Group Title: 2009 Florida Equine Institute proceedings
Title: Suggested worm control program for adult horses in Florida
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Title: Suggested worm control program for adult horses in Florida
Physical Description: Book
Language: English
Creator: Kaplan, Ray
Publisher: Institute of Food and Agricultural Science, University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: September 17, 2009
Copyright Date: 2009
General Note: 2009 Florida Equine Institute proceedings
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Bibliographic ID: UF00095748
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Florida Equine Institute and Allied Trade Show, September 17, 2009



Which Horses? Treat all horses regardless of FEC

Drua(s) of choice: ivermectin or moxidectin, may want to choose a product that contains
praziquantel for tapeworms

Why these drugs? Both ivermectin or moxidectin will kill bots that were acquired since the spring,
the stomach worms Habronema and Draschia that are transmitted by flies and cause summer sores, and
will sterilize Onchocerca females preventing transmission. Pinworms (Oxyuris equi) should be killed as
well, but there are increasing numbers of reports of failures to control pinworms with these drugs. These
drugs will also kill migrating large strongyles and any strongyles (both large and small) that are in the
intestinal lumen.

Is there a reason to use one over the other? Moxidectin has the advantage of also killing large
numbers of the encysted small strongyles and prevents worm eggs from reappearing in the feces for 4-8
weeks longer than ivermectin. However, at this time of the year there is little in the way of larval refugia
on pasture. Therefore some parasitologists are concerned that using moxidectin at this time of the year
may be placing more pressure on drug resistance. If FEC are performed ahead of treatment, using
moxidectin only in horses with FEC >500 or in horses known to have chronically high FEC (High
Contaminators), and using ivermectin on the remainder would be a rational decision.

Should I perform FEC? Yes on all horses. This is probably the single most important FEC to perform
all year (assuming that you follow this program and have not dewormed in the past few months). The
reason for this is that by not deworming for several months, the FEC seen will be a strong indicator of
each horse's strongyle contaminative potential. Based on this FEC you can then categorize your horses
to low (<200 EPG), moderate (200-500 EPG), and high worm egg shedders (>500 EPG). This
characteristic of individual horses has been shown to be repeatable between years.


Which horses? All those with FEC > 500 EPG on October fecal check, horses in the 200-500
range in October may be reasonably treated or can be left untreated.

Drua(s) of choice: Oxibendazole and/or pyrantel (if effective on your farm), or both together at
same time.

Why these drugs? To reduce the amount of ivermectin and moxidectin used thereby helping to slow
down the development of resistance to ivermectin and moxidectin. However, resistance has been shown
to be fairly common to oxibendazole and pyrantel so if using these drugs, before and after FEC should be
performed. Also recent data suggests that using both oxibendazole and pyrantel together at the same
time improves the effectiveness of treatment over treatment with the individual drugs.

Is there a reason to use one over the other? No unless resistance to one of these drugs is

Should I perform FEC? Yes it would be a good idea, but only on the horses that are being considered
for treatment (FEC >200 in October). If you haven't performed FECRT previously for oxibendazole
and/or pyrantel, FEC should be checked again 10-14 days after treatment.

Florida Equine Institute and Allied Trade Show, September 17, 2009


Which horses? Treat all horses regardless of FEC

Drua(s) of choice: ivermectin/praziquantel or moxidectin/praziquantel

Why these drugs? Tapeworm transmission likely peaks in autumn so treatment with praziquantel at
this time will remove all the tapes acquired over the summer and autumn. Since praziquantel is only
available in combination with ivermectin or moxidectin, it makes most sense to just use one of the
combination products. Most bot activity will have ended in northern Florida, so treatment with ivermectin
or moxidectin now will remove all the bots acquired since October and few new bots will infect horses
until next spring when the bot flies become active again (to this authors knowledge, in central and
southern FL the epidemiology of bot transmission has not been studied, so it is difficult to make other
recommendations). Lastly, any of the other worms targeted in the October treatment, that were picked
up in the intervening few months will once again be killed.

Is there a reason to use one over the other? By January small numbers of strongyle eggs have been
deposited on pasture so now there are refugia built up. Therefore the resistance issue is less of a
concern and moxidectin would be a good choice, but ivermectin would also be a rational choice. Treating
with moxidectin now has the added benefit that it provides the longest ERP, so that low egg shedders will
not need to be treated again until October, and this is also probably true for the moderate egg shedders
(who by now with this program may have fallen into the "low shedder" category).

Should I perform FEC? Yes on all horses. It is important to know if the horses with low FEC in
October still have low FEC, and if you have been successful in keeping FEC low in the horses that had
moderate and high FEC in September.


Which horses? If moxidectin was administered in January, the only horses that likely will need to
be treated are the high egg shedders. Remember, that environmental conditions are rapidly changing
such that strongyle worm transmission is nearing an end for the year. A few eggs shed by the low and
moderate shedders will not survive and accumulate on pasture. Treating the high egg shedders makes
sense since the ERP for moxidectin is about up and egg shedding from some of these horses could be

Drua(s) of choice: Oxibendazole and/or pyrantel (if effective on your farm), or both together at
same time. Though probably not needed unless tapeworms seem to be a problem, a double dose of
pyrantel that will also kill tapes might be a reasonable choice. Another alternative for this treatment
would be a 5-day double dose of fenbendazole (power-pack). Since we are only treating the high egg
shedders, we can assume that these horses also have higher levels of encysted mucosal larvae, and the
5-day double dose of fenbendazole does have good efficacy against these larvae (if not resistant).
However, resistance is known to be quite common to fenbendazole at the single dose rate, and there is
mounting evidence that the extended double-dose regimen often fails to provide high levels of control.
Furthermore, evidence suggests that using the 5-day FBZ regimen on all horses where worms already
have FBZ resistance may result in "super" resistant worms being selected. So, this regimen is best used
only in selected animals (as per the recommendations of this April treatment), so that adequate refugia
are maintained, thus maintaining the relative effectiveness of the 5-day fenbendazole treatment

Should I perform FEC? Not necessary

MAY SEPTEMBER: No treatments are needed too hot for transmission. Most worm eggs will not
survive and develop to L3, and those few that do make to L3 will rapidly die. So egg shedding is of no
concern during this time. Save money and reduce the selection for drug resistance by not treating horses
with anthelmintic during the summer months (unless there is a specific clinical need for such treatment).

Florida Equine Institute and Allied Trade Show, September 17, 2009

NOTE: It is critical that all treatment recommendations must be viewed in the proper context. Any
single treatment or non-treatment recommendation made within the context of this "evidence-based"
strategy only makes sense when horses are on this program. Recommendations based on these
principles may not apply to individual horses outside of the context of this program. Furthermore, this
treatment schedule serves only as a suggested program; additional treatments can and should be given
to particular horses if there is evidence for a clinical need for treatment. The main take home message
should be that the program suggested here, should in most cases prevent clinical problems due to
parasites from developing. But if a problem does arise, it needs to be addressed. So, if you are
presented with a thin horse showing evidence of parasitic disease in the summer, then this horse should
be treated and if the horse showing overt symptoms of intestinal parasitic disease then moxidectin
would be the treatment of choice since it is important to kill the mucosal larvae in these animals. In
recent years, every case of severe life-threatening parasitic disease (larval cyathostominosis) that I have
been involved with was due to using drugs that were ineffective due to parasite drug resistance and the
lack of a surveillance system to pick up on this. Thus horses were treated according to a given schedule
that was thought to be adequate, but since many of the treatments were ineffective, large L3 levels built
up on pasture leading to severe parasite loads and subsequent severe clinical disease.


This program is designed to specifically target bots (Gasterophilus), tapeworms (Anoplocephala
perfoliata), spirurid nematodes responsible for producing summer sores (Habronema, Draschia),
Onchocerca, pin worms (Oxyuris equi), large strongyles, cyathostomes. A few other uncommon and
lesser important nematode, arthropod, and cestode parasites likely will also be controlled by this
program, except in rare unusual circumstances when specific measures may be needed. Some horses
with naturally strong immunity to cyathostomes (demonstrated with very low FEC on each fecal exam)
will need no other treatments because the two treatments given have covered the needs of the other
parasites and these horses are naturally protected from cyathostomes. In traditional worming programs,
repeated treatment of these horses accomplishes little to nothing, other than to promote drug resistance.
Some horses in the herd will need a 3rd treatment for cyathostomes, but only a few horses (should be
less than 30% of the herd) should need a 4th treatment. Now compare this to what you are doing now.
Many farms are treating all horses 6 times each year, and likely are getting results that are significantly
less than what will be achieved on the program recommended here. When examining the program
above it is important to remember that the different drugs have differing egg reappearance periods
following treatment. It is important to know these time intervals to understand why the
recommendations are made.

If you are concerned that doing all these fecal egg counts will be too expensive, then think again. A
recent cost analysis performed by veterinary students at the University of Georgia College of Veterinary
Medicine found that the cost of deworming every horse 6 times per year is about the same or more than
treating based upon this schedule and performing the FEC as suggested. In addition, using this system
you know whether your worm control program is working. By treating blindly there is no way to tell and
we know that drug resistance is highly prevalent. Treating a horse with a drug that does not work
because of resistance is very expensive you waste the money spent on the drug and you risk failure of
your worm control program. Treating a horse that does not need to be treated wastes money and
promotes drug resistance, which will have future adverse consequences to the health of your horses.

This program is very different from what most horse owners are doing now (and vets are recommending)
and admittedly is much more complicated. The reason for this complexity is that parasite biology and the
horse-parasite-drug-environment relationship is quite complex. The commonly used rotational program
where all horses are treated on the same schedule with the same drug at frequent intervals may be easy
to manage, but it does not properly address the health needs of the horse, or the biology of the parasites
and parasite-host interactions. That program was designed more than 40 years ago when the parasites
of importance were much different than they are today, and when parasite drug resistance was not an
important concern. A good parasite control program must be based on the most up to date medical and
scientific knowledge. This necessitates a program be used that is evidence-based, not convenience-

Florida Equine Institute and Allied Trade Show, September 17, 2009

based or tradition-based. Furthermore, it should be understood by all livestock owners that any
recommended parasite control program has a finite life-span. New knowledge, spreading drug
resistance, new drugs, changes in parasite prevalence, etc, will cause recommendations to change. The
commonly used calendar-based rotational program has been used for more than 40 years it is time for
a change.


Parascaris equorum (roundworms/Ascarids) are usually the most important parasite of young horses with
cyathostomes next most important. Strongylus vulgaris has its greatest effect on young horses so this
parasite cannot be ignored, but luckily is very uncommon in managed horses. When addressing the
treatment needs of these different parasite species, the issue of drug resistance becomes central.
Therefore, it is critical to perform FECRT on all drugs used in foals, and to monitor the ERP following
effective treatment. Since there is a high prevalence of ivermectin/moxidectin resistance in P equorum,
and there is a high prevalence of BZ and pyrantel resistance in cyathostomes, in many cases it will be
necessary to use both drugs together at the same time. Treatments should begin when the foal is 2
months of age. Most treatments for P equorum, if effective should keep eggs out of the feces for 8
weeks (P. equorum has a 10 week prepatent period). But ERP for cyathostomes for BZ drugs and
pyrantel are only 4 weeks (if they are effective in the first place), and ERP for ivermectin and moxidectin
are less in young horses than in older horses. Additionally there are recent reports of very short ERP with
ivermectin -- as short as 4 to 6 weeks. Therefore, it is difficult to know what the optimal interval for
treatments should be in foals, as it will depend on a number of factors. Treating too often will waste
money and promote drug resistance, but treating too infrequently, or with the wrong drugs can result in
clinical disease problems. Thus it cannot be overemphasized that FEC surveillance in young horses is
critical, including yearly tests of drug effectiveness using FECRT, and monitoring of ERPs. No matter
what the program used for P. equorum and cyathostomes turns out to be, it is very important in foals to
be sure to include an ivermectin or moxidectin treatment at a minimum of 6-month intervals to control
any S. vulgaristhat may be around. Lastly, in young horses, one must be concerned about encysted
larval burdens of cyathostomes. Therefore, all horses between 6 months and about 2 years of age
should be administered a larvicidal treatment in the late spring or summer. Moxidectin would be my
recommendation for this treatment, but 5-day 2X fenbendazole (Power Pack) can also be used.


Drudge, J. and E. Lyons (1977). Pathology of infections with internal parasites in horses. The Blue Book,
Hoechst 267-275.
Drudge, J. H. and E. T. Lyons (1966). "Control of internal parasites of the horse." J Am Vet Med Assoc
148(4): 378-383.
Kaplan, R. M., T. R. Klei, et al. (2004). "Prevalence of anthelmintic resistant cyathostomes on horse
farms." Journal of the American Veterinary Medical Association 225(6): 903-910.
Kaplan, R. M., E. Menigo, et al. (2005). Use of pyrantel pamoate and oxibendazole in combination for the
treatment of equine strongyles American Association of Veterinary Parasitologists, 50th Annual
Meeting. Minneapolis, MN, July 16-19, 2005.
Love, S., D. Murphy, et al. (1999). "Pathogenicity of cyathostome infection." Veterinary Parasitology
85(2-3): 113-121.
Lyons, E., S. Tolliver, et al. (1999). "Historical perspective of cyathostomes: prevalence, treatment and
control programs." Veterinary Parasitology 85: 97-112.
Lyons, E. T., S. C. Tolliver, et al. (2008). "Field studies indicating reduced activity of ivermectin on small
strongyles in horses on a farm in Central Kentucky." Parasitology Research 103(1): 209-215.
Sangster, N. C. (1999). "Pharmacology of anthelmintic resistance in cyathostomes: will it occur with the
avermectin/milbemycins?" Veterinary Parasitology 85: 189-204.
Uhlinger, C. A. (1993). "Uses of fecal egg count data in equine practice." Compendium on Continuing
Education for the Practicing Veterinarian 15(5): 742-748.

Florida Equine Institute and Allied Trade Show, September 17, 2009

Van Wyk, J. A. (2001). "Refugia overlooked as perhaps the most potent factor concerning the
development of anthelmintic resistance." Onderstepoort Journal of Veterinary Research 68(1):


The Horse Magazine 12-part "Parasite Primer" Series Vol. XXI, No.1-12, JAN 04 DEC 04

Florida Equine Institute and Allied Trade Show, September 17, 2009

Figure 1: Graphs of the distribution in fecal egg counts (FEC) on horse farms

Graphs of farms 1, 2 and 3 show FEC of horses on 3 individual farms in Georgia and the 4th graph shows
FEC of all horses on 44 farms in Florida, Georgia, Kentucky, and Louisiana. Farms 1 and 2 only had adult
horses, farm 3 had only yearling horses, and the combined graph represents horses of all ages. Each
square represents the FEC of a single horse which is read on the Y axis. On the combined graph the
large numbers of data points converge to produce what looks like a solid line. Note that in each case the
distribution of FEC is virtually the same. The shape of these graphs shows the aggregated nature of
parasite infections, where a small percent of the animals harbor most of the parasites. Data displayed on
the combined farm graph reveals that horses with FEC of 500 EPG or greater accounted for 88% of total
egg output, yet made up only 31% of the population. In yearlings (Farm 3), because many have not yet
reached their immune potential, the shape of the graph is a little less steep, i.e. fewer horses have very
low FEC. This aggregated pattern of parasite distribution among animals is always seen. The only thing
that changes is the relative magnitude of the parasite level depending on management and parasite
control practices. From these graphs it is obvious that some horses need much more attention to worm
control than do others.

Distribution of FEC in Horses: Farm 1 (Adults)



0 10 20 30 40
Individual Horses

Distribution of FEC in Horses: Farm 3 (Yearlings)


g 10000
CL 4000-
^ 2000-

Distribution of FEC in Horses: Farm 2 (Adults)


O 2000

7 1000D-

0 .... ..
0 10 20 30 40
Individual Horses

Distribution of FEC in Horses:
1350 Horses on 44 Farms


40 50


0 250

500 750 1000 1250 1500
Individual Horses

10 20 30
Individual Horses



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