This information is supplied by Richard Mumm, of Enterprise, Oregon. He did quite a bit of research for this article which first appeared as a post on the Wildhorses mailing list. He is not a veterinarian, but this is excellent information you can use in association with your vet's advice.

Quest is borderline dangerous to give a horse, in my opinion. I've spent 
a fair amount of time on the internet searching out info on horse 
de-worming medicines, and Quest (Quest is the brand/trade name, the 
chemical name is moxidectin) is the least safe of any of the commercial 
worm medicines currently in popular use. There are reports of it killing 
horses who were in marginal condition, undernourished and/or a little 
weak, also foals. 

Quest (moxidectin) is expected to cause physiological problems in a 
healthy horse at 5 times the effective dose. That is, if you give a 
healthy horse 5 times the dose recommended by the manufacturer, according 
to the horse's weight, you could expect to see problems (neurological 
problems would be most likely, with Quest/moxidectin), or have the horse 
die.

By contrast, ivermectin (brand names Zimecterin, Eqvalan, Rotectin 1, 
Equimectrin, etc.) is not expected to cause problems until 60 times the 
effective dose is given.

Oxibendazole (brand/trade name: Anthelcide EQ) is not expected to cause 
problems until 60 times the effective dose is given.

Fenbendazole (brand names Panacur, Safe-Guard) may cause problems at over 
100-200 times the effective dose, although apparently when testing for 
side-effects from fenbendazole, the researchers had difficulty producing 
any problems at all, at any amount of overdose.

Pyrantel pamoate (brand names Strongid P, Strongid T, Rotectin 2) is safe 
for a healthy horse at up to 20 times the effective dose.

Quest's claims to fame are that it will kill bot fly larvae, encysted 
small strongyle larvae, and that it only needs to be given every 84 days. 
Fenbendazole, given at double dose according to the horse's weight for 
five days in a row, will also kill encysted small strongyle larvae, and 
is *much* safer for the horse. As for bots, ivermectin does a great job, 
and is also *much* safer than Quest/moxidectin.  (also, Quest/moxidectin 
only eliminates about 15-25% of encysted small strongyles, where 
fenbendazole eliminates *all* encysted small strongyle larvae - see 
"*Update 8/7/00" at bottom of page, below, and the notes from a lecture 
by Dr. David Laird, also below, for more information)

I have had readers make the comment that it would be difficult to 
mistakenly give a horse 5 times the recommended dose of Quest/moxidectin. 
 That is not the point.  The point is that a worm medicine that is likely 
to cause obvious metabolic/physiological problems at 5 times overdose is 
likely much more toxic to the horse than a worm medicine that may cause 
obvious metabolic/physiological problems at 20, 60 or 100-200 times 
overdose, and is much more likely to cause subtle metabolic/physiological 
problems that may not be immediately apparent to a human observer at the 
recommended dose than another worm medicine that may cause obvious 
metabolic/physiological problems at 20, 60 or 100-200 times overdose.  It 
is a matter of relative toxicity.

Also, when worming foals it would be fairly easy to give several times 
overdose of Quest/moxidectin, and there isn't much room for error.

In spite of the large amount of worm medicine given to the horse during 
the double-dose-for-five-days-in-a-row treatment with fenbendazole, 
experience and research has determined this procedure to be entirely safe 
for the horse, and is the only known way to eliminate *all* encysted 
small strongyle larvae.

Larval cyathostomiasis (or small strongyle infection) is a distinct 
clinical syndrome that occurs with the mass emergence of encysted larvae 
from the intestinal wall. This release happens more in winter and spring. 
Small strongyles are less damaging than large strongyles since the larval 
migration is limited to the intestinal lining. However, the damage 
impairs normal gut motility, leading to signs of colic, colic being the 
most common cause of death in horses.

I believe a good, safe worming program will incorporate fenbendazole or 
oxibendazole, ivermectin and pyrantel pamoate on a rotation basis.

Fenbendazole (or oxibendazole) in fall and again in spring, followed each 
time, at appropriate intervals, by pyrantel pamoate paste, and then 
ivermectin (or followed first by ivermectin, then pyrantel pamoate), is a 
good rotation program.

The frequency of giving your horses worming medicine depends on how 
frequent their contact with other horses they're not pastured with is, 
and how good their pasture management is.  Horses with poor pasture 
management, and/or frequent contact with horses they're not pastured with 
should be given worm medicine on a rotational basis every 6-8 weeks.  
Horses with good pasture management and little or no contact with horses 
they're not pastured with may get by with being given worm medicine every 
3 or 4 months on a rotational basis. [Note: This assumes you worm all 
horses pastured together at the same time.]

Also, some people swear by the daily wormer pyrantel tartrate (brand 
names Strongid C, Strongid C 2x, Equi-Aid CW and Equi-Aid CW 2x). Some of 
the endurance riders say that this wormer can make colic more likely.  
However, most endurance riders ride Arabian horses, which are said to be 
a little more prone to colic anyway, and most of us are not riding our 
horses 50-100 miles in one day, as the endurance riders do.

The daily wormer would need to be supplemented with, at least, ivermectin 
(in fall, after a good freeze), as pyrantel tartrate daily wormer doesn't 
kill bots.  A twice yearly course of fenbendazole, double dose according 
to the horse's weight for 5 days in a row, is needed to eliminate 
encysted small strongyle larvae.  A twice yearly treatment with pyrantel 
pamoate, double dose according to horse's weight for two days in a row, 
*or* 10 times the normal daily dose of pyrantel tartrate daily worm 
medicine for two days in a row, will be necessary to eliminate tapeworms.

Equi-Aid CW and Strongid C are identical products marketed by two 
different manufacturers - the only difference is that Strongid C costs a 
little bit more, and the company that manufactures and markets Strongid C 
(Pfizer) offers a guarantee that if your horse colics while on Strongid C 
(for *any* reason) they will contribute $5000 toward any resulting 
medical bills.  You have to live up to certain requirements in order to 
qualify for the guarantee, such as certain immunizations have to be done 
on a regular schedule, and you have to use a "purge" dewormer, such as 
fenbendazole, oxibendazole, ivermectin or pyrantel pamoate at least twice 
a year. Your vet should have details on how to sign up for the program.

Please note that if you give a horse that has a lot of worms an effective 
purge type worm medicine (any commercially available worm medicine other 
than the daily wormer), so many worms can die at once that it can clog up 
the horses veins, heart, lungs and other parts of the body (what part of 
the body depending on the type of worm) with dead worms and cause the 
horse a lot of suffering, maybe even kill it.

So, horses that are undernourished or weak for some reason, or that are 
suspected to have a lot of worms, should be fed pyrantel tartrate (daily 
wormer) for 2-4 weeks, then the horse should be wormed with ivermectin, 
dosage according to horse's weight, then *6-8 weeks later*, wormed with 
fenbendazole (double dose according to the horse's weight for 5 days in a 
row), and then continue with a regular worm medicine rotation program, 
such as described above.

The pyrantel tartrate daily wormer, according to one of the vets at the 
clinic where I have my horses taken care of, does not have to be 
discontinued to administer oxibendazole, fenbendazole, ivermectin or 
pyrantel pamoate.  He did say that it is probably a good idea to not 
administer Quest/moxidectin while feeding horses the pyrantel tartrate 
daily wormer medicine.  I think it's a good idea to not administer 
Quest/moxidectin at all, ever.


Here's the bottom line:

ivermectin - kills everything except encysted small strongyle larvae & 
tapeworms.

pyrantel (pamoate, tartrate) - kills everything except bots & encysted 
small strongyle larvae (double dose of pyrantel pamoate paste for two 
days in a row, dosage according to horse's weight, is required to 
eliminate tapeworms.  alternatively, giving 10 times the normal dose of 
pyrantel tartrate daily worm medicine for two days in a row will also 
effectively eliminate tapeworms - see "*Update 8/7/00" at bottom of page, 
below).

oxibendazole - wide spectrum, safe and effective dewormer. could not 
determine if oxibendazole has any effect on encysted small strongyle 
larvae or tapeworms. does not kill bots.

fenbendazole - kills everything except bots (double dose according to 
horse's weight for 5 days in a row required to eliminate encysted small 
strongyle larvae & tapeworms). The least expensive way to use 
fenbendazole probably is to purchase Panacur granules, which are 
available through:

 KV Vet Supply
 phone # 1-800-423-8211
 website address: http://www.kvvet.com

KV Vet Supply's most recent price was $1.89 per packet (or $1.69 per 
packet if you buy 10 or more packets at a time). One packet will worm 500 
lbs of horse, one time - single dose. you can mix it right in with your 
horses' feed. you might want to add a couple of tablespoons full of 
molasses to cover the taste.


                     PROBIOTICS IN CONJUNCTION WITH WORMERS



It's a really good idea to give the horse probiotics, especially a day or 
two after you give them worm medicine.  The worm medicines we give our 
horses may kill or upset the horse's natural, beneficial intestinal 
bacteria, and it needs to be replenished somehow.

The probiotics are beneficial intestinal bacteria which are natural to 
the horses instestines.  These can get killed off by worm medicines, 
stressful conditions, etc., and when given to the horse by mouth or added 
to the horses feed will multiply in the horses gut, helping them digest 
their food, and their adequate presence in the gut makes no room for 
infective bacteria.

Among ingredients to look for in whatever probiotic product you select 
may be the following (the more different kinds, the better):

 lactobacillus acidophilus
 streptococcus faecium
 aspergillus oryzae
 bacillus subtilis
 streptomyces
 sacchromyces cerevisiae

The product labeling should indicate that the bacteria are *live*, and 
may say something about how many million colony forming units are 
included per unit volume, unit weight, or per dose.

Most veterinarians and lots of feed & seed stores carry paste type 
probiotics. The paste type probiotics come in a tube-syringe you can use 
to either squirt it into the horse's mouth or you can mix it in with 
their feed.  The paste type probiotics should be refrigerated at the 
vet's or feed & seed store when you buy them and the pull date should not 
be expired.  Also, they should be refrigerated at your home until used up.

Dehydrated probiotics are another good way to replenish the horse's 
beneficial gut bacteria.  It is really easy to add a scoop of dehydrated 
probiotics to the horse's feed every day, and is very good for the horse.

A highly recommended dehydrated probiotic is "Fastrack Probiotic Pack", 
available from:

 Valley Vet Supply
 phone # 1-800-356-1005
 website address: http://www.valleyvet.com

Another good dehydrated probiotic is "Opt-E-Horse", available from:

 KV Vet Supply
 phone # 1-800-423-8211
 website address: http://www.kvvet.com
 (They also carry the Fastrack Probiotic Pack dehydrated probiotic.)

Another source for Opt-E-Horse is:

 Jeffers Equine
 call 1-800-533-3377 for a free catalog.

Another supplement of this type is "Ration Plus", highly recommended by 
John Lyons & crew. This supplement has no live beneficial bacteria in it, 
but "encourages the growth of the beneficial bacteria, allowing the 
normal (beneficial) variety to repopulate the intestines" (p. 122, 
"Veterinary Care For The Perfect Horse" by John Lyons, Eleanor Kellon 
V.M.D., & the
editors of John Lyons' Perfect Horse magazine).

Ration Plus can be obtained by calling 1-800-728-4667, or accessing the 
Ration Plus website at: http://www.rationplus.com

Ration Plus is also available from Jeffers Equine, info listed above.

It could well be that the combination of a probiotic product and Ration 
Plus would create an ideal environment for beneficial bacteria in the 
horses' intestines.

Here are some web site addresses for those interested in doing some research on horse parasites on their own:

Additional information on mimizing the parasite burden of your horses:

(the following was adapted from an article on Equine Internal Parasites 
by the University of Nebraska)

Control and Treatment of Internal Parasites of the Horse:

Horse owners need to understand that an internal parasite control program 
is a continual battle. Management practices include:

        1.Feeding hay in bunks or mangers; avoiding feeding on the 
ground. 
        2.Regular cleaning of stables and paddocks. 
        3.Avoiding overcrowding of pastures. 
        4.Avoiding wet pastures--rain, dew, flooding--especially with 
young animals. 
        5.Not spreading manure where horses can come in contact with it. 
        6.Working dirt lots with a harrow, springtooth harrow, or disk to 
bury manure and destroy weed growth. 
        7.Periodical grazing of cattle in horse pastures decreases 
exposure as equine parasites do not mature in cattle and
breaks the life cycle. 

 Treatment programs consist of: 

        1.Proper drugs at the proper dosage and proper time. 
        2.Deworming all animals in the group or lot. 
        3.Deworming pregnant mares within 30 days of foaling to minimize 
passage to foal. 
        4.Keeping lactating mares and their foals on a rigid treatment 
schedule (every 6 to 8 weeks). 
        5.Rotating anthelmintic classes of drugs to prevent development 
of parasite resistance.

Resistance generally develops with repeated usage over time. 

Number of horses, space allotted per horse, age of horses, contact 
between horses, type of feed (i.e. pasture or dry lot) all have a bearing 
on frequency of deworming.  Your veterinarian should evaluate each 
individual circumstance and determine the proper drug and frequency 
needed. Veterinarians can also provide fecal examinations 7 to 10 days 
post treatment to evaluate efficacy of a control program.

Not only should a deworming program include using an effective dewormer, 
but pasture maintenance is also important.




 (the following information was adapted from an article on worming horses 
by Anne Loiselle)

Despite modern science's best efforts, horsemen have yet to eliminate 
internal parasites, better known as worms, which feed off horses.  
However, today's deworming practices can help prevent parasites from 
robbing horses of the good health.

The most common means of killing parasites is by giving horses a 
deworming drug, which comes in a liquid, paste or pelleted form.  Liquids 
and pastes are usually given every six to eight weeks, while most pellets 
are fed daily.

The number of worms can be kept to a minimum by a combination of 
deworming consistently with effective products, rotating dewormers and 
effective manure removal.

Probably when most people think of parasitism, they think of unthrifty 
horses:  You feed them but they don't look good.  Additionally, a poor 
hair coat may be an outward sign of damage from a heavy worm infestation.

Internal parasites interfere with the horse's ability to use nutrients 
and may damage the digestive tract.  Increasing the quality and amount of 
feed a horse gets won't compensate for a heavy worm infestation.

The damage parasites can cause ranges from colic (the most common cause 
of death in horses), diarrhea and stomach ulceration, to fever, anemia 
and summer sores.

Mindful of these potential effects, the rule of a good deworming program 
is to decrease contamination among horses while using a minimum of drugs.

However, not all dewormers are effective against all types of worms. 
That's where reading the label on the dewormer comes into play, as well 
as consulting your veterinarian.

The label lists the active ingredient that actually kills the worms, as 
well as the types of worms it is effective against.  Restrictions, such 
as whether the product is safe for pregnant mares, should also be checked.

Go into the summer with your horses dealing with as few worms as 
possible. One way to accomplish this is by spreading manure on the fields 
and dragging them with a harrow to break up the manure, exposing more 
worms to the elements to kill the parasites.

Worms are a problem that must be dealt with regardless of where the horse 
lives. It is the temperature extremes that kill them:  The cold winters 
kill worms in the Northern states and the summer heat does the same job 
in the Southern climates.  Dewormers are most effective when used at the 
peak season of infestation, which differs according to the geographic 
area.

Removing manure from pastures is more effective than spreading and 
harrowing the manure, . However, pasture vacuuming, as it is called, is 
also far more expensive and, therefore, not as common as spreading.

Pasture rotation is another common means of controlling worms.

What you're basically hoping for is that the parasites will die while the 
horses are off the field. If you don't put any stock on the field, you're 
not putting more eggs on the pasture.

Ideally you would be able to rotate your horses through several different 
pastures, leaving a pasture idle for several weeks or months.  Another 
option in the fall is to graze the horses on nearby farmland after the 
crops have been harvested.

Although resistance almost guarantees that humans and horses will always 
have to deal with parasites, sound management practices such as manure 
removal and pasture rotation, combined with good dewormers on the right 
schedule, will prevent worms from stealing your horses' health.




(the following information was generously provided by Laura Phelps-Bell 
from personal notes taken during a lecture by Dr David Laird.)

According to Dr. David Laird, the vet who is an expert on encysted small 
strongyles, Quest (moxidectin) in reality will only kill about 15% of the 
encysted strongyles, whereas Panacur (fenbendazole) will kill a much 
higher amount than that and is also a lot safer then Quest/moxidectin 
(note:  Panacur and Safeguard are identical products with different trade 
names - the chemical name for both Panacur and Safeguard is 
"fenbendazole").

Here's some notes from a lecture by Dr. Laird:

Dr. David Laird of Texas is nationally recognized for his expertise on 
internal parasite control and most recently for his work on encysted 
larvicidal treatments for cyathostomiasis, a condition that leads to poor 
performance and colic in horses.

Dr. Laird devoted most of his discussion to the life-cycle of the small 
strongyle worm. When your horse goes out to graze in the pasture or picks 
around in his stall or paddock, he is ingesting thousands and thousands 
of larvae that can stay alive on the ground for up to a year.  The little 
larvae move along your horses digestive tract until it comes to his cecum 
and colon. Then it penetrates the wall of the cecum or colon and stays 
there for a minimum of 45-60 days and as long as 2 to 3 years.  
Impossible, you think.  You worm religiously every 8 weeks, right?  There 
is no way those larvae can live in there if you worm every 8 weeks. 
Besides, there are plenty of wormers that kill small strongyles.  Heck, 
they ALL kill small strongyles.  Right?

Right.....However they only kill the ADULT worms, or larvae that have not 
yet burrowed into the lining of the cecum and colon of your horse.  These 
larvae are known as encysted larvae, and Dr. Laird likened them to a 
hibernating bear.  He explained that they have a very, very slow 
metabolism.  When you worm your horse, that wormer is in your horse's gut 
for about 18 hours.  Because the encysted larvae have a very slow 
metabolism, the wormer simply doesn't do the job over an 18 hour period.  
It doesn't effect the little guys. So, the encysted larvae sit there 
making waste in the lining of your horse's gut, and when they finally 
decide to emerge into your horses stomach, they leave behind all this 
cellular debris, and this is when
your horse can get sick.  This condition is known as Cyathostomiasis 
(small stronyle infection).  Symptoms can include:

        Cow manure-like diarrhea 
        Mild reoccuring colic (2-3 days) 
        Listless, weak 
        Rapid and dramatic weight loss 
        Peripheral edema (swollen legs) 
        May or may not be eating

So you think, I'm a smart and experienced horse owner.  I know when to do 
a fecal egg count. Well, here's the interesting part of trying to 
diagnose this condition.  If you worm your horse every 8 weeks, your 
fecal egg count will very likely come up a big zero -- but your horse can 
still have hundreds of thousands of encysted larvae.  Okay, you say you 
use a daily wormer, such as Strongid C.  Couldn't have "encysted larvae". 
 Right?  Wrong!  Okay you used the ivermectin, do the daily wormer every 
day, except when you were at the show last month, you forgot to bring it 
with you, but that was only 2 crummy days.  Guess what? your horse 
ingested thousands and thousands of larvae those two crummy days, and 
since Strongid C only kills the larvae on the way to the cecum and colon, 
once the little cuties have encysted, your daily wormer has no effect on 
them.  Besides, what about all the encysted larvae that were already 
there before you started using Strongid C?  Remember, they can live in 
your horse for 2 to 3 years. 

Treatment:  Two times the normal dosage of Panacur wormer for five 
consecutive days (note:  Panacur and Safeguard are identical products 
with different trade names - the chemical name for both Panacur and 
Safeguard is "fenbendazole").

He explained what LD-50 means. LD stands for Lethal Dose.  50 stands for 
50%. LD-50 means the dosage of medication that will kill 50% of the 
animals taking it.  Ivermectin has an LD-50 of 15.  This means that if 
you gave 10 horses 15 tubes of ivermectin wormer all at one time, it 
would be likely that 5 of those 10 horses would die. Quest, has an LD-50 
of only 3.  So, if you gave 10 horses 3 Quest wormers, 5 would probably 
die.

Well, interestingly, Panacur (fenbendazole) just doesn't kill a horse, no 
matter how much of the stuff you give it.  Therefore, that hibernating 
baby worm that has burrowed into the lining of your horse's gut gets to 
have the livin' daylights kicked out of it with a double dose of Panacur 
(fenbendazole) for 5 days and it won't hurt your horse.  But it will kill 
all of those encysted larvae and in a nutshell, if you use Panacur 
(fenbendazole) twice a year along with a regular worming program every 8 
weeks, rotating the type of wormers, you will have yourself an Optimal 
deworming program.

He stressed that a high performance horse will benefit tremendously from 
the treatment. 



*Update, 8/7/00:

According to an article by Karen Hayes (DVM, MS, an Idaho-based equine 
veterinarian) in the August, 2000 issue of Horse & Rider magazine (pp. 
88-92), tapeworms should be eliminated by giving your horse 2 times the 
normal dose of pyrantel pamoate (brand names: Strongid P, Strongid T, 
Rotectin 2), dosage according to horse's weight, for 2 days in a row, 
*or* by giving the horse 10 times the normal dose of pyrantel tartrate 
daily worm medicine (brand names: Strongid C, Strongid C 2x, Equi-Aid CW, 
Equi-Aid CW 2x) for 2 days in a row.  This should be done twice a year, 
in spring and fall, about 6 months apart.

This article also points out that fenbendazole (brand names: Panacur, 
Safeguard) is the only worm medicine that eliminates *all* stages of 
encysted cyathostomes (small strongyle larvae), when given at twice the 
regular dose, according to horse's weight, for 5 days in a row.  This 
also can/should be done twice a year, about 6 months apart, perhaps 6-8 
weeks before or after the pyrantel pamoate/tartrate tapeworm treatments.

In addition, information provided by Hoechst, the manufacturer of 
fenbendazole, indicates that encysted early EL3 larvae typically 
represent 75% of the total encysted small strongyle population in an 
infected horse, and that fenbendazole is the only worm medicine available 
that eliminates the EL3 larvae, when given at double the normal dose 
according to horse's weight for 5 days in a row.




A side note here. Burros (donkeys) are often infected by lungworms, which 
they seem to tolerate better than horses. If you bring a burro onto your 
horse farm, it is advisable that you worm the burro with ivermectin 
*before* you bring it onto the farm.