Fripay, DecemBer 19, 1930

No. 1877

Animal Parasites of Man and Their Control: Dr. AutrreD C. REED 611 Obituary: George McLane Wood: Brernarp H. Lane. Me- morials; Recent Deaths 620

Scientific Events: Archeological Field Work of the University of

Minnesota; Work of the Smithsonian Institution ; The Committee of One Hundred at Cleveland;

Statistics of University Registration 622 Scientific Notes and News 625 Discussion:

An Appeal for the Issuance of Reprints of the Tables of Contents of Scientific Journals: Pro- FESSOR CHARLES F. MEYER. Mitochondrial Be- havior: G. E.GatTes. Bentonite in the Ordovician near Collingwood, Ontario: D. C. Mappox. The Relation of the Paranasal Sinuses to the Singing Voice: FRANCIS WHEELER 628

The American Association for the Advancement of

Science: Meeting of the Executive Committee at Cleveland:

Dr. Burton E. LIVINGSTON 631 Scientific Apparatus and Laboratory Methods:

A Method of Injecting the Coelom of Small An-

nelids: Dr. LEONARD P. SAYLES 632

Special Articles:

The Virus of Laryngotracheitis of Fowls: Dr. J. R. Beacu. The Longevity of Bacillus Amylo- vorus (Burr) Trev. in Association with Honey: Proressor H. E. THomas. The Clarification of Plant Juices: D. E. Frear. The Geological Back- ground of Peking Man: Proressor GrEorRGE B.


SCIENCE: A Weekly Journal devoted to the Advance- ment of Science, edited by J. MCKEEN CATTELL and pub- lished every Friday by

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SomE years ago a party of travelers lost its way on a by-road down South, coming finally to a cross- roads store. Against one corner leaned a lethargic, malaria-ridden cracker, meditating over the chewing of a straw. The driver of the automobile hailed him, “Is that the highway yonder?” “I don’t know.” “Where does the road go at the fork ahead?” “I don’t know.” “You don’t know much, do you?” “No, I don’t know much, but I’m not lost.” That is our position to-day with reference to the animal parasites of man. Their tremendous number and fecundity, the mystery of their origin, our lack of specific or any treatment in many cases, our defective knowledge of

1 Presented before the Pasteur Society of Central Cali- fornia, Clift Hotel, San Francisco, September 24, 1930. From the Pacifie Institute of Tropical Medicine within the George Williams Hooper Foundation for Medical es University of California, San Francisco, Cali-


their natural history, our ineffective methods of econ- trol, our feeble prognosis of their future relations to man—these things make us humble in the face of this great broadcast system of parasitism. It is worthy of study for three reasons: beéause of pure academic interest, because of the clinical and public health need of control, and because in it is written the foundation of human history.

Parasitism is a term coming down from the ancient Greek days of parasites—sharers of the feast, or mess- mates, to whom no stigma attached, or reproach or contempt. Religious parasites were attached as as- sistants to the priests to collect the corn dues from farmers on the temple lands or from other sources. It was their province to provide food for temple visitors, to care for certain offerings and to arrange the sacrificial banquets. On the other hand, civil parasites were persons who received invitations to

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dine in the prytaneum as distinguished from those who had the right to dine there ex officio. (The pry- taneum was the official and religious center of the community.) Much later, the term parasite evolved to mean a person who would submit to any indignity or humiliation in order to obtain a good dinner, in other words, a “sponger.” Plautus describes such _ parasites in his comedies. Aleiphron and Athenaeus detail the various insults they endured from hosts and guests alike. Some of these parasites became pro- fessional jesters, like the court fools and buffoons of a later age. Some relied on flattery to secure free feasts. Others practiced various degrading vices to achieve their end.

The evolution of the word “parasite” more than suggests the story of the evolution of parasitism as used in biology. In each case the species begins with a free-living form, independent and with diversity and good development of functions. By natural acci- dent, these free-living forms were placed in temporary close relationship with potential hosts. Some few sacrificed independence, locomotion, individual de- fense and various special senses (moral or physical), in order to secure an abundant food supply and pro- tection. This led to a one-sided development of adaptation or sycophancy, with a stimulation of re- production and simplification of structure and fune- tion. In the end, in each case, we find a parasite incapable of existence aside from its host, and so modified by adaptation to its host that little resem- blanee remains to its free-living ancestors. Thus we consider as parasites such animal organisms as live upon, within or at the expense of other animal organ- isms. Parasitism means the relationship obtaining between the parasite and its host.


Chiefly for want of a better thesis, it is commonly assumed that the animal parasites of man originated from free-living forms. For each parasitic form there is a considerable and often very large number of closely related free-living forms. There is no such thing as a group or class of parasites since almost every animal group contains actual or potential para- sites. Fantham says that parasitism, biologically, is in no sense a matter of structure, but is very definitely a manner of existence. It is a habit which theoreti- cally may be assumed by almost any living organism.

The usually accepted mode of parasitic evolution assumes that free-living forms of animals are older than parasitie forms, since free-living forms must have been in existence before they could find hosts to parasitize. This is almost a paranoid type of argu- ment because it may well prove to be founded on a


LXXII, No. 1877

false hypothesis. No one ever saw a free-living apj. mal change into a parasitic form. The pure theory of mutation of species has proved a serious stumbling block in general evolution and is just as great an obstacle in the evolution of parasitism. We know absolutely nothing as to the real origin of parasitism and processes of change which we observe in fleeting glimpses may be proceeding either forward or back- ward. It is possible to change the habits of a free. living animal so that it adapts itself to a variable degree to a host in a parasitic relation. It is also possible to take a parasitic form and train or adapt it to a more or less independent free-living form. Which is the archetype? We do not know. We assume and infer, and our method is correct because we plant a germ of observation and out of it grows stalk and leaf of hypothesis, and then the blossom of full-grown theory, and a promise eventually of obtain- ing a modicum of fruitful truth. So that while we must theorize, it must be with the effort to improve the theory with wider and keener observation and experiment, realizing that the final truth is far off and possibly very different from the ideas we entertain en route toward it.

Having assumed that animal parasites originated from free-living archetypes, we are next confronted with the difficulties of the general evolution theory itself. Three major types of general evolution have been advanced and each of these offers interesting and self-evident bearings on the problem of the evolution of animal parasites. First of these is the Darwinian theory of mutation through continuous and long- continued variation. The second theory of DeVries considers species to have arisen through sudden and irregular mutation as environmental circumstances for a time became favorable to such changes. The third theory has recently been propounded by the Dutch botanist, J. P. Lotsy, who believes that new species have arisen by crossing of dissimilar parents. Lotsy insists, in spite of the plausibility of the muta- tion theories, that it has never been really proved that a new species arose in this manner. The only new forms produced experimentally, have, been a result of hybridization. He considers it quite possible that the main divisions of organic life originated from hybridization. A fourth theory from a completely new view-point challenges attention for its novelty, close-knit argument and the unique application to biology of the present-day knowledge of atomic physics. Those interested should read the recent volume entitled “What is Life?” by Dr. Augusta Gaskell, and the thought-provoking introduction by Raymond Pearl. Dr. Gaskell applies the principles of atomic physics to the problem of the origin and

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DecEMBER 19, 1930]

nature of life. It is her contention that species may arise de novo. Her argument raises many interest- ing questions in connection with the development of parasitism.

It is usually believed that external or ecto-para- sitism evolved before endo-parasitism, the reason being that the change from free-living te ecto-para- sitie existence seems less radical and abrupt than the change to endo-parasitie existence. for instance, some more occasional or partial parasites took their human host solely at mealtime as in the case of the leech (Hirudo) and the bedbug (Cimex). This is more a predatory relation and is much less difficult to establish than the relation necessary for a parasite in the intestine or the blood stream. The parasitic habit seems to have arisen separately and indepen- dently in most large groups of animals and therefore might be considered of rather common occurrence in the course of general evolution. This is shown by

the presence of free-living as well as parasitic types

in most large animal groups.

We must consider that, in general, parasites belong to animal groups more primitive than their hosts, that is, to groups of a less specialized and less highly developed type. In the ease of protozoa, the rule still holds because protozoa are parasitized by plant organisms. Some ecto-parasites are not limited to one species of host. Others are so limited and some even to a single part of the host’s body. These types are assumed to be evolutionary stages of ecto-para- sitism. The most primitive stage is probably where the ecto-parasite is able to migrate from one species of host to another. With more specialization it be- comes limited to a single host species, and finally to a single part of that host.

The relation of ecto-parasites to endo-parasites affords much food for speculation. Many believe that symbiosis and commensalism lead to parasitism. For instance, an organism that takes its meals in a state of mutualism with another organism, may easily become a food robber, and then a parasite. Or again, symbiotie relations may be disturbed so that one or- ganism gradually becomes pathogenic for the other. Such a situation would help explain the moot question of pathogenicity of Entamoeba coli, for example. In general we assume that endo-parasitism arose (1) from ecto-parasitism, (2) from commensalism, or (3) from symbiosis. An endo-parasite restricted to one species of host is probably no older in race than its host and changes in the parasite have probably been coincident with changes in the host. An endo-para- site found in several species of hosts probably is older racially than these hosts, and has adapted itself


-as hydatids—are usually most highly modified or


to hosts which evolved after its parasitic habit was established.

Faust has introduced the logically correct term “ouest,” for a parasite in relation to its host. in the case of helminths, he suggests that the relation was first one of chance contact only, whereby a free- living guest was carried from one feeding ground to another. Sooner or later the guest began to use food procured by the host, and gradually became more and more dependent on such food Some of the guests then began to consume the tissues or juices of the host itself, either by use of suctorial organs or by actual penetration of and residence in the host’s tis- sues. This resulted in a very degenerate and depen- dent parasite. Forms living in the blood or lymph, such as filaria and blood flukes—in the muscles, such as trichina—or in the pleura and peritoneum, such

adapted and may therefore be considered the oldest | parasitic species. Other forms living in the mouth, bladder and other epitheliated organs, or as ecto- parasites on the skin, are frequently very young in parasitism. This is demonstrated by their relatively slight modification from the archetypes of the group and from related free-living forms. It often happens that the adult parasite undergoes only slight modifi- cation, while the larva, or intermediate form, is highly modified.

Greater length or age of parasitism means greater modification away from the archetype of the species. This means practically greater simplification of the parasite. Change of environment from a free-living existence to one of abundant and assured food sup- ply, and increased protection against enemies, leads to atrophy of certain functions and structures. This is not degeneracy, but evolution; not retrogression, but orderly change. Herein, perhaps, lies a warning to the advocate of return to a so-called simple life. The simple life connotes protection and a free-lunch counter. The result is social parasitism.

Faust states that simplification, which is synony- mous with adaptation, is illustrated in the helminths by reduction in organs of locomotion, except in free- living larval stages. An even more radical reduction takes place in the organs of alimentation. In tape- worms, for example, the digestive tract has entirely disappeared except for possible vestiges in early larval stages. The integument has been simplified to afford protection against digestion and abrasion by the intestinal contents, and to permit attachment to the intestinal wall by suckers, hooks or by penetra- tion. On the other hand, the reproductive system has inereased enormously and this makes a serious



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public health problem. The nervous and excretory systems have suffered little change. There is a strik- ing contrast between the influence of an easy life on reproductivity in parasites and most of the animal kingdom, and in man. In man it is as Adam Smith said, “Poverty seems favorable to generation. But poverty, though it does not prevent generation, is extremely unfavorable to the rearing of children.”

As we approach the problem of the host in relation

to the parasite, it seems probable that parasites using intermediate hosts have evolved from parasites hav- ing but one host. At first, the parasite would have merely tolerated the second host, then would have adapted itself to the second host, and finally estab- lished itself in some stage in the second host. Nature provides numerous examples of the entrance of exogenous stages of parasites into other animals and therefore abundant opportunity exists for parasitism _ to develop and for intermediate hosts to enter the

cycle. Adaptations often of unusual nature are neces- sary to permit the parasite in its larval or oval form to reach the intermediate host. This is nicely por- trayed in the life cycle of the guinea worm, of flukes and of hookworm.

The problem of intermediate hosts is far from settled. R. Leuckardt believed that the present inter- mediate hosts were the original or primitive hosts both for larval and adult forms. Parasites then _ gradually differentiated and developed into later and more numerous stages, which in turn gradually sought out new definitive hosts. The earlier stages remained in the original hosts, now called intermediate hosts. This theory at once meets with objections.

On the contrary, R. Moniez considers the present definitive hosts to have been the original ones, and that asearis, trichinella and enterobius, for instance, have continued this primitive relationship. In most other cases embryos, which were potential parasites, were unable to resist or endure the chemical and mechanical influences of the intestinal tube. They therefore left the intestine by penetrating its walls and burrowing through into the tissues of their ear- riers or the primitive hosts. Here they could attain a fair degree of development, but were mechanically prevented from returning to the intestine where their eggs could be deposited. Most of them died out as parasites, just as their young stages do now if they reach a wrong host. Nevertheless some of them passively were carried into the intestinal tract of beasts of prey. Many perished in the process of digestion, but a few reached the intestine undamaged. These grew, became more resistant and developed into adult forms. Gradually this process became estab- lished by heredity, and became usual and normal.


[Vou. LXXITI, No. 1877

This is an attractive theory and has in it much to commend. |

Under unusual or special conditions parasites may invade unusual hosts. They then tend to aberrant locations and are often very dangerous, as shown by Cysticercus cellulosae, for instance. A common ex. ample is also seen in the so-called larva migrans, or creeping eruption, due to larvae of non-human nema- todes, such as the dog hookworm, penetrating human skin and crawling about in the subcutaneous tissue, It is possible that Loa loa may represent a similar condition.

In general, true or strict parasites have specific hosts. The great exception to this rule is Trichina, which is found in man, pig, bear, rat, mouse, cat, fox, badger, skunk and marten, and can be artificially cultivated in the dog, rabbit, sheep, horse, other mam- mals and in birds. Intestinal and ecto-parasites are usually less dangerous than blood and tissue parasites. The origin of host-parasite specificity is the same as the evolution of parasitism.

Faust emphasizes that, as a host, man, like other animals, suffers more from parasites if they are rela- tively new to him. Long acquaintance brings toler- ance, here as everywhere. In fact, we might well paraphrase the insight of Pope’s famous lines in his “Essay on Man,” and say:

Parasites are monsters of so frightful mien, As to be hated, need but to be seen;

Yet seen too oft, familiar with their face, We first endure, then pity, then embrace.

Negroes are less disturbed than whites by intestinal worms. Chinese are commonly little affected by clonorchis. The Asiatic child is less upset by ascaris. Perfect adaptation of parasite to host means a com- plete absence of symptoms. This principle is seen clinically illustrated in the ease of infection with Entamoeba histolytica. If the parasite is poorly or badly adapted, symptoms arise. Millenniums of asso- ciation in the host-parasite relation definitely seem to decrease pathogenicity. This gradual racial immunity has many counterparts in clinical parasitology. Even California fleas prefer the fresh blood of newcomers. We have no answer, as yet, to the question as to why insects exercise a selectivity at times in the individuals they select as donors for transfusion. Yet such selec- tivity exists at least in the case of fleas, bedbugs, mos- quitoes and sandflies.

Enough has been said to indicate the extreme com- plexity of the world of animal parasites. With so little definite information on the subject of the origin and evolution of parasites, we are foreed to base practical measures of control on studies of para-

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DECEMBER 19, 1930]

sitism as it exists to-day. Certain features of para- sitism are of special interest, therefore, when we come to consider the principles on which specific control may be based.


Granting a condition of parasitism, the optimum habitat for the parasite is provided when there is the least possible or no damage to the host. The host- parasite relation must be considered as a finely ad- justed balance of biology, in which the parasite must preserve itself and perpetuate its race, and in which the host must not be sufficiently irritated or damaged to react in such a way as to prevent achievement of these two objects by the parasite. This is illustrated with nicety in the case of EL. histolytica in relation to its host.

For infection of the host, it is necessary that host and parasite have the same geographie distribution, the habits of the host must bring it in relation with the infective stage of the parasite, and the infective stage in the life eyele of the parasite must be reached at the precise time the host is available to be para- sitized. Numerous remarkable adaptations occur as a result of the need of meeting these requirements. Some of these can be indicated as follows:

1. Guinea worm—Cyclops—ingested by man.

2. Leishmania and Loa in skin (Sandfly and Chry- sops).

3. Leeches (Hirudinei). Limnatis nilotica in drink- ing water, lodges in nose and pharynx. (See Alfred Russell Wallace and Loos, Haemadipsa species. )

4. E. histolyticu—cysts to exterior—to new host.

5. Hookworm—intermediate cycle.

It has been mentioned that parasitology has a con- tribution to make to the prehistoric record of man. This is illustrated by a type of reasoning first devel- oped by von Jhering in 1902.2 In the ease of para- sitie worms von Jhering argues that if the same or nearly the same species of worm parasitizes two or more species of hosts, then the hosts descend from a common ancestor. The similarity of the helminth parasites indicates their origin also from a common ancestor. Thus, apparently, the different species of hosts involved all descended from a primitive ances- tor which was parasitized by the original ancestral parasite.

Zschokke has applied a similar argument to the distribution of certain cestodes in marsupials. He also claims that the original home of the salmon is in the salt ocean because the larger proportion of its endoparasites are marine. Thus the path of salmon

2 See Hegner, Root and Augustine.


migration and its breeding grounds are no index as to» whence it came. Zschokke says, “Each parasite fauna comes to be to some extent a mirror-image of the biology of the host, of the host’s habits of life, and especially of the host’s relations to those creatures that share the habitat with it. Each change of nour- ishment and residence of an animal finds its echo in the changes in the helminthie condition.”

Metealf has discussed the distribution of the family Leptodactylidae. These are frogs characteristic of tropical and sub-tropical America, and also of Aus- tralia and Tasmania. They have been reported from no other parts of the world. Only two explanations of this distribution are possible. (1) Assumption of an earlier land bridge across Antarctica, connecting South America with the Tasmanian-Australian pla- teau. (2) Convergent or parallel evolution. This frog famiiy harbors an opalinid rectal parasite of the genus Zelleriella which is present in both geographic groups of frogs. So nearly alike are these opalinids that their species separation is most difficult. It is possible that either the frogs or the opalinids arose by parallel or convergent evolution. But it seems utterly impossible that both frogs and opalinids de- veloped such a similarity in the two geographie areas. Therefore we must assume a primeval Jand bridge.

Kellogg has applied this reasoning to bird lice, Johnston to trematodes and cestodes. Darling studied the migrations of the human race from data on hook- worm distribution. The method is valid and opens the way for much valuable new work on human geog- raphy and evolution. This is especially hopeful because parasites include the latest known examples of biologie evolution (Metchnikoff).

The principles of parasite control are to-day too specialized, varied, empirical and ineffective because of insufficient data in certain definite lines. These are chiefly: (1) Data on evolutionary development and individual life stages as influenced by external factors. (2) Distribution of parasites and extent to which they can and do enter other hosts than the principal one. (3) The extent to which behavior of parasite and of host influences or determines specific relations between the two (Hegner). (4) The factors in the host which favor invasion and establishment of so-called natural parasites and obstruct similar entry and establishment of foreign parasites (Hegner). (5) Reasons for laboratory, experimental and acci- dental infections in foreign hosts (Hegner). (6) Understanding of immunity and susceptibility in hosts, racially, from heredity, difference between in- dividual hosts of same species, and between young and adult hosts. (7) The reactions between host and parasite which terminate an infection or cause latency

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or relapse. Better data on these points will greatly improve methods of control.

The extremely small number of infective forms which reach a new host necessitate an enormous fecundity in parasites. Transfer to new hosts may be purely passive, by contaminated food or drink, by direct contagion, by inoculation, or may be pre-natal. Therefore simple sanitary measures are usually suff- cient to prevent individual infection.

Before taking up the specific problem of parasite control it will round out the entire subject to review, briefly, current ideas on the antiquity of animal para- sites, especially helminths, in man. Faust states that parasitism was well established before the dawn of human history. All the common species of human parasites are much older than the genus man. This statement is based on the following considerations: (1) Many infections of man are also common in ani- mals, and man is only incidentally or accidentally infected. (2) Some infections are decidedly patho- genie for man but not for other animals. (3) Many parasites which now use two or more hosts, inelud- ing man, probably originally used only one host, the one in which their larval stages now appear. (4) Certain groups of parasites of man and other mam- mals have a similar or identical morphology but widely different physiologic processes. This indicates that the parasite has been established in man long enough to have acquired a relatively fixed adaptation. (5) Strongylus, for example, is a fairly recent para- site of man beeause its free-living stages, which are reproductive, ean probably continue indefinitely out- side the human host. Hookworm has a much longer parasitic history, while ascaris, trichocephalus and enterobius are extremely old in parasitism and their entire life history is carried on in man. Parasites of the blood and lymph are the oldest of all and have undergone much more profound adaptations, espe- cially in physiology.

The ancient story of parasitism is nicely summar- ized by Faust. Long before the dawn of history, at least 100,000 years ago, hunters of wild oxen and boars became infected with tapeworms, ascaris and trichina. Primitive fishermen, eating raw fish, ac- quired Diphyllo bothrium and the liver fluke. Herds- men were infected with hydatids from sheep and dogs. Guinea worm infection and filaria have a sim- ilar antiquity. As man settled down and began to till the soil, he came in closer contact with his own race and bad hygiene exposed him to fecal contami- nation. Hookworm, strongylus, ascaris and trico- cephalus became endemic. In the great valleys of the Nile and Yangtze, fishermen and farmers wading in stagnant water were infected with schistosomes.

[Vou. LXXII, No. 1877

Man’s associate, the rat, conveyed hymenolepis, and became a reservoir for trichina, while the dog flea conveyed dipylidium. Thus at the dawn of history, helminth infection at least was distributed through the inhabited world, but more intensely through the tropics.

Vesical schistosomes have been found in Egyptian

-Imummies of the 13th century B. C. Moses separated

clean from unclean animals on the basis of their visible infection with parasites. This was especially so in the case of goats and kids offered for sacrifice and later eaten. Syrian goats are heavily infected with Fasciola hepatica and persons eating infected raw livers get a disease called “halzoon,” or pharyn- geal fascioliasis. All scavenger birds and beasts were prohibited as food, including hogs, camels, birds of prey, reptiles, snails and others. They were pro- hibited because their flesh was infected with para- sites. All animals not on the prohibited list, whose flesh was found infected, were to be burned. Moses warned against infected water, containing cyclops and cereariae. He taught the people by his brazen images of the fiery serpents how to extract the guinea worm. Later the people were instructed to drink water from their hands instead of lapping it up, possibly to avoid leeches. Many references record the acquaintance of the ancients with animal parasites.


Nearly every group of fauna has in it parasitic species. From the social point of view, this is also eminently true for man himself and brings up the whole field of social human parasitism which has been very insufficiently studied from the biologie point of view and from the standpoint of human geography. In the phyla of the animal kingdom, some have only a few parasitic forms, but three phyla contain a large number, in fact, a majority of animal parasites. These are the protozoa, arthropoda and several phyla of worms.

The general principles on which control must be based have already been discussed. It remains to consider broadly the large parasite groups, comment- ing on control methods in relation to each. We deal with Worms, Protozoa and Arthropods.

I. Worms: The spread of helminths requires the presence of proper intermediate hosts, of suitable climate and moisture, available definitive hosts, and proper interrelations between these and the general environment.

The spread of helminths is determined largely by certain factors which are more or less subject to human control and which therefore offer the practical points of attack on the problem.

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DEcEMBER 19, 1930]

1. Food: Faust gives many examples of the im- portance of the type of local native food. Thus the Chinese and Hindus thoroughly cook most of their food, which is largely eaten hot. (Note also the im- portance of the tea habit.) However, in China and India some cooked food stands in stalls and restau- rants, covered with flies and dust, and exposed to domestic animals, roaches and vermin. To all this the people are oblivious, since they completely lack a sanitary sense. We see analogous food exposure in nearly all countries: in Italy and France, the bread; in England, the milk; in the United States, berries, vegetables, fruits and salads are often exposed to flies, dust, expectoration, dogs, cats, rats and mice, ete. In the orient and most of the tropies, all foods raised in the ground are invariably contaminated. Many are kept fresh in the bazaars by sprinkling with dirty water applied by whisks or blown by the mouth. In China, water chestnuts, the gigantic Chi- nese radishes, lotus roots and bamboo shoots are eaten raw. From Egypt to Iraq sticky sweetmeats, drowned in flies and dust, are consumed by rich and poor alike. Sugar-cane sections are kept fresh in dirty water. China gives a hypo of dirty water to oranges beginning to wither, just as she soaks melons and eucumbers in dirty water after tapping their skins with stiff bristle brushes, and then sells them honestly by weight. Fresh berries, celery, lettuce and other salad plants are anathema in the tropics, yet the plain laws of common-sense and sanitary sense are ignored by native and foreigner alike.

Chinese and Indians eat the water ling and chestnut by peeling off the outer petals with their teeth, and become infected with the cysts of Fasciolopsis buski. In China and Formosa, the same result follows eating certain raw grasses. It is common knowledge among the farmers of central China, where Fasciolopsis is abundant, that hogs raised in the courtyard escape it, while those raised on the hillside get it. Yet the farmers do nothing whatever about it. The situation reminds one of our own ignoring of well-known sani- tary facts in sewage disposal on the farm, as well as in garbage disposal in the city.

In general the Chinese do not eat raw fish and arthropods as do their neighbors around the China Sea, but in some parts they eat large quantities of dried fish. Raw fish and arthropods spread flukes, especially metagonimus, clonorchis and paragonimus, broadeast in Japan, Formosa, Korea and Tonkin. In the wetter parts of China, clonorchis and metagonimus are found in eats and dogs. Several central provinces of China, especially Fukien, Kiangsi and Hunan, have paragonimus infection in tigers, panthers and wild eats, just as in Korea, Siam and Assam. Atten-


tion must be drawn also, in connection with food as a spreader of parasites, to the importance of the dirty fingers or utensils with which the food is eaten. The chopsticks of eastern Asia are more sanitary than the vilely contaminated fingers of Mongolia, the Near East and Arabic countries.

2. Water, as a means of transfer of helminths, is never safe in the orient or tropics, and often unsafe elsewhere. Examples of water transfer are seen in the cereariae of various flukes and the larvae of the guinea worm.

3. Nightsoil is perhaps the chief potential source of human helminth infections. In the orient and tropics nightsoil is universally used as a fertilizer for the fields. The sights and smells of this traffic beset the landscape and offend the senses in city and country alike. By this means virulent bacteria and numerous parasites are propagated and broadcast over the land. Not only are the farmers and coolies subject to infection, but field produce, vegetables, fruits and melons, are all grossly contaminated and carry contagion to those who eat and drink without thought for the morrow. The use of nightsoil as fertilizer is a logical and economical system. In the West the form of fixed nitrogen most valuable for plant use is entirely wasted by our wasteful and short-sighted conservancy methods. The Chinese method is effective, saving, logical, but unesthetic and disease-breeding. Parasite control must go hand in hand with good economies and a reasonable degree of esthetics.

4. Migration and Travel: Negro slaves from Mo- zambique and the Gold Coast probably brought hook- worm and Manson’s schistosome to America. The former needed no adaptation and the latter found Planorbis snails awaiting it as intermediate hosts. The fish-tapeworm was carried to Switzerland a thou- sand years ago by Swedish emigrants and to the United States and Canada by Scandinavian emigra- tion. Wherever Islam spreads, Taenia solium disap- pears and Taenia saginata becomes endemic. Travel and migration are of great importance in spreading all parasites. Conversely, parasites are important in diverting or causing emigration. In the seventh cen-

tury the Moslem invasion of North Africa did not

cross the Sahara because all livestock was inoculated with trypanosomes by the tsetse fly at the southern borders of the desert.

5. Influence of racial and local habits, food customs,

manner of eating, bathing, clothing, housing, conserv- aney, occupation and religion. These things all have an important bearing on helminth spread and puac- tical means of control.

6. Spread of helminths depends to a considerable








degree on the presence of vectors and proper inter- mediate hosts. Plants may act as vectors as where they harbor eneysted fluke larvae, and where heavy fleshy roots are parasitized by plant nematodes and then eaten by man. Invertebrate intermediate hosts belong to the two large phyla of molluses and arthro- pods. Vertebrate intermediate hosts include fish, rep-

tiles, amphibians and oceasionally birds and mammals. -

The public health aspect of parasite control is highly important. Carriers, either as intermediate hosts or as simple vectors, with or without symptoms,

must be prevented from introducing parasites to new

hosts. Studies of reservoir hosts need to be pushed. Human parasites in animals are important and little is known about them. A helminth ean easily become epidemic in an area because of massive dosage or un- usually favoring climatic conditions. Generally hel- minth infection is endemic and when once established with intermediate hosts, if needed, constitutes a vicious eirele extremely difficult to break. One must search for the weakest point locally in the life cycle, with considerable knowledge of the evolution and biology of the parasite, in order to decide the most amenable point of application for preventive mea- sures. In all parasitology, prevention is vastly more important than an attempt at control by cure. Hu- man exposure can be minimized by public education in methods of spread and manner of eritry into the body, as has been done in the ease of hookworm dis- ease. Wholesale treatment of patients has its func- tion (1) in the clinical needs of the patients them- selves, and (2) more vital from the public-health view-point, in protection of intermediate hosts and vectors from infection, and in preserving water and earth from infection. Control measures therefore may be directed to the intermediate hosts as in the schistosome problem of Egypt; to protection of the intermediate host from infection, as in the prophy- lactie use of quinine against malaria; to protection of man from entry of the parasite, as in the ease of guinea worm and numerous other parasites; to cure of the infeetion in man; to improved personal hy- giene, sanitation and conservancy.

I. Hetminto Parasites: This field is fairly fa- miliar to medical workers and can be summarized briefly. Of the parasitic worms, we deal chiefly with the following groups: trematodes; cestodes, including hydatid; round worms, subdivided into the hookworm group ineluding strongylus, asearis and enterobius, and the filariae. The prineiples already reviewed are

applied to each of these in its local setting with refer-

ence to environment both physical, biologic and social. In the ease of filaria, we have to admit little progress and less knowledge. This widespread and dangerous

[Vou. LXXIT, No. 1877

group of worms has so far been refractory to contro] because of unknown factors of evolutionary develop- ment, life history, biology and psychology. The need and opportunity for new experimental work here js obvious.

II. Prorozoa: Turning now to the second great parasitic group, the protozoa, we find practically every point applicable which was mentioned under the general control of parasites. These need not be repeated. The groups of protozoal parasites most important to man are as follows:

1. Malaria: Here it must not be forgotten that it is perhaps more important to protect the mosquito than man against malaria. If human disease is controlled there will be no transmission. If certain species of anophelines alone are controlled, others will take up their duties.

2. Leishmania: Here we have theoretically two methods of control, by eradication of sand-flies and by cure of patients. The former can not be done. The latter is effective on a wholesale scale.

3. Coccidiosis: Coccidial infection is so rare that little ean be said as to its control. Probably, how- ever, it is a late example of parasitism which longer adaptation may spread more widely and make more serious. It therefore needs detailed study to allow exact control to be instituted before it becomes a serious human parasite.

4. Trypanosomiasis: Space forbids even mention of the exceedingly interesting work being done in Africa on the control of trypanosomiasis. Various govern- mental institutes and numerous private organizations and individuals are involved and the outlook finally is hopeful. For American trypanosomiasis, or Chagas disease, not so much can be said.

5. Rabies is world-wide in its distribution and is rampant in the orient and all tropical countries, largely because of religious beliefs or indifference, which allow a large predatory and scavenging dog population. Similar factors are operative in the United States. For human purposes, control lies in dog control. This is to-day an impossibility in most countries, especially in hot latitudes. Even in the United States we are afflicted with that superparasite, highly adapted and late in evolution, the sob-sister of science, Sentimentalism. Her disguises cover Ed- dyites, anti-vivisectionists, maudlin animal lovers and a host of misguided ignoramuses. The total effect of all these differs only in degree from the effect of Buddhist, Hindu and animistic beliefs, so far as dis- ease control is concerned. At present the problem of the dog population, like many another sanitary problem, is insuperable in most parts of the world. Nevertheless, Pasteur Institutes are found in every

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19, 1930]

chief city and treatments are given in wholesale numbers.

6. Intestinal Protozoa of Man: The principles of control have been covered under helminth control and do not need repetition.

ArTHROPopS AND Eoro-Parasires: In this third heterogeneous group of animal parasites are included many familiar pests and strange histories. To some extent the prineiples of control we have re- viewed come into account, but there is much more departure from rule in this group and greater diver- sity and originality in the parasite’s relations to the human host. A brief note, following Ewing’s text, on some of these ecto-parasites is all that time will allow.

1. Mites (Acarina): Of these there are enormous numbers of very small animals. Several million indi- viduals may occupy a few cubic inches. They are found from northern Greenland to the Antarctic. Each group has been derived apparently from a dif- ferent type of