11 – Getting the measure of tapeworms

The distinctive D-shaped egg of the tapeworm (Anoplocephala perfoliata). The dark circles in the picture are air bubbles.
The distinctive D-shaped egg of the tapeworm (Anoplocephala perfoliata). The dark circles in the picture are air bubbles. © Martin Krarup Nielsen

Nematodes have long been considered the bad boys on the equine block. Tapeworms — so named because of the segmented nature of their bodies — appear to be sissies by comparison.

While the king-hitters of the drenching arsenal were being developed, tapeworms managed to keep below the radar.

When you’re a parasite, that’s a good place to be. The health issues caused by tapeworm infection paled against the nasty nematodes and the havoc they created.

These days, we have a much better measure of tapeworms and the news is not altogether good. It transpires they are common and do, indeed, cause their fair share of grief.

Tapeworms — the common form is Anoplocephala perfoliata — spend their day latched on to a horse’s intestinal wall with a mouth-like structure of four specialized suckers, called a scolex.

Each segment of their body is a self-contained unit called a proglottid and they soak up the nutrients they need through their skin. They are typically about 5cm long and 15mm wide, and their preferred real estate in the gut is where the small intestine joins the large intestine.

Tapeworms have a couple of interesting plays in their game plan. For a start, each contains both male and female reproductive organs.

Mature tapeworms don’t produce eggs continuously as with most parasites, instead releasing them in occasional packets from their tail region which then pass out of the horse in its feces.

This means a faecal egg count is not a reliable way to assess whether a horse is carrying tapeworms, as there is no guarantee that a dung sample will necessarily contain eggs.

Even if a packet of eggs is present in a pile of faeces, evidence suggests that the eggs are unlikely to be evenly distributed, meaning that coming across them in a dung sample under a microscope could well be a matter of chance.

But the life cycle of the tapeworm gets even more interesting once it exits the horse.

It actually relies on another animal besides the horse as its host. These are oribatid mites, which are common in pasture. Needless to say, these mites find plenty of edible organic material in horse faeces and this plays into the hands of the tapeworm eggs.

The mites ingest the eggs as part of their diet which, over two to four months, develop in a way that enables them to infect a horse. In something akin to a scene from the movie, Alien, the ingested egg develops into a larva which busts out of the mite’s tiny intestinal tract and parks up in the body cavity to continue months of development.

Horses eat the tiny mites as they graze and, with them, the infective larvae, called cystircercoids, which in six to 10 weeks mature into a tapeworm.

There’s an element of irony here. Oribatid mites and their eating of organic matter play an important role in maintaining healthy soil and pasture. If you pride yourself on the quality of your pasture, there’s a pretty good chance you’ve got an excellent population of mites.

They’re very tiny and even an average pasture is likely to have a population in the hundreds of thousands per square metre. In short, you’re never going to avoid them, or eliminate them – nor indeed would you want to.

The three commonest tapeworms are Anoplocephala perfoliata (found in New Zealand), which grow to about 2.5cm in length. Paranoplocephala mamillana is about half that size, but the giant of the clan is Anoplocephala magma which can grow to 75cm in length.

Mature tapeworms (Anoplocephala perfoliata) in the caecum of a horse. The largest member of the tapeworm family can reach 75cm in length.
Mature tapeworms (Anoplocephala perfoliata) in the caecum of a horse. The largest member of the tapeworm family can reach 75cm in length. © Martin Krarup Nielsen

The presence of tapeworms vary from region to region and country to country, and the commonest form may likewise vary. Climatic factors undoubtedly play a significant part in this uneven distribution.

Because faecal egg counts are unreliable for detecting the presence of tapeworms, most horse owners should assume their horse is infected. It doesn’t require a major leap of faith. Post mortem examinations in several of studies have consistently shown infection rates in adult horses of more than 50 per cent.

Tapeworms for many years escaped the wrath of parasitologists trying to develop effective drenches, with strongyles being the primary target.

Today, we know better. Tapeworms can cause localised inflammation which is likely to be a factor in a colic episode if the burden is heavy enough.

Signs of ill-thrift through tapeworm infestation are unlikely, which is part of the reason that tapeworms didn’t get such a bad rap from scientists. However, the risk of colic is reason enough to tackle tapeworms head-on.

In fact, tapeworms are considered to be the primary cause of a very dangerous colic where the lower part of the small intestine folds inside the part of the large intestine where it joins, which requires surgery to correct.

For a long while, there were no approved products on the market for tackling tapeworms, with many vets recommending high doses of drenches effective against other worms in a bid to kill them.

The first effective weapon was praziquantel, which has proved virtually 100 per cent effective against tapeworms. It is considered safe and is effective at a dose as low as 1 milligram per kilogram of the horse.

Praziquantel is nearly always an ingredient in drenches targeting several different kinds of parasites. It is most commonly combined with ivermectin, moxidectin or abamectin. The precise combination will vary from brand to brand, depending upon how the manufacturer is targeting the drench.

However, to be effective against tapeworms the formulation should contain praziquantel in levels to deliver the minimum above-mentioned dose.

Again, picking up paddock dung will help minimise horse exposure to the infected mites. However, long-term strategies should centre on keeping tapeworm infestation to a level where it is unlikely to pose a danger to the horse.

Orabatid mites are likely to be most active in the warmer months, so there is some sense in drenching against tapeworms going into winter, when the re-infection rate over the cooler months is likely to be lower.

Similarly, a drench in late winter before the weather warms up is likely to play a major part in minimizing the number of eggs being deposited in the pasture. After all, the ingestion of orabatid mites is only a problem if the mite is infected.

The segmented nature of tapeworms poses another interesting problem when dealing with them by drench. Some drenches will, bizarrely, deal only to the tail of a tapeworm.

The head, still firmly attached to the gut by its scolex, will simply regrow a few segments and start reproducing again quite quickly.

The tapeworm is proof that the battle against equine parasites is difficult. If it wasn’t for praziquantel – a comparative newcomer to the drenching arsenal in horses – it’s entirely possible that vets would still be doubling or tripling doses of other drenches to deal with these pests.

» Next: Putting the bite on lice, ticks and mites

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


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