9 – What’s so scary about ascarids?

Foals are cute and adorable. It seems such a shame to be dosing them regularly with a chemical drench. How bad can worm infestations be in young horses?

The antics of the ascarid alone should be enough to convince you that a careful and ongoing drenching of foals and young horses is a very good idea.

Ascarids, also known as roundworms, pose a considerable threat to young horses and their developing immune systems, and they have to potential to kill by triggering colic.

Even if a young horse escapes that fate, a whipper-snapper with a heavy ascarid burden will likely appear depressed and its normal growth will be affected.

The most common ascarid to infect horses is Parascaris equorum. Horses needn’t feel they have been singled out for special attention. Other species of roundworms target plenty of other species, even humans.

Most ascarids are specific to a particular species. They tend to be quite large and the equine varieties are easily visible to the naked eye. In fact, P. equorum is quite enormous. Think of something the width of a ballpoint pen and up to 35cm in length.

The bigger specimens of any given species will almost certainly be female and they spend their days laying eggs furiously, which are destined for your pasture.

Ascarids on their own sound scary enough but their eggs are beautifully adapted for life in the pasture. They pass out into pasture protected by a tough shell which equips them well for survival in this tough new world.

They can withstand drying conditions and even freezing, waiting for that moment when a passing horse ingests them with grass.

Their outer layer is sticky, meaning they stick well to blades of grass, thus increasing their chances of a free one-way ticket into a horse’s gut.

The larvae that develop in ascarid eggs take from 10 days to six weeks to develop to an infective stage, depending upon conditions. If conditions are just right, they can survive outside the horses for a remarkable 10 years or more, although two years would be more typical.

The common P. equorum will naturally be very pleased to find itself a meal. It catches a lift to the small intestine where the larvae are released and burrow into the gut wall.

Being sticky, the eggs can stick to a horse’s leg and even the mare’s udder. A foal will usually be exposed pretty early in its life.

Unfortunately, this is only the beginning of the trouble. The larvae have one destination in their sights: the liver.

They catch a lift in the bloodstream or perhaps the lymphatic system. Once in the liver, it hangs out for a week or so, developing into the next larval stage before catching another lift in the bloodstream to the lungs. It breaks out the capillaries in the tiny air sacs and begins migrating up the airways.

The irritation they cause can trigger coughing in their host — a very handy helping hand to keep the ascarid larvae on the move to their next destination: the upper airways.

Finally, the ascarid larvae will find themselves at the back of the throat and, kind of like a pinball that’s scored maximum points, it then begins to fall back — but this time into the gut.

Once swallowed, the ascarid cruises back to the small intestine, clutches on to the lining, and matures into an egg-laying machine.

A drench is starting to look like a pretty good alternative for a young horse to coma bad-ass bunch of ascarids.

It goes without saying that the migratory habits of ascarids cause damage. How this shows in the horse will depend upon the degree of infection and the whether the larvae are in their migratory phase.

When migrating, a horse may well show signs of respiratory problems. It may have a nasal discharge or a cough and possibly a fever. Antibiotics will make no difference as they don’t kill ascarids.

A heavy burden in the gut will likely show in the classic signs of a poor coat, a loss of weight, a pot-bellied appearance and sluggishness.

Ascarids present one other notable danger. Their size is such that collectively they have the ability to block the intestinal tract of a young horse and trigger a potentially fatal bout of colic.

Ironically, this colic can be triggered by a dose of drench. The worms are killed, fall away from the intestinal lining and cause a blockage. This is the primary reason that you not only need to drench young horses, but do so regularly to ensure there is no potentially fatal build-up of mature ascarids in the gut.

There is one good piece of news about ascarids.

Horses appear to develop quite effective immunity against infection as they grow older. Ascarids are at their most dangerous in horses up to about 15 months of age. Undrenched younger horses can end up with hundreds of ascarids, triggering serious health problems.

Older horses undoubtedly continue to ingest ascarid eggs, but they rarely result in a rate of infection that is likely to cause the animal any particular problems. You will rarely see ascarid eggs when doing a faecal egg count on a mature horse.

It’s a different story when doing faecal egg counts of younger horses. It’s possible to find extremely high egg counts but, conversely, it’s important to remember that only mature ascarids will be laying eggs. A horse may be harbouring ascarids in their migratory phase and no eggs will show in the dung sample.

Fortunately, there are several drenches that are effective against ascarids, but the dose rate is quite high.

Drenches which contain benzimidazoles at a rate of 10mg per kilogram will be effective against ascarids. The label will likely show the active ingredient as fenbendazole, oxfendazole or oxibendazole.

The 10mg dose rate for fenbendazole applies only to younger horses, as this level is needed to kill the commonest ascarid, P. equorum. As older horses should have good immunity to P. equorum, they can be dosed at half this rate to mop up other members of the family.

Drenches that contain tetrahydro-pyrimidines will also do the trick: either pyrantel pamoate at a rate of 6.6mg per kilogram of horse; or you could opt for a pyrantel tartrate, which is used as a daily wormer at a dose rate of 2.64mg per kilogram. At this dose, pyrantel tartrate will kill ascarid larvae emerging from recently swallowed eggs before they cause damage during their migration to the liver and lungs.

These worming agents will not kill larvae migrating through a horse’s liver or lungs.

That being the case, the real stars in this particular fight are ivermectin and moxidectin. Ivermectin is effective against ascarids at a comparatively modest dose a rate of 0.2mg per kilogram of body weight. Its stablemate, moxidectin, is likewise effective, but at 0.4mg per kilogram of body weight.

Ivermectin will deal with ascarids in all their intestinal forms and delivers an effective killer blow against larvae that have migrated to the lungs or liver.

Not all drenches are recommended for use with foals. For example, at least some drench formulations containing moxidectin are not recommended for horses under six months of age, so check the label carefully.

Unfortunately, there are very real risks in drenching young horses with a heavy burden of ascarids. The last thing a horse owner wants to do is trigger impaction colic resulting from a mass kill of ascarids.

The safest option, especially if a young horse is showing signs of heavy ascarid infestation, is to consult a veterinarian. Some may opt for a double course of fenbendazole for young horses, but with a half-strength first dose at 5mg per kilogram of body weight.

This has been shown to kill a proportion of the worms, reducing the worm burden only partially.

A follow-up higher dose can be given at a later stage — perhaps a week later — to mop up the remainder. Such youngsters will need to be monitored closely for any early signs of colic.

The safest longer-term course is to implement a regular deworming programme in youngsters aimed at ensuring ascarids do not reach dangerous levels. If numbers are kept in check, the risk of impaction colic is reduced markedly.

Like strongyles, the life cycle of ascarids presents several strategies to minimise exposure.

In fact, these become very much a recurring theme in your efforts to manage your horse’s exposure to parasites. Picking up dung is important because the more dung you pick up, the few infective eggs will remain to be ingested by horses.

A good compost heap that generates enough heat will kill the eggs.

Feeding horses off the ground, either with feed buckets or through hay nets or a free-standing hay feeder, will similarly reduce exposure.

Ascarids present a particular challenge. Their sticky eggs mean horses can actually become infected without putting their mouth anywhere near pasture.

A mare who enjoys a roll in her paddock can easily end up with ascarid eggs clinging to its coat. A foal who then snuffles its mother’s coat can end up ingesting eggs.

Eggs can similarly transfer from a horse’s coat to the likes of a stable wall or a fence post.

These eggs are not only sticky customers, they’re tricky customers. And let’s not forget they can hang out for 10 years or more and still manage to infect a horse.

Mature ascarids are major eggs factories, so a crucial strategy centres on ensuring your young horses do not harbour ascarids long enough to reproduce. If your worming programme is timed correctly, you can kill them off before they collectively start laying literally millions of eggs to provide you with a decade-long problem.

The magic number in the battle against ascarids is 70: that’s the minimum number of days that an ingested ascarid would take within a horse to mature and begin to generate eggs.

And remember, you’re not counting from when your foal first begins nibbling grass. There is every chance they may have picked up ascarids from nuzzling their mother or just snuffling about in the grass.

The first worming should take place at 60 days after birth to give you a safe 10-day margin. Any of the wormers described above will be suitable, but check the label to ensure they are suitable for use on foals.

The timing of the next dose will depend on which drench you use. It’s important to remember that the benzimidazole or pyrantel drenches will not kill migrating larvae – only those in the gut.

So if we assume that a migrating larvae returns to a horse’s gut just after drenching, how much time do you have before it matures and begins producing eggs? You’ve got eight weeks maximum, so be sure to leave follow-up doses no later than this.

Ivermectin and moxidectin put up a better showing, and are effective against migrating ascarids. This means there is more time to play with between drenching. Aim for 60 days but on no account leave it later than 70.

Ascarids continue to pose a threat until young horses are about 15 months of age, after which they should be capable of dealing with them on their own. You’ll still be deworming, of course, it’s just that you’ll have other enemies in your sights, safe in the knowledge that you’ve seen off the biggest threat from ascarids.

Even though your foal can quite easily pick up ascarid eggs, you should still minimise their exposure. It is good practice to drench a pregnant mare a fortnight or three weeks before foaling (check the label to ensure it’s safe for pregnant mares) and a day or two before the birth wash the mare’s underbelly and udder with soapy water to remove any ascarid eggs.

» Next: What’s so bad about bots?

First published on Horsetalk.co.nz in February, 2009