Malaria and Rome: A History of Malaria in Ancient Italy (8 page)

warmer, with 20–40% more rainfall, in China in the period 6000–2000 .

⁹ Baroni and Orombelli (1996).

¹⁰ Claussen and Gayler (1997).

30

Evolution of malaria

recent research in molecular evolution suggests that speciation in the
A. gambiae
complex in Africa is an active, ongoing process.

Mario Coluzzi has cogently argued that the modern strains of A. gambiae
, which are exceedingly efficient at transmitting malaria, are of recent (Neolithic period onwards) origin.¹¹ The evolution of these extremely efficient vector strains would have permitted an increase in the transmission rate of
P. falciparum
malaria in tropical Africa. Coluzzi has suggested that an increase in the pathogenicity of
P. falciparum
might have accompanied the increased transmission rate. Recent research in molecular diversity indicates that the populations of the
A. gambiae
complex have been increasing recently but have also long had a large effective population size (the size of the breeding population). This suggests that its potential as a vector of malaria in Africa does go back to prehistory and is not a product of human population growth in modern times in Africa, even if it was not quite as efficient a vector then as it is today.¹² One population of Pygmies (hunter-gatherers until very recently) in central Africa has a very high frequency of haemoglobin S, a mutation which confers resistance to
P. falciparum
malaria on heterozygotes for the sickle-cell trait. Cavalli-Sforza argued that this implies that P. falciparum malaria was already widespread in central Africa
before the invention or spread of agriculture in Africa.¹³ Because of the heat, the Neolithic period was the most likely period for the spread of
P. falciparum
malaria into southern Europe. Its previous evolutionary history, as has just been argued, suggests that the small human population sizes of that period would not have prevented it from becoming endemic in southern Europe then. Malaria is a disease that tends to exist in small foci because mosquitoes generally do not fly far from their breeding grounds, not more than five or six kilometres under Mediterranean environmental conditions, although occasional much longer migrations have been recorded.

In 1959 a migration of about 280 kilometres by the Egyptian mosquito species
Anopheles pharoensis
reintroduced malaria to Gaza and the coast of Israel, from which it had previously been eradicated.

¹¹ Byrne and Nichols (1999) on the London mosquitoes; Coluzzi
et al
. (1979); Coluzzi (1999).

¹² Lehmann
et al.
(1998); Powell
et al
. (1999).

¹³ Cavalli-Sforza (1986: 153–5, 416–19). Pygmy populations in forest environments, which A. gambiae
is reluctant to enter, do not have high frequencies of haemoglobin S. Phylogenies based on mitochondrial DNA sequences suggest that some of the Pygmies are among the most ancient human populations.

Evolution of malaria

31

A propensity towards such migrations by this important malaria vector-species in the lower Nile valley might have helped
P. falciparum
to break out of Africa in the distant past.¹⁴

J. L. Angel used to invoke the high frequency of porotic hyperostosis in crania belonging to skeletons recovered from Mesolithic–Neolithic archaeological sites in Greece as evidence for a high frequency of
P. falciparum
malaria in Greece at that time.

There is now a general consensus among those interested in this problem that porotic hyperostosis, whose proximate cause is an iron-deficiency anaemia, has several other possible ultimate causes besides malaria. Consequently porotic hyperostosis cannot be used as evidence for the existence or frequency of
P. falciparum
malaria in Europe in the Neolithic period.¹⁵ However, absence of evidence is not equivalent to evidence of absence. It still remains quite possible that it was spreading in that period. Recently the application of the techniques of molecular biology to ancient biomolecules has opened up new avenues of research. Immunological tests have been used by two different research groups to identify the histidine-rich protein-2 antigen of
P. falciparum
in the mummies of several predynastic individuals from Egypt, dating to
c
.3200 .¹⁶ This constitutes some direct evidence for the existence and activity of
P. falciparum
on the periphery of the Mediterranean world already in the fourth millennium . Further research into ancient biomolecules (especially DNA) from human skeletal remains excavated on archaeological sites in southern Europe offers the best prospect of obtaining direct evidence for
P. falciparum
malaria in Europe in prehistory.¹⁷ There was clearly some contact between Egypt and ¹⁴ M. T. Gillies in Wernsdorfer and McGregor (1988: i. 455) expressed the view that mosquito flight range is a property of the environment, not the species, depending on the availability of breeding sites and food, but it is clear that they generally do not fly far; Garrett-Jones (1962); Halawani and Shawarby (1957) on malaria in Egypt in recent times.

¹⁵ Angel (1966); Borza (1979), Zulueta (1987: 200), Sallares (1991: 275–7), Stuart-Macadam (1992), Grmek (1994), Corvisier (1994: 299–303), and Larsen (1997: 30–40) all agree that porotic hyperostosis does not necessarily indicate malaria.

¹⁶ R. L. Miller
et al
. (1994); Cerutti
et al
. (1999). cf. Marin
et al.
(1999).

¹⁷ The problem with trying to detect ancient proteins is that antibody reactions depend on the conformation of proteins. Since protein conformation would be expected to degenerate over time, it is not clear what degree of specificity could be expected in any particular antibody reaction with degraded proteins. G. M. Taylor
et al
. (1997) unsuccessfully tried to amplify ancient DNA from one of the same individuals studied by R. L. Miller
et al
. (1994), namely the Gurna mummy dating to
c.
700 . There are many possible explanations for this failure. Similarly C. Plowe, reported in
Parasitology Today
, 14 (1998: 9) expressed scepticism about the results obtained by R. L. Miller
et al
. (1994). Consequently further research is need-32

Evolution of malaria

Greece at least as early as the Early Bronze Age in the third millennium . The most striking illustration of this contact was the construction of the small-scale imitations of Egyptian pyramids at Hellenikon and Ligourio in the Argolid in Greece, which Pausanias passed on his travels much later. Consequently it is quite possible that
P. falciparum
could have been transmitted directly from Egypt to Greece at that time.¹⁸

However, there is another, even earlier, possibility. The Neolithic period commenced in Europe with the introduction of agriculture by human populations from the Near East, according to the generally convincing arguments presented in the monumental book by Cavalli-Sforza and his colleagues, a very important contribution to knowledge.¹⁹ Agriculture—specifically the cultivation of cereals and legumes—first developed in the general vicinity of modern Israel, Jordan, and Syria. These regions in antiquity certainly included some significant areas of wetlands, along the Mediterranean coast and in the Jordan valley, which harboured amphibious animals as large as the hippopotamus and permitted the cultivation of aquatic plants like papyrus. In more recent times, until they were drained, these wetlands were intensely malarious.

Similarly in antiquity Josephus described as pestilential in summer the air of the Great Plain around Lake Tiberias and the Dead Sea.²⁰ Consequently the earliest Neolithic farmers lived in a region that included some environments that were extremely favourable for malaria.²¹ This consideration supports Angel’s hypothesis that ed to confirm their results. Sallares and Gomzi (2001) discussed the problems in applying immunological tests to ancient materials. However, the results of the studies in molecular evolution cited earlier based on comparisons of modern DNA sequences make it extremely likely in any case that
P. falciparum
was present in Egypt by the fourth millennium , since such research does not suffer from the same technical problems as research on ancient biomolecules. Schiff
et al
. (1993) described the
Para
Sight test.

¹⁸ Theocharis
et al
. (1997) and Pausanias 2.25.6 on the Greek pyramids.

¹⁹ Cavalli-Sforza
et al
. (1994).

²⁰ Josephus,
de bello Iudaico
4.8.2, ed. Bekker (1855–6): ƒkpuroıtai d† ¿r6 qvrouß tÏ ped≤on, ka≥ di’ Ëperbol¶n aÛcmoı perivcei nos*dh tÏn åvra (The plain is burnt up during the summer season, and extreme drought makes the air unhealthy).

²¹ Tacitus
Histories
5.6–7 also described the Dead Sea region as pestilential with bad air: lacus immenso ambitu . . . gravitate odoris accolis pestifer (a lake with a huge circumference . . . whose oppressive smell brings pestilence to the local inhabitants). Hirsch (1883: 202) noted that malaria affected extensive regions near the Dead Sea in the nineteenth century, as well as the Bekaa valley in Lebanon at an altitude as high as 1200 metres (cf. Leeson
et al.
(1950) ). Fisher (1952) made the interesting observation that mosquitoes are carried up to the Bekaa valley by rising air currents which regularly occur in that region; Amadouny (1997); Filon
et al
. (1995) extracted ancient DNA showing the presence of b-thalassaemia from human skeletal Evolution of malaria

33

all the three species of human malaria under consideration, including the most dangerous,
P. falciparum
, were carried to Europe inside the bodies of the very first Neolithic farmers. The diagnosis of thalassaemia, a human genetic disease that confers some resistance to malaria, in the skeleton Homo 25 (a male sixteen or seventeen years old) from the PPNB (Pre-Pottery Neolithic B) village of Atlit Yam (now submerged off the coast of Israel) supports the idea that malaria was already active in the Levant at the dawn of agriculture.²² Given that the climate in the Neolithic period was actually exceedingly favourable to it, whether
P. falciparum
would have survived in new environments in southern Europe depended, as was noted earlier, on whether it encountered species of mosquito that were capable of acting as efficient vectors. Only a minority of the European species of
Anopheles
mosquito are good vectors for malaria.

This problem leads on to the fourth pillar of the late-introduction theories, namely the question of the possible refractoriness of mosquitoes to infection with
P. falciparum
. Experiments were performed using samples of
A. labranchiae
, originating from the coast of Tuscany near Tarquinia, and of
A. atroparvus
, from the Orcia river valley near Siena and from the upper Volturno valley north of Naples, to see if these Mediterranean populations of mosquitoes could ingest gametocytes from
tropical
strains of
P. falciparum
and successfully transmit sporozoites to new hosts.²³ The results were negative, in agreement with more extensive research of this kind subsequently performed in Russia which indicated that in general tropical strains of
P. falciparum
are not adapted to the mosquito species of Eurasia. Zulueta used these results to argue that a long period of adaptation would have been required to overcome refractoriness on the part of mosquito species in Greece and Italy.²⁴

However, even if this were the case, it would not prove that
P. falciparum
was a newcomer in classical times. Since the new data for its remains from Akhziv in Israel. For hippopotamus and papyrus in the region see Sallares (1991: 26, 370, 400–2). Theophrastus,
HP
9.7.1–2 also mentioned the marshes.

²² Hershkovitz
et al
. (1991).

²³ Ramsdale and Coluzzi (1975); Zulueta, Ramsdale, and Coluzzi (1975). Earlier experiments at Horton Hospital in England had shown that the English malaria vector
A. atroparvus is similarly unable to transmit tropical African strains of
P. falciparum
, although it can transmit Italian strains of
P. falciparum
. However, it could transmit all strains of
P. vivax
that were tested, although it is very inefficient at transmitting
P. malariae
(Shute (1940) and (1951)).

²⁴ Zulueta (1973), (1987), and (1994).

34

Evolution of malaria

presence in Egypt in the fourth millennium  suggest that
P. falciparum
was already present in the Mediterranean world thousands of years before classical times, a long period of time was indeed available for the refractoriness of European species of mosquitoes to be overcome. Moreover the experiments yielded no information about the critical factor of the length of time required for refractoriness to be overcome. It is not clear at the moment whether in this particular case overcoming refractoriness required evolution in the mosquito, evolution in the malaria parasite, or coevolution.

Mosquitoes certainly have several defence mechanisms against intruding foreign bodies in general, and may have genes that specifically respond to invasion by malarial parasites. Melanotic encapsulation or melanization is the most well known of these processes. This process is employed by mosquitoes and other insects to surround and inactivate pathogens. A thick layer of melanin is deposited around the malaria parasite when it tries to cross the mid-gut epithelium of the mosquito,
en route
to the salivary glands for the formation of sporozoites for transmission to another human. Cross-breeding experiments suggest that it is under fairly simple genetic control (no more than about three major loci being involved), with the implication that the expression of the process of melanotic encapsulation can be increased or diminished rapidly in mosquitoes.²⁵

However, there is the complication in the Mediterranean case that the mosquitoes, which transmitted Mediterranean strains of P. falciparum
in the past, are refractory to modern tropical strains, as has just been seen. This suggests that differences between various strains of
P. falciparum
were also important in some as yet un-defined way. Evolutionary processes tend to be very rapid among micro-organisms and, as will be seen shortly,
P. falciparum
has the capacity for very rapid genetic change. If it did not exist in the western hemisphere before Columbus, several species of mosquito indigenous to the western hemisphere quickly became effective vectors of
P. falciparum
, a pathogen which they had never encountered before 1492. The failure of Amerindian populations to develop high frequencies of any of the wide range of genetic mutations that confer degrees of resistance to
P. falciparum
malaria in ²⁵ Lombardi
et al
. (1986); Collins
et al
. (1986); Richman and Kafatos (1996); Yan
et al
.