Counting Critters: The case of the different results in field mates

One question that has popped up a lot lately is how is it possible for horses that graze together to show such different results? It almost seems like people want a consistent set of results across the board that reflects the pasture rather than the individual animals grazing on it. The truth is a little more complicated than that I'm afraid, but makes a lot more sense once understood!

To understand the view that pasture mates should share similar results, we need to go back in time to when blanket worming was the norm. Here it was the pasture that was treated rather than the individual animals. Every horse that shared the pasture was dosed at the same time, with the same chemical, and turned out at the same time. This cycle was repeated leading to a widespread belief that all horses grazing would be equally as likely to pick up worms, and that eradicating these worms once every 6-12 weeks was the end of the story. (The other story here was the widespread increase in resistance due to this- but that's a whole other story that deserves its own blog post or a thousand of them).

Fast forward 20- 30 years and we as horse owners know so much more about parasite control than we did back then, and with a combination of various testing types and strategic dosing using suitable products for the equine's unique circumstances and burden we can target only those who need treatment. The underlying principle behind using worm counts (FECs) is to target those who have a worm burden while avoiding dosing those who don't. Yet.. many horse owners still feel uncomfortable with the idea that one of their horses is full of worms while the other is not. With this in mind we will just briefly introduce some of the reasons why horses have, or do not have, a worm burden at a given time.


Perhaps one of the first things to mention is that equines of different ages have different levels of susceptibility to parasite infection. Foals are more likely to be infected with roundworm (ascarids) but start to develop an immunity to these around 3 years old. Young horses in general are also far more likely to get infected with redworm (strongyles), and as they get older and their immunity strengthens you may find their numbers drop. Sometimes at the opposite end of the scale, older horses also test higher, as their immunity weakens due to old age and possible underlying health conditions as they get older.


Aside from a horses age related immune levels, a horse's immune system can be stressed by different factors, a short term illness, long term conditions such as PPID, even ulcers are now being linked to increased susceptibility to worm burdens. Changes in gut flora can affect how hospitable the environment in the gut is to a parasite. When equines present with repeated medium or high burdens and factors such as pasture management are ruled out, we tend to look towards making the gut stronger against parasites, possibly by using pre and probiotics, or a herbal supplement designed to support the natural immunity towards parasites. These products tend to work better as a preventative than a cure: when used long term they can be beneficial at preventing parasite build up.

Worming history:

Equines with differing worming pasts will usually present with different levels of infection. A horse that had a dose of chemical wormer 8 weeks ago will usually show much less if anything than a horse that hasn't been wormed for 2 years! The length of time a wormer chemical is active within the animal's system varies depending on the chemical in question. The longest acting is moxidectin (found in Equest & Pramox) at around 12 weeks activity time, then ivermectin (such as Equimax or Animec) at 8 or so weeks, from there it gets shorter, with fenbendazole (Panacur) at around 5-6 weeks and pyrantel (Strongid P) the shortest at just 4-6 weeks before worm regrowth can occur. This is not to say you should all rush out and buy an Equest just because it lasts longer, far from it. Moxidectin is so vital in the winter battle against small encysted redworm its important we save it for that time of year, and use it strategically slotted in between tests, in order to keep it working for longer, as when it stops working we are all screwed!

Pasture Management:

One thing that confuses clients is if they have good pasture management, and yet a horse tests high. Poo picking is documented to be the best paddock management method of reducing worm burdens. Complete removal of all muck, 3 times a week minimum, completely removed off the field not slung in the hedges etc is best. Harrowing has been recently documented to spread worm eggs rather than kill them, and we see higher results from harrowed fields. Grazing rotation, resting paddocks and cross grazing with other species such as sheep are all good ways to reduce burdens. Saying this, some horses can graze an unclean pasture for years without getting infected, while others will pick up infection on a twice a day cleared field. In which case we look to the above reasons based on immunity etc to find our reasons why.

Weather: Another thing worth noting is the British weather in recent years. The mild wet winters are no longer killing off any worm eggs, so whereas in past years owners have relied on a good hard frost and possibly stopped performing worm counts during the winter months, this is sadly no longer possible. We are currently testing a lot of horses that haven't been tested or dosed since October time, based on old school opinion that worms cant live in winter. These horses are in a lot of cases testing medium or high for strongyles. A better more modern method is to slot in a dose for encysted small redworm in between 8 weekly worm count tests. Such as test Oct, worm Dec moxidectin/praziquantel combo, Test end Feb, test April, test june etc.

True egg shedders: In the industry, a horse that tests repeatedly positive despite all attempts to break the cycle is whats known as a true egg shedder. These are fortunately quite rare. These would be the ones that infect your other horses, and need close monitoring with tests and dosing before a burden gets out of hand. True egg shedders is really just a way of saying a horse that is really really prone to infection, possibly from a compromised immune system or other reason. If you feel you do have an egg shedder, there are ways to identify patterns and timings to nip in the bud any rises in levels.

Sometimes identifying reasons for different results can make you feel you are sleuthing like Sherlock Holmes. Usually with a bit of digging and looking into past results, doses, pasture, history, health etc we can find reasons, and put in place parasite control strategies to combat further infection and treat where needed. If you feel you need help with your equine's parasite problems, or just want to chat further then do get in touch with us!

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