Veterinary Science | ,5? r Florida Agricultural
Mimeo Report VY66-1 Experiment Stations
June 1966 Gainesville, Florida
A G DEWORMING THE HORSE1
Stanley E. Leland, Jr.2
Successful parasitism is a situation of host-parasite balance.
If either host or parasite completely dominates and eliminates the
other, the state of parasitism no longer exists. Thus, man's efforts
on behalf of the horse is an attempt to shift the host-parasite
balance in favor of the horse. This is accomplished by an appropriate
control program. Control of internal parasites of the horse consists
of both good management practices and the prudent use of anti-para-
sitic drugs anthelminticss). Control programs are designed to
interrupt the life cycle of the parasite and thereby eliminate or
reduce the source of infection for the horse.
The horse has the rather dubious distinction of being host to
more different kinds of worm parasites than any of the other domes-
tic animals. Reference to a recent check list indicated over 50
different species of worm parasites are capable of infecting the
horse. Not only do many different kinds of worms occur in the horse
but total numbers in a single animal are often measured in the
For purposes of both discussion as well as treatment, the
various parasites of major concern fall into convenient groupings
namely: strongyles, ascarids (large round worms), bots, stomach
worms, pinworms, intestinal treadworms (Strongyloides), and tapeworms.
The strongyles are a large group of some 45 closely related
species which, by virtue of size, are further divided into two groups
designated the large and small strongyles. The division is of
particular importance since the large strongyles are, in general,
more pathogenic and more difficult to treat and are, indeed, the most
injurious parasites of the horse. Furthermore, the large and small
strongyle infections can not be differentiated on the basis of worm-
eggs in the feces. Differentiation is possible following fecal
culture but this is a special laboratory procedure and is not done by
laboratories or veterinarians doing routine fecal examinations. Thus,
egg counts can be misleading from the standpoint of detecting the
presence of the pathogenic large strongyles as well as evaluating
i Presented at the Fourth Annual Light Horse Short Course, June 10,
1966, Gainesville, Florida
2 Leland, Associate Parasitologist, Department of Veterinary Science.
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post treatment effects. Therefore a marked reduction. in strongyle
egg count after treatment may be the result of effectiive removal of
small strongyles while the more pathogenic large strcngyles were
The large strongyle, Strongylus vulgaris which is the true
bloodworm, is considered the most pathogenic worm parasite of the
horse. The larvae of this species migrate within the walls of
arteries suppling blood to the small and large intestine causing
inflammation and clots. Portions of the clots often break loose and
are carried to smaller blood vessels where they stop bloodd flow to
tissues of the small and large intestines. This cau'vs death of
intestinal tissue. This is one of the most frequent causes of colic
and death of the horse. The larvae also accumulate in the anterior
mesenteric artery where they cause a ballooning and weakening effect
on this blood vessel called an aneurysm. Obviously a weakened blood
vessel is of concern in an animal whose performance is measured in
speed and endurance.
Larval migration in the arteries and development of S. vulgaris
to the adult egg-bearing stage within the lumen of the large Intestine
requires about 6 months. This observation is extremely important in
the control and treatment of S. vulgaris. This tells us that foals
under 6 months of age harbor 7 vulgaris in migratory phase and not
in the gut. Antistrongyle drugs which remove adult worms from the
gut thus have no bearing on control during this early age. Successful
control can only be achieved in the foal at this age by suppressing
sources of infection. The regular use of antistrongyle drugs in
mares, yearlings, and other older horses passing eggs offers an
effective means of reducing the foals' exposure.
Another pathogenic large strongyle Strongylus edentatus is
similar to S. vulgaris in that the larvae migrile via the blood cir-
culation, 1Tver, and body-wall-lining for 11 months. Likewise
deworming as sucklings and weanlings with drugs now available has
no bearing on control of S. edentatus.
A number of drugs are available which are effective against the
stages of the large strongyles that occur in the gut of the horse.
Many of these drugs are available in combination so that an additive
effect or activity against other parasites is realized. To these
commercially available combinations, still other drugs may be added
to include activity against still more different worm species. The
system of treatment involves periodic administrations of the drugs.
Thiabendazole (EquizoleR) or
Piperazine-carbon disulfide complex (ParvexR) plus phenothiazine
are both over 90% effective against both S. vulgaris and S. edentatus
Phenothiazine plus piperazine, or
Trichlorfon plus phenothiazine plus piperazine (Dyrex T.F.R)
are effective against S. vulgaris but only slightly effective against
S. edentatus and:
Dithiazanine iodide (DizaR) + piperazine is less effective
against both S. vulgaris and S. edentatus.
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Both piperazine or phenothiazine used alone are even less
effective against both parasites.
In addition to the use of the appropriate drug, the period
between treatments is an important factor in designing an effective
strongyle control program. The interval between treatments will
vary from one farm to another and from one geographical area to
another since a number of factors are involved such as the level of
infection, stocking rate, soil and forage type, climate, and pasture
contamination. Unfortunately, information concerning these variables
is not available for Florida. From field studies carried out under
the climatic conditions of Central Kentucky the strongyle egg counts
following treatment are markedly reduced for 4 weeks, moderately
increased after 6 weeks, and markedly increased at 8 weeks. Thus
intervals of 6 to 8 weeks between treatments are required for effective
control in Kentucky and the interval may be even smaller in Florida.
Following a serious outbreak of parasitism, it may be necessary to
treat at 3 to 4 week intervals until the situation is brought under
All horses on the farm should be treated and transients or new
additions should be quarantined and dewormed before being included
with residence horses.
No single drug or mixture should be used exclusively for con-
trol since strains of strongyles resistant to a particular drug have
been observed. There are at least 2 theories as to how drug resistant
worm populations appear and are mentioned here to show the value of
alternating and combining different worm remedies. In one case, it is
felt that certain individual worms of a species population are and
always will be resistant to a particular drug while other members of
the same population are susceptable. Thus continued use of the same
drug has a selective action which produces a population of resistant
parasites. The second theory assumes continued exposure to the same
drug allows the surviving parasites to react and develop increased
tolerance to the drug. In either case, the advantage of alternating
drugs becomes apparent since the odds of a parasite population
becoming resistant to 3 or 4 different drugs are far greater than
they are in the case of one drug. In addition, combining drugs for a
treatment may have greater deworming activity than the sum of the same
drugs used separately. This "compounding" effect is thought to be the
result of each drug having a different point of influence on the
physiology of the parasite.
Another system of strongyle control which has been used extensively
in past years involves the daily administration of small doses of
phenothiazine (2.0 gm. daily 21 days per month, year round, in feed).
Although the low level system has been largely replaced by the more
convenient periodic-treatment system, it is no less effective for the
purpose it was designed. Since some confusion apparently exists
concerning its function and since it can be of value in certain
control programs, the following points should be understood concerning
the low level system: The value of the system lies, not in its
deworming ability in the horses on the system, but in the reduction of
contamination. Thus, the small amount of drug both reduces the egg
production of the female worm and is in sufficient concentration in
the excreted feces to prevent larval development on the pasture.
Horses on the low-level system therefore still require deworming.
Treatment every 6 to 8 weeks for strongyles is also consistent
with an overall control program for the other major parasites.
Ascarid infection is essentially a problem of young animals i.e.
suckling and weanling foals. Since 10 weeks are required by the
ascarid to reach the egg-bearing stage in the gut, treatment should
be started at 8 weeks of age and repeated at 8 week intervals until
they are yearlings which coincides with strongyle control. Piperazines
are highly effective in the removal of both immature and mature
ascarids and should be a component of each treatment in this age
group. Thiabendazole may also be used along with piperazine in these
young animals to control strongyloides.
Bots are actually larval stages of certain flies but commonly
are considered with the worm parasites with respect to treatment.
The use of the broad-spectrum products:
Piperazine-carbon disulfide complex plus phenothiazine or
Trichlorfon plus phenothiazine plus piperazine at intervals in
the basic strongyle control program provides improved bot control.
The trichlorfon in the latter combination and the carbon disulfide
in the former posses the bot activity. The use of a weak acid
solution (600cc of 0.5% hydrochloride acid) with the above complex
appears to improve the release of the carbon disulfide for better
Carbon disulfide is also used in liquid form, primarily in mares,
and can be given with phenothiazine, piperazine mixtures or
thiabendazole. The animals should be pretreatment fasted. Carbon
disulfide is toxic and particularly so in young animals.
Dithiazanine iodide plus piperazine is effective against adults
and immature pinworms. The trichlorfon plus phenothiazine plus
piperazine combination is effective against the adult but less
effective against the immature pinworm. Piperazine or thiabendazole
is effective against the adult but not against the immature pinworm.
Carbon disulfide is used in the treatment of infections of the
large stomach worms (Habronema and Draschia). Since these worms
depend on house and stable flies for transmission a fly control
program is of value in controlling the worm.
The minute stomach worm (Trichostrongylus axei) is controlled
by treatments including phenothiazine or thiabendazole.
Treatment specifically for tapeworms is usually not required
since large numbers of these worms are not ordinarily found in
the horse. Di-phenthane-70- (Teniatol) is effective when specific
treatment is required.
Treat suckling and weanling foals with piperazine-carbon disulfide
or other piperazine containing combination at 8 week intervals start-
ing at 8 weeks of age for ascarid control. Thiabendazole may be
included with piperazine for strongyloides control.
Treat all yearlings and older horses on a regular 6 8 week
schedule alternating use of the following compounds:
Piperazine-carbon disulfide plus phenothiazine
Trichlorfon plus piperazine plus phenothiazine
Other piperazine + phenothaizine mixtures
Alternating the use of these compounds is necessary for maximum
activity against all the major parasites of the horse and also reduces
the possibility of selecting or promoting drug resistant strains of
1. Drudge, J.H. The use of anthelmintics for parasite control in
the horse. Vet. Med. 60, (March 1965): 243-247.
2. Drudge, J.H. Control of internal parasites of the horse. J. Am,
O*t.Med. Assoc. 148, (Feb. 1966): 378-383.