Parasite Profile: Gastrophilus spp of the horse

Told you I was studying for my equine parasitology test.


Gastrophilus spp. of Horses

Common Name: "Horse Bot"


Life Cycle: There are three species of Gastrophilus; Gastrophilus nasalis, Gastrophilus intestinalis, and Gastrophilus hemorrhoidalis. They have very similar life cycles and differ mostly in which part of the horse they choose to lay their eggs on.

Gastrophilus larva are passed with feces and pupate in sandy soil before hatching from the pupal case as a small bee-like fly. The fly lays its eggs on the horse's neck (nasalis), forelegs (intestinalis) and rectum (hemorrhoidalis) and then dies.

The eggs of the Gastrophilus are very sticky and difficult to remove, and the horse will often lick or chew at the intestinalis eggs on its forelegs. These ova are sensitive to saliva and will hatch when exposed to it. From there, they are either ingested as the horse chews the area, or they do as the other two species do, and migrate under the skin to the mouth. Once inside the mouth, Gastrophilus burrows into the tongue and cheeks of the hourse and migrates around in the mouth for 3-4 weeks before being swallowed and passed to the stomach where they attach and settle in. The larva will then spend the winter in their cozy, warm home and pass to the outside world again once spring arrives.

Damage, Diagnosis and Treatment: The adult fly of Gastrophilus has no mouth parts, no way of biting or feeding, and is generally innocuous. The problem is, horses don't know this. Gastrophilus makes an irritating buzzing sound that drives horses absolutely around the bend, and the most common damage from this parasite is injury due to the horse running into or off of something.



This is a pretty mild example of how a horse will react to the buzzing noise Gastrophilus makes, but it gives you the idea. Horses have cold cocked themselves running into trees, broken their legs tripping over objects, and even run straight off of cliffs trying to escape this little fly. Aside from the potential injuries, a heavy load of Gastrophilus can cause some gastric irritation or obstruction in the digestive system.

Diagnosis is made by identifying the ova laid on the horse, and treatment is twofold. Eqvalan or Zimectrin are used to kill the larval stage and the ova must be removed from the horse with warm water and a razor. Fly sprays, masks and other common fly control measures can also be helpful in controlling Gastrophilus.