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MALARIA IN MEXICO C. Johnson
'Mr. Johnson was a graduate student at the University of Mi nnesota when he d id the research for this paper.
The traditional picture of Mexico, at least to the average tourist, is never complete without a peon, in sombrero and serape, sleeping in the noonday sun, or putting off to manana whatever physical exertion is required. Actually, when seriously considered, this picture is found to be a myth, no more representative of the Mexican than the traditional cowboy or gangster image is of the American . What is interesting in the image of the peon, however, is that he is an excellent example of a population debilitated by malaria.
To most North Americans, the existence of endemic malaria in an industrialized, modern nation is unthinkable -it is considered to be a lowland tropical disease of undeveloped nations. As recently as 1957, however, it was in Mexico the leading cause of death from infectious disease, accounting for 519.6 deaths per million population, far exceeding such killers as tuberculosis (302.1 per million), influenza (270.9 per million), dysentery (188.6 permillion) and typhoid (117.8 permillion).1 Doubly interesting is the fact that, by that year, both smallpox and yellow fever had been vanquished, evidence of the progress of modern medicine in Mexico. Since 1957, malaria also has declined in its toll in human lives, but a question is raised of how extensive that toll may have been historically.
28
Population D ecline in Mexico
The phenomenal decline of the native population of Mexico in the years following the Spanish Conquest is a matter of record. The cause of the decline has been a subject of controversy, but examination of the literature available seems to indicate that disease played a major role. The problem ishow much of the decline may be attributed to malaria?
No actual figures are available on the population of the Valley of Mexico before the Conquest and during the early years of the colonial regime. Many estimates have been made, but only recently have they been based on such records as tributary lists or taxation data. Such is the case with the estimates of the Berkeley school;2
Estimated Year Population
1519 25,200,000 1532 16,800,000 1548 6,300,000
1568 2,650,000 1580 1,900,000 1595 1,375,000 1605 1,075,000
Opinions as to the lowest point of the decline vary-sufficient data are not yet available-but there is general agreement that it was some time in the seventeenth century.
In th ei r studies of population decline, Borah and Cook divide Mexico into ten ecologically-unique regions, grouped as either Highland or Lowland. Comparing subtotals, they note that, while the total population of Mexico was declining at a rate of about 3% to 6% per year, the rate of decline was nearly twice as great in Lowlands as in Highlands. They interpret this difference as being disease-related. By extending this type of analysis to the
29
individual regions, interesting differences are revealed between them, as well as between Highlands and Lowlands. (Tables 1 and 2) Mapping the rates of decline suggests that no single disease could have been responsible for the decline, but a series of diseases, affecting different areas at different times, or with different degrees of severity (Maps 1 to 4).
The data obtained also reveal that recovery began in some regions during the late sixteenth century-areas along the Pacific Lowlands with some associated highlands (i.e., Nayarit, Colima, Jalisco, Guerrero, and part of Michoacan), plus the northern Atlantic Lowlands (i.e., parts of Tamaulipas, San Luis Potosi, and Hidalgo, and northern Veracruz). In southern Veracruz, however, population appears to have stabilized briefly about mid-century (1568-1580), and then resumed the decline (1580-1595). This suggests the introduction of a new disease at that time in the area around the chief port of colonial Mexico.
Characteristics of Malaria
Research into the epidemiology of malaria reveals that, of the four species of Plasmodia, three are found in Mexico. All may be fatal, especially if untreated or if treated with inadequate quinine therapy. Falciparum malaria, which is not usually recognizable by the classic chills and fever, may kill within a few hours. Repeated attacks of the milder vivax malaria also may kill. Under epidemic conditions, as when introduced into a non-immune population, pernicious malaria (particularly falciparum) is more common, and large numbers of people of all ages may be killed in a relatively-short outbreak. During such epidemics, there may be intestinal complications or hemorrhaging, but death is most often due to cerebral involvement, so
TABLE 1
DECLINE OF POPULATION (by regions)
Population Population Percentage of Pre-Conquest 1532 Pre-Conquest
Region (mi ll ions) (millions) Population
10.9 7.99 73.3 II 2.8 1.53 54.6 Iia } 1.9 .88 {
.17 46.3 III .71 IV 1.7 1.56 91.8 V .8 .68 85.0
VI } 4.6 2.10 { .86 45.7 VII 1.24 VIII 1.3 1.03 73.1
IX } 1.2 1.07 { .46 89.2 X .61
Total 25.2 16.87 67.1
Source : Woodrow Borah and Sherburne F. Cook, " The Aboriginal Population of Central Mexico on the Eve of the Spanish Conquest," Ibero-Americana : 45 (Berkeley and Los Angeles : University of California Press, 1963), p. 157.
TABLE 2
DECLINE OF POPULATION IN HIGHLANDS (A ) AND IN LOWLANDS IN COASTAL REGIONS (B)
(by regions)
Percentage of Percentage of Percentage of Percentage of 1532 Population 1568 Population 1580 Population 1595 Population
Remaining Remaining Remaining Remaining Region in 1568 in 1580 in 1595 in 1605
A Highlands I 21.3 72.1 62.5
Iia 18.1 68.7 90.9 IV 14.2 67.0 97.3
VIII 18.0 85.6 60.2 IX 17.5 80.2 140
Subtotal 19.8 73.1 69.9 75.7
B Lowlands in Coastal Regions II 4.8 56.7 109.5
III 5.3 84.2 56.2 V 9.9 82.3 66.0
VI 7.4 68.7 77.2 VII 9.1 56.1 110.9
X 9.9 34.4 195.2 Subtotal 7.4 62.6 94.1 89.9 Total 16.7 71.5 72.5 78.3
30
)
o 10:) E4 E""9
5c'lle in '.!iles
31
SURVP.TAL ~ TE
0:'" POPULA '!'ION IN COLONIAL !O~EXICO
20:) I
13:v Regions
Prp.conqup.st t o 1532
IT:] [IIJ ,...,-, L.:..-'..J
ITZJ &\'SI
Man 1
Least decline - 99 .0 to 99.9% 713 .0 to 87 .9% 73 . 0 to 77 . 9% 50.0 to 59 .9%
~.Iost decline hO . f) to hc}.9% £J
" )
~ ( ) L. . .,
I c._ . 1'"
i~·· - · \ . C ~ ~ i . ( "I. ." . .-J , . \ r
/ - ~ ~ ' \ ") i 'l ~
I / I 1
.,- ) \ ../ I" ! ,... . 6 ; (
", ' • c.. ~ -'\ ~ . I c... II ""I 1[: ' - .
'\). I " ~j" '- . . " . Iil' ._. ii " , .--' J! I I ~/ . . '\ .
I' . . ~ . -' " J II . "1. f I 1" ' --. .
~ -' (_. " 1" . '1 .. -... . -- ~ I ) .. ~
. I ·1 r ·...}
I l ~rr
I I . :IX. . I - I I " . , ,
~~I VIII .. -W
. (
\1.1 11 . :EtA h, IIlliI I I . . . . _IE: 1:::. ..... I . . . ~Df I I' I . ..
• ... I - flj ...
I I I I1iiW , I I I I I I "'1: , . \ 1111 I I I
" I I ' I N II I I W I I I 1\
I I I \I I '--1[11 1111 I ~ I ~ I . t-
I ..,;,
'1L l ,!!! ---.....
Sl' l1 'TI7AL qATE OF POPULATION IN COLONIAL ~ICO
9y ~egions
1532 to 1568
HE Over 20% ( Leas t decline)
EH3 15. 0 to 19.9% 0 100 200 M 9.0 to l u . 9% E4 F3 F"1 I
SC'll e in }Ul es ~ Under 9t (Greatest decline)
tJ
V.an 2
32
33
.J"
I )
i~ ' - " . \ . { i. ' , \
._ . ,/ :r'\ /' (
,-'
SU:PTI7AL ~ATE OF POPULATION IN COIDNIAL MEXICO
'3y ttegions
o 100 200
1568 to 1590
m 78 . 0 to 87.9% (Least decline) o 60. 0 to 72 .9~ m 50.0 to 59 .9%
Scale in Miles a 30.0 to 39.9% ( Greatest decline)
lJ
Map 3
\ )
r o-. • . , \
f' . . \ . I ". \
. ?\ / I
.--- \ <;
(
\
--- . ~ . l.'" '\, ." ~.-
(
SURnVAL ltATE OF POPULATION IN COWNIAL MEXICO
o 100 200 E3 F?"3 E3 I
Scale in Miles
3y ~e~ions
1580 to 1595
D [L] ~ l.!....:....:J
EZl W2l
Over 100% 88 .0 to 99.9% (Least decline) 71.0 to 77 .9% 60 .0 to 72 .9'1, 50.0 to 59 .9% (Greatest
decline )
Man 4
34
...
L
that the disease may resemble encephalitis or meningitis. Thi s fact leads to a suspicion that the puzzling death of Ponce de Leon could have been due to malaria. The puzzle then becomes why the epidemic spread no further than the hundred persons stated to have been on the ship involved.
Origin and Diffusion of Malaria
Historically, malaria was common around the Mediterranean. The Atlas of Diseases3 shows Plasmodium vivax (tertian malaria) as above average in prevalence in this area, and Plasmodium fa/ciparum below average. In Africa, however, P. falciparum is above average, and P. viva x below average in prevalence, partly the result of a genetic immunity to the latter, found only in certain groups of Negroes and in some hill people of India. Such resistance indicates the process of natural selection operating over a long period of time, hence long association with the disease. Africa is, therefore, the probable source area of malaria, with P. vivax representing the earliest form of the disease.
The outbreak of an epidemic requires only: (-I) a non-immune population ; (2) a source of disease organisms (in the case of malaria, gametocytes) which may initially be small; (3) a vector (in the case of malaria, any of numerous species of anopheline mosquitos). All three factors appear to have been present in the New World. The migration of small groups of peoples across the Siberian land bridge during glacial ages would effectively screen out diseases such as malaria in which temperature is a crucial factor in the development of both the parasite and the vector. This argues for the existence in America of a non-immune population. Records of the Spanish Conquest show that the first Negro reached Mexico as early as 1520 with
35
Narvaez' army, so it is probable that sources of different gametocytes (from Spain and from Africa) were present, at least sporadically, from the date of the Conquest or very soon after. The distribution of anophelines therefore becomes the critical factor to trigger epidemics of malaria in the NewWorld.
Vectors of Malaria
In the laboratory, many species of mosquitoes may be made to carry disease organisms, but, under natural conditions, they may not do so. A species must first of all be widespread and exist in large numbers to be important as a vector. Distribution of anophelines is limited by ocean barriers, each species tending to be restricted to a si ngle continent unless a land bridge exists, such as that between North and South America. Most species are neotropical, but a few are found in northern latitudes or high altitudes, and these are obviously of great interest in a study of the Mexican Plateau.
The Atlas of Diseases locates the Mexican species as follows: in Yucatan and the southern lowlands, Anopheles pseudopunctipennis p., and A. darlingi ; on the Gulf Coast, A. quadrimaculatus, and A. albimanus; on the plateau, A. pseudopunctipennis, and A. aztecus (a subspecies of the series maculipennis). Komp (4) adds to these A. crucians, and A. atropos. Russell et al (5) add A. argyritarsis and A. punctimacula (extending from southern Mexico to Argentina), and A. freeborni (which extends from the west coast of Canada to Northwest Mexico).
Komp ' s classification shows A. quadrimacu/atus, A. maculipennis, A. crucians, A. pseudopunctipennis, and A. eiseni in a closely-related group, and A. albimanus, A. argyritarsis, and A. darlingi in different series of another sub-genus of anophelines. Any of these can be vectors of malaria. They
are not equally abundant, however, so that the most likely vectors in Mexico are A. aztecus, A. quadrimaculatus, A. pseudopunctipennis, and A. albimanus. As these are all domestic mosquitoes, their distribution would account for the presence of the disease everywhere in Mexico, which was in fact the case.
Boyd6 has noted that the ecology of anophelines varies among species. These differences explain some of the peculiarities of malaria. A. albimanus, believed to be the single most important vector, is highly adaptable, but not found over 3000 feet elevation, generally breeding below 1300 feet (400 meters) . It tends therefore to extend inland along river valleys. A marked reduction in its numbers is noted during the dry season in Mexico. In the areas where A. albimanus is the vector, therefore, malaria tends to occur during the rainy season.
Anopheles quadrimaculatus is believed to be the vector of malaria on the Gulf Coast from around Tuxpan north into the southern United States. This mosquito appears to be inhibited by high temperatures.
Anopheles pseudopunctipennis, the principal vector in highland areas, also appears to sp read along river valleys, is distributed throughout the mountains from the southwest United States into South America, and inhabits most of the area between Sierra Madre Occidental and Oriental. It appears to prefer mountain streams which are not being flushed out by heavy rains and is responsible, therefore, for outbreaks of malaria during the dry season, and, obviously, for most of the incidence of the disease in arid and semi-arid areas.
One characteristic that A. albimanus and A. pseudopunctipennis have in common is their preference for resting high on the inside of palm-leaf-thatch
roofs. This habit makes both dangerous as vectors, because of the proximity of victims.
The last major vector of malaria in Mexico is Anopheles aztecus, a variety (or sub-species) of A. maculipennis. This species is not as particular as most mosquitoes about clean water, and, therefore, tends to replace A. pseudopunctipennis around populated areas such as Mexico City. It breeds in canals, irrigation channels, and pools containing algae and protozoa, and has been found surviving even under ice. This species is believed to be responsib le for malaria around Xochimiico, where clinical histories have been recorded since 1875.
Anophelines are not noted for longdistance flights, but may be dispersed for considerable distances downwind. Wind velocities at sunset and sunrise, when mosquitoes fly, may therefore be important factors in malaria. Conversely, by creating waves, wind action may be destructive to anophelines in the aquatic stage.
The individual susceptibility to malaria of different species of Anopheles varies, and some seem to be more susceptible to one species of Plasmodia, e.g., A. crucians seems to carry P. falciparum. Species which have been infected experimentally in the laboratory do not always seem to transmit malaria in nature. This suggests that a period of adjustment or adaptation is necessary before a vector can transmit infection efficiently. If this is so, the lapse of time between the arrival of the first Spanish conquistadores and the first outbreak of malaria, as well as the strange death of Ponce de Leon, may be explained (i.e., the disease was contracted elsewhere by all the people on the ship, but did not spread further because, in Mexico, no vector was yet adapted to the disease organism).
36
....
Physical Factors in The Incidence of Malaria
The most malarious area in Mexico is reported to be the south, but the disease occurs in highlands and arid areas as well as on the coast and in the rainforest. Official rates have been listed as: for the entire Republic, 140.7; Distrito Federa l, 6.16; Michoacan, 9.43; Chihuahua, 12 .55; Tabasco, 502.29; Oaxaca, 563.02. In 1939, an average figure, covering the period from 1922 to 1938, was set for morbidity at 576 per 100,000 population ; for mortality, 145.5 per 11 ,000,000 population?
Temperature appears to be the single most important climatic factor, since the optimum condition for development of the parasi te in a mosquito is 25 °C (45 ° F) for P. vivax, 30°C (54° F) for P. falciparum, and 22 ° C (39.6 ° F) for P. malariae. With lowered temperature, the time variable for development of the parasite increases, so that factors which affect temperature (i.e., winds, altitude, cold currents) will affect the occurrence of malaria, possibly even extend the period of development beyond the life expectancy of the vector.
The effects of wind and rainfall have already been noted, so that it is obvious how climatological factors may determine the type and number of mosquitoes in an area, their life expectancy, and the extent of their dispersal. However, since different species are adapted to different conditions of the natural environment, there is usually a vector present in Mexico which may transmit malaria.
Human and Cultural Factors in The Incidence of Malaria
Man is the only known carrier of malaria parasites. His movements are thus important in the spread of the
37
disease, through pilgrimages, work patterns, migrations, trade missions, even holiday travel , and the subsequent pattern of disease should suggest such movements. In Mexico, the predominant types of malaria are: on the p lateau, P. vivax; in Yucatan and the adjacent lowlands, P. falciparum (though both of these plus P. malariae appear in each area). Tertian malaria (P. viva x), endemic in Spain, is believed to have been brought to America by the conquistadores, this being at least partly responsible for the rapid initial disappearance of the aborigines of the coast and Caribbean islands. Malignant malaria (P. falciparum) appears to have come to the New World later, from Africa, with the importation of increasing numbers of Negro slaves. The distribution appears to corroborate this theory, as Negroes were more frequently employed on plantations in lowland areas.
Man's activities provide new breeding areas for the vectors of malaria by deforestation, the breaking of ground for agriculture, irrigation, or the construction of such reservoi rs as fish and mill ponds, or even stock-watering tanks.
Various cu l tural habits are also contributory. Examples cited in the Atlas of Diseases include the type of housing (e.g., on the ground or on piles), cooking facil ities (indoors or out), and the proximity of domestic animals which might be alternate victims. The use of screening, repe ll ents, and insecticides in modern industrial countries is important, but the conservatism of primitive peoples may lead to rejection of such modern innovations. Human attitudes are thus crucia l to the incidence of malaria.
The selective nature of diseases in Mexico is one basis of the Black Legend (i.e., the theory of deliberate extermination). Examination of the incidence of malaria reveals something
I I I
else. Early sources refer to the host of insects encountered, but we cannot be certain that the people of Colonial Mexico were aware of the capacity of vectors as carriers of disease. In the sources examined, only GageS refers to the use of pabelfons. The extent of such use of mosquito nets or mosquiteras is not known, but their origin dates back to the canopea of the Roman Empire. Since the Romans were aware of the connection between mosquitos and malaria, when they drained marshes, it is probable that the Spaniards also knew.
Among the Indians, the one known custom which would have discouraged mosquitoes- the continual censing with copal that was practiced on the teocalfi and elsewhere- was banned by the Spaniards. Thus the incidence of disease tended to be increased in one group and decreased in the other by cultural practices, above the differences produced by centuries of exposure to disease in the Mediterranean and Africa on the one hand and America on the other, with no immunity at all .
Conclusions
In nature, there is no immune state for malaria, although it has been induced experimentally. This is partly due to the large number of strainsthere appears to be no limit9-and immunity gained from an infection by one strain gives no protection from others, or, indeed, from the same strain after a lapse of time. The sole exception appears to be the genetic resistance associated with the abnormal blood type S (i.e. , the sickle-cell trait).
In Spain and in Africa, where it apparently originated, malaria is endemic because of long exposure of the population. In the New World, exposure of a non-immune population would result in the outbreak of epidemic as
soon as a vector was capable of transmitting the disease. At least two such epidemics seem probable-one of tertian malaria, one of malignant-but there could also have been an epidemic of quartan (P. ma/ariae) as it may easily be masked by the more obvious symptoms and severity of the other types.
These characteristics of malaria appear to fit into the observed pattern of decline - recovery - decline of population during the sixteenth century in Mexico. The outbreak of fever reported in Yucatan in 1527 was probably malaria (not yellow fever, as reportedL and it seems probable that it was one of the many diseases which broke out in Mexico just before and after 1532. Malaria was probably the great epidemic which followed Guzman 's expedition to the west. Certainly malaria was established on the Gulf Coast in 1536, as, in that year, Zumarraga wrote the Council of the Indies regarding the need for hospitals on the highway from Veracruz to Mexico City. From 1599 to 1618, one source10
reports yellow fever in Yucatan, but most others believe this disease to have made its initial appearance considerably later, in 1648.11 It is probable, therefore, that this epidemic was the first appearance of malignant malaria (P. fa/ciparum), since slaves were being imported in the late sixteenth century. Following this outbreak, disease appeared on the plateau at Xochimilco, in 1601, in a pattern typical of malaria.
Translated, the Aztec term cocoliztli means merely painful fever. Because of the tendency of the Aztec language to describe qualities rather than specifics, this name, which appears frequently in the literature, seems to have been applied to different diseases at different times. It is probable that malaria was an important member of this
38
...
..
group. The fact that it does not seem to have been recognized by the Spaniards may be ascribed to the changed pattern of the disease, which had been
FOOTNOTES
I I ) World Heal lh Organizalion, Annual Epidemio logical and Vital Statistics, 1957 (Geneve : 1960).
(2) Sherbu rne F. Cook and Woodrow Borah, " Indian Population of Cenlral Mexico 1531-1610," IberoAmericana : 44 (Berke ley and Los Angeles : Universily of California Press , 1%0). Woodrow Borah and Sherburne F. Cook, " The Aboriginal Population of Centra l Mexico on Ihe Eve of the Spanish Conquest," Ibero-Americana : 45 (Berke ley and Los Angeles : University of California Press , 1963). Woodrow Borah and Sherburne F. Cook, " The Population of Central Mexico in 1548," Ibero-Americana: 43 (Berkeley and Los Angeles : University of California Press, 1960).
(3) American Geographical Society, Atlas of Diseases (New York).
(. ) W. H. W. Komp, "The Classification and Identification of (he Anopheles Mosquitoes of Mexico, Central America , and the West Indies," A Symposium on Human Malaria , ed. Forest Ray Moulton (Washington : American Association for the Advancement of Science, 1941) .
39
produced by the interaction of factors -a non-immune population, different vectors, and the different physical and climatic characteri sti cs of Mexico.
(5) Paul F. Russe ll , Luther S. West, Reginald D. Manwe ll , and George MacDonald, Practical Malariology (London: Oxford University Press, 1963).
(0 ) Mark F. Boyd (ed.), Malariology (Philadelphia and London: W. B. Sau nders Company, 1949).
(7) Ernest Carroll Faust, " The Distribution of Malaria in North America , Mexico, Centra l America , and the West Indies," A Symposium on Human Malaria, ed. Forest Ray Moulton (Washington : American Association for the Advancement of Science, 1941) .
(S) Thomas Gage, Travels in the New World, ed. J. Eric S. Thompson (Norman : University of Oklahoma Press, 1958) .
(' ) W . B. Redmond, " Immunity to Human Malaria : Characteristics of Immun ity," A Symposium on Human Malaria , ed. Forest Ray Moulton (Washington : American Association for the Advancement of Science, 1941).
(101 Gordon Schendel , Medicine in Mexico (Austi n and London : University of Texas Press, 1968) .
(1 1) George Cheever Sha ttuck, The Peninsula of Yucatan: Medical, Biological, Meteoro logical and Sociological Studies (Washington: Carnegie Institute, 1933) .