Warbles (Grubs)

Causative agent: Hypoderma bovis, Hypoderma lineatum
During the spring and summer warble flies, the adult stage of hypoderma, lay their eggs on the legs and lower body of cattle, bison, some species of deer and occasionally horses (9). The eggs hatch into larvae which penetrate the skin and migrate to the esophagus (H. lineatum) or the spine (H.bovis), where they continue growing. As spring approaches the larvae migrate to a position just below the skin along the back. They make an air hole in the skin and then emerge and fall to the ground where they pupate. Adult flies emerge from the pupae to complete the cycle (9).
There is some evidence that hypoderma larvae may not be able to penetrate through the skin and dense hair on the back of bison. In Montana and Yellowstone National Park, bison were found to have dead, discolored larvae from previous years infestations underneath the hide along the back (42).
Hypoderma larvae have been found around the esophagus of bison in Montana (42), Yellowstone National Park (42), and Oklahoma (43). The location of the larvae suggests that the species of hypoderma that infected these bison was H. linatum. In Michigan, hypoderma larvae were found along the spinal chord in bison (44), suggesting that the species of hypoderma in this case was H.bovis.

Clinical signs:
In cattle, the clinical signs of Hypoderma spp. infestation are associated with poor growth and swellings along the back that are present during the spring (9). Clinical signs associated with hypoderma infestation in bison have not been reported.

Postmortem findings:
Postmortem findings of hypoderma infestation in bison include larvae migrating adjacent to the esophagus, along the spinal chord, and underneath the skin of the back (42,43,44).

Palpating the midline of the back of bison during the spring for swellings containing hypoderma larvae may make the diagnosis.

Organophosphates and ivermectins kill Hypoderma spp. larvae. The timing of treatment is very important. Treatment should be instituted before the larvae reach critical areas such as the esophagus and the spinal chord. If the larvae are killed when they are in these areas the dead larvae may cause serious harm to the bison. In Michigan, 23 out 70 bison were killed as a result of improper timing of the treatment of bison with an organophosphate (44). The timing will vary from area to area depending on the time that the peak of the hypoderma fly season occurs. In Michigan, mortalities were associated with organophosphate treatments that were applied to bison in December (44). Bison producers should consult with local veterinarians as to the optimal time for the application of parasiticides to their bison.

There have been no programs reported for the control of hypoderma in bison. In endemic areas, yearly applications of parasiticides for the control of hypoderma will be required.


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