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Parasite resistance occurs in two forms and is now well recognized. This changes how we should deworm our horses and requires a new and smarter approach. Adult horses are resistant to parasites at variable levels, in addition to parasites developing resistance to all dewormers currently in use at variable levels. The best way to utilize this new information is to deworm the right horse at the right time with the right product at the right dosage.

Resistance by horses to parasites occurs. Young horses are susceptible to all parasites. Adult horses can be grouped into three categories by how easily infected they are, and by how many eggs are then produced (shed) by the female worm. High shedders are the most susceptible to parasitic infestation, followed by medium shedders, followed by low shedding horses being the most resistant to parasitic infestation. Fecal egg counts determine which group to place a horse in. These counts are taken after the last deworming has “worn off.” Low shedders should only be dewormed twice a year; the others with increasing frequency.

Resistance of parasites to the deworming medication also occurs. This is on the rise now and no “new” dewormers are on the horizon. Somewhat like antibiotic resistance, dewormer resistance is due to overuse of dewormers, under dosing and incorrect timing. Using fecal egg counts taken at the proper time, we can accurately determine the best way to deworm each horse. Ultimately this will improve the effectiveness of current deworming medications, eliminate unnecessary treatments, reduce costs and the negative impact on the environment. Over time, an appropriate program will reduce contamination of pastures and re-exposure of our horses. Initially the improved program will require some effort but it will soon become quite routine.

There are additional factors in controlling equine parasites. In northern climates, temperatures below 45 degrees F do not support the hatching of eggs or larval development. Therefore, winters here are relatively free from exposure to infective strongyle larvae. High shedders have worse infestation of parasites AND they serve to contaminate pastures much more significantly. Pastures may be dragged in the summer or rotated with resting periods to reduce pasture contamination. Horses that have little exposure to pastures (stalled with dry lot paddocks) will need less deworming on average.

However, there are always exceptions and additions to the rules. Each new horse should be dewormed and/or tested with a quantitative fecal egg count before putting them out on pasture. Foals do not have natural resistance to parasites and benefit from a regular monthly deworming program. Moxidectin (Quest) can not be used on horses until they are at least 6 months of age. Yearlings are also not mature enough to resist parasites like adults, so deworming every other month is appropriate. After about 18 months of age, young horses can go on the same program as adults. Debilitated, sick, heavily parasitized horses or those with chronic diarrhea will require special management in a deworming program.

The laboratory gives us information from the fecal tests. Fecal egg counts (FECs) are tests that are done to determine the level of infestation on a per gram of feces basis. Horses are now being divided into LOW (0-200 eggs/ gram manure), MEDIUM (201-500 eggs/ gram manure) and HIGH (over 500 eggs/ gram manure) shedders. The fecal egg count reduction (FECR) is used at 14 days after deworming to demonstrate how effective the treatment was. This is used to determine the resistance of the parasites to the deworming product used. Egg reappearance period (ERP) is used to determine how resistant the horse is to parasites. It is used after the dewormer has worn off so we see the natural resistance. The ERP varies from product to product. The ERP ranges or when a certain dewormer “wears off” for some are:

DEWORMER EGG REAPPEARANCE PERIOD (ERP)
MOXIDECTIN (QUEST) 12 WEEKS
IVERMECTIN (ZIMECTRIN) 8 WEEKS
PYRANTEL (STRONGID) 6 WEEKS
FENBENDAZOLE (PANACUR) 4 WEEKS

The first fecal sample should be collected at the end of the ERP of the last dewormer administered. Fecal samples should be fresh (same day), consist of 2 fecal balls, and legibly labeled with horse’s name, date last dewormed and product used. The ERP can be checked to make sure the timing is correct. Call our office with the date and product if you have a question. The sample should be refrigerated if it isn’t submitted the same day.

We are using the following deworming schedules for normal adults:

Low Shedders
April – MOXIDECTIN (not to be used in horses less than 6 months in age, small ponies, or minis)
November – MOXIDECTIN + PRAZIQUANTEL COMBINATION (Quest Plus) or
IVERMECTIN + PRAZIQUANTEL COMBINATION (Zimectrin Gold, Combo Care Plus) can be substituted

Medium Shedders
April – MOXIDECTIN (not to be used in horses less than 6 months in age, small ponies, or minis)
July – IVERMECTIN
September – PYRANTEL PAMOATE
November – MOXIDECTIN + PRAZIQUANTEL COMBINATION (Quest Plus) or
IVERMECTIN + PRAZIQUANTEL COMBINATION (Zimectrin Gold, Combo Care Plus) can be substituted

High Shedders
February – IVERMECTIN
April – MOXIDECTIN (not to be used in horses less than 6 months in age, small ponies, or minis)
June – IVERMECTIN
August – FENBENDAZOLE
September – PYRANTEL PAMOATE
November – MOXIDECTIN + PRAZIQUANTEL COMBINATION (Quest Plus) or
IVERMECTIN + PRAZIQUANTEL COMBINATION (Zimectrin Gold, Combo Care Plus) can be substituted

This program overlaps logically with a manageable rotation schedule for horses in different groups in the same barn. Fecal egg counts will be taken throughout the year. How often these need to be performed is determined by what category of shedder your horse is. For example, the low and medium shedding horses will just need one additional fecal egg count in July to ensure that they are in the right category and that the program is working for them. Once your horse is determined to be in a certain category and is set on a deworming schedule, subsequent years will contain minimal (if any) fecal egg counts.

Frequently Asked Questions
Q. Why do the extra deworming on high shedders?
A. It appears that this group comprises only 20% of the horse population but 80% of the parasite infestations on pastures.

Q. My horse has been on a daily dewormer. Should I stay on that program?
A. A fecal egg test could be used to help decide, but a rotation such as outlined above is probably better. Horses with recurrent colic, diarrhea or weight loss may need to stay on the daily dewormer.

Q. Is there a need for Panacur Power Pac in the new program?
A. Yes. New horses, or horses with high FEC could benefit from a Power Pac treatment.

Q. Do all dewormers treat all parasites?
A. No. For example, pyrantel or praziquantel treat tapeworms (other products do not.) Fenbendazole in a single dose does not treat migrating parasites, but fenbendazole 10x (Power Pac) does. Moxidectin and ivermectin treat bots (other products do not) and some migrating larval parasites.

Q. Isn’t this going to be expensive?
A. Once in place, this program could reduce deworming costs by as much as 78%.