Hypoderma bovis and H. lineatum

Hypoderma bovis and Hypoderma lineatum are the common warble flies of cattle, and occasionally other hosts, in the northern hemisphere.  They used to be common in cattle in western Canada, but effective treatments and legislated control programs have greatly reduced their occurrence.  There are, however, still localized hot spots of infestation.

Adult Hypoderma are free-living and do not feed on animals. During the summer, after mating the females lay eggs on the hairs of cattle, usually on lower parts of the body.  After a few days, the larva that has developed in each egg hatches and penetrates the skin.  With H. bovis subsequent larval migrations follow the nerves to the epidural fat, where the larvae remain for the winter,  With H. lineatum, larval migrations follow connective tissue towards the diaphragm and then on to the sub-mucosa of the oesophagus, where overwinter.  In the late winter the larvae of both species move into the subcutaneous tissues along the back and perforate the skin.  After several weeks they drop to the ground and pupate.  The adult flies subsequently released from the pupae are most active in warm weather, and mate and die within a couple of weeks.

Warble flies are significant because of the damage they do to cattle hides, especially the perforations they cause and the inflammatory changes they create in the subcutaneous tissues.  The behaviour of the adult female flies during egg laying can also be very troublesome, especially for dairy cows.  The flies make a buzzing noise, dive bomb the cattle, and often make repeated attacks.  Affected animals panic and run about and weight gains, and milk production can be reduced.  Also, in regions where Hypoderma is common, treatment too late in the fall can kill the migrating larvae in the spinal canal (H. bovis) or oesophagus (H. lineatum), with the possibility of paresis or bloat.

Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda
Order: Diptera
Suborder: Cyclorhapha
Family: Hypodermatidae

As well as H. bovis and H. lineatum of cattle, the genus includes the holarctic H. tarandi of caribou, reindeer and some deer species, and H. actaeon and H. diana of elk and some deer species in the Palearctic.

Adult Hypoderma species of cattle are robust flies, up to approximately 15 mm in length, with the general appearance of bumble bees with yellowish hairs over most of their body. They have only vestigial mouthparts.

Third-stage larvae of Hypoderma located beneath the skin of the backs of cattle are also robust, measure up to approximately 30 mm in length, and have transverse rows of spines. Initially the larvae are yellowish, but become brown or black before emergence.

Hypoderma bovis and H. lineatum occur in cattle in many parts of the northern hemisphere, and although introduced into the southern hemisphere with imported cattle, reports of their establishment in that part of the world are very rare. In western Canada and elsewhere, Hypoderma larvae are sometimes found in horses, usually in the saddle region, and very occasionally in people.

In cattle in western Canada, Hypoderma species are less common than twenty to thirty years ago. This reduced occurrence is probably because of government-sponsored control programs and the widespread use of very effective endectocides. There are, however, still hot-spots of infestation.

Adult Hypoderma species are free-living and do not feed. In summer, the females lay eggs on the hairs of cattle. Hypoderma bovis lays eggs singly on hairs of the rump and upper hind limb; H. lineatum lays eggs in groups on the hairs of the lower parts of the limbs and the ventral body surface. The eggs of both species are equipped with a specialized attachment organ which fixes them to the hair.

The eggs hatch spontaneously within a week and the first-stage larvae penetrate the skin and migrate through the body. Details of the larval migration routes are not fully known, but H. bovis larvae travel through the spinal canal, and those of H. lineatum through the wall of the oesophagus.

In late winter, larvae of both species arrive beneath the skin of the back. Here they develop through the second to the third larval stage and make a breathing hole in the overlying skin. In the spring, the third-stage larvae emerge from the back, drop to the ground, pupate, and the adults flies hatch. In Canada there is one generation of flies each year.

Life Cycle: Hypoderma bovis

Life Cycle: Hypoderma lineatum

Hypoderma species will do well where there are susceptible cattle, where there are suitable conditions for the survival and development of the parasite’s free-living stages on the ground, and where treatment and control measures are inadequate or absent. In Canada, treatments are usually in the fall during the early stages of migration by the larvae, and before they have reached the spinal canal or oesophagus.

The flies, especially H. bovis, are extremely irritating as they approach the cattle and lay eggs, and they may interfere with feeding and production, and cause the cattle to stampede ("gad about"). The larvae of H. bovis in the spinal canal, and those of H. lineatum in the oesophagus, may be associated with neurological signs or bloat, respectively, especially if they are killed by treatment while in these locations. These complications are much less common in Canada than previously, presumably because the incidence of the infestation has declined. The breathing holes in the hide caused by the larvae are a cause of economic loss.

The characteristic swellings and then holes along the back, in the late winter and spring are indicative of infestation, particularly if a larva can be extracted. Be careful not to rupture the larvae while they are still in the cattle, as this may result in an allergic reaction. There are also serological tests for the detection of Hypoderma, especially on a herd basis, but they are not used in the field in Canada.

There are several products approved in Canada for treating the migrating larvae of Hypoderma species. It is important to time treatments to minimize the risk of killing the larvae in the spinal canal (H. bovis) or the oesophageal wall (H. lineatum).

Products for Hypoderma species

Drug	Product
Doramectin	DECTOMAX
Eprinomectin	EPRINEX
Ivermectin	VARIOUS
Moxidectin	CYDECTIN

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

In the past, many jurisdictions in the northern hemisphere have introduced government sponsored control programs for Hypoderma species. These usually involve some mandatory combination of checking for the infestation, treatment (in the fall) and restrictions on the movement of infested animals. Because Hypoderma has a great capacity for the regeneration of populations, control really means eradication. In addition to these efforts, considerable research has been directed to the development of a vaccine for Hypoderma species in cattle, but none is yet commercially available anywhere.

A recent study in western Canada demonstrated, using serological testing to detect infestation, that Hypoderma can maintain a high prevalence in cattle subjected to a rigorous, multi-year program of fall treatments for warbles.

Human infections with larvae of Hypoderma species have been reported very rarely, and sometimes involve the eye, with very serious consequences.

Cortinas R et al. (2006) Ectoparasites of cattle and small ruminants. Veterinary Clinics of North America Food Animal Practice 22: 673-693.