Conquest and Pestilence in the Early Spanish Philippines

CHAPTER 2 The Role of Disease

10 C H A P T E R 2 The Role of Disease S outheast Asia is generally considered to have been part of the Eurasian disease pool, with Old World diseases spreading to the islands as trading contacts with the mainland developed in the Christian Era.1 Hence, the lower level of depopulation in the early colonial Philippines compared to the Americas is often attributed to its populations having acquired some immunity to Old World diseases, such as smallpox and measles, prior to Spanish arrival. However, the issue of whether or not Filipinos had acquired such immunity has not been investigated directly. Rather it is has generally been inferred from the relatively low level of depopulation in the early colonial period. The issue is important because if Filipinos lacked immunity it could be argued that population decline in the early colonial period could be attributed to Old World diseases rather than other causes, such as conflict or colonial rule. This chapter will show that any immunity to acute infections the Filipinos had in pre-Spanish times was very limited because the population of the Philippines was small and dispersed. But that low population density may have been a product, at least in part, of the presence of chronic infections, which took a regular toll of the population. This factor has not been considered in explaining low population densities in Southeast Asia in general, for which warfare and low fertility are usually held responsible.2 This chapter will therefore examine what diseases were present in the Philippines when the Spanish arrived and explore their demographic implications. Some Key Concepts Critical to an understanding of the demographic impact of Old World diseases is the distinction between chronic and acute infections.3 Chronic diseases do not kill their hosts or provoke an immune response that results in the death of the disease organisms, which means they can persist for long periods even in small populations. Diseases such as dysentery and typhoid are often related to sanitary or environmental conditions, while others, such as such as leishmaniasis, malaria, filariasis, and schistosomiasis, are spread by non-human vectors.4 Among the few diseases that are spread by human contact and that can persist in small populations are tuberculosis, leprosy, and treponemal infections. While these infections may affect individuals, the parasites do not threaten the survival of the population as a The Role of Disease 11 whole and their persistence is not contingent on the presence of a large population. They remain in the population indefinitely, though they may occasionally erupt as epidemics. Acute infections, such as measles, rubella, and smallpox, on the other hand, require the presence of a large population before they can become endemic. These infections are spread from human to human and are characterized by short periods of infection that result in high mortality. They normally confer lifelong immunity on survivors and are characterized by short periods of communicability, generally less than two weeks. As a consequence, they only become endemic where human populations are of sufficient size to generate enough susceptibles, generally children , to maintain the disease indefinitely, hence they are often termed “crowd infections .” Maurice Bartlett has estimated that for measles to become endemic in U.S. and U.K. cities it is necessary to have a population of between 250,000 and 300,000 or one where there are more than 4,000 to 5,000 new cases a year.5 However, for densely settled island populations Francis Black suggests a higher figure of 500,000.6 Frank Fenner and others have estimated that since smallpox spreads less rapidly than measles,7 to sustain the disease a smaller population of between 100,000 and 200,000 is required.8 Below these thresholds and where the population is dispersed the spread of disease is slow and “fade outs” are common.9 This is especially true in pre-industrial societies, where the rate of natural increase is too low to generate a sufficiently large pool of susceptibles to sustain the disease indefinitely. Indeed the birth rate may be critical in determining whether or not an acute infection can become endemic.10 Small communities may therefore remain relatively disease-free for long periods, but their lack of exposure to infection leads to a build-up of susceptibles , so when a disease is re-introduced it is associated with high mortality that affects adults as well as children. The fact that adults are affected may have significant implications for the economic and...