Factors affecting a horse's worm load
Unfortunately, there is no "one size fits all" deworming program. Every horse's situation is different, even for those within the same barn or the same pasture. An effective deworming program needs to consider a number of factors, including:
- Age - Age-foals and young horses are more susceptible to certain parasites
- Location - Location-certain parasites are more common in certain areas or climates
- Season - certain parasites, like bot flies are only active during certain seasons
- Travel - horses who travel to shows may be exposed to infected horses
- Pasture Load - horses grazing in a given area may increase parasite exposure
- Pasture Pals - other animals can carry parasites that may infect your horse
In addition, recent research has revealed that not all horses are infected to the same degree. In fact, we have learned that 20% of the horses in a herd are responsible for 80% of the parasite problem.
Identifying your horse's deworming needs
The best way to determine your horse's deworming needs is through having your veterinarian perform a fecal exam in which the feces is examined for the presence of worm eggs, and the eggs are counted. A fecal egg count will enable you and your veterinarian to develop a targeted deworming program.
In addition to performing egg counts prior to deworming, it is helpful to also perform an egg count 10-14 days post deworming to evaluate the effectiveness of the dewormer that was used. A fecal egg count reduction test like this, performed every 1-2 years, will help identify if there is resistance developing to a certain dewormer. If there is resistance, a different dewormer can be selected.
In general, foals and younger horses, horses under stress due to exercise or travel, and those with infections or certain metabolic diseases will have higher egg counts and need more frequent deworming.
Resistance to dewormers
In the past, it was common to deworm horses up to every two months. Many veterinary parasitologists believe this may be one reason we see resistance developing to some dewormers. A resistant parasite is one that is not killed by a specific dewormer. Because resistance to dewormers is developing in some herds, it is advised to strike a balance between deworming often enough to keep your horse healthy, but not too often, which would increase the likelihood of resistance developing.
One suggested parasite control strategy is to deworm all adult horses on the premises in the spring and in the fall with either ivermectin or moxidectin (with the addition of praziquantel). Further dewormings of adult horses with higher fecal egg counts could be carried out with different
products during the season when parasite transmission is most likely (summer in the north and winter in the south).
Since foals are much more susceptible to the effects of worm burdens, periodic dewormings throughout the year are recommended. It is also important to perform fecal egg count reduction tests in foals as well.
Other strategies to decrease worm burdens
Deworming is important, but so is decreasing the likelihood that your horse will become infected with worms. It is not secret that internal parasites are primarily transferred through manure. To that end, proper care of your paddocks, pastures, and fields is an important - but often overlooked - component of any effective worm control program. To reduce the exposure of your horse to worms:
Clean regularly - remove and dispose of manure at least twice per week Harrow pastures - during the appropriate seasons, break up manure piles to expose eggs and larvae to the elements Rotate pastures - move horses between pastures to naturally break parasitic life cycles Group horses - group horses by age to maximize deworming schedules Reduce pasture load - fewer horses per acre means reduced fecal contamination Use elevated feeders - lift grain and hay off of the ground where parasites thrive
Also known as daily deworming, continuous dewormers are in the form of a daily feed additive to constantly protect your horse against internal parasites. Most importantly, however, daily dewormers help prevent parasite re-infestation. They contain pyrantel tartrate, which attacks worm larvae before they can reproduce and migrate into body tissue and begin to damage internal organs. Many daily dewormers also help break the parasite life cycle by rendering manure inhospitable to parasite eggs. Usually, this method is combined with a twice-yearly broad-spectrum purge wormer to ensure complete parasite control. It is important, however, that your horse consumes the recommended daily wormer dose, which is easily top dressed or mixed with his daily grain ration. Missed doses decrease wormer levels in your horse's system and greatly affect the wormer's potency. A typical continuous wormer schedule includes:
- Clean regularly - remove and dispose of manure at least twice per week
- Harrow pastures - during the appropriate seasons, break up manure piles to expose eggs and larvae to the elements
- Rotate pastures - move horses between pastures to naturally break parasitic life cycles
- Group horses - group horses by age to maximize deworming schedules
- Reduce pasture load - fewer horses per acre means reduced fecal contamination
- Use elevated feeders - lift grain and hay off of the ground where parasites thrive
By: Dr Foster & Smith Educational Staff - www.drsfostersmith.com