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

vicinity of Rome and concluded that the periodic fluctuations which he thought he could discern in the severity of malaria in the Roman Campagna could only be explained in terms of variations in the virulence of P. falciparum malaria. However, now that it is known that the physical environment has been changing continuously, it is possible to entertain other explanations for any periodic variations in the degree of endemicity of malaria. It is essential to follow this approach because, as was noted in Ch. 3 above, the most recent scientific research indicates that extreme virulence is adaptive for P. falciparum. There is no reason for supposing, pace Celli, that its virulence ever diminished in historical times. Moreover, the frequency of malaria could also have been influenced in the past by changes in the balance between different mosquito species. Mario Coluzzi expressed to the author his view that there is intense competition between different species of mosquito and that the balance between different species of mosquito is constantly changing at the local level in Italy. Evidently there could have been countless changes in the balance between different species in the past as well.

Although the details remain obscure, and will probably always be obscure, it is inevitable that major alterations in the habitats available to mosquitoes for breeding purposes will have a huge ¹¹⁰ Pliny, Ep. 2.17.25; Celli (1900: 77, 80); F. Giordano Condizioni topografiche e fisiche, in Monografia (1881: xxxiv); Vitruvius 8.3.2; Pliny, NH 31.6.10; Seneca, QN 3.20.4; LeGall (1953: 51); Quilici (1979: 64–5).

¹¹¹ Hackett (1937: 4).

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impact on species which are so sensitive to environmental parameters. Mosquitoes themselves can sometimes modify their own patterns of behaviour to their own advantage. The contemporary example is the tiger mosquito, Aedes albopictus, the vector of the virus of dengue fever in its homeland in south-east Asia. This denizen of tropical rainforests has started to use rubber tyres as a breeding site.

It is thought to have been transported around the world in this way and is now spreading in Mediterranean countries, even though it does not fly further than about two hundred metres from its breeding sites. It was first observed in Italy at Genoa in 1990, and has since spread to Campania, Lazio, Tuscany, Veneto and other regions.¹¹²

Today the various species belonging to the Anopheles maculipennis complex prefer fresh water. However, the three anthropophilic species which are dangerous vectors of human malaria in Europe ( A. labranchiae, A. sacharovi, and A. atroparvus) have a greater tolerance of slightly brackish conditions than the other, more zoophilic species. A. sacharovi can tolerate higher salinity levels than the other two vector species. Lancisi in the eighteenth century reckoned that coastal marshes which sometimes received seawater were particularly dangerous (e.g. at Ostia, Ferrara, and Ravenna).¹¹³ Consequently the anthropophilic species have an adaptive advantage over the zoophilic species in coastal environments, so long as the degree of salinity of the water is not too high. This explains why malaria in Italy tended to have a predominantly coastal distribution. The dangerous anthropophilic species certainly also occurred inland, especially in salty regions such as Diamantina in the Po valley, but faced greater competition there for suitable breeding sites from the harmless zoophilic species.¹¹⁴

¹¹² Sabatini et al. (1990).

¹¹³ Missiroli (1938: 14); Lancisi (1717: 21). Sambon reported finding larvae of what was then called A. maculipennis very plentiful in brackish water at Ostia, ‘amongst large brown clumps of floating algae and seaweed’ (Sambon 1901 a: 198).

¹¹⁴ Hackett (1937: 26, 60, 106). Koella et al. (1998) produced some evidence that malaria parasites can alter the feeding behaviour of mosquitoes by inducing them to bite humans more frequently, another illustration of the complexity of the phenomena under consideration.

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Ecology of malaria

4. 3 M   S

On Sardinia A. labranchiae exploits a lack of competition to breed in environments, such as mountain streams above 500–1000 metres, where it is rare on the mainland of Italy, since several other species of mosquito ( A. atroparvus, A. claviger, A. maculipennis s.s. , A. subalpinus, A. superpictus) which are common on the mainland of Italy are either rare or absent from Sardinia.¹¹⁵ This helps to explain the greater intensity of malaria on Sardinia compared to the mainland of Italy in the past. Since the intensity of malaria on Sardinia was already obvious to Roman authors two thousand years ago, it may be inferred that the distribution of mosquito species on Sardinia at that time already matched the modern distribution patterns.¹¹⁶

Nevertheless the chronology of the introduction of malaria to Sardinia is as controversial as the chronology of its spread in mainland Italy, and even more difficult to resolve, given the lack of evidence. Brown advocated a late-spread theory. He suggested that P.

falciparum malaria was not significant on the island during the time of the nuraghic culture in the Bronze Age and Early Iron Age, even though he accepted that the malaria vector species A. labranchiae has probably been present there for several million years. Brown argued that malaria first became a major problem in the fifth century  owing to deforestation, a new agricultural system in the plains, and imports of slaves infected with malaria during the period of Carthaginian domination of the island. However, there is no positive evidence for this interpretation of the origin of malaria on Sardinia.¹¹⁷ It is generally assumed that the nuraghi were designed for defensive purposes. Since the highest densities of nuraghic settlement were in lowland regions with intense malaria until it was eradicated after the Second World War, Brown ¹¹⁵ Coluzzi and Sabatini (1995); Ramsdale and Snow (2000).

¹¹⁶ On the unhealthiness of Sardinia see Cicero, Epist. ad familiares 7.24.1; Livy 23.34.11; Pausanias 10.17.11; Pomponius Mela, de situ orbis 2.123, ed. Parroni (1967): ceterum fertilis et soli quam caeli melioris, atque ut fecunda ita paene pestilens (otherwise Sardinia