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Parasites - Bot fly

Horse licks or rubs the skin on legs, chest and flanks and allows eggs to break open and hatch. Larvae (maggots) burrow into lips and tongue for three to four weeks before emerging again at the back of the throat to be swallowed and attach to the stomach wall.

Adult bot grubs live attached to the wall of the first part of the horse's stomach for several months.
Adult flies hatch from pupae in the warmer months. Adult flies live two to four days, do not sting, but annoy horses while trying to lay eggs. Eggs on the horse's hair may remain infective for about 7 weeks. Regular removal of bot eggs from coat is an effective control measure at this point. Bot grubs pass out in the manure usually in the warmer months and find a protected spot to pupate. Bot pupae cannot survive cold conditions once they are passed out of the horse.

Although an important internal parasite of horses, the bot is not a worm as are some other parasites. The bot is the instar or larva of a fly - the bot fly. There are three types of bot fly in Australia (Gasterophilus intestinalis, Gasterophilus nasalis and Gasterophilus haemorrhoidalis), but since their behaviour and effects on the horse are similar, they will be discussed as a group.


The life cycle of the bot is illustrated above.

Adult female flies appear in summer and early autumn (the precise time may vary from state to state). They lay their eggs on the hairs of the horse's coat. One type of fly (G. intestinalis) lays its eggs on the flanks and forelegs of the horse. The adherence of the eggs to hair causes considerable irritation to the horse. Larvae emerge from the eggs in a few days in response to the horse's licking, and penetrate the horse's mouth and tongue. The other two types of fly (G.nasalis and G. haemorrhoidalis) lay eggs around the horse's face. These eggs hatch out spontaneously into larvae which migrate through the skin to the tissues of the mouth. During the period of egg laying, the flies, which are large, dark yellow and hairy, cause considerable annoyance to the horse, although they do not bite.

The larvae migrate and develop for a variable period (some weeks) in the mouth, where they may cause the development of ulcers and pusy discharges, particularly in the gums between the teeth. From the mouth they eventually reach the stomach where they develop into the characteristic bot attached to the stomach lining. The bot is about 1 cm long, plump and segmented. It varies in colour from pink to dark brown.

At the end of the winter, the bot releases its hold on the stomach lining where it has been feeding and passes out in the horse's droppings. 

On the ground, the bots burrow into the soil and form pupae. It is from these pupae that the adult flies emerge in the warmer months, and so complete the cycle.


Apart from the annoyance caused to the horse by the adult fly and the damage caused by the larvae migrating through the mouth, bot infection has also been associated with ulceration of the stomach, and occasionally penetration of the stomach wall resulting in peritonitis and death.

Treatment with oral wormers, is a simple and effective way to control bots.

Treatment Chart Click here


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