Bot infection in horses

October 29, 2007


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

They wing their way in with just one thing in mind - causing your horse grief. Neil Clarkson discusses strategies to deal with the parasitic botfly.

Computer geeks know bots as the web robots that crawl the internet indexing websites. For horses owners, they're a much older creation that which can cause a lot of grief for their equine friends.

The life cycle of the botfly is designed, naturally enough, to ensure survival of the species.

Unfortunately, part of that lifecycle 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 your enemy. We'll begin with the botfly on the wing.

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 hoon 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 headed for the horse's gut, but have different strategies 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 may well be caused by botfly larvae.

Gasterophilus nasalis have the same destination in mind, but adopt 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 for the stomach and burrow into the surface of the gut wall. Interestingly, intestinalis prefers the top part of the stomach, while nasalis heads for the lower part. 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 bad enough infestation will impact on a horse's ability to digest its food, and result in a fall-off in condition. But by far the biggest concern is the damage done to the stomach lining. The borrowing 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.

Stomach ulcers are a big enough problem in horses without botflies helping the problem along.

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 and take a ride through the intestines, emerging in the faeces. The larvae burrow into the ground for six weeks 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 is 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.

So what are your options?

Insect repellents

Botflies work outdoors, so repellents will have to be effective in the great outdoors. They will be partially effective at best.

Scrubbing the horse's legs with warm water

This 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

This technique 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 won't 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 have been infected. It's an excellent strategy, but it won't remove the need for a de-worming agent.

Using a de-worming agent

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 borrowed into the gums.

There are many proprietary horse wormers, but each is likely to fall into one of three key families of drenches.

The drench effective against bots are in the ivermectin, abamectin and moxidectin family.

The drench label should be clear on whether the particular product is effective against bots. Be sure to follow the dosage recommendation, for which you may need to calculate the weight of your horse.

Dosing intervals are also important. They aren't just "made up" by the drench manufacturers. They are there to ensure they break the life of parasites as effectively as possible.

Drench resistance is a growing problem. You're encouraged to rotate your drenches to prevent this. How then, you ask, do I rotate drenches when not all are effective against bots?

It's a good question, and one which is best answered by your veterinarian. The precise behaviour of botflies vary around the world, depending upon climate. Much depends on the severity of winter and its timing.

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

These drenches are effective against other parasites, and worming for control of bots will only be part of your overall strategy for controlling the wider worm problem.

Discuss your drenching strategy with your veterinarian.


Video showing how horses react to bot flies.