23 – The perils of youth

Ascarids, or roundworms, pose the biggest threat to young stock.
Ascarids, or roundworms, pose the biggest threat to young stock. © Horsetalk.co.nz

Equine age has its virtues, and one of them is an ability to fend off several of the parasites that commonly afflict horses.

We already know that ascarids (roundworms) prefer to hang out in youngsters and are rarely found in horses over 15 months of age.

Threadworms and pinworms are similarly most prevalent in young horses.

In fact, threadworms (Strongyloides westeri) can infect your newborn foals in the blink of an equine eye. This parasite can be transferred to the foal as migrating larvae are well adapted to migrating to breast tissue when hormones indicate that the time is right.

Threadworm larvae can even find their way into the mare’s colostrum and thereby infect the foal.

It is for this reason that some experts recommend worming a foal within a few days of birth. Ivermectin is often used for this purpose.

Threadworms succumb quickly to ivermectin so another possible strategy is drenching the mare shortly before foaling to ensure all larvae in the breast tissue – or on their way there – are killed.

A heavy threadworm infection in a foal can cause diarrhoea.

A threadworm infection is easy to identify as the eggs can be seen during a faecal egg count.

Threadworm larvae mature once they get into the digestive track and a foal, with its naïve immune system, is the perfect host. The threadworm larvae will mature within a fortnight and begin producing.

The risk of infection from most other parasites increases after weaning, but even the youngest of foals will explore the pasture beneath their feet and are likely to pick up infective larvae.

When conditions are absolutely right, after rain and when temperatures are mild, threadworm larvae have been known to burrow into a horse’s skin, triggering severe irritation.

The risk from another foe, ascarids, cannot be under-estimated. Youngsters are particularly prone and the build-up of mature ascarids in the gut can be so substantial that they can trigger potentially fatal colic.

Drenching foals with a heavy ascarid load has the potential to trigger colic. When dead ascarids in heavily infected youngsters let go of the gut wall, the sheer numbers are capable of causing a blockage.

Aside from preventing this, you don’t foals happily depositing ascarid eggs around the pasture. Fortunately, the ivermectin given for threadworms will work its magic on ascarids.

While several drenches will kill adult ascarids, ivermectin will also kill the migrating larvae (see original ascarid story). Given the life cycle of ascarids, drenching with ivermectin at no more than 60-day intervals should ensure the ascarids do not mature and begin producing eggs.

And, come 15 months of age, your horse’s immune system will be tuned to the point where ascarids should no longer be a problem.

If you suspect a foal may have a heavy burden of ascarids, consult your veterinarian. He or she is likely to recommend a reduced drenching dose which will kill only some of the ascarids. By killing off the population a bit at a time, the chance of an impaction colic is greatly reduced.

Strongyles – both large and small – will be dealt with adequately by the use of ivermectin for the other worm threats.

The lack of resistance show nby young horses is a key driving factor in implementing a drenching programme early. Always check labels to ensure a drench is suitable for use in foals or youngsters.

And don’t forget to calculate the weight of the youngster and drench accordingly.

Monthly, or even fortnightly, faecal egg counts will help determine whether the strategies you’re employing are working.


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




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