Bot Flies: Summer Fly, Winter Parasite with Dr. Luke Wells-Smith

This article is from the July 2020 Horse Deals magazine.

Every horse owner likely knows what a bot fly is — a large, bee like fly that horses really hate. Bot flies are common in the warmer months in Australia such as late spring, summer and early autumn.

There are two common types of bots - Gasterophilus intestinalis and Gasterophilus nasalis. Gasterophilus intestinalis typically lay eggs on the limbs whereas Gasterophilus nasalis lay eggs around the face and throat.

The life cycle of bots start by the fly laying eggs on the hair of the horse. As a horse owner, you have likely seen bot eggs on a horse at some point — bot eggs are small yellow eggs.

Gasterophilus intestinalis bot eggs need to be stimulated by the horse itching and biting at the eggs, which pass into the stomach. As Gasterophilus nasalis lay eggs around the face and throat, the eggs do not need to be stimulated by itching or biting as the horse can’t easily reach these areas. Instead Gasterophilus nasalis bots hatch and burrow through the skin, into the mouth where they pass into the stomach. Any eggs that do not hatch are killed during the first frost of the season or cold weather.

Once the bots are in the stomach, the parasite starts to grow. This is where the bot can cause irritation to the stomach lining and in severe cases are thought to cause a blockage at the pylorus which can lead to colic.

As veterinarians, one way we can tell if your horse has bots, is to perform a gastroscope. A gastroscope is a long (three metre) endoscope that we pass down the nose of the horse into the stomach. The gastroscope has a high resolution camera at the end and can be used to visualise the stomach to check for diseases of the stomach such as gastric ulcers. If it is late autumn, winter or early spring, if your horse has bots, we should be able to visualise them in the stomach. The number of bots in the stomach vary from one to two parasites to over 50 in severe cases.

The bots tend to live in the stomach throughout winter. During spring, the bot is passed through the gastrointestinal system, into manure and on to pasture. Again, as veterinarians, this is a time where we can perform what’s called a faecal egg count to see if your horse is passing any bots in their manure. As the weather starts to warm up, the parasite develops into a fly and the lifecycle is complete.

As you can see, the best time to treat your horse for bots is at the end of Autumn and the start of Spring, particularly in temperate environments. This will kill any bots that are in the stomach.

Wormers that contain Ivermectin and Moxidectin are effective against bots so it is important to consult with your veterinarian to discuss which wormers you should use as part of a year round worming program.

During the warmer months, another way to limit the chances your horse will develop bots is to remove the bot eggs from the hair on a regular basis. There are a number of combs and blocks designed to remove bots from the hair. That being said, bot eggs are sometimes difficult to see and even removing all the visible eggs doesn’t mean that your horse won’t develop bots in the stomach.

Regularly picking up manure from the paddock will also reduce the chances of the flies hatching near your horse and laying eggs. That can help break the cycle. If you are spreading manure on your pasture, the manure needs to be composted. This can help kill the parasite and allow you to use manure as a fertiliser. There are also many different types of insect repellents that can be used around your stable to help reduce the number of bot flies.

In conclusion, as we are in winter, it is important to discuss your horse’s year round worming strategy and make a plan to reduce the impact of bots when coming into the warmer months.

As winter progresses, a faecal egg count is an important procedure to see what your horse’s worm burden is and to choose the appropriate dewormer. If your horse has episodes of colic, performing a gastroscope can rule in or out the presence of bots in the stomach.

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