Hypoderma bovis, H. lineatum: Ox warbles or cattle grubs; heel or bomb flies.

General Description: Adults are hairy flies without functioning mouth parts. Thus they do not feed. The abdomen has three bands of hairs, light yellow hairs toward the front, dark hairs in the middle and orange-yellow hairs at the rear. The name "bomb flies" comes from the adults´ habit of diving at the cattle. H. bovis is 15mm long. H. lineatum is 13mm. The larvae or grubs are large with a segmented surface tapering at both ends.

Life Cycle: Adult flies are most active on warm days of summer months, attacking cattle to deposit eggs, 1mm long, on hairs of the leg, sometimes on the body. Females may lay 100 eggs on one animal. Tiny larvae hatch in about 4 days and crawl down the hair and penetrate the skin. Once in the skin the larvae migrate through the subcutaneous tissue toward the back of the animal, growing as they move. Hypoderma bovis larvae usually go directly toward the back and are 27 to 28mm long when they reach it. Larvae of H. lineatum travel to the wall of the esophagus and rest there through autumn before continuing on to the back, reaching a size of 25mm. Swellings about 3cm in diameter form around the larvae, which perforate the skin in order to breathe. This stage lasts about 1 month. In spring the larvae leave through the holes, drop to the ground, and burrow into soil to pupate. Flies emerge 34 to 36 days later. Life cycle generally spans 1 year.

Location: Adult flies lay eggs on hairs of legs and occasionally the body. Larvae live in the subcutaneous tissue of the back.

Geographical Distribution: Many countries of the northern hemisphere.

Significance: Migrating larvae cause much damage to skin and underlying flesh. Affected areas of the carcass must be discarded at slaughter. Annual losses in the U.S. are $200 to $400 million. Adult flies cause sufficient annoyance to cattle to decrease feeding, and cause weight loss. Cattle attempt to escape fly attacks by running to shade or water. Stampedes may result from the activity of only a few flies. Poor quality hides due to grub damage cause serious economic loss.

Effect on Host: Enzymes are released by the larvae to break down tissue and ease their movement as they migrate to the back. The tissue through which larvae travel becomes greenish-yellow. Allergic reactions to larvae may occur if they die under the skin naturally or if killed during attempts to remove them manually. The larvae emerge through breathing holes in the skin, leaving a large hole which makes the hide worthless. The disease is referred to a cutaneous myiasis.

Diagnostic Information: Eggs may be found on hair of the animal´s legs. Larvae in cysts or lumps under the skin indicate ox warble infestation.

Control: Insecticides are used to kill migrating larvae, but infections and allergic reactions may occur when the larvae die. Systemic use of some chemical agents (eg. ivermectin) has been effective when applied after a frost, when flies have died.

Hypoderma – adult fly		Eggs attached to hair – H.Lineatum		Hypoderma – larvae leaving through breathing hole on back
Back of cow severely infested with Hypoderma		Hide showing damage by Hypoderma		Hypoderma – pupa on grass
Larva in epidural fat in spinal canal