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[Illustration: An artificial lake, nearly dry and partly filled with
rubbish, has become a breeding-ground for dangerous mosquitoes.]
American Nature Series
Group IV. Working with Nature
INSECTS AND DISEASE
A POPULAR ACCOUNT OF THE WAY IN WHICH
INSECTS MAY SPREAD OR CAUSE SOME
OF OUR COMMON DISEASES
WITH MANY ORIGINAL ILLUSTRATIONS FROM
PHOTOGRAPHS
BY
RENNIE W. DOANE, A.B.
_Assistant Professor of Entomology
Leland Stanford Junior University_
LONDON
CONSTABLE & COMPANY LIMITED
1910
COPYRIGHT, 1910,
BY
HENRY HOLT AND COMPANY
_Published August, 1910_
THE QUINN & BODEN CO. PRESS
RAHWAY, N.J.
PREFACE
The subject of preventive medicine is one that is attracting world-wide
attention to-day. We can hardly pick up a newspaper or magazine without
seeing the subject discussed in some of its phases, and during the last
few years several books have appeared devoted wholly or in part to the
ways of preventing rather than curing many of our ills.
Looking over the titles of these articles and books the reader will at
once be impressed with the importance that is being given to the subject
of the relation of insects to some of our common diseases. As many of
these maladies are caused by minute parasites or microbes the
zoölogists, biologists and physicians are studying with untiring zeal to
learn what they can in regard to the development and habits of these
organisms, and the entomologists are doing their part by studying in
minute detail the structure and life-history of the insects that are
concerned. Thus many important facts are being learned, many important
observations made. The results of the best of these investigations are
always published in technical magazines or papers that are usually
accessible only to the specialist.
This little book is an attempt to bring together and place in
untechnical form the most important of these facts gathered from sources
many of which are at present inaccessible to the general reader, perhaps
even to many physicians and entomologists.
In order that the reader who is not a specialist in medicine or
entomology may more readily understand the intimate biological relations
of the animals and parasites to be discussed it seems desirable to call
attention first to their systematic relations and to review some of the
important general facts in regard to their structure and life-history.
This, it is believed, will make even the most complex special
interrelations of some of these organisms readily understandable by all.
Those who are already more or less familiar with these things may find
the bibliography of use for more extended reading.
My thanks are due to Prof. V.L. Kellogg for reading the manuscript and
offering helpful suggestions and criticisms.
Unless otherwise credited the pictures are from photographs taken by the
author in the laboratory and field. As many of these are pictures of
live specimens it is believed that they will be of interest as showing
the insects, not as we think they should be, but as they actually are.
Mr. J.H. Paine has given me valuable aid in preparing these photographs.
R.W.D.
Stanford University, California,
March, 1910.
CONTENTS
CHAPTER I
PAGE
PARASITISM AND DISEASE 1
Definition of a parasite, 1; examples among various animals,
2; _Parasitism_, 3; effect on the parasite, 4; how a harmless
kind may become harmful, 5; immunity, 6; _Diseases caused by
parasites_, 7; ancient and modern views, 7; _Infectious and contagious
diseases_, 8; examples, 9; importance of distinguishing,
9; _Effect of the parasite on the host_, 9; microbes everywhere, 10;
importance of size, 11; numbers, 11; location, 11; mechanical
injury, 12; morphological injury, 13; physiological effect, 13;
the point of view, 14.
CHAPTER II
BACTERIA AND PROTOZOA 15
_Bacteria_, 15; border line between plants and animals, 15;
most bacteria not harmful, 15; a few cause disease, 15; how
they multiply, 15; parasitic and non-parasitic kinds, 17; how a
kind normally harmless may become harmful, 18; effect of the
bacteria on the host, 18; methods of dissemination, 18; _Protozoa_,
19; _Amoeba_, 19; its lack of special organs, 19; where
it lives, 19; growth and reproduction, 19; _Classes of Protozoa_,
20; the amoeba-like forms, 20; the flagellate forms, 20; importance
of these, 21; the ciliated forms, 22; the Sporozoa or spore-forming kinds,
22; these most important, 23; abundance, 23; adaptability, 23; common
characters, 24; ability to resist unfavorable conditions, 24.
CHAPTER III
TICKS AND MITES 26
_Ticks_, 26; general characters, 27; mouth-parts, 27; habits,
27; life-history, 27; _Ticks and disease_, 28; _Texas fever_, 28; its
occurrence in the north, 28; carried by a tick, 29; loss and
methods of control, 31; other diseases of cattle carried by ticks,
31; _Rocky Mountain spotted fever_, 32; its occurrence, 32;
probably caused by parasites, 32; relation of ticks to this disease,
33; _Relapsing Fever_, 33; its occurrence, 34; transmitted
by ticks, 34; _Mites_, 35; _Face-mites_, 35; _Itch-mites_, 36;
_Harvest-mites_, 37.
CHAPTER IV
HOW INSECTS CAUSE OR CARRY DISEASE 40
Numbers, 40; importance, 41; losses caused by insects, 41;
loss of life, 42; _The flies_, 43; horse-flies, 43; stable-flies, 44;
surra, 45; nagana, 45; black-flies, 46; punkies, 46; screw-worm
flies, 47; blow-flies, 48; flesh-flies, 48; fly larvæ in intestinal
canal, 49; bot-flies, 50; _Fleas_, 52; jigger-flea, 53; _Bedbugs_, 54;
_Lice_, 54; _How insects may carry disease_, 55; in a mechanical
way, 55; as one of the necessary hosts of the parasite, 56.
CHAPTER V
HOUSE-FLIES OR TYPHOID-FLIES 57
The old attitude toward the house-fly, 57; its present standing,
58; reasons for the change, 58; _Structure_, 59; head and
mouth-parts, 60; thorax and wings, 61; feet, 62; _How they
carry bacteria_, 62; _Life-history_, 63; eggs, 63; ordinarily laid in
manure, 63; other places, 63; habits of the larvæ, 64; habits of
the adults, 64; places they visit, 65; _Flies and typhoid_, 65;
patients carrying the germs before and after they have had the
disease, 65; how the flies get these on their body and distribute
them, 66; results of some observations and experiments, 66;
_Flies and other diseases_, 68; flies and cholera, 68; flies and
tuberculosis, 69; possibility of their carrying other diseases, 70;
_Fighting flies_, 71; screens not sufficient, 71; the larger problem,
71; the manure pile, 72; outdoor privies, 72; garbage can,
72; coöperation necessary, 72; city ordinances, 73; an expert's
opinion of the house-fly, 73; _Other flies_, 75; habits of several
much the same but do not enter house as much, 75; the small
house-fly, 75; stable-flies, 75; these may spread disease, 75.
CHAPTER VI
MOSQUITOES 76
Numbers, 76; interest and importance, 76; eggs, 77; always in water, 77;
time of hatching, 77; _Larvæ_, 78; live only in water, 78; head and
mouth-parts of larvæ, 78; what they feed on, 78; breathing apparatus,
79; growth of the larvæ, 80; _Pupæ_, 80; active but takes no food, 80;
breathing tubes, 80; how the adult issues, 81; _The Adult_, 81; male and
female, 81; how mosquitoes "sing" and how the song is heard, 82; the
palpi, 82; _The Mouth-parts_, 83; needles for piercing, 83; _How the
mosquito bites_, 84; secretion from the salivary gland, 84; why males
cannot bite, 84; blood not necessary for either sex, 84; _The Thorax_,
85; the legs, 85; the wings, 85; the balancers, 85; the breathing pores,
86; _The abdomen_, 86; _The digestive system_, 86; _The salivary
glands_, 87; their importance, 87; effects of a mosquito bite, 87;
probable function of the saliva, 88; _How mosquitoes breathe_, 89;
_Blood_, 90; in body cavity, 90; heart, 90; _Classification_, 91;
_Anopheles_, 91; distinguishing characters, 92; eggs, 92; where the
larvæ are found, 93; _Yellow fever mosquito_, 94; its importance, 94;
the adult, 95; habits, 95; habits of the larvæ, 95; _Other species_, 96;
some in fresh water, others in brackish water, 96; Natural enemies of
mosquitoes, 97; how natural enemies of mosquitoes control their numbers,
98; mosquitoes in Hawaii, 98; _Enemies of the adults_, 99; _Enemies of
the larvæ and pupæ_, 100; _Fighting mosquitoes_, 101; fighting the
adult, 102; _Fighting the larvæ_, 103; domestic or local species, 104;
draining and treating with oil, 104; combatting salt-marsh species by
draining, 105; by minnows or oil, 105.
CHAPTER VII
MOSQUITOES AND MALARIA 106
Early reference to malaria, 106; its general distribution, 106;
theories in regard to its cause, 107; insects early suspected, 107;
_The parasite that causes malaria_, 108; studies of the parasite,
108; _Life-history in human host_, 109; its effect on the host, 110;
the search for the sexual generation, 111; _The parasite in the
mosquito_, 112; review of whole life-history, 115; malaria transmitted
only by mosquitoes, 115; _Summary_, 117; experimental proof, 118.
CHAPTER VIII
MOSQUITOES AND YELLOW FEVER 120
A disease of tropical or semi-tropical countries, 120; outbreaks in the
United States, 120; parasite that causes the disease not known, 121;
formerly regarded as a contagious disease, 122; _The yellow fever
commission_, 123; Dr. Finlay's claim, 124; experiments made by the
commission, 125; summary of results, 129; what it means, 130; _results
in Havana_, 131; _the fight in New Orleans_, 132; _In the Panama canal
zone_, 135; _in Rio de Janeiro_, 136; claims of the French commission,
138; _habits of stegomyia_, 139; breeding habits, 139; possible results
of war against the mosquitoes, 139; Danger of this disease in the
Pacific Islands, 140.
CHAPTER IX
FLEAS AND PLAGUE 142
Great scourges, 142; the "black death," 142; old conditions
and new, 143; _How plague was controlled in San Francisco_,
143; _Indian Plague commission_, 144; Dr. Simond's claim,
145; The advisory committee and the new commission, 146;
_Results of Dr. Verjbitski's experiments_, 147; _Results of various
investigations_, 150; _structure and habits of fleas_, 151; feeding
habits, 152; _Common species of fleas_, 153; _Ground squirrels and
plague_, 155; squirrel fleas, 156; _Remedies for fleas_, 157; cats
and dogs, 159.
CHAPTER X
OTHER DISEASES, MOSTLY TROPICAL, KNOWN OR THOUGHT TO
BE TRANSMITTED BY INSECTS 161
_Sleeping Sickness_, 161; its occurrence in Africa, 161; caused by a
Protozoan parasite, 162; the tsetse-fly, 163; _Elephantiasis_, 164;
caused by parasitic worms, 164; their development, 165; how they are
transferred to man, 165; effect on the patient, 166; _Dengue_, 168;
other names, 168; probably transmitted by mosquitoes, 170;
_Mediterranean fever_, 171; cause, 171; may be conveyed by mosquitoes,
171; _Leprosy_, 171; caused by a bacteria parasite, 171; possibilities
of flies, mosquitoes and other insects transmitting the disease, 172;
_Kala-azar_, 173; transmitted by the bedbug, 173; _Oriental sore_, 174;
the parasite may be carried by insects, 174.
BIBLIOGRAPHY 175
Parasites and parasitism, 175; Protozoa, 176; Bacteria, 177; Insects and
disease, 178; Mosquitoes--systematic and general, 179; Mosquito anatomy,
182; Mosquitoes--life-history and habits, 183; Mosquito fighting, 183;
Mosquitoes and disease, 185; Malaria, 186; Yellow fever, 189; Dengue,
192; Filarial diseases and elephantiasis, 193; Leprosy, 193; Plague,
194; Fleas, 198; Typhoid fever, 199; House-flies--anatomy, life-history,
habits, 200; House-flies and typhoid, 202; House-fly and various
diseases, 203; Human myiasis, 207; Stomoxys and other flies, 208;
tsetse-flies, 209; Trypanosomes and Trypanosomiasis, 210; Sleeping
sickness, 211; Rocky mountain fever and ticks, 212; Ticks and various
diseases, 213; Kala-azar and bedbugs, 216; Text or reference books, 216;
Miscellaneous articles, 218.
ILLUSTRATIONS
AN ARTIFICIAL LAKE, NEARLY DRY AND PARTLY
FILLED WITH RUBBISH, HAS BECOME A BREEDING-GROUND
FOR DANGEROUS MOSQUITOES _Frontispiece_
PAGE
FIG. 1. A LAMPREY 2
FIG. 2. _Sacculina_ 2
FIG. 3. _Trichina spiralis_ 2
FIG. 4. AN EXTERNAL PARASITE, A BIRD-LOUSE (_Lipeurus
ferox_) 3
FIG. 5. AN INTERNAL PARASITE, A TACHINA FLY (_Blepharipeza
adusta_) 3
FIG. 6. WORK OF AN INTERNAL PARASITE, PUSS-MOTH LARVA
PARASITIZED BY A SMALL ICHNEUMON FLY 3
FIG. 7. TYPHOID FEVER BACILLI 20
FIG. 8. _Amoeba_ 20
FIG. 9. _Euglina virdis_ 21
FIG. 10. _Spirocheta duttoni_ 21
FIG. 11. _Paramoecium_ 22
FIG. 12. _Vorticella_ 22
FIG. 13. PATHOGENIC PROTOZOA; A GROUP OF INTESTINAL
PARASITES 22
FIG. 14. CASTOR-BEAN TICK (_Ixodes ricinus_) 28
FIG. 15. TEXAS FEVER TICK 28
FIG. 16. TEXAS FEVER TICK (_Margaropus annulatus_) 29
FIG. 17. _Amblyomma variegatum_ 29
FIG. 18. _Ornithodoros moubata_ 36
FIG. 19. THE FOLLICLE MITE (_Demodex folliculorum_) 36
FIG. 20. ITCH-MITE (_Sarcoptes scabiei_) 37
FIG. 21. HARVEST-MITES OR "JIGGERS" 37
FIG. 22. HORSE-FLY (_Tabanus punctifer_) 44
FIG. 23. STABLE-FLY (_Stomoxys calcitrans_) 44
FIG. 24. A BLACK-FLY (_Simulium sp._) 45
FIG. 25. SCREW-WORM FLY (_Chrysomyia macellaria_) 45
FIG. 26. BLOW-FLY (_Calliphora vomitoria_) 45
FIG. 27. BLUE-BOTTLE FLY (_Lucilia sericata_) 50
FIG. 28. FLESH-FLY (_Sarcophaga sp._) 50
FIG. 29. "THE LITTLE HOUSE-FLY" (_Homalomyia canicularis_) 51
FIG. 30. HORSE BOT-FLY (_Gastrophilus equi._) 51
FIG. 31. OXWARBLE FLY (_Hypoderma lineata_) 51
FIG. 32. SHEEP BOT-FLY (_Gastrophilus nasalis_) 51
FIG. 33. CHIGO OR JIGGER-FLEA, MALE (_Dermatophilus penetrans_) 54
FIG. 34. CHIGO, FEMALE DISTENDED WITH EGGS 54
FIG. 35. BEDBUG (_Cimex lectularis_) 55
FIG. 36. BODY-LOUSE (_Pediculus vestimenti_) 55
FIG. 37. ONE USE FOR THE HOUSE-FLY 57
FIG. 38. THE HOUSE-FLY (_Musca domestica_) 58
FIG. 39. HEAD OF HOUSE-FLY SHOWING EYES, ANTENNÆ AND
MOUTH-PARTS 60
FIG. 40. PROBOSCIS OF HOUSE-FLY, SIDE VIEW 60
FIG. 41. LOBES AT END OF PROBOSCIS OF HOUSE-FLY SHOWING
CORRUGATED RIDGES 61
FIG. 42. WING OF HOUSE-FLY 61
FIG. 43. WING OF STABLE-FLY (_Stomoxys calcitrans_) 62
FIG. 44. WING OF HOUSE-FLY SHOWING PARTICLES OF DIRT
ADHERING TO IT 62
FIG. 45. LAST THREE SEGMENTS OF LEG OF HOUSE-FLY 62
FIG. 46. FOOT OF HOUSE-FLY 63
FIG. 47. LARVA OF HOUSE-FLY 63
FIG. 48. BARN-YARD FILLED WITH MANURE 64
FIG. 49. DIRTY STALLS 65
FIG. 50. PUPA OF HOUSE-FLY 76
FIG. 51. HEAD OF STABLE-FLY 76
FIG. 52. MASS OF MOSQUITO EGGS (_Theobaldia incidens_) 76
FIG. 53. MOSQUITO EGGS AND LARVÆ (_T. incidens_) 77
FIG. 54. MOSQUITO LARVA (_T. incidens_), DORSAL VIEW 77
FIG. 55. EGGS, LARVÆ AND PUPÆ OF MOSQUITOES (_T. incidens_) 78
FIG. 56. LARVA OF MOSQUITO (_T. incidens_) 78
FIG. 57. MOSQUITO LARVÆ AND PUPÆ (_T. incidens_) 79
FIG. 58. ANOPHELES LARVÆ (_A. maculipennis_) 79
FIG. 59. MOSQUITO PUPÆ (_T. incidens_) 80
FIG. 60. MOSQUITO PUPA (_T. incidens_) 80
FIG. 61. MOSQUITO LARVÆ AND PUPÆ (_T. incidens_) 80
FIG. 62. A FEMALE MOSQUITO (_T. incidens_) 81
FIG. 63. A MALE MOSQUITO (_T. incidens_) 81
FIG. 64. HEAD AND THORAX OF FEMALE MOSQUITO (_Ochlerotatus
lativittatus_) 82
FIG. 65. HEAD AND THORAX OF MALE MOSQUITO (_O. lativittatus_) 82
FIG. 66. HEAD OF FEMALE MOSQUITO 83
FIG. 67. CROSS-SECTION OF PROBOSCIS OF FEMALE AND MALE
MOSQUITO 83
FIG. 68. WING OF MOSQUITO (_O. lativittatus_) 86
FIG. 69. END OF MOSQUITO WING HIGHLY MAGNIFIED 86
FIG. 70. DIAGRAM TO SHOW THE ALIMENTARY CANAL AND
SALIVARY GLANDS OF A MOSQUITO 87
FIG. 71. SALIVARY GLANDS OF MOSQUITOES 87
FIG. 72. HEADS OF CULICINÆ MOSQUITOES 90
FIG. 73. HEADS OF ANOPHELINÆ MOSQUITOES 90
FIG. 74. WING OF _Anopheles maculipennis_ 90
FIG. 75. WING OF _Theobaldia incidens_ 90
FIG. 76. A NON-MALARIAL MOSQUITO (_T. Incidens_), MALE,
STANDING ON THE WALL 91
FIG. 77. FEMALE OF SAME 91
FIG. 78. A MALARIAL MOSQUITO (_A. maculipennis_), MALE,
STANDING ON THE WALL 91
FIG. 79. FEMALE OF SAME 91
FIG. 80. EGG OF _Anopheles_, SIDE VIEW 92
FIG. 81. EGG OF ANOPHELES, DORSAL VIEW 92
FIG. 82. ANOPHELES LARVÆ 92
FIG. 83. ANOPHELES LARVÆ 93
FIG. 84. ANOPHELES LARVA, DORSAL VIEW 93
FIG. 85. ANOPHELES PUPÆ RESTING AT SURFACE OF WATER 93
FIG. 86. SALT-MARSH MOSQUITO (_Ochlerotatus lativittatus_);
MALE 98
FIG. 87. SALT-MARSH MOSQUITO (_O. lativittatus_); FEMALE 98
FIG. 88. TOP-MINNOW (_Mollienisia latipinna_) 99
FIG. 89. DRAGON-FLIES 99
FIG. 90. THE YOUNG (NYMPH) OF A DRAGON-FLY 100
FIG. 91. THE CAST SKIN (_exuvæ_) OF A DRAGON-FLY NYMPH 100
FIG. 92. DIVING-BEETLES AND BACK-SWIMMERS 101
FIG. 93. KILLIFISH (_Fundulus heteroliatus_) 102
FIG. 94. STICKLEBACK (_Apeltes quadracus_) 102
FIG. 95. AN OLD WATERING-TROUGH, AN EXCELLENT BREEDING-PLACE
FOR MOSQUITOES 103
FIG. 96. HORSE AND CATTLE TRACKS IN MUD FILLED WITH
WATER 108
FIG. 97. A MALARIAL MOSQUITO (_Anopheles maculipennis_)
MALE 108
FIG. 98. A MALARIAL MOSQUITO (_A. maculipennis_) FEMALE 109
FIG. 99. DIAGRAM TO ILLUSTRATE THE LIFE-HISTORY OF THE
MALARIAL PARASITE 110
FIG. 100. MALARIAL MOSQUITO (_A. maculipennis_) ON THE WALL 111
FIG. 101. MALARIAL MOSQUITO (_A. maculipennis_) STANDING ON
A TABLE 111
FIG. 102. SALT-MARSH MOSQUITO (_O. lativittatus_) STANDING ON
A TABLE 118
FIG. 103. ANOPHELES HANGING FROM THE CEILING 118
FIG. 104. YELLOW FEVER MOSQUITO (_Stegomyia calopus_) 122
FIG. 105. RAT-FLEA (_Læmopsylla cheopis_); MALE 152
FIG. 106. RAT-FLEA (_L. cheopis_); FEMALE 152
FIG. 107. HEAD OF RAT-FLEA SHOWING MOUTH-PARTS 153
FIG. 108. HUMAN-FLEA (_Pulex irritans_); MALE 153
FIG. 109. HUMAN-FLEA (_P. irritans_); FEMALE 156
FIG. 110. MOUSE-FLEA (_Ctenopsyllus musculi_); FEMALE 156
FIG. 111. TRYPANOSOMA GAMBIENSE 164
FIG. 112. TSETSE-FLY 164
INSECTS AND DISEASE
CHAPTER I
PARASITISM AND DISEASE
PARASITES
The dictionary says that a parasite is a living organism, either animal
or plant, that lives in or on some other organism from which it derives
its nourishment for a whole or part of its existence. This definition
will serve as well as any, as it seems to include all the forms that
might be classed as parasites. As a general thing, however, we are
accustomed to think of a parasite as working more or less injury to its
host, or perhaps we had better say that if it does not cause any
irritation or ill effects its presence is not noted and we do not think
of it at all.
As a matter of fact the number of parasitic organisms that are actually
detrimental to the welfare of their hosts is comparatively small while
the number of forms both large and small that lead parasitic lives in
or on various hosts, usually doing no appreciable harm, often perhaps
without the host being aware of their presence, is very great indeed.
Few of the higher animals live parasitic lives. The nearest approach to
a true parasite among the vertebrates is the lamprey-eel (Fig. 1) which
attaches itself to the body of a fish and sucks the blood or eats the
flesh. Among the Crustaceans, the group that includes the lobsters and
crabs, we find many examples of parasites, the most extraordinary of
which is the curious crab known as _Sacculina_ (Fig. 2). In its early
stages this creature is free-swimming and looks not unlike other young
crabs. But it soon attaches itself to another crab and begins to live at
the expense of its host. Then it commences to undergo remarkable changes
and finally becomes a mere sac-like organ with a number of long slender
root-like processes penetrating and taking nourishment from the body of
the unfortunate crab-host.
The worms furnish many well-known examples of parasites, whole groups of
them being especially adapted to parasitic life. The tapeworms, common
in many animals and often occurring in man, the roundworms of which the
trichina (Fig. 3) that causes "measly" pork is a representative, are
familiar examples. These and a host of others all show a very high
degree of specialization fitting them for their peculiar lives in their
hosts.
[Illustration: FIG. 1--A lamprey. (After Goode.)]
[Illustration: FIG. 2--_Sacculina_; _A_, parasite attached to a crab;
_B_, the active larval condition; _C_, the adult removed from its host.
(After Haeckel.)]
[Illustration: FIG. 3--_Trichina spiralis_ encysted in muscle of a pig.
(From Kellogg's Elementary Zoöl.)]
[Illustration: FIG. 4--An external parasite, a bird-louse (_Lipeurus
ferox_).]
[Illustration: FIG. 5--A tachina fly (_Blepharipeza adusta_) the larva
of which is an internal parasite.]
[Illustration: FIG. 6--Work of an internal parasite, puss-moth larva
parasitized by a small ichneumon fly.]
From among the insects may be selected interesting examples of almost
all kinds and degrees of parasitism, temporary, permanent, external,
internal (Figs. 4, 5, 6). Among them is found, too, that curious
condition known as hyperparasitism where one animal, itself a parasite,
is preyed upon by a still smaller parasite.
"The larger fleas have smaller fleas
Upon their backs to bite um,
These little fleas still smaller fleas
And so _ad infinitum_."
Coming now to the minute, microscopic, one-celled animals, the Protozoa,
we find entire groups of them that are living parasitic lives, depending
wholly on one or more hosts for their existence. Many of these have a
very remarkable life-history, living part of the time in one host, part
in another. The malarial parasite and others that cause some of the
diseases of man and domestic animals are among the most important of
these.
PARASITISM
Among all these parasites, from the highest to the lowest the process
that has fitted them for a parasitic life has been one of degeneration.
While they may be specialized to an extreme degree in one direction they
are usually found to have some of the parts or organs, which in closely
related forms are well developed, atrophied or entirely wanting. As a
rule this is a distinct advantage rather than a disadvantage to the
parasite, for those parts or organs that are lost would be useless or
even in the way in its special mode of life.
Then, too, the parasite often gives up all its independence and becomes
wholly dependent on its host or hosts not only for its food but for its
dissemination from one animal to another, in order that the species may
not perish with the host. But in return for all this it has gained a
life of ease, free from most of the dangers that beset the more
independent animals, and is thus able to devote its whole time and
energy to development and the propagation of the species.
We are accustomed to group the parasites that we know into two classes,
as harmful or injurious and as harmless, the latter including all those
kinds that do not ordinarily affect our well-being in any way. But such
a classification is not always satisfactory or safe, for certain
organisms that to-day or under present conditions are not harmful may,
on account of a great increase in numbers or change of conditions,
become of prime importance to-morrow. An animal that is well and strong
may harbor large numbers of parasites which are living at the expense of
some of the host's food or energy or comfort, yet the loss is so small
that it is not noticed and the intruders, if they are thought of at all,
are classed as harmless. Or we may at times even look upon them as
beneficial in one way or another. "A reasonable amount of fleas is good
for a dog. They keep him from brooding on being a dog."
But should these parasites for some reason or other increase rapidly
they might work great harm to the host. Even David Harum would limit the
number of fleas on a dog. Or the animal might become weakened from some
cause so that the drain on its resources made by the parasites, even
though they did not increase in numbers, would materially affect it.
Perhaps the most serious way in which parasites that are usually
harmless may become of great importance is illustrated by their
introduction into new regions or, as is more often the case, by the
introduction of new hosts into the region where the parasites are found.
Under normal conditions the animals of a given region are usually immune
to the parasites of the same region. That is, they actually repel them
and do not suffer them to exist in or on their bodies, or they are
tolerant toward them. In the latter case the parasites live at the
expense of the host, but the host has become used to their being there,
adapted to them, and the injury that they do, if any, is negligible.
But when a new animal comes into the region from some other locality the
parasites may be extremely dangerous to it. There are many striking
examples of this. Most of the people living in what is known as the
yellow fever belt are immune to the fever. They will not develop it even
under conditions that would surely mean infection for a person from
outside this zone. Certain of our common diseases which we regard as of
little consequence become very serious matters when introduced among a
people that has never known them before. The cattle of the southern
states are immune to the Texas fever, but let northern cattle be sent
south or let the ticks which transmit the disease be taken north where
they can get on cattle there, and the results are disastrous.
Another striking example and one that is attracting world-wide attention
just now is the trypanosome that is causing such devastation among the
inhabitants of central Africa. With the advent of white men into this
region and the consequent migration of the natives along the trade
routes this parasite, which is the cause of sleeping sickness, is being
introduced into new regions and thousands upon thousands of people are
dying as a result of its ravages.
DISEASES CAUSED BY PARASITES
Some two hundred years ago, after it became known that minute animal
parasites were associated with certain diseases and were the cause of
them, it rapidly came to be believed that all our ills were in some way
caused by such parasites, known or unknown. Further study and
investigation failed to reveal the intruders in many instances and so it
began to be doubted whether after all they were responsible for much
that had been laid at their doors. Then after it was discovered that
minute plant parasites, bacteria, were responsible for many diseases
they in turn began to be accused of being the cause of most of the ills
that the flesh is heir to.
In later years we have come to adopt what seems to be a more reasonable
view, for we can see and definitely prove that neither of these extreme
views was correct but that there was much truth in each of them. To-day
we recognize that certain diseases, such as typhoid, cholera,
tuberculosis and many others, are caused by the presence of bacteria in
the body, and it is just as definitely known that such maladies as
malaria and sleeping sickness are caused by animal parasites.
Then there is a long list of other epidemic diseases, such as smallpox,
measles and scarlet fever, the exact cause of which has not been
determined. Many of these are believed to be due to micro-organisms of
some kind, and if so they will almost certainly sooner or later be
found. Curiously enough most of the diseases in this last class and many
of those in the first are contagious, while all that are caused by
animal parasites are, as far as is known, infectious but not contagious.
INFECTIOUS AND CONTAGIOUS DISEASES
It is important that we keep in mind this distinction. By contagious
diseases are meant those that are transmitted by contact with the
diseased person either directly, by touch, or indirectly by the use of
the same articles, by the breath or effluvial emanations from the body
or other sources. Small-pox, measles, influenza, etc., are examples of
this group. By infectious diseases are meant those which are
disseminated indirectly, that is, in a roundabout way by means of water
or food or other substances taken into or introduced into the body in
some way. Typhoid, malaria, and yellow fever, cholera and others are
examples of this class. Thus it is evident that all of the contagious
diseases may be infectious, but many of the infectious diseases are not
as a rule contagious, although some of them may become so under
favorable conditions.
Just one example will show the importance of knowing whether a disease
is contagious or infectious. Until a few years ago it was believed that
yellow fever was highly contagious and every precaution was taken to
keep the disease from spreading by keeping the infected region in strict
quarantine. This often meant much hardship and suffering and always a
great financial loss. We now know that it is infectious only and not
contagious, and that all this quarantine was unnecessary. The whole
fight in controlling an outbreak of yellow fever or in preventing such
an outbreak is now directed against the mosquito, the sole agent by
which the disease can be transmitted from one person to another.
EFFECT OF THE PARASITE ON THE HOST
We have seen how a few parasites in or on an animal do not as a rule
produce any appreciable ill effects. This is of course a most fortunate
thing for us, for the parasitic germs are everywhere.
There is perhaps "more truth than poetry" in the following newspaper
jingle:
"Sing a song of microbes,
Dainty little things,
Eyes and ears and horns and tails,
Claws and fangs and stings.
Microbes in the carpet,
Microbes in the wall,
Microbes in the vestibule,
Microbes in the hall.
Microbes on my money,
Microbes in my hair,
Microbes on my meat and bread,
Microbes everywhere.
Microbes in the butter,
Microbes in the cheese,
Microbes on the knives and forks,
Microbes in the breeze.
Friends are little microbes,
Enemies are big,
Life among the microbes is--
Nothing '_infra dig_.'
Fussy little microbes,
Millions at a birth,
Make our flesh and blood and bones,
Keep us on the earth."
While of course most of these microbes are to be regarded as absolutely
harmless and some as very useful, many have the power to do much injury
if the proper conditions for their rapid development should at any time
exist. While the size of the parasite is always a factor in the damage
that it may do to the host the factor of numbers is perhaps of still
greater importance because of the power of very rapid multiplication
possessed by so many of the smaller forms.
Certain minute parasites in the blood may cause little or no
inconvenience, but should they begin to multiply too rapidly some of the
capillaries may be filled up and trouble thus result. Or take some of
the larger forms. A few intestinal worms may cause no appreciable effect
on the host, but as soon as their numbers increase serious conditions
may come about simply by the presence of the great masses in the host
even if they were not robbing it of its nourishment. Many instances are
known where such worms have formed masses that completely clogged up the
alimentary canal. Such injuries as these may be regarded as mechanical
injuries. Some parasites injure the host only when they are laying their
eggs or reproducing the young. These may clog up passages or some of
them may be carried to some more sensitive part of the body where the
damage is done. The guinea-worm of southwestern Asia and of Africa lives
in the body of its host for nearly a year sometimes attaining a great
length and migrating through the connective tissue to different parts of
the body causing no particular inconvenience until it is ready to lay
its eggs when it comes to the surface and then great suffering may
result. The African eye-worm is another example of a parasite causing
mechanical injury only at certain times. It works in the tissues of the
body sometimes for a long while, doing no harm unless it finds its way
to the connective tissue of the eyeball.
It is known that many of the germs which cause diseases cannot get into
the body unless the protecting membranes have first been injured in some
way. Thus the germs that cause plague and lockjaw find their way into
the system principally through abrasions of the skin. Many physicians
have come to believe that the typhoid fever germ cannot get into the
body from the intestine where it is taken with our food or drink unless
the walls of the intestine have been injured in some way. It is well
known that of the many parasites that inhabit the alimentary canal some
rasp the surface and others bore through into the body cavity. This in
itself may not be a serious thing, but if the mechanical injury thus
caused opens the way for malignant germs, baneful results may follow.
Even that popular disease appendicitis is believed to be due sometimes
to the injury caused by the work of parasites in the appendix.
Parasites may cause morphological or structural changes in the tissues
of their hosts. The stimulation caused by their presence may result in
swellings or excresences or other abnormal growths. Interesting examples
of this are to be found in the way in which pearls are formed in various
mollusks. In the pearl oysters of Ceylon occur some of the best pearls.
If these are carefully sectioned there may usually be found at the
center the remains of certain cestode larvæ whose presence in the oyster
caused it to deposit the nacreous layers that make up the pearl. Other
parasites cause similar growths in various shellfish. The great
enlargements of the arms or legs or other parts of the body seen in
patients affected with elephantiasis is an abnormal growth due to the
presence of the parasitic filaræ in some of the lymph-glands where they
have come to rest.
Finally, the parasite may exert a direct physiological effect on the
host. This is evident when the parasite demands and takes a portion of
the nourishment that would otherwise go to the building up of the host.
Sometimes this is of little importance, but at other times it may be a
matter of life or death to the infected animal. The physiological
effect produced may be due to the toxins or poisonous matters that are
given off by the parasite while it is living in the host's body. Thus it
is believed that the malarial patients usually suffer less from the
actual loss of red blood-corpuscles that are destroyed by the parasite
than they do from the effects of the poisonous excretions that are
poured into the circulation when the thousands of corpuscles break to
release the parasites.
One other point in regard to the relation of the parasite to its host
and this part of the subject may be dismissed. From our standpoint we
look upon the presence of parasites in the body as an abnormal
condition. From a biological standpoint their presence there is
perfectly normal; it is a necessary part of their life. We think that
they have no business there, but from the viewpoint of the parasites
their whole business is to be just there. If they are not, they perish.
And when we take a dose of quinine or other drug we are killing or
driving from their homes millions of these little creatures who have
taken up their abode with us for the time being. But they interfere with
our health and comfort, so they must go.