Climate change, zoonosias, and human expansion: Virolution, Hendra Virus, Plague and Malaria

Human population growth, deforestation and epidemics, Frank Ryan's theory in Virolution: Recently I listened to Lord Monkton whilst looking at his slides about global warming. Why did I listen to Lord Monkton? Because I was irritated at the ad hominem and unscientific religious terminology of some of those trying to combat global-warming-denialist arguments. Before he ever got to global warming science, however I became frustrated at Lord Monkton’s simplistic opinions about the use of DDT and malaria rates. Didn’t he realize that the application of DDT failed with malaria as the mosquitoes adapted their habits to avoid the use of the poison and as they developed immunity to its effects? But the disease of malaria and that of other zoonosias is far more complicated than that because these kinds of disease involve a wide range of mosquito and other vectors with many different habits, in a wide range of landscapes among a wide range of humans with many different habits. Cites some little-known research from French epidemiologist historian Chantal Beauchamp. I hope the reader will find the following discussion interesting and possibly contribute more discussion. Much of this article comes from a book in progress by Sheila Newman (me) about population numbers and capitalism.


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Virolution

Since the mapping of the human genome we have discovered that virus material comprises a substantial portion of animal and plant cells, and is treated by organisms as part of themselves, because it has, in fact, evolved with that organism. Frank Ryan’s book, Virolution, Harper-Collins, UK, 2009, advances a theory about one of the roles of the virus material that all species carry as part of their genomes. Because their presence is deeply symbiotic – indeed it is just another part of the animal - so the animal is “immune” to the virus.

However, different animal populations carry different viruses as part of their genome, and sub-populations of species may carry viruses that are dangerous to other members of the same species. Ryan discusses this prospect in a study of different populations of koala in Australia, for instance. [1]

Ryan goes on to write about how it is likely that certain diseases, including diabetes mellitus and multiple sclerosis, result when this mutually beneficial relationship breaks down in humans for reasons that need more exploration. This is also an explanation for cancer and the increase in cancers in elderly people whose systems, as they become more disorderly, are unable to maintain an effective immune system.

Back to how the endemic viral material of different populations and species can be dangerous for that of other populations and species: The fact that viruses embodied in animal cells are dangerous to other populations which are not immune to those viruses, means that where one population – of the same species but from a different area - encroaches on another population’s territory, the encroaching population runs the risk of succumbing to viruses carried by the first. Other species can also be affected.

Hendra Virus, bats and invading horses and humans

It seems likely that recent infections of horses, humans and a dog by the Hendra Virus in Queensland demonstrate the Virolution theory. Under Queensland government policy, stable natural environments have been rapidly cleared for agriculture, tourist infrastructure and housing, to accommodate intrastate, interstate and international human immigration.
Humans and their horses have encroached on the territory of wild species, and have exposed themselves to viruses from the species they have invaded.

The species which are suspected of having transmitted the viruses will themselves have been impacted by the diseases that travel with humans and their domestic animals. We should be studying our effect on the bats, in order to learn more.

Australian natural history is a sad tale of vegetation and animal species depletion. For instance, native cats (quolls) have virtually disappeared from some states, probably succumbing to parvovirus borne by imported cats and dogs that accompanied land-clearing humans.

Hendra virus is not the only case where the forest has fought back. Australians have for some time coexisted uncomfortably with Barma Forest virus and Murray Valley Encephalitis. As we continue to push our ecological environment beyond the limits, we can expect this mix to become more ferocious and that this will be compounded by the absolute whirlpool of human population movements and activities globally.

Other places, other plagues, other times

This tale of the Australian ecological disaster can also be told of every other country where humans have, through the development of agriculture and new ways of traveling, rapidly and massively extended their footprint.

The Black Plague and ecological theory

There are theories that the Black Plague of the 14th century in Europe was not really rat-borne bubonic plague, but a zoonotic hemorrhagic fever. There are a few reasons to think this might be so, including an absence of reports of dying rats preceding the black plague, and the mortality rate, which was not really typical of bubonic plague. [2] Note, however, that several kinds of forest animal can carry plague and that rats may live in forests as much as they live in cities and on ships.

The plague, whatever it really was, greatly reduced population numbers in Britain and permitted forests to grow back and damaged soil to regenerate. The risk of malaria in England declined during this arboreal renaissance, probably because of a concomitant reduction in well-lit still-water pools. It would also have declined due to decline of the infection pool.

By the time of King Henry VIII, England's forests inspired envy elsewhere.

“The forests in the south of England, decimated by Roman iron smelting centuries before, had long since regenerated. England’s relatively rich endowment of wood provoked the admiration of the Venetian ambassador to King Henry VIII’s court whose country was then, for the most part, bare of trees. He wrote of the great bounty with which nature had provided England, allowing the English to have firewood in abundance. Indeed, in contrast to Venetian territory, ‘no lack of timber was felt or feared’ when Henry VIII ascended the throne.” (From John Perlin, A Forest Journey) [3]

Henry, however, saw Britain as backward and made haste to clear the forests for industry, importing skilled workers and capital to develop new industries, many of which depended on cheap fuel from burning plentiful wood, the prospect of which attracted capital.
With the addition of new immigrants, the human population began to grow and its activities to expand again in England. Where the trees were cleared, the water would rise and new swamps formed.

Little Ice Age and Malaria (1500-1750): exploiting the coastal niche in the Netherlands and England:

Adding opportunity for malaria in Britain was the rash of canal building, led and financed by immigrants from the Netherlands, where there was also a high rate of malaria. Malaria, which 20th century people tend to associate exclusively with the tropics, became a chronic scourge in England and continental Europe and would have impacted both on population numbers and on life expectancy. [Remark about Henry VIII edited out] [3A]

This was the time of the “Little Ice Age” (1500-1750), which Malaria Epidemiologist Otto S. Knottnerus describes as “the high days” of malaria around the North Sea in Europe, coinciding with this rush to exploit the “coastal niche” (made available by falling sea-level).

“... but its high-days were the Little Ice Age. After 1750 the disease retreated until it disappeared in the 1950s. The hotbeds of malaria were largely restricted to brackish coastal zones, where the mosquito Anopheles atroparvus could thrive. In these zones death-rates were 25-50 pct higher than in inland areas.” [4]

He attributes the increase in malaria in the Netherlands in part to resettlement around the polders (dykes), where malaria carrying mosquitoes which could live in quite salty water found conditions conducive. [5] This resettlement in the Netherlands was associated with massive land and water disturbance of a relatively chronic nature in conjunction with rapid population growth and the accumulation of high densities around cities. Around this time, as well, swamp draining and the building of sea-walls was ongoing across the sea, in England. [6]

Was the rising incidence of malaria in the late middle ages due to the new habit of digging canals and the denser settlement that brought relatively dense populations of humans in contact with malarial mosquitoes or the mosquitoes in contact with malarial humans including new immigrants from the Netherlands, where similar conditions prevailed?

Disease, mortality and environment in early modern England

Using data sets of demographic indices ranging from 1600 to 1800 AD for more than 1000 parishes in three counties of South East England, Mary Dobson, in “Disease mortality and the environment in early modern England,” [7] showed that death rates in marshland parishes were much higher than they were for other places in England. Mortality ran at over 50 in 1000 in such places and infant mortality was as high as 250 to 300 per 1000. These figures are comparable for death rates in tropical African countries where malaria prevails. They also reflect seasonal and annual changes in weather consistent with mosquito activity.
Paul Reiter, in “Defoe and Beyond,” [8] cites Defoe,

“a strange decay of the [female] sex here … it was very frequent to meet with men that had had from five to six, to fourteen or fifteen wives… the reason… was this; that they [the men] being bred in the marshes themselves, and seasoned to the place, did pretty well with it; but that they always went into the hilly country … for a wife: that when they took the young lasses out of the wholesome and fresh air, they were healthy, fresh and clear, and well; but when they came out of their native aire into the marshes… they presently changed their complexion, got an ague or two, and seldom held it above half a year or a year at most; and then …[the men] would go to the uplands again, and fetch another; so that marrying of wives was reckoned a kind of good farm to them.”

Diseases associated with marshes in England

Malaria was not the only disease associated with water in England.

“Altitude and drainage: water-borne diseases and the role of human pollution. One striking and repeatedly observed characteristic of the Southeast England data as the apparent significance of altitude and natural drainage in determining variations in death rates. Low-lying communities, especially those close to rivers and streams, while not as mortal as coastal and estuarine marshland parishes, nevertheless had consistently higher death rates than ‘dry upland’ settlements, defined as those situated above three or four hundred feet where there was often an absence or scarcity of surface drainage and water was obtained from wells or natural springs. …Low lying riverine parishes has average background mortality rates of the order of 30 to 40 per 1000; infant mortality rates between 150 and 200 per 1000; life expectation at birth in the thirties…” [9]

Why did Malaria disappear in Europe?

Malaria rates dropped after 1750 and slowly receded to almost vanishing point around the 1950s.

Decline in malaria incidence is often attributed to the introduction of quinine. Others, however, suggest that the doses and administration of quinine were probably epidemiologically ineffective in areas with high rates of endemicity. [10]
Among the technologies and practices known to Rome but reintroduced over the early 17th C was the technology of draining swamps. This technology, imported from Italy via Holland, came to England around 1614. [11] It is frequently claimed that this did much to reduce the very high mortality from malaria in England, but it has also been pointed out how mosquitoes did not always disappear. [12]

Another factor cited is climate change.

“Only two hundred years ago ague (malaria) was prevalent in the marshy lands of Norfolk, England. At that time the climate was more continental, giving very hot summers and cold winters. The disease as thus able to develop in the vector, Anopheline mosquitoes, in the summer and so spread from host to host, while it overwintered in man. The climate has changed now and the cooler summers are not favourable for the mosquito’s development.” [13]

Swamp draining and malaria in humans

The draining of swamps to enlarge agricultural land is often considered largely responsible for the reduction of mortality from malaria, in addition to other changes to agriculture. Some of these changes were the introduction of root crops such as mangel-wurzels and turnips as winter fodder for animals, which supplemented herd diets and meant carrying capacity was augmented. It is thought that this herd-boosting effect may have given mosquitoes a wider choice of targets. The mosquito is ‘zoophilic’, meaning that it has an eclectic taste in species. (The African anopheles mosquito is considered an exception, having seemingly developed a strong preference for human hosts in cohabitation with humans since farming in Neolithic times. This may be a reason it seems much harder to eradicate in Africa. On the other hand, continuous friction between humans and their disrupted natural environment might have more to do with this, as Robert Desowitz suggests, exposing more water to sunlight and therefore improving fertility opportunities for mosquitoes.) The introduction of machinery took men out of the fields and would therefore have lessened their availability to mosquitoes. New materials and construction methods made houses more mosquito-proof. More medical care and lower cost for quinine (the first treatment for malaria) reduced the amount of mosquito parasites in the blood and the amount of time they remained in the blood stream. This would have reduced the opportunities for mosquitoes to obtain infected blood from hosts. [14]

Chantal Beauchamp’s contribution to the epidemiological debate on malaria decline

Explanations do not always coincide with the epidemiological reality though. Malaria remains a mystery. When swamps are drained, water often appears elsewhere. The actual mosquito population may not diminish, but the incidence of malaria does.

War and malaria

In Chantal Beauchamp, “Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,” [15] Beauchamp describes how, between 1920 and 1930 in Spain, there was a massive organized fight against malaria, employing researchers and doctors in central and regional locations. Malaria rates fell until the time of the Spanish Civil War, when a new wave of malaria was largely blamed on infections originating in Morocco, plus disorganization of epidemiological medical services by the war. The anti-malarial war was relaunched with more services in 1944. In 1946 DDT was used in a few regions to kill anopheles mosquito vectors, although these mosquitoes were not common in Spain. Statistics of the time collected by Fernandez Maruto (see Figure, “Life and Death of a Disease”) showed a massive decrease between 1900-1918, followed by a couple of blunt spikes, then renewed decline after 1920, with this decline continuing until about 1938 when rates of malaria in Spain spiked to 1922 levels in 1943 or so, after which they declined precipitously to where the graph ends in 1960. Maruto attributed this decline to the medical approaches which had preceded it.

European Commission to fight malaria

As Chantal Beauchamp comments, however, it is obvious that the decline in malaria had begun well before 1920 when the European Commission to fight malaria and its [medical] services had first been organized and deployed. So, what really caused the decline? she asks. Perhaps anti-malaria combat simply needs sustained efforts and the interruption by the Spanish Civil War interrupted these? No, says Chantal. Malaria came back in Corsica at the same time, despite there being no civil war there and no mass arrival of troops from Morocco.

Quinine and malaria

Beauchamp also discusses the role of high and low doses of quinine in France, Corsica and Spain, finding that, high or low, quinine had impact, but cannot explain the whole story. Later drugs also had impact, but are still not sufficient. The success of campaigns against anopheles and other mosquito vectors is undermined by comparison with other countries which can be cited as ‘controls’. There are so many problems in identifying species of mosquitoes and determining their impact over many different regions that conclusions are difficult.

Herd-keeping and malaria

The role of changes to ways keeping herds is examined and found to be contradictory. In some countries the practice of putting animals in stables seems to have been accompanied by a decline in malarial predation on humans. In other countries, however, where herds were not enclosed, malaria also declined in humans.

Marsh-draining and malaria

Next, elimination of sundry water-sources such as marshes, ponds and puddles, coincided with a decline in malaria. Unfortunately for this theory, malaria also declined where water remained as it was.

On page 255 Beauchamp gives this example of how malaria declined in some regions of France whether or not swamps and similar water-sources were drained. Furthermore, in these cases, variations in the density of anopheles populations fails to coincide with variations in the rate of malarial infection in humans.

“In France, in three regions justly reputed as malarial because they were marshy – Brenne, Sologne and Dombes – health measures involving draining marshes were only temporary in Dombes, very patchy in Sologne, and non-existent in Brenne. Despite this, malaria disappeared in these areas between 1880 and 1890. Marsh draining, limited or absent, had practically no impact on the anopheles mosquito population, of which the density today is similar to the density it would have had in the 19th century, when malaria was rife. In some cases the ponds and marshes have become even more extensive than they were last century [19th], and the presence of anopheles mosquitos is even greater. This is also true of the dead arms of the Rhine River and the length of the Il in Alsace. We can say the same for the Languedoc, where, despite the growth in rice-paddies and the expansion of larval niches from 1914, no expansion of local malaria has been observed.” [16]

Human vectors and malaria

Beauchamp then notes that human malarial infection ultimately depends on human vectors. High birth rates tend to coincide with high rates of malaria, and declines in malaria rates with declines in birth rates. Unfortunately this coincidence doesn’t explain why the rate of malaria jumped over the period of 1937 and 1943 in different places.

Improvements in living standards and malaria

Next she notes how improvements in living standards have often been cited as responsible for declines in disease rates – for malaria and for many other maladies. The details of cause and effect, however, are lacking and the evidence is often contradictory:

“In its detail however, the relation between economic standard and standard of health is much less clear. From 1948 malaria mortality rates declined to the levels of 1936, but in 1955 agricultural production was 18% less than in 1930 and the availability of cereals per inhabitant was 35% less! It is only in 1962 that agricultural production in Spain again attained the level of 1931-1935. To explain the regression of a disease after the Second World War on economic development in Spain seems a little quick. In that period it seems that organized epidemiological approaches were the decisive factor in the eradication of the disease.” [17]

Ecological degradation and malaria

She refers (p.258) to Pérez Moreda’s ecological theory that changes to the landscapes of agricultural holdings in the 18th century, with neglect of huge ranches [latifundios] in the South and the center of the country, coincided with the rise of malaria in the same period. Deforestation caused by overgrazing, and soil degradation through environmentally destructive farming practices, were two important factors in the expansion of the disease.

For Moreda, ecological degradation with thinning of vegetation layers assisted the development of anopheles larval niches in the soil, while deforestation provoked mosquito migration on a grand scale.

Presence of humans and their activities and malaria

So, what are we to conclude?

That we are invisible to ourselves, perhaps. That we lack insight into our own presence on the planet.

Beauchamp concludes that the presence and activities of humans are not properly taken into account in the history of malaria.

My reason for exploring the relationship between population numbers and malaria was because, when I started a book I am currently writing about population numbers and the rise of capitalism, I had assumed that population numbers had risen in Britain because of the availability of coal as fuel. I discovered, however, that the numbers had begun to rise just before the actual industrial revolution. One reason for this seems to have been a decrease in rates of malaria.

“Retreat (1750-1950): The retreat of malaria has not yet been sufficiently explained. During the 18th century mortality rates in some regions were falling rapidly, whereas in others they remained largely the same. Particularly in Southeast England and the western districts bordering the Wadden Sea (Friesland, Groningen and East Friesland) population growth started early. But in Holland, Zealand and many German districts figures remained stagnant until the 19th century, whereas mortality rates were high up to the 1850s (Dobson 1998:81-159; Knottnerus 1997:38; Norden 1984). Moreover, in the German Lower Rhineland as well as in the Baltic unprecedented outbreaks of malaria took place during the first half of 19th century (Jaenson & al. 1986; Anderson 1980; Kortenhaus 1928; Wesenberg-Lund 1920-21:172). In general, tertian fevers got a more epidemic character instead of remaining endemic, whereas quartan fevers tended to become rare. As malaria outbreaks became more uncommon, seasonal peaks shifted from late summer to early spring (Swellengrebel and De Buck 1938; Seventer 1969).” [My translation][18]

What caused the decrease? It is possible that Britain's environment became more stable after 1750, and that between 1600 and 1750, destruction of the natural environment was at its height, but after 1750 there was either very little natural environment left and/or human settlements and activities took on an urban character which did not bring large numbers of people into contact with mosquitoes, soil and water. Around this time, enclosures reduced land for peasants, and forced people to seek work in cities.

London and Queensland: Plague and Hendra Virus

At the time of the civil wars in Britain, people in London went into the countryside and attacked the forests for wood to burn for fires. This was part of an orgy of forest destruction that went on preceding and during the time of the last big black plagues in England, which hit London badly.

Was the forest fighting back?

Is the forest fighting back again against the insightless destruction by humans of their natural environment in Queensland? Should we modify our behaviour?

NOTES

[1] Ryan also makes a remarkable mistake about the numbers of koalas in Queensland, which I brought to the attention of the publishers and the Koala Foundation of Australia. I think that he confused the number of Queenslanders with the number of koalas somehow, because he came up with 4 million!
[2] Source: G. Christakos, Interdisciplinary Public Health Reasoning and Epidemic Modelling: the Case of Black Death (2005), pp. 110-14.

[3] John Perlin, A Forest journey, the story of wood and civilisation, The Countryman Press, Woodstock, Vermont, USA, 2005, p. 163.

[3A] On 21 April 2013 I edited out the following remark because I cannot find my original source for it and because it seems to be contradicted by dates of usage of malaria: "Henry VIII, by the way, was an early user of quinine and may have suffered from malaria."

[4] Otto S. Knottnerus, “Malaria Around the North Sea: A Survey,” in Gerold Wefer, Wolfgang H. Berger, Karl-Ernst Behre, Eynstein Jansen (ed.), Climatic Development and History of the North Atlantic Realm: Hanse Conference Report. Berlin Heidelberg: Springer-Verlag, 2002, pp. 339-353 http://www.xs4all.nl/~ottoknot/werk/Malaria.html

[5] http://www.xs4all.nl/~ottoknot/werk/Malaria.html

[6] From historical records, we know that a malarious illness referred to as “the ague” or “intermittent fever” caused high levels of mortality in the British marshlands and fens from the 15th to the 19th century (4, 5). Robust evidence that the illness was malaria emerged in the early 19th century, when the increasing use of quinine and advances in fever diagnosis and pathology created a distinct separation from other acute fevers. Source: Katrin Gaardbo Kuhn, Diarmid H. Campbell-Lendrum, Ben Armstrong, and Clive R. Davies, “Malaria in Britain: Past, present, and future,” National Academy of Sciences, 2003, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC188345/

[7] Dobson, Mary J., “Contours of Death: disease, mortality and the environment in early modern England”, Health Transition Review, Vol.2, Supplementary Issue, 1992, p.81.

[8] Paul Reiter, “From Shakespeare to Defoe: Malaria in England in the Little Ice Age”, Emerging Infectious Diseases, Vol.6, No.1, Jan-Feb 2000, Center for Disease Control and Prevention, San Juan, Puerto Rico, p.7.

[9] Dobson, Mary J., “Contours of Death: disease, mortality and the environment in early modern England”, Health Transition Review, Vol.2, Supplementary Issue, 1992, p.82

[10] Chantal Beauchamp, “ Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,” Économies, Sociétés, Civilisations, Volume 43, Issue 1, 1988, pp. 249-275, p.258, and J. Callot, “Un problème complexe : la régression du paludisme en France,” Annales, Économies, Sociétés, Civilisations, Vol2, Issue 3, 1947, pp. 328-335, p.332.

[11] Joan Thirsk, “The Rural Economy”, in Ed. Jerome Blum, Our Forgotten Past, Seven centuries of Life in the Land, Thames & Hudson, 1982, p.86.

[12] Chantal Beauchamp, “ Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,” Économies, Sociétés, Civilisations, Volume 43, Issue 1, 1988, pp. 249-275, p. 255.

[13] The Larousse Encyclopedia of Animal Life, Hamlyn, London, 1972, p.27

[14] Paul Reiter, “From Shakespeare to Defoe: Malaria in England in the Little Ice Age”, Emerging Infectious Diseases, Vol.6, No.1, Jan-Feb 2000, Center for Disease Control and Prevention, San Juan, Puerto Rico, p.7. and Robert S. Desowitz, New Guinea Tapeworms and Jewish Grandmothers,W.W. Norton and company, New York, London, pp20-21 and 50-51.

[15] “Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,”Économies, Sociétés, Civilisations, Volume 43, Issue 1, 1988, pp. 249-275

[16] Chantal Beauchamp, “ Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,” Économies, Sociétés, Civilisations, Volume 43, Issue 1, 1988, pp. 249-275, p.258, and J. Callot, “Un problème complexe : la régression du paludisme en France,” Annales, Économies, Sociétés, Civilisations, Vol2, Issue 3, 1947, pp. 255.

[17] Chantal Beauchamp, “ Fièvres d'hier, paludisme d'aujourd'hui. Vie et mort d'une maladie,” Économies, Sociétés, Civilisations, Volume 43, Issue 1, 1988, pp. 249-275, p.258, and J. Callot, “Un problème complexe : la régression du paludisme en France,” Annales, Économies, Sociétés, Civilisations, Vol2, Issue 3, 1947, pp. 257-258.

[18] Otto S. Knottnerus, “Malaria Around the North Sea: A Survey,” in Gerold Wefer, Wolfgang H. Berger, Karl-Ernst Behre, Eynstein Jansen (ed.), Climatic Development and History of the North Atlantic Realm: Hanse Conference Report. Berlin Heidelberg: Springer-Verlag, 2002, pp. 339-353 http://www.xs4all.nl/~ottoknot/werk/Malaria.html

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Comments

Don't forget - FISH and malaria

Sheila,

Interesting speculations about malaria in your piece.

But an absolutely crucial 4 letter word is missing:

FISH.

Fish are the great predators upon mosquito wrigglers. Almost all normal fish (other than those too large or too specialised in bottom feeding or algae scraping) eat mosquito wrigglers in preference to almost all other foods. (Get yourself an aquarium with guppies or white cloud minnows -- add wrigglers at the same time as other fish foods, and watch.)

Wrigglers are rapidly and utterly eliminated by fish from open waters -- unless the water is so choked with vegetation that fish can't penetrate. Or some fool has being spraying DDT on pools that would normally contain fish.

Some supposed authorities are so ignorant of this basic ecological fact that they talk as if it was the amount of water around that determines mosquito populations.

In general wrigglers need temporary pools where fish can't survive. That's why you can often harvest wrigglers at the beach -- but only in fish-less rockpools well above normal high tide.

Marshes may or may not contain such fish-free areas.

Cheers,

Mark

Fish, mosquitoes, land disturbance in ecological theory

You are absolutely right, Mark. How could I forget and how come this factor is so little mentioned?

If you have more on it, please write more.

I have mosquito fish and goldfish in ponds at home and it is true that there are no mosquitoes in the ponds, however there are mosquitos in the grass around the ponds, morning and evening. Without taking away from your comments and the importance of fish, let us consider how land disturbance in agriculture, war, and canal digging, create new opportunities in the soil for mosquitoes to rear their young away from the usual presence of fish and other creatures that dine of mosquitoes. This ties in with Pérez Moreda’s ecological theory that changes to the landscapes of agricultural holdings in the 18th century, with deforestation caused by overgrazing, and soil degradation through environmentally destructive farming practices, were two important factors in the expansion of the disease.

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