animal health consulting

Faecal egg count (FEC) is a lab test that estimates the number of parasitic worm eggs being passed in the horse’s manure (faeces). To be specific, it counts the number of strongyle eggs.

Strongyles include the once-prevalent large strongyles such as the bloodworm or redworm (Strongylus vulgaris) and the now-predominant small strongyles, a group of more than 40 different species we collectively call the cyathostomins or cyathostomes.

Strongyles, large and small, are the most important internal parasites in adult horses, so they’re the main target of deworming programs in adults.

Tapeworms, pinworms, roundworms, threadworms, and bots can all be addressed within strategic deworming programs that focus on the strongyles.

In horses on grass, even if only for a few hours a day, FEC is an important estimate of how many strongyle eggs a horse may be ‘shedding’ onto the pasture. These eggs contaminate the pasture and continue the parasitic life cycle:

* the eggs are shed in the horse’s manure

* the eggs hatch into larvae which move out over the grass

* the larvae are swallowed by the grazing horse

* the larvae mature into egg-laying adults in the horse’s digestive tract, and their eggs are then shed in the horse's manure

There are a few reasons for having a FEC done instead of simply deworming your horse on a schedule as we’ve all been taught at one time or another:

1. You’re not paying to deworm the horses who don’t need it.

According to research, somewhere between 50% and 75% of healthy adult horses don’t need deworming any more often than once or twice a year, as long as they and their pastures are well managed (not overstocked or overgrazed).

Let me say that again, because it’s really important: half to three-quarters of all healthy adult horses do not need deworming more often than once or twice a year.

When well managed, these horses are able to maintain a low level of strongyle infection and remain healthy without frequent deworming, and they shed relatively few strongyle eggs onto the pasture. As long as they are not grazing really ‘wormy’ pasture, their immune systems keep their strongyles in check.

There’s even some human research which indicates that persistent, low levels of intestinal worms encourage immune system tolerance, something that is lost in allergic conditions such as eczema and asthma. In other words, having at least some strongyles may be beneficial to the horse. Now, that’s a mind-bending concept!

Of the other healthy adult horses, somewhere between 10% and 20% are ‘on the fence’; they’re still relatively healthy, but they’re shedding a lot of strongyle eggs onto the pasture.

These individuals may need deworming more often than the majority of adult horses, but it all depends on several factors, including their diet, levels of physical and psychological stress, the size of their pasture and the number of other horses grazing with them, facilities for rotating and resting or cleaning pastures, climate, season, etc.

And then there are the last 15–30% of healthy adult horses who shed large numbers of strongyle eggs, even when they’re dewormed regularly. Deworming with an effective product temporarily drops them into the ‘low-shedding’ group, but within weeks or months they’re back to shedding large numbers of eggs.

These horses — at most, 3 out of 10 adult horses — are the ones who need frequent deworming because their immune systems seem unable to keep their strongyles in check. Even so, these high-shedders may need strategic deworming only 3 or 4 times a year — not every 6 to 8 weeks throughout the year.

By the way, I keep using the term ‘healthy adult horses’ for all these groups because that’s what the conventional wisdom says. Even the high-shedding horses may be able to hold their weight and have a decent coat on a good diet and in a low-stress environment. In other words, they may look healthy to the casual observer.

From my holistic perspective, though, both the moderate- and high-shedding groups are not as healthy as they could be, and there are usually signs of rumbling inflammation somewhere in the body. So, deworming is not the only thing we can be doing for these horses. But I digress…

Although FEC costs money, identifying the moderate- and high-shedders, and targeting them for more frequent deworming is the most economical long-term strategy. Here’s why:

2. By being selective and strategic with anthelminthic (dewormer) use, you won’t be indirectly applying as much of it to your pastures.

By selective, I mean only some horses; and by strategic, I mean only at certain times of the year. I could just as well have said seasonal instead of strategic; that’s how important climate and weather are to effective deworming strategies.

Ivermectin, moxidectin, abamectin, and other anthelminthics in that family are passed out onto the pasture in the horse’s manure, where they may kill dung beetles and various other beneficial pasture and soil organisms. If you want to have healthy pastures that feed your horses well, you need healthy soil. One of the keys to having and keeping healthy soil is the vast and vibrant web of soil life, a veritable microcosmos of its own.

The more your pastures can sustain your horses throughout the year, the less feed and supplements you need to buy. So, the relatively small cost of having periodic FEC tests done is offset by the greater long-term benefit to soil and pasture health.

3. Deworming only the horses who need it slows down the inevitable development of anthelminthic resistance.

We are now at the end of ‘the golden age of ivermectin’. It was once a wonder-drug, as it virtually wiped out the very common and often deadly bloodworm. Ivermectin and its more recent cousins are still highly effective against most equine internal parasites.

Tapeworms have never been susceptible to the 'mectins/dectins. These 'flat' worms are an entirely different class of parasite from the 'round' worms, which include the strongyles, roundworms (ascarids), pinworms, and threadworms.

However, resistance to that entire family of anthelminthics is widespread in sheep and goats, and while still uncommon overall, resistance has been steadily increasing in equine internal parasites as well.

Fenbendazole (e.g., Panacur®) hasn’t been effective in horses at previously reliable dosages for decades now, and other anthelminthics in that family have suffered the same fate. Over time, resistance to one drug in the family generally becomes resistance to all, as all drugs in a family work along the same basic lines to inactivate or kill the target parasite.

Fenbendazole is still somewhat effective against the larval stage of the small strongyles that hides out in the lining of the bowel — but only at twice the previously effective dosage (which was 5 mg/kg body weight) and repeated daily for 5 days.

Resistance has now been reported for every one of the commonly used anthelminthics in horses, and there are no good alternatives waiting in the wings. We must preserve the effectiveness of the ones we have for as long as we can, otherwise we’ll be in real trouble. That’s where FEC comes in.

Every time we deworm a horse with an effective product, most of the strongyles will die but some will survive, as not even the highly effective ivermectin-moxidectin class of anthelminthics is 100% effective. So, every time we deworm a horse, we’re inadvertently ‘selecting for’ resistance in that worm population, similar to how we select for the traits we want or don’t want when breeding horses.

Perhaps a better example is the development of multiple antibiotic resistance in bacteria because we’ve been using antibiotics too liberally for too long. The more times the worm population on a farm is exposed to a dewormer, the faster resistance to that drug develops in that worm population.

It is logical to assume that rotating among different classes of anthelminthic would slow the development of resistance, but studies have shown that not to be the case, for at least two reasons.

First, not all anthelminthics are equally effective against the target parasites. Every time an anthelminthic is used that is less than 90% effective against the target parasite, more resistant worms remain, and so more are produced in the next generation.

Second, we’ve been deworming too often within these rotational programs. By giving one or another type of anthelminthic on a schedule, such as deworming every 6–8 weeks, we end up encouraging the development of a population of ‘super-worms’ which are resistant to all available classes of dewormer. Under-dosing when deworming does the same thing.

Use a weight-tape to estimate your horse's body weight before deworming; and if you ever get the opportunity to have your horse weighed on a livestock scale, take it! Knowing what your horse weighs is important when deworming and when giving medications and supplements. If you don't have a weight-tape, ask your local produce store to order you one if they don't already have any in stock.

Real-world example

Here is a real-world example of the power of selective deworming based on FEC results.

Researchers in the US took the FEC results from 261 horses on 12 farms and examined what would happen if only the horses with a FEC of 200 eggs per gram (epg) or higher were dewormed with a highly effective anthelminthic such as ivermectin or moxidectin.

Parasitologists generally divide horses into three groups based on their FEC results:

* low shedders have a FEC below 200 eggs per gram (epg) of manure

* moderate shedders have a FEC between 200 and 500 epg

* high shedders have a FEC greater than 500 epg

The horses in the low-shedding group accounted for 55% of all horses in the study but were responsible for only 4% of the total strongyle eggs shed by the group.

The horses in the moderate-shedding group accounted for 18% of all horses in the study and were responsible for shedding 13% of the total strongyle eggs.

The horses in the high-shedding group accounted for 27% of all horses in the study but were passing a whopping 83% of the total strongyle eggs.

By deworming only the horses with a FEC of 200 epg or above, only 45% of the horses would be dewormed, yet this approach would result in a 96% reduction in total egg shedding by the group!

That’s a stunningly good result, for less than half the usual amount of anthelminthic (because less than half of the horses would be dewormed). That study was conducted in Georgia, a southeastern US state with a climate much like that in southeast Queensland and northeastern New South Wales.

Study conclusions

Here's what the authors of that study had to say about using FEC results:

"Costs of performing FECs must be viewed as a necessary expense for maintaining optimal horse health. Owners must be warned against embracing the mistaken notion that since the price of a tube of dewormer is the same or less than the price of a FEC, it is cheaper to just go ahead and treat.

"Millions of tubes of anthelmintic are being administered to horses every year that are killing very few parasites either because there are very few worms in the horse to kill, or because the drug is ineffective as a result of resistance. Furthermore, there are future costs to over-treating in the form of worsening drug resistance.

So, not only do current practices of over-treating horses waste money and promote drug resistance, but by not monitoring the success of the programme using FEC, there is no way to gauge how successful the programme actually is.”

RM Kaplan and MK Nielsen. An evidence-based approach to equine parasite control: It ain’t the 60s anymore. Equine Veterinary Education 2010; 22 (6): 306–316.

Brief comment on natural dewormers

I have not found natural dewormers such as diatomaceous earth (DE), 'natural occurring mineral supplement' (NOMS), and various herbal products particularly reliable. In fact, some of the wormiest horses I've seen since the introduction of ivermectin have been those fed DE or NOMS as a daily dewormer by well-meaning owners who did not want to use any synthetic anthelminthics.

I applaud the goal of using natural products when we can, but an effective natural dewormer may be an unrealistic expectation in the horses who most need to be dewormed. The unnatural way most horses are kept contributes to the problem of internal parasites. What makes us so sure there is an effective natural solution to this unnatural situation?

I live in hope and continue to explore and experiment, but here's what I've concluded thus far:

No dewormer, natural or synthetic, is a match for poor nutrition and poor pasture management (overstocking, overgrazing, and inadequate manure management).

If you are using a natural dewormer of some sort, then check its effectiveness by running a FEC on your horse periodically.

© Christine M. King, 2020, 2022. All rights reserved.

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