H bovis and H lineatum (warble flies) are occasional parasitic flies of horses.
These flies, primarily affecting cattle, are found between 25° and 60° latitude in the northern hemisphere in >50 countries of North America, Europe, Africa, and Asia. In North America, H lineatum, the common cattle grub, is found in Canada, the USA, and northern Mexico; H bovis, the northern cattle grub, is generally found north of the 35th parallel. Occurrence in cattle and American bison is common. Larvae of Hypoderma spp also have been reported in horses. The prevalence of both Hypoderma species has declined dramatically in North America.
Adult Hypoderma, known also as heel flies, are ~15 mm long, hairy, and bee-like in appearance. In late spring or early summer, they attach their eggs on the hair of cattle, particularly on the legs and lower body regions. The eggs hatch in 3-7 days, and first-stage larvae travel to the base of the hair shaft and penetrate the skin. Normally, the first-stage larvae travel through the fascial planes between muscles, along connective tissue, or along nerve pathways. They secrete proteolytic enzymes that facilitate their movement. During fall and winter, larvae migrate toward 2 different regions, depending on the species. H lineatum larvae migrate to the submucosal connective tissue of the esophageal wall, where they accumulate for 2-4 mo. H bovis larvae migrate to the region of the spinal canal, where they are found in the epidural fat between the dura mater and the periosteum for a similar period.
Beginning in early winter, the larvae arrive in the subdermal tissue of the back of the host where they make breathing holes through the skin. Cysts or warbles form around the larvae, which undergo 2 molts (second and third stage). The warble stage lasts 4-8 wk. Finally, third-stage larvae emerge through the breathing holes, drop to the ground, and pupate. Flies emerge from the pupae in 1-3 mo, depending on weather conditions. Adult flies, which do not feed, live <1 wk. The life cycle is complete in 1 yr.
For the 2 species, seasonal events are similar except that those for H lineatum occur ~6-8 wk earlier than those of H bovis . These events vary from year to year but correlate with local and regional climatic conditions. Larvae first appear in backs of cattle about mid September in southern USA but not until late January or later in northern USA. Grubs first emerge from the back during the last half of November in Texas and during the first half of March in Montana. When both species are present, grubs may appear in the back for ~5-6 mo; when only one species is present, for ~3-4 mo. The activity of ovipositing (by female flies) is at its height from January to March in southern USA and from May to July in northern USA.
During periods of sunshine on warm days, cattle may run with their tails high in the air when chased by female heel flies, particularly H bovis. Not all stampeding or “gadding” of this kind is the result of heel fly attacks, as this activity has been seen in the absence of heel flies.
In otherwise normal cattle, H bovis larvae and their secretions in the epidural fat of the spinal canal are associated with dissolved connective tissue, fat necrosis, and inflammation. Sometimes, the inflammation extends to the periosteum and bone, producing a localized area of periostitis and osteomyelitis. Occasionally, the epineurium and perineurium may become involved. In rare severe cases, paralysis or other nervous disorders may occur. Similarly, H lineatum in the submucosa of the esophagus may cause sufficient inflammation and edema in the surrounding tissues to hinder swallowing or eructation. It is unusual, however, for clinical signs of parasitism to be evident during the migratory phase.
Penetration of the skin by newly hatched larvae may produce a hypodermal rash, most often in older, previously infested cattle. The points of penetration are painful and inflamed and usually exude a yellowish serum. Warbles may occur in the back from tailhead to shoulders, and from topline to about one-third the distance down the sides. Usually, the cysts are firm and raised considerably above the normal contour of the skin. In each cyst, there is a breathing hole, ranging in size from a small slit to a round hole (3-4 mm in diameter) for more mature larvae. Generally, secondary infection is depressed; however, cysts may occasionally develop in large, suppurating abscesses. The emergence of the grub, its forced expulsion, or its death within the cyst usually results in healing of the lesion without complications. Carcasses and hides of cattle infested with cattle grubs show marked evidence of the infestation and are reduced in value.
An infested animal may have 1 to ≥300 warbles but generally <100; infested herds often have individual animals with no grubs. Young animals are most heavily infested.
If migrating Hypoderma die in esophageal tissue (H lineatum) or near the spinal cord (H bovis), they can cause severe reactions that are sometimes fatal. These reactions appear to be related to the numbers of grubs but are rare in any case.
Death of first-stage larvae of H bovis in the spinal canal of cattle after systemic insecticide treatment has resulted in stiffness, ataxia, muscular weakness, and paralysis of hindlimbs. Recovery is usually rapid and complete, but occasionally, paralysis may be permanent.
Death of first-stage larvae of H lineatum in the submucosal connective tissue of the esophagus causes inflammation of the esophageal wall, dysphagia, drooling, and bloat. Again, recovery is usually rapid and complete (48-72 hr after treatment), but in severe cases, the bloat may be fatal. Rupture of the esophagus may be caused by attempted passage of a stomach tube in an affected animal.
Third-stage larvae can be easily differentiated. H bovis is generally larger and has no spines on the tenth segment and a funnel-shaped spiracular plate; H lineatum is smaller and has spines on the tenth segment and typically a flat spiracular plate. In cases of bloat or paralysis, the presence of disintegrating grubs and the associated hemorrhage and tissue damage distinguishes animals that are parasitized from those that are not.
Systemic insecticides in various formulations are available for treatment. Pour-on treatments of the organophosphates famphur and fenthion or the macrocyclic lactones doramectin, eprinomectin, ivermectin, or moxidectin are poured evenly along the midline of the back. Fenthion in a 20% formulation is applied to a single spot on the midline. Some products must not be applied when the skin or hair coat are wet or when rain is expected to wet cattle within 6 hr. The application site should be free of skin lesions, mud, or manure.
Doramectin and ivermectin are systemically active against cattle grub larvae when administered as a SC injection. Famphur and ivermectin are also available as an oral paste. The injectable and pour-on systemic treatments are approved for control of Hypoderma and other myiasis-causing flies in many countries. Coumaphos may be applied as a whole body spray for control of Hypoderma ; however, in the USA the practices of dipping or spraying cattle for cattle grub control have been replaced by the pour-on and/or injectable treatment methods.
No organophosphate systemic agent should be used in conjunction with another because their actions may be synergistic. Horses stressed by castration, overheating, vaccination, or shipping should not be treated.
On small groups of tractable animals, extraction by instrument or hand expulsion (by squeezing) of the individual grubs is effective. Rarely, when this procedure is performed carelessly, the grub is crushed in its cyst and an anaphylactic reaction results.