Two species of Hypoderma, H bovis (common cattle grub) and H lineatum (northern cattle grub), are economically important and primary pests of cattle. They 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 is found in Canada, the USA, and northern Mexico, whereas H bovis is generally found north of the 35th parallel. Occurrence in cattle and American bison was historically common. However, widespread use of macrocyclic lactones have decreased the prevalences of H lineatum and H bovis in cattle in North America. H tarandi parasitizes native Cervidae and reindeer in arctic and subarctic regions. Larvae of Hypoderma spp also have been reported in horses, sheep, goats, and people.
Adult Hypoderma, known also as heel flies or gad 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 two 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 (central punctum) through the skin. Cysts or warbles form around the larvae, which undergo two 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 two 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, third-stage larvae may appear in the back for ~5–6 mo; when only one species is present, the larvae may appear 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. This behavior in cattle is referred to as "gadding" and is a strategy to avoid female flies and their attempt to deposit eggs. Not all stampeding or gadding of this kind is the result of heel fly attacks, because this activity has been seen in the absence of heel flies. Gadding in cattle may result in loss of production, altered reproduction, self-injury, or death.
In otherwise healthy 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 be found in the back from tailhead to shoulders, and from topline to about one-third the distance down the sides. Usually, the warbles are firm and raised considerably above the normal contour of the skin. In each warble, 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, warbles may occasionally develop in large, suppurating abscesses. The emergence of the third-stage larvae, their forced expulsion, or their death within the cysts usually results in healing of the lesions 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 larvae. 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, sometimes fatal, reactions. These reactions appear to be related to the numbers of larvae but are rare in any case.
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.
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.
Concurrent with the decrease in the prevalences of H lineatum and H bovis in cattle, widespread use of macrocyclic lactones has also led to a decline in the clinical relevances of these parasites in North America. However, serologic surveys have demonstrated that cattle are still exposed to Hypoderma spp. Should treatment programs stop or move away from products that have efficacy against Hypoderma spp, clinical signs due to infestation will likely reappear.
Third-stage larvae of Hypoderma spp are found in warbles, the furuncle-like nodules or cysts along the back of cattle. On recovery from a warble, third-stage larvae can be easily differentiated. H bovis is longer (27–28 mm) and has no spines on the tenth segment and a funnel-shaped spiracular plate. H lineatum is slightly shorter (25 mm) 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. ELISA-based tests have been developed to detect anti-Hypoderma antibodies in sera and milk. These ELISA tests are often used to measure risk of exposure and do not correlate well with observation of clinical hypodermosis in cattle.
Systemic insecticides in various formulations are available for treatment. Pour-on products containing macrocyclic lactones (doramectin, eprinomectin, ivermectin, or moxidectin) are poured evenly along the midline of the back. 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. Cattle stressed by castration, overheating, vaccination, or shipping should not be treated.
In the USA, registrations of most, if not all, systemic organophosphate insecticides for cattle grub control have been cancelled, and the practices of dipping or spraying cattle for cattle grub control have been replaced by pour-on and/or injectable treatments.
Doramectin and ivermectin are systemically active against Hypoderma larvae when injected SC. Ivermectin is 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.
Eprinomectin and moxidectin pour-on formulations are approved for treatment of both beef and dairy cattle. Otherwise, use of drugs for cattle grub control is prohibited in dairy animals of breeding age. Because residues may be present in cattle for varying periods after treatment, withdrawal times for all treatments must be observed.
In areas where Hypoderma spp are prevalent, cattle, especially calves, should be treated as soon as possible after the end of the heel fly season. They should not be treated later than 8–12 wk before the anticipated first appearance of grubs in the backs, because adverse reactions may occur when migrating larvae are killed.
Where systemic insecticides cannot be used, Hypoderma spp larvae can be controlled by applying tetrachlorvinphos dust to the warbles in the back. The dust should be applied to the animal’s back and worked into the grub holes. Because new grubs continue to appear in the back, treatment must be repeated every 30–45 days during the warble season.
On small groups of tractable animals, mechanical extraction by instrument or hand expulsion (ie, squeezing) of the individual larva is effective. Larvae can also be removed by carefully injecting 1 mL of 3% hydrogen peroxide with a blunt cannula through the central punctum. However, care must be used to not rupture or pierce the larvae during removal. Rarely, when these procedures are performed carelessly, larvae are crushed or pierced, inducing an anaphylaxis-type reaction. This anaphlyaxis is believed to result from the overwhelming and sudden release of Hypoderma spp antigens. Experimental studies have also demonstrated that this anaphylaxis-type reaction is due to the release of toxins from Hypoderma larvae.
Research to develop vaccines against Hypoderma spp infestations was conducted as early as the 1950s. Even though some natural and recombinant antigens were demonstrated to have efficacy against Hypoderma spp infestations, the high efficacy and ease of use of macrocyclic lactones have precluded modern vaccine development.