By Billie
Zeller-Barrett, DVM
When
it comes to deworming many horses owners believe more is better. For many
years, horse owners have used the standard 4-6 weeks deworming protocol with an
anthelmintic (dewormer) of choice. Most horse owners rotate anthelmintics while
others find what seems to work and stick with it. Equine scientists have
recently been cautioned that these current deworming strategies may cause
parasite resistance for our horses.
Over
the last 40 years, researchers have witnessed parasite resistance to
anthelmintics in sheep, goats, and cattle. It is only natural to assume that
the same would happen in horses. There have been many studies documenting drug
resistance on equine farms with a history of frequent deworming, especially
when using the same class of dewormer, rather than rotating. Because there are
no new anthelmintic drugs on the horizon preventing parasite drug resistance
must be a priority for veterinarians and horse owners.
There
is not a universal or “cookbook” deworming protocol available since parasite
prevalence depends on so many factors (geographic location, pasture exposure
and rotation, individual horse immunity, etc.). There are several suggested
deworming protocols that have recently been made. You as a horse owner should
educate yourself on the various protocols and work with your veterinarian to
determine what works best for your farm.
A
valuable tool your veterinarian can use when planning a deworming strategy for
your horse farm is a fecal egg count. Fecal egg counts can be used to identify
what types of worms infest your horse, the shedding status of your horse and
with repeated use can help identify anthelmintic resistance. Initially, a fecal
egg count should be performed on all horses on the premises. If you have a high
number of horses you can sample horses from different categories (weanlings,
yearlings, 2 year olds, adult horses, broodmares, geriatric, etc).
When
the fecal egg count results are available, your veterinarian can classify your
horses into 1 of 3 shedding categories; low, medium and high. The cut off
number for these categories is not universally agreed upon but are similar.
Horses that fall in the low shedding category will have less than 250 Eggs Per
Gram, medium shedders 250-500 EPG and high shedders will have greater than 500
EPG. High shedders are thought to make up about 35% of the horse population but
are responsible for approximately 85% of the pasture burden. For this reason
high shedders should be dewormed more frequently.
When
your veterinarian is using fecal egg counts, for the purpose of determining
drug resistance, it is necessary to do an initial fecal to get an egg count,
administer the anthelmintic and then do a repeat fecal egg count 10-14 days
later. If the drug is still effective against the parasite you should see at
least a 90% reduction in the egg count.
The
most practical approach to deworming is a quarterly rotation program
recommended by researchers at Texas Tech University. This quarterly rotation
protocol includes different anthelmintics targeting adult parasites, larval
parasites, tapeworms and bots. It is important to base your deworming strategy
on the range of parasites that may possibly infest your horse. It is important
to consider the season or time of year when choosing the anthelmintic each
quarter. There are 4 major categories of anthelmintics that we rotate through;
Praziquantel, macrocyclic lactones (moxidectin, ivermectin), pyrantel salts,
and benzimadozoles (fenbendazole, oxybendazole). Each of these categories has a
slightly different target and should be used at different times of year.
Praziquantel is primarily used to control tapeworms (Anoplocephala) and should
typically be administered in the Fall. Macrocyclic lactones (Ivermectin and
Moxidectin) are used to control large strongyles and bots (Gasterophilus) and
should be administered in the spring and fall. The Panacur Power pak, which is
a benzimidazole, should be saved to treat encysted cyathestomes (redworms) and
should only be administered when the fecal egg count is above 500 EPG. This
rotational program was developed by taking the egg reappearance period of each
drug into consideration.
Each
anthelmintic is effective for a specific amount of time. The egg reappearance
period is the amount of time it takes the parasite eggs to reappear in the
feces after the anthelmintic is administered. The egg reappearance period for
each commonly used drug is as follows: Moxidectin (Quest) 12 weeks, Ivermectin
8weeks, Pyrantel salts 4-6 weeks, Benzimadozoles 4 weeks. For high shedders the
quarterly rotation may not be enough. These horses may need a 6 time a year rotation,
still waiting the appropriate egg reappearance period.
Good
management practices should be combined with any deworming schedule. Management
practices focus on decreasing the worm population in the environment. These
practices include removing manure from grazing areas weekly, rotating pastures,
planting a winter annual grass such as rye or wheat grass, rotating different
livestock within your horse pastures, and holding newcomers in a quarantine
area. A common practice of spreading or disking manure in pastures may need to
be reconsidered. The current recommendation is to only disc pastures or spread
manure in the summer when the temperature is above 85 degrees and the horses
can be held off of the pasture for at least a week. When the parasite eggs are
exposed to this high of a temperature for 3 consecutive days they will die.
Unlike the various deworming protocols, these recommended management practices
are agreed upon universally.
The
rotational strategy is just one of several strategies that can be utilized when
deworming your horses. Using a rotating schedule and waiting the appropriate
egg reappearance period or longer (for low shedder) can help preserve the
anthelmintics we currently have available. There are several factors that
should be evaluated when deciding on a deworming protocol that fits your farm.
The number of horses and frequency of horses entering and leaving your farm,
the amount of pasture exposure, genetics and the age of horses you are treating
are all factors that should be considered when choosing a deworming protocol.
Working with your veterinarian to discuss the best possible option for your
individual situation is the best way to ensure preservation of our current
anthelmintic effectiveness.
Contact
Surgi-Care Center for Horses at 813-643-7177 or
email lkuebelbeck@surgi-carecenter.com
with any questions regarding this topic.
This article was originally published in Horse & Pony magazine and is reprinted with their permission.