Germs, parasites, insects and nematodes
In the 1870s, French chemist Louis Pasteur (1822—1895) and German physician Robert Koch (1843-1910) introduced the germ theory which revolutionised ideas about disease causation, medical research and medical practice.
They successfully developed new forms of medical research which focussed on the identification of microbes which were now seen as the cause of disease. Microbiological research took place in medical laboratories and was conducted by physicians and other scientists (pharmacists, chemists and of course the first microbiologists) aided by microscopes and Petri dishes. The new approach to disease assumed that specific diseases were the result of the intrusion of specific foreign pathogens. When such pathogens were identified, preventive action could be undertaken while further research could identify possible vaccinations and even cures. Medical investigation no longer focussed on diffuse factors such as the climate or miasmas, but on the identification of micro-organisms. This new approach to disease led to several significant discoveries, in particular the identification of disease vectors. In a number of diseases common to the tropics, the pathogenic micro-organisms that caused them were transmitted between infected individuals by a third agent (mosquitoes or rats, for example), which served as intermediary hosts. Mosquitoes, lice, rats, ticks, nematodes and other parasites were now seen as the carriers of pathogenic agents. The tropics were now viewed as unhealthy because they were unusually favourable to their proliferation.In 1879, Patrick Manson (1844-1922), a British physician working in Amoy, China, discovered the mechanism though which lymphatic filariasis, which is caused by a parasite, was transmitted. The most noticeable symptom of this disease is elephantiasis, which is the swelling of the lower limbs through the thickening of the skin and underlying tissues.
The parasite causing this condition was transmitted by common mosquitoes that served as intermediate hosts for the parasite. Manson thought that the same mechanism might also be responsible for the spread of malaria, a much more significant and highly debilitating disease that caused intermittent high fevers. In 1879, the protozoa causing malaria, Plasmodium, also a parasite, was identified. Research demonstrated that the parasites in the infected person’s blood divide almost simultaneously at regular intervals; at those moments patients suffer from attacks of high fever. In the early 1890s, Manson suggested that insects were the main vector of transmission. This inspired Ronald Ross (1857-1932) to undertake further research, which, in 1897, demonstrated that the anopheles mosquito transmitted malaria among birds. Ross demonstrated that the mosquitoes not only ingested the malaria parasite from the human bloodstream, but also acted as a true host: once the parasites entered the mosquito’s system, they developed further and migrated to its salivary glands from where they could be transferred to a new human carrier through bites. Even though these insights did not lead to a cure for malaria, they indicated how efficient preventive measures could be undertaken. These included screening doors and windows, using mosquito nets at night and draining swamps to kills mosquito larvae. Killing mosquitoes and preventing them from biting humans became central to preventing malaria.In 1881, Carlos Finlay (1833-1915), representing Spanish Cuba at a sanitary conference in Washington, described the transmission of yellow fever through a blood-sucking mosquito (Aedes aegypti). Between 1898 and 1902, the government of the United States of America sent the Havana Yellow Fever Commission, a group of experts in public health led by Walter Reed (1851-1902), a military physician, to the newly acquired territory of Cuba to study the epidemiology of yellow fever and to take appropriate measures to prevent the disease from being transported by ship.
In the experiments Reed conducted, it was proven that a mosquito transmitted the disease. Yellow fever had ravaged the Spanish troops before and had lost nothing of its lethal potential, posing definite obstacles for Cuba’s economic development. By isolating infected individuals and eradicating the mosquito responsible for transmission, William C. Gorgas (1854—1920), military doctor and chief sanitary officer for the city of Havana, eliminated the disease from the city in a matter of months in 1901. He reproduced his programme on the Panama Canal’s construction site, once again with success, which later inspired Ross to state that the ‘canal had been dug with a microscope.’18In 1894, the French physician Alexandre Yersin (1863—1943) and the Japanese bacteriologist Shibasaburo Kitasato (1853—1931)—theJapanese Empire was growing at the time, including ‘tropical countries’ such as Taiwan—simultaneously discovered the bacillus that caused bubonic plague in Hong Kong. It was also found that the same bacillus was present in rats at times of epidemics. This indicated that rats had an important role in spreading the disease.19 Measures were now taken to limit the spread of the plague, such as increased medical surveillance of populations at risk, the isolation of afflicted individuals, the killing of rats, the rapid disposal of corpses and the destruction of indigenous houses, which were often made of bamboo, which provided an excellent place for rats to hide. In 1900, an epidemic of sleeping sickness occurred in British Uganda, killing around 250,000 individuals in five years. A commission for the Royal Society of London confirmed that the disease was caused by a parasite (a protozoa: Trypanosoma brucei) and spread by tsetse flies. As the epidemic began to spread to other colonies in East and Central Africa, a number of imperial powers conducted research and implemented efforts to control the disease vector. Because the tsetse fly appeared to be limited to specific areas, villages were forcibly evacuated while control policies developed by entomologists were implemented.
Finding ways to kill tsetse flies was the most effective method to prevent the disease from spreading.The medical research conducted in the colonies during the second part of the nineteenth century demonstrates how the colonies had become large medical laboratories for research and experimentation on parasites, infectious diseases and other tropical ailments. For instance, in 1897, Christiaan Eijkman (1858—1930), working in the medical laboratory of the Batavia Military Hospital, linked beri-beri to a nutritional deficiency which, in Java and elsewhere in the colonial world, was caused by eating white rather than brown rice. His research later led to the discovery of vitamins.20 Near the end of the nineteenth century, a network of Pasteur Institutes (Instituts Pasteur d’Outre-mer) had started to take shape in the colonies. In 1891, only three years after the founding of the Pasteur Institute in Paris, the Pasteur Institute of Saigon opened its doors. Thirteen more institutes followed. In the following decade, laboratories for interdisciplinary research opened in several other places: the Institute of Medical Research (Kuala Lumpur, 1900), the American Public Health Laboratory (Manila, 1902) and the Wellcome Laboratory of Tropical Medicine (Khartoum, 1904) conducted medical research, developed and produced vaccines (against smallpox, typhoid, cholera, plague and later tuberculosis), analysed food and water and investigated the toxicology of drugs. These laboratories soon became connected through medical journals focussing on health and disease in the tropics, by scholarly associations and by conferences (including those of the Far Eastern Association for Tropical Medicine, which was founded in 1908 in Manila). Through these new connections, physicians working in the tropics could exchange information and benefit from each other’s experiences.
Because of parasitology, older representations of the tropics as inherently pathogenic were reinforced and rephrased in the terminology of the new medical specialty: ‘just as the fauna and flora of the tropical world are infinitely richer in species than those of colder climates, so there is a corresponding distribution of pathogenic organisms’, as Manson wrote in the first edition of his authoritative manual, Tropical Diseases.“21 A number of diseases, including yellow fever, typhoid, cholera, plague, leprosy and malaria, were no longer found in the West; they had therefore become tropical diseases.
Although few medical researchers focussed on the climate as an explanation of the high mortality rates in the colonies, the new tropical medicine continued to focus on the tropics as an area unusually dangerous to health. Through new public health initiatives, physicians targeted the control and elimination of vectors and introduced a variety of hygienic measures. They turned their attention to the indigenous population as non-symptomatic carriers of disease and, because of habits considered unhygienic, as a threat to the health of colonial settlers.22In fact, it did not matter much whether the cause of ailments suffered by Europeans in the colonies was a miasma or microbes. At the time of the discovery of the plague bacillus in 1894, debate in Hong Kong and Calcutta focussed on where the bacillus lurked and how it was transmitted between humans. In both cities, many physicians and lay people held that the most likely place to find bacilli was in the houses in the ‘native quarters’, and on the goods and the bodies of working-class people.23 Bacteria were thus found where other epidemic and endemic diseases had been located earlier. Moreover, if microbiology gave scientific proof that bacteria were to blame for a number of infectious diseases, it did not necessarily say who could be infected and why. It was thus possible to continue explaining disease prevalence by referring to the natives’ lack of hygiene. In a logic that was simultaneously biomedical, colonial and discriminatory, different forms of control would continue to be imposed in the name of the public good and civilisation. These representations would reinforce a fundamental, hierarchised distinction, the one between the Self and the ‘Other’.