10 – What’s so bad about bots?

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The eggs of gasterophilus intestinalis are most likely found on the horse's legs, shoulders, and possibly the mane. The eggs of gasterophilus nasalis will be seen around the mouth.
The eggs of gasterophilus intestinalis are most likely found on the horse’s legs, shoulders, and possibly the mane. The eggs of gasterophilus nasalis will be seen around the mouth.
© Horsetalk.co.nz

Computer geeks know bots as the web robots that crawl the internet indexing websites. For horses owners, they’re an annoying pest that can cause a lot of grief for their equine friends.

It hardly seems fair that horses not only have to endure the nematodes and cestodes lurking in their paddock, but an aerial invasion as well.

The life cycle of the botfly is designed, as with any parasite, to ensure survival of the species. Part of that life cycle involves living as a parasite off horses. In fact, most of the life cycle of the bot is spent holed up inside your horse.

As with any pest, the first step is to understand the enemy. We’ll begin with the botfly on the wing, which looks a little like a small honeybee.

You’re unlikely to see these little flies that grow to about a centimetre in length, but with a sharp eye you’ll easily pick up their calling card.

They fly in on a pleasant, warm day, and lay their eggs on the horse’s coat. If you look closely you’ll see you the pale yellow eggs clinging on. The location of the eggs will give a strong clue as to which botfly has come calling.

The eggs of gasterophilus intestinalis are most likely found on the horse’s legs, shoulders, and possibly the mane. The eggs of gasterophilus nasalis will be seen around the mouth. Both are found in New Zealand.

In both cases, the bot eggs are headed for the horse’s gut, but each has a different strategy for getting there.

The eggs of gasterophilus intestinalis are playing a waiting game. They’re waiting for the horse to attend to an itch by nibbling it or rubbing it with their mouth – or, of course, another horse kindly doing the scratching for them.

This rubbing means the horse will pick up a few eggs in its mouth and this is all that’s needed to prompt immediate action. The eggs, spurred on by the moisture and warmth of the mouth, immediately hatch a pinhead-sized larva which sets about burrowing into the gums, or beneath the tongue.

Inflammation and ulcers in the mouth which are not attributable to teeth problems could potentially be caused by botfly larvae.

Gasterophilus nasalis has the same destination in mind, but adopts a different strategy. A week after being laid, they hatch of their own accord and crawl into the mouth, again burrowing into the gums and beneath the tongue.

These larvae are known as first instars.

They take up residence for about three weeks and grow, emerging as second instars about 5mm to 6mm long. They head down the throat to the stomach and burrow into the wall.

Interestingly, G. intestinalis prefers the top part of the stomach, while G. nasalis heads for the lower part, around the duodenum. Three weeks to a month later they moult again to become third instars, and may then stay in the horse’s stomach from two to 10 months, feeding off the swirling contents of the horse’s stomach.

A really severe infestation will impact on a horse’s ability to digest its food, and result in a decline in condition. Bots can also damage the stomach lining. Their burrowing can cause ulceration and abscesses, and these can trigger colic. The ulceration is aggravated by the digestive juices and can get worse, potentially causing a breach of the stomach lining, which can prove fatal through peritonitis. Fortunately, this is extremely rare.

Stomach ulcers are a big enough issue for horses without botflies helping the problem along. That said, bots are down the list when it comes to the most dangerous parasites, and only in rare or extreme cases will disease or discomfort occur.

Isolated bots can even burrow through the stomach wall and cause havoc elsewhere.

Pleasant as the stomach environment may be for the third instar larvae, there’s the small matter of completing the life cycle.

They finally release their grip and take a ride through the intestines, emerging in the faeces. The larvae burrow into the ground for three to nine weeks, depending on the temperature, to pupate, the end result being a botfly ready to begin the cycle again.

The flies are active only during the warmer months, and typically only one generation will “roll over” in a year. Needless to say, the aim of the cycle is to have the larvae ride out the winter months inside the warm stomach of the horse.

Bot infection can be hard to detect. You’ll be lucky to spot the third instars in your horse’s dung, and you’d never want to wait until a lack of thrift or colic before suspecting the bot as a cause.

Research has shown that horses develop only limited resistance to bot infections. It’s up to owners to lead the charge.

Using a bot knife to scrape bot eggs off the horse's hair will help destory many bot eggs.
Using a bot knife to scrape bot eggs off the horse’s hair will help destory many bot eggs.

Horses appear to be able to tolerate quite a heavy burden of bots in their stomach and upper duodenum, but their burrowing into the gums and tongue can cause nasty and painful ulceration.

So what are your options?

Insect repellents may be more effective indoors, but are likely to have a limited effect in the outdoors.

Scrubbing a horse’s legs with warm water was a common strategy in years gone by. The logic went that the warm water and moisture tricked the eggs into hatching. Yes, you’ll get some, but plenty won’t be fooled. As a strategy for keeping your horse’s legs clean, it’s great. For controlling bots, it’s pretty much considered a waste of time.

Scraping or sanding the coat, or using a bot knife, is a technique that has been used for a good many years. You can certainly destroy a lot of bot eggs, but the problem is that you’ll always miss some which the horse may well ingest.

Also, you’re unlikely to be on hand to get all the eggs all the time. By the time you take to the little blighters, there’s every chance your horse may already be infected. It’s an excellent strategy, but it won’t remove the need for a de-worming agent.

Dealing with internal bot infection was once a problem for horse owners. Then, the macrocyclic lactones came on the scene – that’s the likes of ivermectin, moxidectin and abamectin.

This is the winner by a long shot. Earlier worming agents had a limited effect on bots, but now, thanks to ivermectin-based drenches, mankind certainly has the upper hand. Ivermectin will kill bots in all larval stages, including when burrowed into the gums.

The life cycle of the bot poses opportunities for horse owners.

In New Zealand, with a winter running from June to August, the use of an ivermectin, moxidectin or abamectin-based drench in late-May/early-June and again in early August will be particularly effective.

Be sure that the dose rate on your selected drench is sufficient to kill bot larvae. It is possible the drench you select may have the right active ingredient but not at a sufficient dose rate to kill all bot larvae. The label should be it clear whether a particular drench is effective against bots.

» Next: Getting the measure of tapeworms

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


2 thoughts on “10 – What’s so bad about bots?

  • September 24, 2016 at 7:19 am

    I’ve been noticing bot eggs on our horses lately. Even though we remove them with a bot knife, those we’ve missed that make it through the digestive tract, & are excreted through feces, landing on the ground, & then burrowing into the soil, are my worry. The horses seem to “go” in the same places, so I wonder, if after worming them, we should spray an insecticide on the ground, & feed them elsewhere? How often should we worm them with an Ivermectin product, or is there something else better out now?
    We usually worm them on a regular basis, 4 times a year. Thanks!

  • October 22, 2016 at 10:39 pm

    Ivermectin is great but worm every 2-3 months and just switch it up with the wormers different ones do different things and almost all are good. Bot eggs happen; scrape regularly use fly spray, and clean up the poo or separate them from the contamination for a short time (about a week).


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