There’s a lot for horse owners to know when deciding when to deworm and how often to do so.
They need to know how their horses pick up parasites and what resistance means. They must learn how to rotate through families of dewormers by reading the active ingredients. They must know when to not deworm and instead wait until higher levels of parasites are seen.
The best measurement these days is a fecal egg count, which can be performed by a veterinary technician skilled at doing them. They set the base line level for an individual horse.
Owners should bring in a couple fresh fecal balls in a sealed bag labelled and kept cool.
Many clinics use the McMaster fecal test, which is repeatable, uses a finite amount of manure and can be performed in five to 10 minutes at most veterinary clinics.
Another test called the modified Wisconsin will also detect tapeworms.
There is also another test for lungworms if owners suspect an infection.
The routine fecal count will come up with a number for an individual horse that should tell owners if the animal is a low, moderate or high shedder.
A fecal egg count of less than 150 eggs per gram is considered a low shedder, 150 to 500 is a moderate shedder and greater than 500 is considered a high shedder. Most times low shedders are not dewormed unless it is a time of year such as after the first frost when bots will be treated.
It’s known that 80 percent of internal worms will be found in 20 percent of horses. Individual fecals on horses can identify the high shedders or horses in the high range.
This is important because these high shedders are the big source of infection for the horses that may be low shedders. We identify the moderate and high shedders, and then working with a veterinarian, can come up with a deworming schedule tailored to an individual horse based on the egg count, age, time of year and pasture conditions.
Parasites are much harder to control when the density of the horses on pasture is high.
Owners can also ask their veterinarian to check two weeks after deworming to see how many of the worm eggs have been removed. If less than 85 to 90 percent have been removed, it may mean that resistance is an issue or many encysted parasites are present.
Several species become encysted in the intestinal tract and may emerge once the adults are killed. These encysted ones are hard to kill, and some veterinarians may recommend dewormings in succession to bring the egg count down.
Worms are very prolific, and certain species produce thousands of eggs a day, so transmission is inevitable.
Eggs hatch into larvae in spring and summer, which get out onto the grass, are ingested and become adults. This life cycle could take months, depending on the specific species involved.
There are three main families of dewormers for the regular nematode worms that horses acquire. One must rotate between these families of the benzimidazoles (Safe-Guard is an example), pyrimididines (Strongid) and the macrocyclic lactones (Eqvalan).
The benzimidazoles are best used in spring and summer, while the macrocytic lactones work in the fall because they also get the bots.
Please read the active ingredients because many trade names have the same active ingredient, which means horse owners are essentially giving the same thing. This is a very common mistake in the equine industry.
Resistance has been found to all the families of anthelmintics (dewormers). A positive fecal count is necessary in some European countries before a dewormer can be purchased. This is to try and slow down or prevent resistance from developing.
Young horses can pick up roundworm eggs directly without being on grass and get severe infestations to the point where the numbers of worms block intestines.
As a result, definitely do fecal counts on foals and yearlings to prevent these severe infestations.
It’s unlikely that horses housed in dirt stalls or internal box stalls for long periods will need to be dewormed.
Work with your veterinarian to establish individual protocols based on egg output. Understand which products work against what parasites.
Owners can also kill larvae by putting horses in new fresh grass, removing manure in small pastures or harrowing and allowing manure to desiccate. Our winters help because although the larvae can overwinter, eggs essentially burst with the freezing and parasite transmission is slowed or halted.
Fecal counts cost a few dollars but they tell us a lot and in some cases actually save us from buying dewormer or at least allowing us to give the dewormers to the horses that actually need it.
One fecal count done at the right time — when horses are potentially on pasture and have not been de-wormed for awhile — can set the stage for potentially the next few years, and deworming decisions can be made accordingly.
Let’s work together to minimize resistance and maximize the benefit that a strategic deworming with the right product can give to a horse.
Roy Lewis works as a technical services veterinarian part time with Merck Animal Health in Alberta.