Beyond Deworming – PT 2


If you caught My Stable Life\’s Beyond Deworming post last week, you may have seen that reader, Valerie Smith posted some very good questions for further discussion. Those questions were as follows:

1. What dewormers do we use at what time of year? There are so many kinds out on the market right now, it can be confusing. Is there a chart? or Website?
2. Does your geographic location matter?
3. Does your weather factor into what type of dewormer you use, like if in Oct you are moderately dry – does that factor into it against someone that is very wet in the same time of year?

Today, Western Horse Review Health Editor, Susan Kauffmann helps answer some of Smith\’s queries. (For broodmare deworming considerations, check out Vet Q&A For Pregnant Mares):

“A few years ago, WHR published exactly the information you are looking for, Valerie. These recommendations were put forward by Dr. Ray Kaplan, a leading parasitologist at the University of Georgia. Here is what he had to say, from our previous article:”

Recommendations for Worm Control in Adult Horses in a Cold Northern Climate

Worm control programs are best viewed in the context of a yearly cycle starting at the time of year when worm transmission to horses changes from negligible to probable.  In the northern states with cold harsh winters, this is in the spring when temperatures begin to moderate and grass begins to grow.  Eggs of intestinal strongyles of horses do not survive and develop on pasture to any significant level during the cold winter months, although small numbers of infective L3 already on pasture in the late fall may survive into the spring.  However, L3 that survive over the winter, will die fairly rapidly once the temperatures rise in the spring.  Since worm transmission begins again in the spring, this is the time when the worm control cycle begins.

The objective of a worm control program is to optimize the health of horses — not to kill all worms.  Small numbers of small strongyle worms cause little harm and treating low level infections can actually cause more harm to the horse than not treating.  With this in mind, the goals of the worm control program laid out here are to: (1) keep FEC low thereby reducing future worm transmission, (2) control all important parasites by using the right drug in the right horse at the right time of the year, and (3) reduce the development of drug resistance so that the drugs we have now will remain effective into the future. Horses classified as high, moderate and low egg shedders will be managed differently since their treatment needs are quite different.  Please keep in mind that this suggested program is just one of many possible programs and there is room for differences of opinion among parasitologists and veterinarians. Ultimately, each farm (with veterinary guidance) should
develop its own program tailored to the needs of the farm.  Furthermore, program recommendations must be modified to fit the needs of different regions because differences in climate and management practices can have a major influence on program requirements.  The take home message must be that there is no such thing as a one size fits all program, but there are general principles that do not change.

Before a rational program can be developed, one must know which drugs work and which drugs do not.  Therefore, a prerequisite for establishing a rational program is performing a fecal egg count reduction test (FECRT).  This is rarely done but is of utmost importance.  Extremely high levels of resistance to dewormers are present in the important small strongyles of horses.

The program outlined below is for adult horses in a northern climate.  Foals will require a different program.  The first treatment of the worm control cycle should be given in April and the last treatment given in October or November.  [NOTE: In Kentucky, we do not know the precise epidemiology of strongyle transmission, but it is possible that in years with warm winters it is significantly greater than the typical situation in the northern states.  Likewise, in years with cooler summers, transmission might be significantly greater than the typical situation for southern states in the summer.  However, in most years it is likely that from June through August and again from December through February, temperatures are sufficiently high or low to prevent any significant transmission.  However, because the necessary research has not been done on the seasonal transmission in the “border” states, and weather may vary considerably from year to year, it is difficult to make precise recommendations for this region.]



Which Horses?    Treat all horses regardless of FEC

Drug(s) of choice:    ivermectin or moxidectin with or without praziquantel

Why these drugs? These drugs will kill any migrating large strongyles and any strongyles (both large and small) that are in the intestinal lumen. These drugs are also very broad spectrum so will kill a variety of other worms that might be infecting the horses.  They also have the highest efficacy against strongyles, ensuring that egg shedding will be extremely low during this critical time (mid to late spring) of the worm transmission cycle.  If the farm has a history of tapeworm problems or even periodic positive fecals for tapeworm eggs, now is a good time to give a treatment for tapes using one of the products containing praziquantel.  This will prevent shedding of tapeworm eggs at a time when the orbatid herbage mites are coming active once again.

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.  Also worth considering is the fact that late winter/early spring is the time when larval cyathostomosis is most common.  Moxidectin is the only drug that can prevent this serious but rare disease.  Therefore, any horse that has had chronic diarrhea over the winter or early spring should be treated with moxidectin.

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 innate immunity of small strongyles (strongyle contaminative potential).  Based on this FEC you can then categorize your horses to low (500 epg).  This characteristic of individual horses has been shown to be highly repeatable between seasons and years.  The exception to this is if worm control has been severely inadequate over the past year(s).  In such cases, many horses may show up as high egg shedders.  However, once a good parasite control program is instituted and monitored for effectiveness, then the FEC results can be used to profile the horses into their respective egg shedding groups.


Which horses? All those with FEC > 500 epg on the September fecal check.  Any horses that had a FEC between 150-500 epg in April should have a FEC performed and then only treated if epg >100.  Any horses that had a FEC of 500 epg) and low ( 500 epg in April, any horses that had FEC > 150 epg in June, and any additional horses with FEC > 150 epg now.

Drug(s) of choice:    ivermectin or moxidectin

Why these drugs? These drugs will kill bots that were acquired since the spring, kill the stomach worms Habronema and Draschia that are transmitted by flies and cause summer sores, sterilize Onchocerca females preventing transmission, and kill pinworms (Oxyuris equi). 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? By July, small to moderate numbers of strongyle eggs have been deposited on pasture by the low egg shedders that have not been treated since April, so now there are refugia built up.  Therefore the resistance issue is less of a concern and moxidectin would be a reasonable choice as would ivermectin. Again, using moxidectin in horses classified as high egg shedders and ivermectin in the other horses would be a good option, but either drug could be used in all horses.

Should I perform FEC?    Yes – on all horses.  It is important to know how well your program is working – it is important to keep egg shedding down to low levels at this point of the summer.


Which horses?  Based on FEC.  Only treat horses with FEC >150 epg unless the horse is known to be a High Contaminator.

Drug(s) of choice: Different drugs can be used depending on the horse and what has been used previously.  Moxidectin would be a good choice for all horses that are High Contaminators (or consistently have had the highest FEC), but only if moxidectin was not already administered in July.  Oxibendazole and/or pyrantel can be used on the horses that have FEC >150 but have not shown high FEC through the year.

Is there a reason to use one over the other? Horses with chronically high FEC likely also will have large numbers of encysted larval cyathostomes and moxidectin has the greatest efficacy against these encysted stages.  Oxibendazole and/or pyrantel for all the same reasons these drugs were recommended for the June treatment.

Should I perform FEC?    Perform FEC on all horses with >150 epg on the July fecal exam.  Any horse with FEC 150 epg.

Drug(s) of choice:    Will depend largely on which drugs were used for previous treatments.

Should I perform FEC?    Yes – on all horses that were not treated in September, or were treated in September with oxibendazole and/or pyrantel.


Which horses?  Treat all horses (regardless of FEC)

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

Why these drugs?  Tapeworm transmission peaks in late summer and autumn so treatment with a drug that kills tapeworms at this time will remove all the tapes acquired over the summer and autumn.  Praziquantel is the most effective drug for tapeworms and is only available in combination with ivermectin or moxidectin.  Another good reason to choose one of these combination products is that bot transmission will have ended with the onset of cold weather (hard freezes) so treatment with ivermectin or moxidectin now will remove all the bots acquired since the July treatment and no new bots will infect horses until next spring when the bot flies become active again.  Also any of the other worms targeted in the July treatment that were picked up in the intervening few months will once again be killed.  One of the pyrantel products labeled for tapeworms could also be used together with ivermectin or moxidectin.  Using ivermectin will prevent significant egg shedding until January and using moxidectin until February.  Because no worm transmission will be occurring in the winter months, treatments can stop until the April 01 treatment.

Is there a reason to use one over the other?    As per previous discussion .

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


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 (Strongylus spp), and small 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.  Treatments in April and November with ivermectin or moxidectin (+/- praziquantel) should control all of the worms listed above for the entire year (with the exception of cyathostomes, which will require additional treatments in some horses).  Some horses with naturally strong immunity to cyathostomes (demonstrated with very low FEC on each fecal exam) will need no other treatments because the spring and fall treatments 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.  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 or 5th treatment and only 5% or less should need more than this.  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.


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