866-1936) working at the Institut Pasteur de Tunis, demonstrated that epidemic typhus was transmitted by a rickettsia carried by the body louse, a finding for which he was awarded the Nobel Prize in 1928. This process of an intermediate carrier, itself not harmed by the pathogenic agent which it transmits, saw a further extension in the field of toxicology. In this latter, it was demonstrated that a poison or toxin could pass along one or more intermediate steps in an ecological food chain, before causing disease in creatures which form the penultimate or final links in such a chain. The most common example of this phenomenon is that of ciguatoxin - whose food-chain passage from invertebrates to lower vertebrates and then to humans is the cause of clinical ciguatera. This food-chain with its intermediate links was first suggested in 1958. This paper is an analysis of the evolution of scientific thought concerning the concept of an "intermediate agent", usually an arthropod, as an essential link in what from 1876 became known as a chain of disease transmission. The word "vector", to describe such an "intermediate agent", was first used (in the Encyclopaedia Britannica) in 1922. One task of medical history is to analyse the causes and perspective of those paradigm shifts which alter knowledge about disease. One such advance was the discovery of the phenomenon of "vectors" - living creatures which themselves are unaffected, but which might transmit pathogenic microorganisms from human victim to victim. Self-evident and taken for granted today, this milestone in the history of medical thinking constituted an advance which occurred over three decades at the end of the nineteenth century. It was a qualitative concept which extended the former sixteenth century two-stage concept of "contagion" to a three- or multiple-link chain with vector intermediates. That first use in 1876 of the term "intermediate host" and later "vector" referred to arthropods - especially mosquitoes, flies, lice and mites. By the first decade of the twentieth century, it was appreciated that arthropods could transmit viruses, rickettisias, bacteria, protozoans and multi-cellular parasites. This new paradigm of disease pathogenesis came late in the chronology of the history of medicine. This paper is a review of that development of thought concerning "vectors", discoveries which led ultimately to the potential control of a series of diseases which have figured greatly as killing epidemics throughout the chronology of medical history