Earlier in her career, Professor Linda Newson, director of the Institute of Latin American Studies, wrote extensively on the devastating demographic impact of ‘Old World’ diseases in colonial Latin America following the arrival of Europeans. In the current Covid-19 pandemic she is constantly reminded of how many of the issues we face today have parallels with those she studied in the past.
Epidemic diseases that result in high mortality have one feature in common – they all have their origins in animals, normally domesticated animals. Hence, smallpox is related to cowpox, influenza to swine flu and measles probably to rinderpest and/or canine distemper. It has been suggested that the Covid-19 virus may have originated in bats and perhaps spread to humans via a pangolin in a wet market in Wuhan, China.
When new diseases emerge, they cause high mortality because most humans do not have immunity to them due to the lack of previous exposure. This scenario was repeated multiple times in the past beginning in the Christian era when populations in China, India and the Mediterranean grew sufficiently large to sustain acute infections. With the development of transoceanic trade and colonial expansion from the 15th century their incidence increased dramatically. The regions that were most profoundly affected at that time were those that in the pre-colonial era had been relatively isolated, notably the Americas, Australasia and the Pacific islands.
While the initial impact of acute infections on non-immune populations is generally accepted, my research suggested that it is difficult to generalise about their spread and impact. Pathogens differ significantly in the way they spread, for how long they remain infectious, and what environmental conditions they require to survive. A parasite may depend on an insect or animal vector to reach a new host or rely on direct contact, while all have environmental limits beyond which they cannot survive. For example, malaria causing mosquitos can only survive in warm climates.
Parasites also change over time and this is especially true of viruses, such as influenza, that mutate more rapidly than other pathogens. Although caused by a bacterium rather than a virus, it is thought that the incidence of scarlet fever, once a deadly disease, declined due to changes in the pathogen itself. Each parasite has its own history.
In modelling the spread and impact of acute infections epidemiologists have recognised that population size and density are crucial factors. However, less explored, especially historically, has been the influence of the economic and socio-political characteristics of the society into which it is introduced. This may not only affect the spread of a disease, but also a society’s ability to develop immunity to it and recover. History has demonstrated that these cultural influences can result in considerable geographical variations in the impact of a single epidemic.
This is a large topic, which I have explored in several books and papers (see list below) particularly in the context of the colonial experiences of Central America, Ecuador and the Philippines. Here I will focus on two issues that are particularly pertinent to the Covid-19 pandemic – the significance of cultural factors in enabling societies to develop immunity to a pathogen and the importance of leadership in crisis management.
Some people have innate immunity that derives from their genetic, bio-chemical or physiological make-up, but most acquire it through exposure to an infection. Acquired immunity cannot be inherited, so historically a society could only develop immunity where the parasite remained in the community and continually infected susceptible individuals. Now, it is more often induced through vaccination.
A pathogen needs to reach a suitable host within a specified time otherwise it dies out. Acute infections associated with epidemics are only infectious for short periods, so they can only circulate where populations are large or there are intense contacts with an external source of infection. However, the period for which a disease is infectious varies and the shorter the infectious period, the chances of ‘fadeouts’ occurring increase.
Epidemiologists calculate that for measles to become endemic (infectious for eight days), it needs a population of between 200,000 and 300,000, or even 500,000, so that it can generate 7,000 ‘susceptibles’ (children) to maintain the parasite. On the other hand, smallpox remains infectious for two weeks and can survive on clothing and bedding for up to 18 months and is associated with a lower threshold population of 200,000. This can explain why smallpox epidemics were more frequent than those of measles and especially influenza, during the first century of European presence in the Americas.
The ability of a society to develop what epidemiologists have called ‘herd immunity’ depends on the size and density of the population and the intensity of outside contacts. However, it also depends on the fertility rate that produces a new generation of non-immune hosts in the form of children.
Evidence from colonial Andes indicates that epidemics resulted in reduced fertility as marriages were broken up by the death or flight of a partner, as economic conditions deteriorated and as the psychological effects of an epidemic encouraged smaller families. And from colonial Ecuador evidence suggests that it took around one hundred years for the population to acquire ‘herd immunity’, such that smallpox and measles become diseases of childhood. It is worth noting that in pre-industrial non-contracepting societies, due to high mortality and low life expectancy, population growth is not normally more than 1 percent a year. Even a small change in fertility can therefore affect its ability to recover demographically.
The current dilemma, which in many respects parallels the colonial experience, is that in the absence of vaccines ‘herd immunity’ can only be acquired through continued exposure that results in high mortality over an extended period. Such a strategy has probably never been socially and politically acceptable and, given our greater knowledge of how epidemics operate and can be controlled, it is perhaps even less so today.
Where populations are small and the fertility rate is low, ‘fade outs’ often occur. This means that they are unable to acquire immunity and therefore any re-introduction of the disease results in high mortality. Hence the impact of the same disease may be different according to the characteristics of the society and at different points in time.
For Ecuador I have argued that populations in the highlands were able to develop immunity to a number of acute infections and recover at an early stage, with some becoming endemic by the beginning of the 17th century, whereas societies in the Amazon headwaters continued to suffer high mortality due to reinfection at regular intervals throughout the colonial period. This resulted in their continued decline that was only effectively reversed with the introduction of scientific medicine in the 20th century. Nevertheless, the lack of immunity among Amazonian populations means they continue to be particularly susceptible to new infections.
Disease control and management in history
In the current Covid-10 pandemic many states have sought to contain the disease through reducing social contact by confining people to their homes. Historically quarantining was used as a method of inhibiting the spread of an epidemic, for example with bubonic plague and leprosy, though measures generally focused on improving sanitary methods rather than reducing the level of contact.
Quarantining might be effective where diseases are spread through face-to-face contact, but not where they are spread by an animal vector such as a mosquito or rat. However, in the past, and as we are experiencing today, when a new pathogen emerged, knowledge of how it is spread was limited, so that many measures taken were inappropriate and did little to contain them.
For diseases spread though face-to -face contact, population density and distribution clearly influenced the initial impact of disease, but the nature of interpersonal contacts were also affected by a range of cultural factors. These may include the extra communal trade and travel, religious pilgrimages and congregations, bathing practices or residence in extended rather nuclear family households. Less frequently discussed in the literature is how cultural factors may influence the ability of a society to recover from an epidemic, which also varies according to the cohort affected.
The death of able-bodied adults has a much greater impact on recovery, especially where those dying possess specific and scarce skills. In the colonial Andes, where the indigenous population may have been reduced to only a tenth of its size in the 16th century, it led to the widespread abandonment of irrigation works, terracing and raised fields. Large-scale losses can also affect food supplies and result in reduced levels of nutrition, while the death of fertile women can result in an immediate loss of reproductive capacity that cannot be restored until a new generation of reproductive women emerges. Where only one partner dies, demographic recovery may also be held back by opposition to remarriage and/or by marriage rules that determine the suitability of spouses, factors which are more significant in smaller communities where marriage pools are small.
Leadership can play a critical role in determining the outcome of an epidemic. State societies generally have greater administrative capacity to organise practical responses to them. However, where leadership is weak or a leader dies or fails to command confidence in the measures being taken, this can lead to social unrest and conflict.
This can be illustrated most dramatically in the death of the Inca ruler, Huayna Capac, probably from smallpox, about 1524. It precipitated a widespread dynastic civil war between his sons Huascar and Atahualpa, that together with the Inca wars of conquest are estimated to have cost 100,00 lives before the Spanish arrived. In addition, where the cause of the disease is unknown and there is a lack of strong leadership, blame is often placed on an outside group or class, leading in the case of Amazonian groups, to intertribal conflict and raised mortality.
It is clear that each new disease poses multiple questions, some of which relate to the nature of the disease itself, its infectiousness, how it is spread and who it infects. Scientists are learning more about the characteristics of the Covid-19 virus as it spreads. However, we should expect that the impact of the disease, and indeed a society’s ability to recover, will not be same throughout the world. History demonstrates that cultural factors, including the actions taken by leaders, will be important in explaining geographical variations in its impact.
This article first appeared on the Institute of Latin American Studies’ blog. You can read the original here.
Professor Linda Newson is director of the Institute of Latin American Studies (ILAS) at the School of Advanced Study, University of London. Her publications include Conquest and Pestilence in the Early Spanish Philippines, Patterns of Life and Death in Early Colonial Ecuador, Indian Survival in Colonial Nicaragua and The Cost of Conquest: Indian Societies in Honduras under Spanish Rule.