ANIMAL PARASITES
Animal parasites are common in all countries, but are especially
abundant in the tropics, where almost every native is host for one or
more varieties. Because of our growing intercourse with these regions,
the subject is assuming increasing importance in this country. Many
parasites, hitherto comparatively unknown here, will probably become
common.
Some parasites produce no symptoms, even when present in large
numbers. Others cause very serious symptoms. Only those which have
clinical interest will be considered here. The illustrations will give
a better idea of their appearance than any description. They belong to
three classes: I. Protozoa. II. Vermes. III. Arthropoda.
I. PROTOZOA
1. Amoeba Coli Dysenteriæ.--This organism is found, often in large
numbers, in the stools of tropical dysentery and in the pus and walls
of hepatic abscesses associated with dysentery, and is generally
regarded as the cause of the disease. It is a colorless, granular
cell, 20 to 40 µ in diameter (Fig. 91). It contains one or more
distinct vacuoles; a round nucleus, which ordinarily is obscured by
the granules; and frequently red blood-corpuscles and bacteria. When
at rest, its shape is spheric; but upon a warm slide it exhibits the
characteristic ameboid motion, {245} constantly changing its shape or
moving slowly about. This motion is its most distinctive feature.
Other amebæ, resembling the pathogenic variety but smaller (10 to 15 µ
in diameter), are sometimes found in normal feces.
[Illustration: FIG. 91.--Amoeba coli in intestinal mucus, with
blood-corpuscles and bacteria (Lösch).]
When the presence of amebæ is suspected, the stool should be passed
into a warm vessel and kept warm until and during the examination. A
warm stage can be improvised from a plate of copper with a hole cut in
the center. This is placed upon the stage of the microscope, and one
of the projecting ends is heated with a small flame. Amebæ are most
likely to be found in grayish or blood-streaked particles of mucus.
Favorable material for examination can be obtained at one's
convenience by inserting into the rectum a large catheter with roughly
cut lateral openings. A sufficient amount of mucus or fecal matter
will usually be brought away by it.
2. Trichomonas Vaginalis.--The acid discharge of catarrhal vaginitis
sometimes contains this parasite in {246} abundance. It is oval or
pear-shaped, one to three times the diameter of a red blood-corpuscle
in length, and has a cluster of flagella at one end (Fig. 92). It is
not unlike a pus-corpuscle in size and general appearance, but is
actively motile. When in motion the flagella are not easily seen. No
pathogenic significance is ascribed to it. Other varieties of the
genus have been found in the feces, the urine, and the sputum.
[Illustration: FIG. 92.--Trichomonas vaginalis (after Kölliker and
Scanzoni).]
A similar but somewhat smaller organism, _Cercomonas hominis_ (Fig.
93), has been found in the feces in a variety of diarrheal conditions
and in from 10 to 25 per cent. of healthy persons in tropical regions.
[Illustration: FIG. 93.--Cercomonas hominis: A, Larger variety; B,
smaller variety (Davaine).]
In urine or vaginal discharges these organisms might be mistaken for
spermatozoa by one who is entirely unfamiliar with the appearance of
either.
3. Paramoecium Coli (Balantidium Coli).--This parasite is an
occasional inhabitant of the colon of man, and sometimes produces
diarrhea. It is an oval organism, about 0.1 mm. long, is covered with
cilia, and contains {247} a bean-shaped nucleus, two contractile
vacuoles, and variously sized granules (Fig. 94).
[Illustration: FIG. 94.--Balantidium (Paramoecium) coli (Eichhorst).]
4. Hemosporidia.--This is a large group of parasites with two
life-cycles: one in the blood-corpuscles or plasma of a vertebrate
host--man, mammals, birds, reptiles; the other in the body of some
insect. The malarial parasite, already described; the organism
(_Pirosoma bigeminum_) producing Texas fever in cattle; and the
questionable parasite (_Piroplasma hominis_), which has been described
as the cause of "tick fever" of Montana, belong to the group.
5. Trypanosomes have been mentioned (p. 195).
II. VERMES
1. Cestoda.--Tape-worms are very common parasites of both man and the
animals. The most important are _Tænia saginata_, _Tænia solium_,
_Bothriocephalus latus_, and _Tænia echinococcus_. They all pass a
larval stage in the body of an intermediate host. In the adult stage
they consist of a linear series of flat, rectangular segments
(proglottides), at one end of which is a smaller segment, the scolex
or head, especially adapted for attachment to the host. The series
represents a colony, of which the {248} scolex is ancestor. The
proglottides are sexually complete individuals, derived from the
scolex by budding. With exception of the immature segments near the
scolex, each contains a uterus filled with ova. The three tape-worms
first mentioned are distinguished from one another mainly by the
structure of the scolex and of the uterus (Fig. 100). The scolex
should be studied with a low-power objective. The uterus is best seen
by pressing the segment out between two plates of glass.
[Illustration: FIG. 95.--Tænia saginata (Eichhorst).]
(1) Tænia Saginata or Mediocanellata (Fig. 95).--This, the beef
tape-worm, is the common tape-worm of the United States. Its length
sometimes exceeds twenty-five feet. The middle segments measure about
one-fourth by one-half inch. The scolex is about the size of a
pin-head, and is surrounded by four sucking discs, but has no hooklets
(Fig. 96). The uterus extends along the middle line of the segment and
gives off about twenty branches upon each side (Fig. 100). The larval
stage is passed in the muscles of various animals, especially cattle,
where it lies encysted (cysticercus stage).
The larva is ingested with the meat, its capsule is {249} dissolved by
the digestive juices, and it attaches itself to the intestinal wall by
means of its suckers. It then develops into the mature worm.
The ova are present in the stools of infected persons, often in great
numbers. They are spheric or ovoid, yellow in color, and have a thick,
radially striated shell (Fig. 101). Their greatest diameter is 30 to
40 µ (about four or five times the diameter of a red blood-corpuscle).
Vegetable cells, which are generally present in the feces, are often
mistaken for them.
[Illustration: FIG. 96.--Head of Tænia saginata (Mosler and Peiper).]
[Illustration: FIG. 97.--Head of Tænia solium (Mosler and Peiper).]
(2) Tænia solium, the pork tape-worm is very rare in this country. It
is usually much shorter than _Tænia saginata_. The scolex is
surrounded by four sucking discs, and has a projection, or rostellum,
with a double row of horny hooklets (Fig. 97). The uterus has only
seven to ten branches (Fig. 100).
The ova closely resemble those of _Tænia saginata_, but are a little
smaller (Fig. 101).
(3) Bothriocephalus latus, the fish tape-worm, is the largest parasite
of man, sometimes reaching fifty feet in {250} length, although
generally not more than half so long. It is common in some countries
of Europe and in Japan, but is very rare in this country. The head is
not unlike the bowl of a spoon in shape. It is unprovided with either
suckers or hooklets, but has two longitudinal grooves which serve the
same purpose (Fig. 98). The uterus, which is situated in the center of
the segment, is roset-shaped (Fig. 100).
[Illustration: FIG. 98.--Head of Bothriocephalus latus: _a_, _a_,
Bothridies; _b_, neck (Blanchard).]
The larval stage is found in fish, especially the pike.
[Illustration: FIG. 99.--Egg of Bothriocephalus latus, showing lid and
yolk granules (photograph by F. C. Wood).]
The ova are characteristic. They measure about 45 by 70 µ, are brown
in color, and are filled with small spherules. The shell is thin, and
has a small hinged lid at one end (Fig. 99).
{251} Bothriocephalus latus is interesting clinically because it often
causes a very severe grade of anemia.
[Illustration: FIG. 100.--Segments of--(1) Tænia saginata; (2)
Bothriocephalus latus; (3) Tænia solium, showing arrangement of
uterus.]
[Illustration: FIG. 101.--Comparative size of eggs of intestinal
parasites: _a_, Tænia solium; _b_, Tænia saginata; _c_, Ascaris
lumbricoides; _d_, Trichocephalus dispar; _e_, Oxyuris vermicularis
(after Strümpell).]
(4) Tænia Echinococcus.--The mature form of this tape-worm inhabits
the intestine of the dog and wolf. The larvæ develop in cattle and
sheep ordinarily, but are {252} sometimes found in man, where they
give rise to echinococcus or "hydatid" disease. The condition is
unusual in America, but is common in Iceland and Australia.
[Illustration: FIG. 102.--Tænia echinococcus; enlarged (Mosler and
Peiper).]
The adult parasite is 2.5 to 5 mm. long, and consists of only four
segments (Fig. 102). It contains many ova. When the ova reach the
digestive tract of man, the embryos are set free and find their way to
the liver, lung, or other organ, where they develop into cysts, thus
losing their identity. Other cysts, called "daughter cysts," are
formed within these. The cyst-wall is made up of two layers, from the
inner of which develop larvæ which are identical with the head, or
scolex, of the mature parasite. These are ovoid structures about 0.3
mm. long. Each has four lateral suckers and a rostellum surmounted by
a double circular row of horny hooklets. The rostellum with its
hooklets is frequently invaginated into the body.
[Illustration: FIG. 103.--Contents of echinococcus cyst, showing
hooklets, scolices, and cholesterin crystals (Wood).]
Diagnosis of echinococcus disease depends upon detection of scolices,
free hooklets, or particles of cyst-wall, which is characteristically
laminated and usually has {253} curled edges. These can be found in
fluid withdrawn from the cysts, or, less frequently, in the sputum or
the urine when the disease involves the lung or kidney (Figs. 55 and
103).
The cyst-fluid is clear, between 1.002 and 1.010 in specific gravity,
and contains a notable amount of sodium chlorid, but no albumin.
2. Nematoda.--(1) Ascaris Lumbricoides.--The female is 20 to 40 cm.
long and about 6 mm. thick; the male, a little more than half as
large. Their color is reddish or brown. They are the common
"round-worms" so frequently found in children. Their habitat is the
small intestine. Large numbers are sometimes present.
[Illustration: FIG. 104.--Ascaris lumbricoides (female) (Mosler and
Peiper).]
The diagnosis is made by detection of the worms or ova in the feces.
The latter are generally numerous. They are elliptic, measuring about
50 by 70 µ, and have an unsegmented protoplasm (Fig. 105). The shell
is thick, and is surrounded by an uneven gelatinous envelop which is
often stained with bile.
[Illustration: FIG. 105.--Eggs of Ascaris lumbricoides (Mosler and
Peiper).]
(2) Oxyuris Vermicularis.--This is the "thread-worm" or "pin-worm"
which inhabits the colon and rectum, especially of young children. Its
presence should {254} be suspected in all unexplained cases of
pruritus ani. The female is about 1 cm. long; the male, about 0.6 cm.
(Fig. 106).
[Illustration: FIG. 106.--Oxyuris vermicularis and egg: _a_, Natural
size; _b_, egg (after Heller).]
The worms are not infrequently found in the feces; the ova, rarely.
The latter are best found by scraping the skin at the margins of the
anus, where they are deposited by the female. They are asymmetrically
oval, about 50 µ in length, and often contain a partially developed
embryo.
(3) Filaria Sanguinis Hominis.--A description of this worm will be
found in the chapter upon the Blood, p. 194. The embryos are sometimes
found in urine and chylous fluids from the serous cavities. Their
motion is then usually less active than when in blood. That shown in
Fig. 107 was alive sixty hours after removal of the fluid. Embryos
were present in the blood of the same patient.
{255} [Illustration: FIG. 107.--Filaria sanguinis hominis (embryo) in
chylous hydrocele fluid; length, 300 µ; width, 8 µ. A number of red
blood-corpuscles also appear (studied through courtesy of Dr. S. D.
Van Meter).]
[Illustration: FIG. 108.--Trichinella spiralis (larvæ) from head of
right gastrocnemius muscle; seventh week of disease (two-thirds
objective; eye-piece 4) (Boston).]
(4) Trichina (Trichinella) Spiralis.--This is a very small worm, not
exceeding 3 mm. in length when fully developed. Infection in man
occurs from ingestion of insufficiently cooked pork, which contains
encysted embryos. These reach maturity in the small intestine. The
female produces great numbers of young, which {256} migrate to the
voluntary muscles, chiefly near the tendinous extremities, and there
become encysted.
Trichiniasis is generally accompanied by a marked eosinophilia. The
diagnosis is made by teasing out upon a slide a bit of muscle,
obtained preferably from the outer head of the gastrocnemius, the
insertion of the deltoid, or the lower portion of the biceps. The
coiled embryos can easily be seen with a two-thirds objective (Fig.
108).
[Illustration: FIG. 109.--Uncinaria duodenalis: _a_, Male (natural
size); _b_, female (natural size); _c_, male (enlarged); _d_, female
(enlarged); _e_, head; _f_, _f_, _f_, eggs (after v. Jaksch).]
(5) Uncinaria.--The two varieties of this worm, _Uncinaria duodenalis_
and _Uncinaria Americana_, are among the more harmful of the animal
parasites. They inhabit the small intestine, usually in great numbers,
and commonly produce a severe and often fatal anemia. Infection is
common in subtropical regions, notably in Egypt, in some European
countries, and, especially, in Porto Rico and the West Indies, where
about 90 per cent. of the rural population is infected. It is much
more {257} common in the United States than was formerly supposed.
The adult worms are seldom found in the feces, but may appear after a
dose of thymol followed by a brisk purge. They resemble _Oxyuris
vermicularis_ to the naked eye. Ova are usually present in enormous
numbers. Those of _Uncinaria duodenalis_ measure about 30 by 50 µ; of
_Uncinaria Americana_, somewhat more. They have a thin, smooth,
transparent shell, and their protoplasm is divided into 2, 4, 8, or
more rounded segments (Fig. 110).
[Illustration: FIG. 110.--Four eggs of the New World hook-worm
(Uncinaria Americana), in the one-, two-, and four-cell stages. The
egg showing three cells is a lateral view of a four-cell stage.
Greatly enlarged (after Stiles).]
(6) Strongyloides Intestinalis.--Infection with this worm is by no
means so rare in this country as the few clinical reports would
indicate. It is very common in subtropical countries, notably in Italy
and in southern China. It seems probable that the parasite is the
cause {258} of "Cochin China diarrhea," although some authorities
regard it as harmless.
The adult worm, which reproduces by parthenogenesis, is about 2 mm.
long. It inhabits the upper portion of the small intestine, but
neither it nor the ova appear in the stool unless an active diarrhea
exists. Ordinarily the eggs hatch in the intestine, and when infection
is severe, embryos can be found in the feces in large numbers. These
are the "rhabditiform embryos," which measure about 0.40 by 0.02 mm.
They are actively motile, and are best found by making a small
depression in the fecal mass, filling it with water, and standing in a
warm place (preferably an incubator) for twelve to twenty-four hours.
The embryos will collect in the water, and can be easily found with a
two-thirds objective.
[Illustration: FIG. 111.--Strongyloides intestinalis: A, Mature
female; B, rhabditiform larva; C, filariform larva (after Braun).]
Outside the body the rhabditiform embryos develop into a free-living,
sexually differentiated generation. The young of this generation are
the more slender "filariform embryos" (Fig. 111). Infection can occur
either through these embryos of the free-living generation, or by
direct transformation of rhabditiform into filariform embryos and
these into the parthenogenic parasitic adult.
(7) Trichocephalus Dispar (T. Trichiurus).--This, {259} the
"whip-worm," is 4 or 5 cm. long. Its anterior portion is slender and
thread-like, while the posterior portion is thicker (Fig. 112). It is
widely distributed geographically, and is one of the most common of
intestinal parasites in this country. It lives in the large intestine,
especially the cecum, with its slender extremity embedded in the
mucous membrane. Whip-worms do not, as a rule, produce any symptoms,
although gastro-intestinal disturbances, nervous symptoms, and anemia
have been ascribed to them. They, as well as many other intestinal
parasites, are probably an important factor in etiology of
appendicitis, typhoid fever, and other intestinal infections. The
damage which they do to the mucous membrane favors bacterial invasion.
[Illustration: FIG. 112.--Trichocephalus dispar; _a_, Female; _b_,
male (natural size) (Heller).]
The number present is usually small. The worms themselves are rarely
found in the feces. The ova, which are not often abundant, are easily
recognized. They are brown, ovoid in shape, about 50 µ long, and have
a button-like projection at each end (Fig. 101).
III. ARTHROPODA
Most of these are external parasites, and the reader is referred to
the standard works upon diseases of the skin for descriptions. The
itch-mite (_Acarus scabiei_) and the louse (_Pediculus capitis_,
_corporis_, _vel pubis_) are the more common members of the group.
{260} A number of flies may deposit their ova in wounds or in such of
the body cavities as they can reach, and the resulting maggots may
cause intense irritation. Ova may be swallowed with the food and the
maggots appear in the feces. Probably most important is the "screw
worm," the larva of _Compsomyia macellaria_, infection with which is
not rare in some parts of the United States. The ova are most commonly
deposited in the nasal passages, and the larvæ, which may be present
in great numbers, burrow through the soft parts, cartilage, and even
bone, always with serious and often with fatal results.
{261}