Chapter 10: Disease and Death (Cont.)

Other Eurasian diseases

One of the oldest human diseases is leprosy, which, although it originated with the water buffalo, only affects humans. It has been eliminated in 113 countries and is now found mainly in Brazil, India, Madagascar, Mozambique and Nepal, where there are about 400,000 new cases a year.

One of the consequences of the rise of tuberculosis in Europe, mainly caused by a diet high in milk from tubercular cows, was the development of scrofula or ‘The King’s Evil’. It was rarely fatal, unlike the pulmonary tuberculosis that became common later.
Typhus was one of the major factors that ensured that in every war until the early 20^th century more soldiers died of disease than from combat with the enemy.
The disease can remain latent in patients for decades after recovery and a new epidemic could easily develop if louse numbers ever multiplied.

The second great transmission of diseases

Until the 15^th century the Americas remained isolated from Eurasia and Africa. The large cities such as Teotihuacan and the Aztec capital of Tenochtitlan are likely to have produced endemic intestinal diseases but it is clear that the lack of domesticated animals, in particular the complete absence of cattle, meant that the diseases of Eurasia (in particular smallpox and measles) were unknown. After several millennia of exposure to these diseases Europeans had developed a considerable degree of natural immunity. The native Americans had no such immunity and the impact of these diseases was therefore catastrophic.

The first disease to strike the Americas was smallpox, which reached the Antilles in 1519. It reached the mainland and Tenochtitlan with the relief expedition sent to help Cortés after he and his group of adventurers were forced to flee the capital.
There were so many dead that the survivors were unable to bury them all and the bodies rotted in the streets.
The Spaniards were left with only a demoralized wreck of a society to conquer. The same impact of disease occurred when the Spaniards moved on to the conquest of the Incas in the 1520s.
The impact of smallpox was compounded by the arrival of measles in 1530-31 and then typhus in 1546.

The overall scale of destruction brought about by these Eurasian diseases is almost unimaginable. The population of central Mexico (roughly equivalent to the Aztec empire) was 20 million in 1500, equivalent to a quarter of the European population and four times as big as that of Britain. Within a century there were less than 1 million people in the area. The population of the Inca area fell from 11 million in 1500 to less than a million in 1600. The population of the Caribbean (about 6 million in 1500) was almost entirely exterminated. A population collapse of about 90% may seem unbelievable but it is consistent with other evidence of the impact of disease on populations with no natural resistance. How many people actually died? It is unlikely that the figure is much less than 100 million during the 16^th century. Death on this scale is the greatest any society has ever had to bear. In addition it was combined with immense cultural shock as the Europeans looted the area, imposed Christianity and effectively enslaved the remnants of the native population.

The other major disease taken to the Americas was yellow fever. The disease had long been endemic in west Africa where it had passed to humans from monkeys and had established itself in a mature form of low death rate and high levels of immunity among the local population
Death rates in human populations newly exposed to the disease are around 60%, often higher.
Its impact meant that Europeans could establish no more than a foothold in west Africa for centuries – a series of small trading posts along the coast.
For every 10 soldiers sent to west Africa between 1695 and 1722 6 died in the first year, 2 more died between years two and seven and only one man survived to be discharged in Britain.
The transmission of the disease to the Americas did not take place for more than a century and was the direct result of the European slave trade which developed in the early 17^th century. It first reached Barbados on a slave ship in 1647.
Although the Africans taken as slaves to the Americas had considerable immunity (which increased demand for them as slaves) it ravaged the European populations of the Caribbean.
When it reached Memphis in 1878-79 the death rate was over 50% and the city almost ceased to function.
Occasionally it reached European ports but did not take hold because the mosquito could not survive.
An effective vaccine was developed in the 1930s but it is difficult to eliminate the disease because it survives in the monkeys of the tropical forest.
By the early 21^st century there were 200,000 cases a year with a death toll of 30,000 but this is likely to be a severe under-estimate because of large-scale under-reporting.
The disease now seems to be increasing again and the much greater speed of travel makes it far more likely that it could spread from one area to another even though an outbreak ought to be containable with an effective vaccination campaign.
A disease related to yellow fever that was also taken to the Americas was dengue fever (also known as ‘breakbone’). It is spread by mosquitos and because the disease exists in four subtypes and immunity is only obtained after all types have been contacted it is very difficult to produce a vaccine.
The first outbreak of syphilis in Europe is unusual among diseases in that it can be dated precisely to the early 1490s.
There is little doubt that syphilis was a new disease in the 1490s because of its extreme virulence.
The initial impact in the early 16^th century in a population with no resistance was terrifying and it usually reached its tertiary and terminal form very quickly.

The perennial diseases: malaria

Malaria is one of the longest-established human diseases and has proved to be one of the most difficult to conquer.

It was the development of two new chemicals that seemed to promise the elimination of the disease. The pesticide DDT, first used in the 1940s, killed mosquitoes and could be sprayed on to walls where it remained active for up to 3 months and therefore broke the cycle of infection.
At the same time the development of the highly effective chloroquinine attacked the disease once it had been contracted.
By the 1960s the highly toxic nature of DDT became apparent and its use had to be abandoned. At the same time strains of malaria that were resistant to chloroquinine began to emerge.

The situation worsened in the 1980s when strains of multiple-drug-resistant malaria emerged along the Cambodia-Thailand border. By the early 21^st century there was only one drug that could cope with all types of malaria and the WHO warned that if it continued to be used on its own then it would not be long before resistant strains emerged. In these circumstances the world would be back in the situation found before the 1940s of having no effective drug treatment.

The situation would, however, be far more serious than 80 years ago. Rapid population growth, resistant strains of mosquitoes and climate change mean that whereas 10% of the world’s population was at risk from malaria in 1960 that figure has now risen to 40%. Malaria is now the main cause of death in the developing world with over 90% of infections taking place in sub-Sahara Africa. Between 300 million and 500 million people are infected with the disease and the death rate is between 1.5 million and 2.7 million people a year, of whom 1 million are children. In the long interaction between humans, the environment and disease, malaria remains one of the most significant human failures.

The endemic disease: influenza

One of the most persistent diseases in the world has been influenza. Like so many other diseases it originated with animals, and now develops new strains in domesticated pigs and chickens; it can also be spread by wild birds. The virus responsible mutates easily and rapidly and new strains constantly arise. As an airborne disease it is easily spread between humans through sneezing and coughing. In most years the death rate is low – about 1% – and it is usually the young and the old who are most affected. There have, however, been numerous pandemics when a new strain emerges because previous infection provides only limited immunity against the new variants of the disease.

In the 19^th century there were three great pandemics across the world: in 1830-31, 1833 and 1889-90.
By far the most serious of all influenza pandemics was that at the end of the First World War.
It first appeared in the spring of 1918 in the United States and appeared to be very mild before mutating in August 1918 into a much more virulent form.
It appeared almost simultaneously in three ports – Boston (Massachusetts), Freetown in west Africa and Brest – and was clearly linked to troop movements in the latter stages of the war.
Half the victims were healthy adults between the ages of 20 and 40. The death toll across the world was between 30 and 40 million.
The problem in dealing with the disease is that vaccines have only limited effectiveness and their use is usually restricted to vulnerable groups in the population.
Because the influenza virus can mutate so easily a new pandemic is inevitable at some time.

The diseases of industrialization and urbanization

A Green History of the World Part 9

Chapter 10: Disease and Death (Cont.)

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