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PUERTO RICO AGRICULTURAL EXPERIMENT STATION
MAYAGUEZ, PUERTO RICO
Under the supervision of the
UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 37
Washington, D. C. August 1935
PARASITES AND PARASITIC DISEASES OF HORSES
IN PUERTO RICO
By H. L. VAN VOLKENBERG, parasitologist
Introduction ..---------------------------- 1 Internal parasites.......-----------------.............- 5
General characteristics and effects of para- Large stomach worms.------------------ 5
sites .---..----------------------------- 1 Large strongyles ------------------------ 7
Abundance and location of parasites----.. 1 Small strongyles-__ --------------------- 10
Symptoms and damage produced by Perfoliate tapeworm-___.-.---------..-- 11
parasites--------------------------- 2 Other internal parasites ---------------- 11
General control measures for parasites ------- 2 External parasites... ----------------------- 13
Disposal of manure-------.-------------. 2 The tropical horse tick ---------------- 13
Management of pastures and grass fields. 3 Psoroptic mange, sarna, or piojillo....... 15
Other sanitary measures-.--------------- 3 Sarcoptic mange --.--------------------- 16
Medicinal treatment-------.-----------. 4 Biting or bloodsucking flies------------ 17
Other external parasites--------------- 18
The subject of parasites of the horse as presented in this bulletin
includes also the parasites and parasitic diseases of the mule, ass,
and burro. The kinds of the common worm parasites of the horse in
Puerto Rico are similar to those found in the continental United
States, differing somewhat in their manifestations. The parasitic
arthropods, which are mostly external parasites, vary considerably
from those found in the more temperate climates. Apparently these
latter are more greatly influenced by climatic and other external
GENERAL CHARACTERISTICS AND EFFECTS OF PARASITES
ABUNDANCE AND LOCATION OF PARASITES
Horses are usually infested with many different kinds and very
large numbers of parasites. The skin may be attacked by numerous
ticks, mange mites, and certain flies. The large intestine, which is
the favorite location of the internal parasites, may harbor thousands
of worms. Wandering larval worms may invade practically all
organs and tissues of the body.
i Acknowledgment is made especially of the constructive and very helpful criticism of the manuscript
by Maurice C. Hall and F. C. Bishopp of the U. S. Department of Agriculture, and W. A. Hoffman of the
School of Tropical Medicine, San Juan. P. R.
BULLETIN 37, PUERTO RICO EXPERIMENT STATION
SYMPTOMS AND DAMAGE PRODUCED BY PARASITES
The symptoms of parasitic infestation are unthriftiness, weakness,
emaciation, anemia, tucked-up flanks, rough coat, and digestive
disturbances such as diarrhea and colic.
The external parasites abstract blood from the host and irritate
the skin. Ectoparasites, especially the flies attacking the horse,
may also act as transmitters and intermediate hosts of disease germs
and parasitic worms. The internal parasites of the horse cause
damage principally by abstracting blood and by mechanically injur-
ing tissues and organs. Parasites are especially injurious to young
growing animals, causing stunting and lack of development. Mature
horses do not lose their susceptibility to external parasites and to
many of the gastro-intestinal parasites and are frequently injured by
them. The losses from parasites are represented largely by poor
development, decreased capacity for work, uneconomical use of feed,
and the loss of working time from indigestion and colics.
GENERAL CONTROL MEASURES FOR PARASITES
DISPOSAL OF MANURE
The eggs of many of the worm parasites of horses are passed with
the manure and undergo some development on the soil. The infective
eggs or larvae are usually ingested by the horse with the food and
drinking water. Thus the proper disposal of manure is an essential
control measure, as manure is the source of infestation of most of the
common internal parasites. Fresh manure should not be spread on
pastures or grass fields to be used for horses; pastures so manured
will be dangerous even months later, as the larvae of many parasites
are long-lived. It may be spread on areas used by other kinds of
livestock without much danger, as the parasites of equines are rarely
transmissible to other animals, and vice versa. Fresh manure may
be spread on heavy soils which are to be plowed, as the eggs and larvae
are buried and destroyed. Plowing under manure spread on light
soils may be less effective as a control measure. Recent experiments
made by the Zoological Division of the Bureau of Animal Industry
of the United States Department of Agriculture have shown that
some of the larvae of horse parasites buried under 5 inches of sandy
loam migrated to the surface, while there was practically no upward
movement in clay soil.2
An accumulation of horse manure in an open pile undergoes a
spontaneous heating process in the center but remains cold on the
surface. The more compact manure well below the surface may be
used after a sufficient time as fertilizer on pastures or grass fields for
horses with considerable safety as the parasitic material has been
destroyed by the heat. However, the outer or exposed layer to a
depth of 4 or 5 inches may be teeming with live parasitic ova and
larvae. This portion should be separated from the remainder and
handled the same as fresh manure or placed as the beginning of another
pile in order to subject it to heat. Another method of handling is to
turn over the outer layer of the pile every week or so and bury it
under the inner material to sterilize it by heating.
* MoHLER, J. R. PABABITES or HOBSas. U. 8. Dept. Agr., Rept. Chief Bur. Anim. Indus. 1938: 47.
PARASITES AND PARASITIC DISEASES OF HORSES 3
A method of storage of horse manure so that the heat can be
utilized to destroy all the worm eggs and larvae of parasites has been
developed by the Bureau of Animal Industry. A double-walled,
double-floored box made of tongue-and-groove lumber with a 4- to
6-inch space between the double parts of the walls is constructed.
The space between the double parts of the walls is filled with sawdust
for insulation. A well-fitting cover consisting of two thicknesses of
lumber is provided to retain the heat. Parasite worm eggs and
larvae are destroyed in a period of 2 weeks in horse manure stored in
MANAGEMENT OF PASTURES AND GRASS FIELDS
Heavy stocking of pastures should be avoided. Otherwise the
larger amount of manure deposited on a given area results in a
heavier concentration of parasite eggs and larvae, and the larger
number of animals grazing on such pastures are more certain to pick
up parasites. Pastures on hillsides are more desirable than on the
level areas, since the rains have a cleansing action in washing down
the manure and parasitic material from the hillsides. For the same
reason the bottom 'of a slope and the lowland may be dangerous
because of the higher concentration of eggs and larvae and the more
abundant moisture. Bogs in pastures around springs, watering
troughs, and along streams provide a favorable environment for
parasites. Streams which drain land on which infested animals are
maintained may carry the infection. Animals drinking water from
these streams may become infested or the infective material may be
deposited on grass fields or pastures if flooding occurs.
Rotation of pastures, stock rotation, and alternating pastures
with field crops are all valuable control measures for parasites. The
more often horses can be moved into clean pastures, the more effec-
tively infestations will be kept down.
SBesides the infestations acquired on pastures, infestations with
parasites may be acquired from fresh forage fed in the stable. The
feeding of fresh, wholesome, and clean grass is very important in this
climate, where green roughage is available throughout the year. At
present malojillo (Para) grass is probably the most common source
of infestation, as it thrives best on swampy land which provides a
favorable environment for the development and perpetuation of
parasites. This grass, because of its habitat, cannot be protected
from contamination by the usual precautions since infective material
may be washed on it with every fall of rain. Malojillo grass should
not be fed to either young or old horses, especially during the wet
season. The green forage for horses should be obtained from well-
drained fields which have not been fertilized by fresh manure and
where there is no danger of contamination by drainage from adjacent
land. Elephant grass, cane tops, Guatemala grass, the sorghums,
and dry hay are usually safe to feed so far as parasites are concerned,
if they are protected from contamination by manure after harvesting.
OTHER SANITARY MEASURES
Horses should be fed from overhead racks or mangers well raised
above the ground or floor. The feeding of horses on the ground is
wasteful and dangerous to the animals. The feed becomes mixed
with manure and ultimately the infective larvae are ingested with the
4 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
Concrete floors in stables are preferable to wooden or earth floors
from the stand omt of sanitation. Moisture-soaked standings pro-
vide a favorable medium for the development of parasites. Manure
from stables should be removed often, daily if possible. If a paddock
or corral is used instead of or in connection with a stable the soil
sho-ild be well drained. Feeding from the ground should be pre-
vented by daily removal of all feed dropped from the mangers and
by destroying any grass or weeds that may grow. Contamination
of the drinking water with manure should be prevented by providing
sanitary watering troughs. Animals should never be allowed to
drink from stagnant pools. A horse stable, corral, or manure pile
located in or draining into a pasture or grass field used by horses is
dangerous. The fly nuisance can be reduced by storing manure and
other decaying vegetable wastes in closed containers or by spreading
them thinly on fields; also by using fly traps and fly sprays.
The control of parasites by preventive measures is not always
practical or is inefficient, and treatment is usually necessary. The
Bureau of Animal Industry has developed several effective treatments
for the intestinal parasites of the horse.
A veterinarian is by training and experience best qualified for the
task of treating animals. All of the drugs which have been demon-
strated by critical tests to be effective for worms in horses are poi-
sons, and great care must be used in their administration and i
calculating the dose according to the weight and condition of the
animal. The various kinds of worms require different and more or
less specific treatments. A knowledge of the parasites present as
indicated by microscopic examinations of the feces may be necessary
to determine the drug to be used. Besides the general contraindi-
cations for the use of worm remedies, such as extreme youth or age,
greatly weakened condition, or febrile conditions, there are specific
contraindications for nearly all the drugs used in the treatment of
The safest method of administering drugs to horses is by the
stomach tube. Capsules are often used but these have the disad-
vantage of occasionally breaking in the mouth, and the contents, if
volatile or irritant, may be drawn into the windpipe and lungs and
cause serious consequences.
The administration of worm remedies with the food is unsatisfactory
even with drugs otherwise known to be effective for worms. Prior
to the administration of drugs, animals should be fasted. The general
rule in connection with treatments for parasites of the stomach and
small intestine is to fast the horse for 18 to 24 hours, and.with treat-
ments for those in the large intestine to fast 36 hours. Water may
be given during the period of fasting, but both food and water should
be withheld from 4 to 6 hours after the administration.
The necessity for treatment is indicated by physical examination
of the animal or by examination of the feces for worm eggs. Treat-
ment should always be given before an animal becomes weak and
emaciated. An animal that is in a condition of extreme debility
caused by parasites may show a striking improvement after medica-
tion. However, the task of treating such an animal is difficult, and i
even if an animal survives the treatment and it is effective it may
PARASITES AND PARASITIC DISEASES OF HORSES 5
never entirely recover from the effects of the parasites. It is best
to adopt a program of regular treatments. For some kinds of para-
sites of the horse, treatment once or twice a year is sufficient. For
others treatment should be given every month or two depending on
the amount of the infestation.
Treatment not only renders the infested animals more serviceable,
but also decreases the stable and pasture contamination by reducing
the output of parasite eggs.
LARGE STOMACH WORMS
The stomach worms, Habronema muscae, H. microstoma, and
H. megastoma (fig. 1, A), are slender and whitish in color. Two of
these species may attain a length of nearly 1 inch, whereas the smallest
is about one-half inch long. These worms may occur free in the
stomach, embedded in the mucosa, attached to the wall of the stom-
ach, or in tumors in the stomach wall.
Importance.-Infestations with these worms usually are not notice-
able except for the injuries produced by the larvae which get into the
skin of horses and produce a skin disease known as "summer sores."
This skin disease seems to be more common among horses in the dry
Life history.-The eggs which are passed with the feces are swal-
lowed by maggots of house flies, stable flies, or other flies which breed
in horse manure. The larvae develop with the maggots and com-
plete their growth in the flies after they emerge from the pupae.
Horses become infected by swallowing live or dead flies which are
infested with the larvae or by swallowing the larvae which have
escaped while the flies are feeding about the mouth and lips of the
Symptoms and lesions.-Except for the summer sores no definite
symptoms are associated with these parasites. The worms may cause
injury to the stomach by their attachment and by penetrating into
the wall and also by the formation of nodules which interfere with the
proper functioning of this organ.
Treatment.-Carbon disulphide should be given in doses of 6 fluid
drams (24 cubic centimeters) for a 1,000-pound horse or at the rate
of 6 cubic centimeters for each 250 pounds of live weight. No
purgative should be used with this treatment. Fats and oils should
be avoided as they promote absorption of the drug and increase the
toxicity of the carbon disulphide.
The treatment is more effective if this drug is preceded by 8 to 10
quarts of a 2-percent solution of sodium bicarbonate. This alkaline
solution tends to remove the thick layer of tenacious mucus which
normally covers the lining of the horse's stomach. This solution
should be siphoned off with a stomach tube, or otherwise an interval
of 20 minutes should elapse before administering the carbon disul-
Prevention.-Stomach-worm infestation in the horse can be con-
trolled by any effective measures which prevent flies from breeding
in horse manure. Fresh manure should be spread at once or stored
in closed containers.
--- ,,- 1
6 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
Summer sores are pronounced skin lesions which seem to occur
more frequently at the joints on the outer surface of the front legs,
FIGURE 1.-The more common internal parasites of Puerto Rican horses (natural size): A, Large stomach
worms; B, large strongyles; C, perfoliate tapeworms; D, small strongyles; E, threadworms of the abdom-
inal cavity; F, bots of the throat botfly.
on the neck, and around the withers. The sores on the body are
usually small, but on the legs they may attain a diameter of 2 inches
or more. These indolent sores consist of a brownish-red pulpy
PARASITES AND PARASITIC DISEASES OF HORSES 7
material in the midst of which are round granulations, firm in texture.
In an old sore these granulations become calcified and are hard.
Apparently the larvae of the stomach worms escape from the mouth
of infested flies which are feeding on wounds in the skin. These
larvae remain in the wounds and cause irritation so that, instead of
healing, the wound may increase in size, and a chronic lesion is
produced which may persist for several months.
Treatment--Treatments recommended for these sores are as follows:
The use of a 5- to 10-percent solution of formalin, applied by means
of a cotton pad which is left on the sores for 2 or 3 hours every day;
or covering with an astringent powder consisting of plaster of pars,
100 parts, alum, 20 parts, naphthalene, 10 parts, and quinine, 10
parts; or washing the sores with ether or chloroform and then painting
For the old chronic lesions, the center material may be softened by
applying a caustic paste consisting of arsenious oxide, 1 or 2 parts,
and flour, 5 parts. In a few days this growth will separate from the
normal tissues and can be removed surgically, and a bandage and
antiseptic dressing should then be applied until healing occurs.
Prevention.-Skin injuries may be protected from flies by use of
pine-tar oil. Breeders of race horses who have had difficulties with
this disorder are now controlling it by applying ordinary wound
dressings to all wire cuts and abrasions on the skin as soon as they
occur. These fresh wounds heal readily whereas the summer sores
do not or are slow to respond to ordinary treatment. Incidentally
the flies on these horses are being controlled by the use of kerosene-
pyrethrum extract sprays.
The strongyles, located in the large intestine, include two closely
related groups of roundworms which are called large and small
strongyles. The large strongyles, also known as bloodworms, sclero-
stomes, or palisade worms, include three common species, viz, Stron-
gylus equinus, S. edentatus, and S. vulgaris (fig. 1, B). The largest
of these may reach a length of nearly 2 inches while the smallest
species is from one-half to 1 inch long. These worms are bloodsuckers
and feed on the blood by attaching themselves to the walls of the
Importance.-At present the large and small strongyles are the
most common and widely distributed parasites of horses in Puerto
Rico. Strongylidosis, caused by these worms, is probably the most
serious disease of horses occurring in Puerto Rico, although it is
receiving less attention than some of the other diseases.
The following statements regarding life history, symptoms and
lesions, treatment, and prevention of the large strongyles are quoted
from a circular of the United States Department of Agriculture.3
Life history.-The eggs of blood strongyles are discharged into the intestine
of the host and are eliminated from the body with the feces. Under favorable
conditions of temperature and moisture the eggs develop rapidly on the ground
and on pastures and hatch in a day or so. The larva which emerges from each
egg which develops normally, feeds on the contents of the manure in which it
finds itself and after it undergoes two molts in more or less rapid succession it
becomes infective. This entire development on the ground or on pasture takes
3 SCHWARTZ, B., InES, M., and WRIGHT, W. H. PARASITES AND PARASITIC DISEASES OF HORSES. U. S.
Dept. Agr. Circ. 148; 20-24, illus. 1933. (Revised.)
8 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
place in a week or so during the warm months and is delayed considerably during
the cold months. Lack of moisture is unfavorable to the developing eggs and
larvae, but ordinarily there is sufficient moisture in horse feces to favor normal
development, provided the temperature is favorable. Shade affords protection
to the eggs and larvae and it is likely that direct sunlight is more or less injurious
The infective larvae are very resistant to unfavorable conditions and are
probably capable of maintaining themselves on pastures for long periods. They
remain ordinarily in or near the manure where they hatch; however, they are
doubtless scattered by rain and wind and may reach places on pastures relatively
remote from their original locations. When the air is sufficiently moist so that
the grass becomes covered with a film of moisture, as happens in times of rin,
dew, or fog, the larvae migrate up the grass blades and this brings them to a
favorable situation to be swallowed by horses while grazing. So far as is known
these larvae do not penetrate the skin and must infect horses by being swallowed
with grass, water, or dry food which has become contaminated with the larvae.
The course of development of these parasites after they get into the body of
horses has not been definitely ascertained; it is known, however, that not all
larvae, and perhaps none of them, go down directly into the cecum and colon and
settle there and grow to maturity. On the contrary, the larvae, after entering
the body of the horse, undergo extensive migrations which bring them to various
organs and tissues, such as the liver, pancreas, spleen, lungs, kidneys, and other
organs and tissues, from which many of the larvae probably fail to get back to the
large gut. However, those larvae which return to this organ become attached
to its wall and develop to fertile maturity. The eggs which are produced by
the female worms and eliminated from the horse's gut with the manure start
the cycle of development once more.
From this account of the life cycle of these parasites it is evident that horses
infested with blood strongyles contaminate the pastures on which they feed with
the eggs produced by the worms, and that the larvae which issue from the eggs and
develop to the infective stage may be swallowed by these and other horses.
When horses are kept on the same pastures year after year the number of eggs
and larvae gradually increases, and this contamination, accumulating from year
to year on a given pasture, may be highly damaging to horses which are grazed
there. Foals, in particular, suffer from the effects of gross parasitism acquired
in this manner. *
Symptoms and lesions.-The blood strongyles injure the wall of the gut to
which they-are attached. These worms suck a tuft of the inner wall of the gut
into their mouth cups and abstract blood from this delicate lining. As the worms
move from one place to another within the gut they expose its injured wall to the
entrance of disease-producing bacteria. Bloodworms abstract blood from the
finer blood vessels in the lining of the gut, and when many worms are present
in a horse at the same time the loss of blood may be considerable and may lead
to anemia with the usual consequences of weakness and of watery swellings
(edema) in various parts of the body. These worms also produce injuries of
various sorts in the parts of the body to which they wander, such as the liver,
pancreas, and other organs.
One species, the single-toothed strongyle, is especially injurious because as an
immature form it settles in certain arteries, especially in the anterior mesenteria
artery which supplies blood to the large gut. As a result of the presence of the
worms in this blood vessel, the wall of the artery, where the worms accumulate,
becomes thickened and stretches considerably to form what is known as an
aneurism. An aneurism is a spindle-shaped, cylindrical, or globular dilation of
a blood vessel and often contains a heavy deposit of fibrin inside. In the horse
it may attain the size of a child's head.
An aneurism interferes to a considerable extent with the circulation of blood
through the affected artery, because the heavy deposits of fibrin inside may
almost obliterate the lumen of the blood vessel. This condition results in a
diminished blood supply to the large intestine; when the intestine becomes anemic
as a result of this it becomes predisposed to colic, twist, and intussusception, the
last being a condition in which part of the gut slips into an adjoining part. When
a piece of fibrin deposit in the aneurism breaks loose, it may be carried in the
circulation to a terminal portion of an artery and may lodge there as a plug. As
a consequence, the circulation to a part of the large gut may be completely shut
off. Such a condition interferes with the functions of the large gut, produces an
anemic condition with the consequences noted above, and in extreme cases may
produce death. It is believed that most cases of colic in horses result form such
disturbances in the blood circulation of the gut. If the plug forms in a hind leg
PARASITES AND PARASITIC DISEASES OF HORSES
it may cause a forth of intermittent lameness. Aside from the injuries described,
mixed parasitic infestation, known as strongylidosis, results when blood stron-
gyles occur in large numbers and in association with other species of roundworms
in the large gut, particularly when horses are kept on permanent pastures.
This condition is widespread in horses and is usually more injurious to foals
and young horses than to older horses. Strongylidosis is frequently mistaken
for infectious anemia or swamp fever and cases diagnosed as swamp fever, in
many instances, have cleared up following the removal of worms by medicinal
treatment. The common symptoms of strongylidosis are diarrhea, weakness,
and emaciation. The digestive disturbances result from the irritation to the
lining of the gut produced by the parasites. At first the diarrhea is slight and
the soft manure has a bad odor and contains poorly digested food material;
later the diarrhea becomes more pronounced, with softer feces. The appetite,
which is irregular at first, becomes poorer and finally the horses are off feed.
As a result of their weakened condition, affected horses find difficulty in chewing,
may throw out mouthfuls of feed, and then reject feed altogether. This leads to
further emaciation which becomes very marked. As the disease progresses the
bones become prominent, the coat becomes rough, the eyes are sunken, and the
animal becomes greatly weakened. With these symptoms there are marked
changes in various tissues and organs of the body.
The working capacity of horses which are suffering from strongylidosis is
decreased considerably even before the symptoms become very pronounced, and
in the absence of treatment such horses are able to do less and less work as the
disease progresses. It has been found that when horses are treated for the
removal of parasites many of the symptoms described above disappear, the
animals take on weight, regain their working capacity, and make an all-round
Treatment.-It is advisable to fast the animal 36 hours before treating for large
strongyles. Oil of chenopodium is effective for their removal. This drug
should be given in a dose of from 4 to 5 fluid drams (16 to 20 cubic centimeters)
for a 1,000-pound animal, or at a dose rate of 1 fluid dram (1 cubic centimeter)
for each 250 pounds of weight, immediately preceded or followed by 1 quart of
raw linseed oil or by an aloes ball. Cases of excessive purgation have been
reported in some instances following the use of raw linseed oil. It is possible
that this undesirable action is due to impurities in the product; consequently a
good grade of oil should be used.
A mixture, said to be without the undesirable effects of raw linseed
oil, has been recommended as follows:
For weanlings, castor oil, 4 to 6 ounces, and neutral oil, 1 pint; for yearlings
and 2-year-olds, castor oil, 6 to 8 ounces, and neutral oil, 1 pint; for 3-year-olds
and older, castor oil, 8 to 10 ounces, and neutral oil, 1 pints.
Carbon tetrachloride is also of value for the removal of large strongyles. It
should be used in treating pregnant mares as oil of chenopodium is dangerous
for such animals. It may be given in doses of 6 to 12 fluid drams (25 to 50 cubic
centimeters) for a 1,000-pound animal.
Normal butylidene chloride is another drug that is effective for
the removal of large strongyles. It should be given in a dose of 3 fluid
ounces (90 cubic centimeters) for a 1,000-pound animal, and followed
in 5 hours by raw linseed oil at the rate of 1 quart per 1,000 pounds of
weight. It is probable that the dose of normal butylidene chloride
could be reduced to 2.5 ounces (75 cubic centimeters) for a 1,000-
pound animal, without materially reducing the efficacy of the drug.
One treatment with any drug will not always remove all worms present and,
if necessary, treatment may be repeated in from 4 to 6 weeks.
Prevention.-Preventive measures designed to control strongyles consist (1) in
rotation of pastures, so far as possible, avoiding low and wet pastures, and (2)
sanitation of stables to prevent larvae from developing to the infective stage and
from contaminating the feed and water. This is accomplished by daily removal
of manure from stables, supplying the feed in boxes and racks well raised above
the floor, and supplying clean water. The disposal of stable manure is an import-
ant preventive measure. Little can be accomplished in the way of
pasture sanitation except on farms wherethoroughbred or other valuable horses
10 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
are raised. On these farms the removal of manure deposits Trom pastures may
be practiced, as this procedure will remove almost all the parasite material before
it can develop and spread. Such a procedure is necessarily troublesome and
expensive and can be undertaken only by breeders to whom the question of
expense is of secondary importance. The average breeder will have to resort
to simpler and less radical measures, such as avoiding the overstocking of pastures,
frequent rotation of pastures, and special attention to foals. Where overstocking
and the use of wet pastures are unavoidable, and rotation is impossible, reliance
must be placed on treatment as often as necessary for control.
Horse breeders, and even the general farmer, should pay considerable attention
to the sanitation of paddocks in which the newly born foals are kept. The foals
should be kept there for several weeks before they are put on pasture. The
removal of manure from the paddocks, at least once a week, will cut down the
supply of eggs and larvae to which the foals would otherwise be exposed. This
precaution will help to tide the foals over the most critical period of their lives.
Young animals of all sorts are special cases and require special care. In the
last analysis the saving of young livestock involves the same precautions which
are used in connection with the prevention of sickness in children. Above
everything else, a wholesome food supply and clean surroundings are the best
safeguards against disease. Special precautions to prevent foals from becoming
parasitized are essential parts of sound management in horse-breeding establish-
ments and on the average farm.
The small strongyles (fig. 1, D), including the cylicostomes, consist
of several groups of worms and many different species. Whereas
the large strongyles are red, the small strongyles are usually white or
grayish white in color. These worms are either attached to the walls
or are found free in the contents of the caecum and colon. They do
not feed, at least ordinarily, on the blood, but they may feed on the
mucosa or lining of the intestine. Many of these worms are very
small, while others of the many species are as large or nearly as large
as the smaller palisade worms. The large strongyles may occur m
infestations of a few hundred worms while the small strongyles may
number thousands in the same animal. The immature forms of some
of these worms occur in nodules in the walls of the large intestine.
One species produces rather severe ulcers on the walls of the colon.
The frequent obstipations or so-called colics among horses and
mules, which occur at certain seasons each year both in St. Croix of
the Virgin Islands and in Puerto Rico, seem to be caused by abrupt
and massive invasions of the larvae of the small strongyles, either
alone or associated with the large strongyles. These larvae passing
through the intestinal walls into the organs and tissues of the abdo-
men in numbers, injure these tissues and may carry with them patho-
genic organisms. Acute strongylidosis or infestation with the larval
phase of the strongyles are terms that may be used to describe this
condition. It has been demonstrated both in Puerto Rico and in
St. Croix that these epizootics of colic can be prevented by following
the recommendations given for the control of parasites.
Treatment.-The same anthelmintics are used for both the large
and small strongyles. Carbon tetrachloride is less effective than
either oil of chenopodium or normal butylidene chloride.
Prevention.-The same control measures should be used for both
the large and small strongyles. Preventive measures are more
necessary during the 2 or 3 months immediately following the tor-
rential showers. In the southern coastal section precautions should
be used during the entire rainy season.
The most important recommendations for the prevention of
strongyle colic in horses and mules are: (1) Do not feed forage on
PARASITES AND PARASITIC DISEASES OF HORSES 11
the ground; (2) do not spread horse manure on grass fields or pastures,
and prevent the drainage of yards, stables, and manure piles into
Pastures or grass fields used by horses; (3) feed only wholesome grasses
selected from the higher and better drained fields; (4) avoid low, wet
pastures; (5) treat horses as often as necessary for parasites.
The tapeworm Anoplocephala perfoliata (fig. 1, C) is found in the
caecum and also in the lower portion of the small intestine. This is
a white, flat segmented worm from 1 to 3 inches long.
Life history.-The ripe or gravid end segments which contain the
eggs become detached from the rest of the chain and are expelled
from the body with the manure. The further development and
manner of infestation is unknown.
Importance.-Very heavy infestations have been found in horses
and mules in the district of San Sebastian. In the vicinity of Maya-
guez and on the southern coast infestations among horses seem to be
Symptoms and lesions.-Heavy infestations may cause intestinal
catarrh, digestive disturbances, emaciation, and anemia. This
worm is said to cause ruptures of the intestinal walls. Ulcers on the
walls of the intestine are produced at the points of attachment of
Treatment.-Critical tests to determine effective treatments for
tapeworms in the horse have not been performed. The following
drugs have been recommended: Oil of turpentine, areca nut, kamala,
and oleoresin of male fern.
Oil of turpentine is given in a dose of 2 fluid ounces (60 cubic centi-
meters) in capsules and followed every second day by 1 ounce until
5 or 6 doses have been given. One quart of raw linseed oil is given
with the last dose.
Areca nut, freshly ground, may be given to adult horses in doses
of from 1 to 1.5 ounces (30 to 45 grams) in capsules after fasting the
animal 24 to 36 hours. If the bowels do not move within 4 or 5 hours,
it is advisable to administer 1 to 2 pints of raw linseed oil.
Kamala is given in doses of 1 ounce (30 grams) after fasting for
from 24 to 36 hours. A purgative is not usually necessary with this
The dose for oleoresin of male fern is 3 to 6 drams (10 to 20 grams)
with a fasting period of 24 hours. It should be followed immediately
by 1 quart of raw linseed oil.
All of these drugs should be used with caution. They should be
given only to animals presumably able to withstand any poisonous
effects of the drugs.
Prevention.-As the life history is unknown, no recommendations
other than the proper disposal of manure can be recommended.
OTHER INTERNAL PARASITES
The large intestinal roundworm or ascarid, Ascaris equorum, is
found in the upper part of the small intestine. This worm may equal
or exceed an ordinary lead pencil in size. Ascarids are more common
and cause more damage among foals and young horses. This worm
is occasionally found, usually as small, immature specimens, in adult
horses in Puerto Rico, but its incidence among colts is not known.
12 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
Because of its large size, the toxic secretions produced by it, and
the wandering habits of the larvae, it has the capacity for causing
considerable damage, and heavy infestations may cause illness and
even death. Unthriftiness in foals that is not a result of poor feeding
or poor breeding may be caused by ascarids, and microscopic exami-
nation of the feces for the worm eggs should be made in such cases.
Carbon disulphide (p. 5) is an effective remedy for the removal of
ascarids. The dose of this drug should be carefully judged and
measured according to the size and age of the animal to be treated.
The small stomach worm, Trichostrongylus axei, occurs in the lining
of the stomach. It is only about one-fifth of an inch in length. It
causes small tumors, thickening of the stomach wall, and other
injuries to the stomach by embedding in the lining. These worms
are removed to some extent by the same treatment as that for the
large stomach worms.
The pinworm, Oxyuris equi, inhabits the large intestine. It is from
3 to 6 inches in length, with a long thin tail. Yellowish crusts con-
sisting of masses of eggs may be seen around the anus of infested
horses. Usually the only symptom noticed is the irritation produced
by presence of the worms in the region of the anus and by some irri-
tant property of the eggs, which cause the horse to rub the tail and
hind parts against any convenient object. Apparently these worms
are of little importance in this climate.
Oil of chenopodium as recommended for the large strongyles is an
effective drug. Oil of turpentine in a dose of 2 ounces (60 cubic
centimeters) for a 1,000-pound animal is also effective. Both the
oil of chenopodium and the oil of turpentine should be preceded or
followed immediately by 1 quart of raw linseed oil or by the proper
dose of the mixture of castor oil and neutral oil, as given under the
treatment for large strongyles (p. 9).
The lungworms, Dictyocaulus arnfieldi, are long, slender, whitish
worms found in the bronchi and bronchioles. They are from 1 to
2 inches in length. So far as known, lungworms are not a serious
pest of Puerto Rican horses. It is doubtful if medicinal treatment is
of any value against lungworms. Nursing treatment and prevention
are the most feasible measures that can be recommended for lung-
A bot, Gastrophilus nasalis (fig. 1, F) is found in the stomach and
small intestine. This bot is the larva or maggot of a fly, which is
known as the throat botfly or chin fly. Apparently this fly is the
only one of several common species of botflies of the horse that has
been able to establish itself under climatic conditions such as exist in
The ova of the chin fly are deposited by the fly on the hairs under
the jaw of the horse. The oviposition period in Puerto Rico has not
been determined. These ova hatch and the young bots crawl to
and enter the mouth. Eventually they pass down and attach them-
selves to the lining near the exit of the stomach and also in the upper
part of the small intestine. They remain here for 8 to 12 months
undergoing development, and finally pass out with the manure and
pupate on the ground. These bots are often found on necropsy but
so far always in very light infestations. The throat botfly has the
reputation of being very annoying to horses while depositing its eggs
but trouble from this source has not been noticeable, probably due
PARASITES AND PARASITIC DISEASES OF HORSES
to the scarcity of the fly. However, this fly will probably increase
in abundance. Because of the large mouth hooks used for attach-
ment and the spines which project from the body, bots may cause
irritation and injury to the stomach and small intestine if present in
Carbon disulphide (p. 5) is the most effective drug for the removal
of the bots. Judging from the size of the bots found on necropsy
the drug should be administered during January or February.
The threadworm, Setaria equina (fig. 1, E), is a white, slender
worm from 2 to 5 inches long. It is found in various locations out-
side the alimentary canal but usually occurs free in the abdominal
cavity. A few of these worms are usually found on autopsy. These
worms in the abdominal cavity do not appear to cause any damage
and are not known to produce any symptoms. If an immature
worm gets into the eye, which apparently occurs only rarely, it may
be removed surgically under local anesthesia.
The neck threadworm, Onchocerca cervicalis, is a long, slender
worm found in the large neck ligament. Sand flies (Culicoides) have
been shown to carry these worms. This worm apparently irritates
the tissues and thus lowers their resistance to disease germs, and may
lead to the development of poll evil and fistulous withers. This worm
seems to be scarce and this infestation and also disease of the withers
and poll are uncommon among local horses. No treatment for the
destruction of the worms is known.
The liver fluke, "lingua" or "cucaracha", Fasciola hepatica, which
is a common parasite in cattle and goats and to a less extent in swine,
is also found in the horse. Apparently the horse is not very suscep-
tible to this worm as infestations are uncommon. Carbon tetra-
chloride as given for the large strongyles should be effective for the
destruction of these flukes.
THE TROPICAL HORSE TICK
The tropical horse tick, Dermacentor nitens (fig. 2), which is the
common tick on horses in Puerto Rico, is reddish brown in color and
without any markings. This tick prefers the inside of the ears as a
place for attachment. Colonies are found also in protected areas such
as the mane, tail, around and below the anus, and under the jaw.
Long-haired horses, especially foals, may have the entire body infested
with these ticks. Occasionally colonies of young ticks are found on
Importance.-These ticks are widely distributed over the island
but infestations seem to be more severe among horses of the mountain
regions and in the dry sections. The environment seems to be less
favorable for the tick in the wet coastal areas. These ticks are
causing considerable injury to horses and very little is being done to
Life history.-According to the Bureau of Entomology and Plant
Quarantine, United States Department of Agriculture, the life history
may be summarized as follows: The engorged female drops from the
animal to the ground where it may deposit from a few hundred up
to 3,000 eggs. The eggs hatch in 24 days or longer. After attach-
ment the tick may become mature within 26 days. The entire life
cycle may be completed within 8 weeks.
BULLETIN 37, PUERTO RICO EXPERIMENT STATION
Symptoms and lesions.-The tick causes injury by drawing blood
from the host and by irritating the sensitive ears. The female of this
tick during engorgement excretes a substance which, when dry, re-
sembles coagulated blood. The ears may be filled with crusts of
FIGL'RE 2.-The tropical horse tick infesting ear and eyelids.
this excreta, and the hair, especially about the head, becomes matted
with a mass of material. The filth in the ear may result in suppuration.
In the dry sections this excreta is less noticeable since it dries and
scales off as it forms.
PARASITES AND PARASITIC DISEASES OF HORSES
Treatment.-The arsenical solutions as used for the common cattle
tick are effective for killing this tick and may be applied to the body
as dips or sprays, but should not be used as local applications inside
First clean the ears with a wire loop, being careful not to injure the
animal, and then inject into the canal of each ear .about one-half
ounce of a mixture consisting of 2 parts commercial pine tar and 1
part cottonseed oil. A mixture of 1 part kerosene and 3 parts lard
is also effective. The first mentioned mixture has the advantage
of protecting the ears from reinfection for a month or more.
Prevention.-Keeping horses from an infested pasture for 4 months
is said to be sufficient to insure the starvation of the seed ticks on
PSOROPTIC MANGE, SARNA, OR PIOJILLO
The mite, Psoroptes equi, which causes psoroptic mange, lives on
the surface of the skin (fig. 3). This is a very small mite, barely
visible to the naked eye if placed on a dark background.
Importance.-Infestations are common, but usually do not cause a
serious disease in this climate. The more serious infestations have
been found in the more closely confined horses used as draft animals
in the towns. In the country districts very little is being done to
control this mange except among the larger herds of horses.
Symptoms and lesions.-This mange mite prefers the more protected
parts of the body. Usually the first lesions are noticed on the head
and neck near the mane or at the base of the tail. From here mange
spreads slowly to other parts of the body. The lesions on the body
are usually localized in spots or areas which may gradually increase
in size. The mites biting or pricking the skin cause a slight inflamma-
tion and intense itching. The animals bite and rub themselves to
BULLETIN 37, PUERTO RICO EXPERIMENT STATION
relieve this itching. Later scales or scabs are formed and there is a
loss of hair. Finally the skin becomes thickened and hard and thrown
into wrinkles or folds. Among pastured animals this last stage of
swelling and tumefaction of the skin may be long delayed. A horse
which is in the open throughout the year may harbor the infestation
for several years, and the only lesions which will be noticed by the
casual observer are a loss of hair in spots and a slight thickening of
the skin. Sometimes these hairless spots are found over the entire
body, including the legs.
Treatment.-The most effective preparations for mange are lime-
sulphur or nicotine dips which are applied as a spray or by dipping in
a tank. Proprietary brands of these preparations, usually with di-
rections for dilution on each container, are available. The coal-tar
creosote disinfectants sold under various trade names are also effective.
These should be mixed or diluted with soft water in order not to
injure the animal. Two applications of any of these preparations,
from 10 to 12 days apart, usually cure ordinary cases. Four or more
applications may be necessary for chronic cases.
Unprocessed crude petroleum or crank-case draining will cure
mange if applied before the disease becomes chronic. These are
usually applied to the infested areas of the skin at intervals of one
week. However, these oils cause a loss of hair and often blister the
skin. As an added precaution, animals should be kept in the stable
or otherwise protected from the direct rays of the sun as long as the
oil remains on the skin.
Prevention.--As this mange is very contagious to all classes of horses,
it is important that mangy horses be isolated and all equipment
kept separate until the disease is eradicated. It is a good sanitary
practice to clean and disinfect all stables and small enclosures which
have been occupied by mangy horses, also all harnesses and other
objects used on horses, before using them for clean animals.
The mite, Sarcoptes equi, which causes sarcoptic mange, is slightly
smaller than the psoroptic mite. This mite penetrates the skin and
excavates or burrows through the upper layer of the skin. It is more
serious than the psoroptic mange mite as it causes more irritation
to the skin and is more difficult to eradicate. However, sarcoptic
mange is an uncommon disease among horses in Puerto Rico.
As psoroptic mange and sarcoptic mange may be confused with
each other, examinations should be made for the mites. This is
done by scraping the skin of an infested area until the blood oozes
and then examining the scrapings under a microscope.
The treatment is the same as given for psoroptic mange, but it
requires persistent, thorough, and frequent applications of the prep-
arations recommended. From 4 to 6 dippings, 5 to 7 days apart,
may effect a cure in ordinary cases, especially if all the affected areas
are soaked with warm dip and scrubbed with a brush just prior to
the first dipping. Animals which have been neglected until the
affected skin has become greatly thickened and leatherlike are usually
PARASITES AND PARASITIC DISEASES OF HORSES
BITING OR BLOODSUCKING FLIES
All the more common biting flies found on the Continent of North
America are represented in Puerto Rico by the same or similar species.
However, none of these is as serious for the horse as those occurring
in parts of the continental United States, although two or more
species are common and injurious during certain seasons of the year.
The stable fly, Stomoxys calcitrans, resembles somewhat the com-
mon house fly, but it has a long slender proboscis projecting down
and forward from the head and not terminating in the enlargement
present in the house fly. The stable fly is widely distributed over
the island and occurs throughout the year. As in the case of many
S other flies, both insufficient moisture and excessive moisture are
unfavorable to its increasing in numbers, so that it is usually more
abundant following a period of rains. Probably the stable fly causes
more injury than any of the other flies attacking the horse in Puerto
i The horn fly or la mosca del ganado, Haematobia irritans, prefers
S cattle as a host but frequently attacks horses. This fly is about one-
S half the size of the house fly and can be identified by its habit of feed-
ing with the head down, in line with the hairs of the body and with
its wings spread. The horn fly is found in the coastal areas, and
occurs in great abundance at different times during the summer in
the dry southern coastal area. Usually the horn fly does not attack
the horse in swarms as it does cattle.
The deer fly or ear fly, Chrysops variegatus, is a long slender fly
with a yellowish color. Distinguishing features of this fly are the
conspicuous black eyes and the broad smoky bands in the wings.
S This fly usually attacks the horse on the head about the ears. It is a
S vicious biter but it is not so numerous as to be a noticeable pest to
horses. This fly is more common following the rainy season and is
usually found in the low wet coastal areas. It prefers heavily shaded
areas as resting places and horses located in or near these places are
more subject to attack.
Four species of the large black horseflies, Tabanus spp., have been
reported. These flies are capable of causing considerable injury to
horses because the long piercing mouth parts of the female produce
a painful wound. Specimens are hard to find on the horse or in the
field. Because of their scarcity these flies are usually unfamiliar to
Various species of mosquitoes attack animals, especially horses.
Small swellings caused by the bites of mosquitoes are often noticed
on tender-skinned horses. A few stockmen consider the mosquito an
important pest of the horse.
The black flies or majes, Simulium spp., of which there are at least
three species in Puerto Rico, are more abundant in the higher eleva-
tions near streams. As these flies are carried by the wind, swarms
S of them may be found in other localities. These flies are a serious
S pest in many places, killing numbers of animals, but because of lack
of abundance they are not considered of much importance.
The sand flies, or plagas, of which Culicoides furens is the most
important member, are very abundant in the vicinity of the seashore.
As these flies are carried by the wind they are often found farther
inland. These bloodsucking midges are very annoying to man, but
18 BULLETIN 37, PUERTO RICO EXPERIMENT STATION
horses do not seem to be greatly disturbed even in the presence of a
swarm of them.
Treatment.-For the prevention of stable fly and horn fly breeding
all collections of manure, straw, old hay, and other vegetable matter
about the stable should be cleaned up and spread immediately or
stored in enclosed boxes or pits. For mosquitoes, destroying the
breeding places by drainage or treatment with oil or paris green or
by impounding water and using certain species of small fish which
eat the wrigglers are always important control measures. Detailed
information, including preventive measures, concerning several of
these flies is available in publications issued by the Bureau of Ento-
mology and Plant Quarantine, United States Department of Agricul-
OTHER EXTERNAL PARASITES
The screwworm, or el gusano de la herida, Cochliomyia homino-
rorar (Syn., C. americana), is the maggot of a blowfly which is found
in wounds of animals. Recent investigations made by the Bureau of
Entomology and Plant Quarantine indicate that this fly is a primary
invader of wounds, while another fly, C. macellaria, difficult to
distinguish from this species, is a secondary invader and also breeds in
The screwworm usually attacks cattle and swine but also occurs in
the horse. The maggots of this blowfly are dangerous in that they
are capable of penetrating practically sound tissues. There are
several other species of blowflies but the maggots are located usually
in old infected wounds or those already infested with screwworms.
The eggs of the screwworm fly deposited in a wound hatch and the
larvae or maggots burrow into the wound, grow rapidly, and complete
their development in 4 or 6 days. The other blowflies have a similar
life history. Because of the eating away of the tissues there is serious
danger from hemorrhage and also from infection. Attacks by screw-
worms may occur at any season of the year, but are more frequent
immediately following the rainy season.
For treatment of screwworms pour benzol or very small amounts
of chloroform into the infested wound, remove the easily accessible
maggots, and then apply pine-tar oil. Removing the deeply embed-
ded maggots or probing or opening the burrows is not recommended.
All fresh wounds of animals should be coated with pine-tar oil to
prevent fly blow.
The chigger, Trombicula tropica, which is a very small red larva
of a mite, attacks horses and other livestock. This mite is common
in the coastal areas during and immediately following the wet season.
Its attacks on the horse are especially noticeable about the face and
head. Because of the rubbing from the intense itching produced by
attachment and feeding of these larvae, the hair may be rubbedoff
the face. Dusting sulphur on the skin or applying it in an ointment
is an effective control measure.
The gnats or mimes, Hippelates spp., of which there are several
species, are very abundant especially in dry localities. These very
4 BitHOPP, F. C. THE STABLE FLY: HOW TO PREVENT ITS ANNOYANCE AND ITS WLOSsS TO IUVYLTOK.
U. S. Dept. Agr. Farmers' Bull. 1097, 18 pp.. illus. 1931. (Revised.)
HOWARD L. 0. and BISHOP, F. C. MOSQUITO REMEDIES AND PREVENTIVE. U. S. Dept Ag. Ar.FuM
Bull. 1570. 11 pp. 1932. (Revised.)
BISHOP, F. C. THE CONTROL OF FLIES ON DAIRY CATTLE AND AROUND DAIRY BUILDING U. U. D S.
Agr., Bur. Ent. E267, 3 pp. I1928.] [Mimeographed.]
- BLACK FLIES OR BUFFALO GNAT. I. S. Dept. Agr., Bur. Ent. E321, 2 pp. 1934. [Mimegraphed.]
PARASITES AND PARASITIC DISEASES OF HORSES
small flies do not bite but if abundant they cause some annoyance
to horses and other livestock by constant humming, swarming about
the body, and darting into the eyes, ears, and nostrils. They can
be controlled by using the kerosene-pyrethrum extract sprays. As
the flies are attracted to sores on the skin all abrasions and wounds
should be coated with pine-tar oil.
Both bloodsucking and biting lice occur on the horse. Lice occur
more frequently among horses in the temperate climates and are
serious only during cold weather when the hair is long. In the cli-
mate of Puerto Rico they seem to be rare. If present, they probably
occur in such small numbers that they do not cause annoyance and
are not easily detected. Several verbal reports of the finding of
lice on horses have been received, but specimens have not been ob-
tained for identification of the variety or species. The treatment for
lice is to apply either an arsenical solution as used for the cattle tick 9
or one of the coal-tar preparations, following the directions given on A. .
the container. To eradicate lice it is necessary to give two dippings 4
at an interval of from 14 to 16 days, as one application will not kill &
all the eggs.
I. S. GOVERNMENT PRINTING OFFICE: 1935
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