BOTS are a common parasite that can affect horses. They are the immature form (larvae) of the adult flies named Gasterophilus Intestinalis, which look similar to small bumble bees.

The adult flies are not parasitic and cannot feed. They only exist long enough to mate and lay eggs on the horse’s forelimbs and shoulders, in late summer and autumn. Sometimes the eggs can be seen on the abdomen and other areas of the horse’s upper body.

Horse owners will recognise bot eggs as little yellow eggs which are unsightly, particularly if they appear on dark-coloured show horses. The eggs hatch after approximately one week when stimulated, usually by the horse licking.

DANGER

The larvae then remain embedded in the tongue or the mouth for a number of weeks, where they can cause mouth ulcers. The larvae then pass to the stomach where they attach themselves to the stomach wall. Bots can lead to mild gastritis and other gastric problems, stomach ulcers, poor body condition and low grade colic.

After development in the stomach for approximately 8–10 months, they pass out in the faeces and develop into maggot-like pupae, remaining in the soil for three to five weeks, after which the adult bot fly emerges.

The buzzing sound made by the adult flies when they lay their eggs on the horses’ legs etc, may alarm or annoy horses causing them to over-gallop or bolt in the field, increasing the risk of injury.

Evidence of bots cannot be found in faecal egg counts for worms because, the eggs are laid externally (on the horse’s legs, etc) by the adult bot fly. There are no eggs laid by the bot worm internally to be passed out in the horse’s faeces externally.

If there are bot eggs on the horse’s coat, then it is reasonable to assume that the horse has been exposed to bot larvae.

TREATMENT

It is still very important to remove bot eggs as this will help break the bot fly’s life cycle. A bot egg knife (blunt knife) or a bot egg comb may be used for this purpose.

Most deworming programmes for other intestinal parasites will also be effective against bots.

A dewormer such as ivermectin or moxidectin, used twice a year, will usually eliminate bots. Dosing in August/September will target the migratory phase of bots.