Do I really need to worm my horse?

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Endoparasites;  the gut-wrenching villains that terrorise our horses from their tums to their bums, but how big an issue are they for the average horse? Which worms do we need to be aware of? Is wormer resistance really that big an issue? So many questions, so many drug names.

What is a worm?

A worm, or an endoparasite, is an organism that lives inside of your horse, to your horse’s detriment. We have all seen the adverts for ‘good bacteria’; this is known as a synergistic relationship, where both host and occupier benefit. With parasites, only the parasite gains.

  1. Cyathostomes. Why did the cyathostome always get what he wanted? Because he was so encystant… Excuse my awful jokes; it’s been a long day. Cyathostomes are a type of nematode, or round worm, known as small encysted redworm.  The adults, when in the large intestine, produce eggs that the horse will excrete onto their pastures; the eggs then hatch, and the larvae are eaten by the horse. It is also the larvae that are capable of encysting (hiding) in the walls of the large intestine. So the saying really is true, don’t eat where you… These nasty critters can encyst in the mucosal lining of the large intestine of horses; the larvae are capable of ‘hypobiosis’; they stay in a state of arrested development (a bit like hibernation).  It is when they emerge that they can cause potentially fatal damage and diarrhoea, known as larval cyathostomosis.
  2. Strongyles (large redworms). These nematodes are detrimental to your horse in a different way than their smaller namesakes. Eggs in the pasture have a moult phase, referred to as Larval stage 1, L1, and moult to form, logically, L2. The L3 are consumed by an unwitting grazer; it is the L4 stage that migrate from the gut to the arterial supply of the intestine (the cranial mesenteric artery if you are curious). This can cause a compromised blood supply to the large intestine with inflammation of the arteries known as verminous arteritis, and can cause the dreaded colic.
  3. Parascaris Equorum (ascarids); another nematode. Are you the owner of a young horse? This one is for you (sorry!).  Thankfully, our equid amigos develop a resistance to these worms; however, young-stock in the 6 month – 2-year old bracket are highly susceptible. The eggs are passed out in excrement, and moult to L1 and then L2; unlike large redworms, it is the second larval stage that is ingested. They, too, have a damaging migratory pathway; from the intestines, they migrate through the liver and moult to L3, before progressing to the lungs.  From here, they can be coughed up, swallowed, and moult to L4 and then adulthood in the small intestine before starting the whole cycle again. Liver and lungs may be damaged, but impacted colic from a heavy worm burden, along with ill-thrift and a pot-belly, are common signs.
  4. Dictyocaulus arnfieldi (lungworm) is another nematode. To my horror, it is donkeys that are particularly affected by lungworm, and carry it, as the life cycle is not actually completed in the horse.  These larvae are ingested, and burrow out of the intestinal wall into the bloodstream, where they penetrate the lungs. They will cause reactive changes in the respiratory system, such as coughing, increased mucus production and irritation of the bronchi. Chronic pneumonia, secondary infections and pulmonary oedema (fluid on the lungs) may be other features in heavy burdens.
  5. Anoplocephala (tapeworms); a cestode, not a nematode. It requires an intermediate host to develop to a larva from an egg, and it finds a host in the oribatid (harvest) mite. When the horse eats the mite with the parasite, the adult tapeworms can then settle in the caecum (the huge fermentation chamber of the horse gut) and small intestine. Tapeworms pose a real threat to your horse by associations with spasmodic colic; it can cause food impaction and intussusception, when the colon ‘telescopes’, folding in on itself. Inception may be about a dream within a dream, so think of intussusception as a colon within a colon.

Other parasites include Oxyurius equi (pinworms, causing itching and irritation around the anus) and Gasterophilus (bots; actually a fly larva, and not known to cause many problems despite settling in the stomach).

What can we do about worms?

I have been on many yards with rigid worming routines as a means of prevention as much as treatment: this is called interval dosing. Is it necessary? If I had to fall in a strict ‘yes’ or ‘no’ camp, I would be in the latter. Wormers, known as “anthelmintics”, are becoming less efficacious; that is to say, anthelminthic resistance is becoming a real problem. The more that worms are exposed to wormers, the more the wormer becomes a selection pressure; some worms will have innate features which allow them to survive despite these chemicals specifically designed to kill them – pesky mutants. The more that we use wormers when we may not need to, the stronger this selection pressure is; we kill the worms which are susceptible to the wormers, allowing the few worms which can survive to reproduce in an environment with less competition. Thus, new wormers need to be developed all the time; a laborious and long task. How can we slow or stop this resistance developing? By being responsible owners and avoiding ‘over-worming’ – saving our horses and wallets in the process!

Moving away from wormers, we need to look to management. As we can see in the life-cycles, it is the output of eggs in faeces that are responsible for providing a suitable environment for parasites. Poo-picking fields is one of our biggest weapons in the battle of the bugs; deploy it often! Quarantining new horses prior to turn-out can help to minimise worms on a busy yard; moxidectin and praziquantel can be used 24 hours prior to turn-out.

What wormers are available?

There are macrocyclic lactones (ivermectin and moxidectin), tetrahydropyrimidines (pyrantel) and benzimidazoles (fenbendazoles) and pyrazinoisoquinolines (praziquantel). We must treat with what is most efficacious for the type of worm, and also only when it is needed. Faecal egg counts (FECs) give a picture of what worm eggs are being put out in your horse’s faeces; when the FEC exceeds 200 eggs per gram, it may be justification for worming. If that sounds a lot, we need to get our heads around the fact that horses will always have worms; whilst this is not a pleasant idea, unfortunately our horses will never have a totally worm-free body, and we shouldn’t strive for that in our worming regimes. Further to this, we want to keep a certain worm population ‘in refugia’; this means we want to keep some worms unexposed to wormers, because then we are not selecting for worms resistant to the wormers. It is only when worm burdens get too high and will damage our horses’ well-beings that we should use wormers. FECs can reduce the selection pressure that help those resistant worms to thrive, as well as being a cost-effective means of targeting the individual horses who need it most.

Is there an ideal worming regime?

Perhaps not. FECs will not give an accurate representation of encysted populations, and are not deemed specific enough for tapeworm counts. Fecal egg flotation or ELISA (enzyme-linked immunosorbent assay) blood tests can be used for tapeworms, but are more expensive. Furthermore, an ELISA detects the antigen (the immune response to a parasite) level, thus the burden may appear high as antibodies are still circulating against the old burden, even if the worms are now dead.

However, there are means of interval dosing that do not require administration of a wormer, regardless of whether the horse needs it. We must focus on what burdens are of concern and when…

SPRING: Performing a faecal egg count (FEC) for strongyles; ivermectin or single dose pyrantel can be utilised if there are over 200 eggs per gram. Stronglyes were previously a huge concern for causing colic, but thanks to ivermectin, they have become less of a menace, hence the need to protect the efficacy of this wormer by responsible use. Additionally, treatment for tapeworms in the form of praziquantel or double dose pyrantel may be used in spring.

SUMMER: FEC for Strongyles and treatment when the FEC indicates, again with ivermectin or pyrantel.

AUTUMN: we must treat for any encysted cyathostomes. Remember the larval cyathostomosis? Commonly these larvae will encyst, and emergence can occur in late winter/early spring. Treatment of a heavy burden is advisable; a five day course of fenbendazole, or a single dose of moxidectin are licensed for encysted cyathostomes. However, a large amount of dead worms and a huge inflammatory reaction can spell out a disaster in the form of colic, so if there’s a heavy burden your vet may recommend using the older (and less potent) but “gentler” course of fenbendazole first, and then following up with moxidectin 4-6 weeks later to “mop up” any survivors.

Tapeworms can be treated with praziquantel or double dose pyrantel again at this time of year.

WINTER: The same treatment (or not!) for strongyles when indicated; if bot flies were a problem over the summer, ivermectin or moxidectin will kill the larvae in the stomach.

From all of us here from VetHelpDirect, we hope your horses have a wonderfully worm-free year ahead!

There is nothing so good for the inside of a man as the outside of a horse. ~John Lubbock

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