Malaria and Rome: A History of Malaria in Ancient Italy, Robert Sallares [reading a book TXT] 📗

within these lower levels. The effects of these temperature fluctuations can be analysed by studying periodic advances and retreats of the glaciers in the Alps, which have occurred throughout the Holocene. The time of the Roman Empire was mostly a warm period, relative to the periods immediately before and immediately afterwards. The results from the glaciers can be corroborated by various types of proxy data, for example the spread of viticulture into Roman Britain as demonstrated by the recent archaeological finds at Wollaston in the Nene Valley in Northamptonshire. The time of the Roman Empire was warmer than the period of the ‘Little Ice Age’, c. 1500–1800, which encompasses much of the data that are available for studying the distribution of malaria in Italy in the early modern period. Consequently the climate was more favourable for the spread of malaria in Italy during the Roman Empire than it was, say, in 1782, when malaria occurred all over Lazio, except at high altitudes, as shown by Bonelli’s map.¹⁴⁹

Climatic change, then, is a very important factor that creates the expectation of a wider distribution of malaria in Italy during the Roman Empire than during the early modern period. It is not surprising, as has already been noted (see Ch. 4. 2 above), that ¹⁴⁹ Huntington (1910: 672–5); Fraccaro (1919: 66–70); Röthlisberger (1986: 60–1, 70–4); Bianchi and McCave (1999); Brown and Meadows (2000).

102

Ecology of malaria

P. falciparum malaria occurred in Umbria, far inland, by the fifth century . Temperature is very important in relation to malaria because of the requirements of the parasites for sporogony inside the mosquito. This developmental process, which is essential before the mosquito can infect anyone, takes only about 9 days at a mean temperature of 30°C, 10 days at 25°C, but up to 23 days at 20°C, in the case of P. falciparum. The time scale in the case of P. malariae is even longer, about 20 days at 25°C, and 4–5 weeks at 20°C. Even though on the surface such temperature levels are not maintained for long periods in Europe (taking account of the drop in temperature that occurs every night), nevertheless the longevity of P. malariae infections in humans assured its survival until the next favourable transmission period came along, and there are many references to quartan fever in historical sources from Europe. In the case of P. vivax the process takes only 9 days at 25°C, but 30 days at 16°C. The slowing down of sporogony caused by low temperatures impedes the transmission of malaria because few adult mosquitoes actually live as long as 3 or 4 weeks. Hackett noted that over 50% of mosquitoes are dead after a week in Italy during the summer.¹⁵⁰

The scale of the temperature changes in the Roman period can be roughly estimated. One recent study suggested that the mean July temperature in the Arctic about 411 ± 70 years  (by C14

dating), at the peak of the Little Ice Age, was about 0.7°C lower than today. Wider studies have estimated a cooling of about 0.4–0.6°C during the Little Ice Age, explained by periodic variations in annual-mean radiation from the sun.¹⁵¹ Consequently it is likely that mean summer temperatures in Italy during the Roman Empire were a minimum of about 0.5°C higher (and quite possibly more) than the temperatures of the early modern period, which were already sufficient for P. falciparum malaria to be widely distributed in central and southern Italy. A temperature change of this magnitude may appear to be quite modest, but it is well known in ecology that small changes of this kind can have considerable effects on the distributions of living organisms. Moreover the effects of even small temperature changes would be most significant in geographical areas on the periphery of the distribution of P. falciparum, such as southern Europe, rather than in the tropics. Modern ¹⁵⁰ Hackett (1937: 67–9); Gilles and Warrell (1993: 111, 126–31).

¹⁵¹ Havström et al. (1995); Wigley and Kelly (1990).

Ecology of malaria

103

global warming is already creating more favourable conditions for the mosquito vectors of malaria and other diseases in many parts of the world.¹⁵² Some studies have argued that parts of the Roman period were even hotter, up to about 2°C warmer than the temperatures of the Little Ice Age in the early modern period, as well as being wetter than today. If so, the prospects for malaria during the time of the Roman Empire were that much better.¹⁵³

4. 6 A    

Increasing temperature favoured the spread of both P. falciparum malaria itself and its vector mosquitoes in Italy during the time of the Roman Empire. There were other factors which were also of fundamental significance. Heat is good for malaria, but the mosquitoes still need to find suitable habitats for breeding sites. Human activity in the first millennium  unwittingly made a crucial contribution to the spread of malaria by altering the hydrology, geomorphology, and vegetation cover of lowland areas on a scale which transcended anything achieved in earlier periods. In Etruria the combination of the different types of evidence yielded by archaeological field surveys (e.g. the Tuscania and Veii surveys), palaeobotany, and palynology indicates that very substantial population growth occurred during the development of the Etruscan cities. This human population growth was supported by Mediterranean polyculture (the triad of cereals, olives, and vines), a new agricultural system which developed in central Italy for the first time in the Early Iron Age. The same sort of intensification of land use occurred in Latium as well.¹⁵⁴ Deforestation of hills and mountain slopes caused increased run-off of rainwater. This raised the water table and increased the chances of flooding in the lowlands and eroded soil to be redeposited as alluvial deposits in the lowlands. These areas of alluvial deposition were very likely to become marshy. This process happened all over Italy. The Greek colony of Metapontum in southern Italy is a well-studied example.

Archaeological investigations have shown that the water