Bot fly larvae.

Asked February 4, 2013, 12:45 AM EST

I just purchased a new horse that has rubbed her tail head enough to have broken off a lot of the hair at the tailhead. I suspected a wormload so I dewormed her with a full tube of ivermectin. The next day I found many plump red larvae in her manure. Some of the larvae were dead but sone were alive. Should I deworm again, and with what product and how soon? My newly purchased horse is an ex racehorse who has been out to pasture for two years. She is 6 years old and about 1100 lbs.

Hays County Texas horses parasites deworming

2 Responses

The best way to tell if the first dose was effective in getting rid of all parasites would be to wait the appropiate dosing range and then have your veterinarian do a fecal egg count to decide if they should be dewormed again. For Ivermectin this range is 4-8 weeks. By testing instead of just doing another dewormer it will help decrease the risk of creating resistant parasites.

Hope this helps.

I expect the plump red larvae were larvae of the Horse Bot Fly; the larvae are called Horse Bots and are maggots which are segmented with spines, elongated, and pea-size

The Horse Bot Fly is normally active in Hays County Texas during August laying eggs on horses' front legs. When a horse licks the eggs, the larvae hatch into the mouth, burrow in the mucosa, and migrate to the stomach for larval development of 8 months, after which time pass in the manure for pupal development into adult flies. The ivermectin killed all the bots, and the bots passed from the stomach. Your horse will not be reinfested with bots until next fly season in August.

If worms were passed, they are not plump but are small and worm-like, are normally adults of Small Strongyles. Since reinfection is common on contaminated pastures by ingestion of larvae on wet grass throughout the year, a deworming program will include deworming at 2-6 month intervals, depending on level of exposures to contaminated pastures with multiple head of horses.