Bots are not worms contrary to popular belief, as they actually belong to the fly family. They are an internal parasite of the horse as part of their lifecycle involves internal development. The fly lays its eggs on the abdomen, legs and throat of the grazing horses in late summer, when they are then licked off by the horse the larvae are stimulated, hatching and burrowing into the lining of the horses gums.
They then migrate to the stomach after about one month, which becomes inflamed and ulcerated. Eventually, around 10 months later, the bots are passed out in the dung. Bot eggs can be seen on the horses coat during summer and early autumn, and can be removed with special bot knives or bot scraping blocks.
The entire population of Bots reside in the stomach over winter, so that is why traditional thinking was to treat with Ivermectin or Moxidectin based products preferably in December or January, as the adult bot flies are killed off by freezing temperatures. However, with weather patterns changing leading to milder winters without early frosts it might be advisable to treat twice per year.
If treatment is given before the bot flies have stopped egg laying, the larvae will still be capable of causing damage throughout the winter and into spring. Additionally, treatment being delayed until after the first frost gives the larval stages burrowing in the horse’s mouth and stomach the chance to cause damage. Therefore it is important to use products capable of killing all stages of bots perhaps during summer and autumn whilst the flies are still active, and then again later in the winter after a true hard frost.