1. General Considerations

   1. Hypodermosis is also referred to as “warbles” or “grubs.” It is a worldwide disease.
   2. It is common in cattle and goats but occurs occasionally in horses and sheep.
   3. Hypodermosis is an economically important problem in cattle, goats and sheep.
   4. Cattle grubs cause significant economic loss from damage to the meat and hide.
   5. Cattle are the normal host and have the most severe infestations.
   6. Hypodermosis can occur in horses that are in close proximity to cattle.
   7. Younger horses and those in poor condition seem to be more susceptible.

2. Cause and Pathogenesis

   1. Warbles or grubs are the larvae of the flies Hypoderma bovis (northern states) and Hypoderma lineatum (southern states).
   2. Adult flies are active in the hottest part of the summer and lay their eggs by attaching them to the hairs.
   3. The life cycle is completed in 12 months.
   4. The adult female resembles a hairy bee. She lays eggs on the hair of the legs and abdomen of cattle.
   5. The eggs hatch in 2 to 6 days and the first stage larvae migrate along the hair and penetrate into the host’s skin and subcutaneous tissue.
   6. The larvae then migrate to the abdominal cavity and reach the site for their further development in 4 to 6 weeks.
   7. Larvae of Hypoderma lineatum migrate to the submucosal connective tissue of the esophagus and Hypoderma bovis larvae migrate to the region of the spinal canal and epidural fat.
   8. The first stage larvae remain in this location for 5 to 8 weeks (winter).
   9. During January and February, the now second stage larvae migrate to the subcutaneous tissues of the back where they make a hole to breathe through the skin.
   10. They remain in this location for 5 to 8 weeks (the spring resting site) where they mature to third-stage larvae.
   11. Subcutaneous swellings form and the skin over each swelling becomes perforated (breathing pore). This is the “warble” stage of infestation and it lasts 30 days.
   12. In spring, the mature larvae wriggle out of their cysts and fall to the ground to pupate.
   13. Most grubs fail to reach normal size and complete their life cycle in aberrant hosts such as the horse.
   14. Economic losses result from the following: (1) larval damage to the hide and the resulting carcass trimming at slaughter; (2) decreased weight gain and milk yield due to the annoyance from adult flies; (3) occasional cases of bloat and posterior paralysis; and (4) anaphylactic shock resulting from migrating larvae.
   15. Migrating larvae in subcutaneous tissues cause areas of edema.
   16. Secondary infection with bacteria can occur and cause abscesses to form.

3. Clinical Signs

   1. Bovine:
      1. Multiple skin nodules along the back with small breathing holes. Pain is variable.
      2. Some may abscess.
      3. Older cattle have fewer grubs than younger cattle.
      4. Signs related to the path of larval migration such as esophagitis, bloat, and complete or partial paralysis of the hind limbs can be seen.
   2. Equine:
      1. Small nodular swelling develops on the dorsal surface of the animal, frequently in the region of the withers.
      2. Most affected horses have only one or two grubs. Occasionally large numbers develop.
      3. If Hypoderma spp. is the cause of the nodules, most lesions should have a breathing pore.
      4. Migration to the brain with acute neurologic disease may develop.

4. Diagnosis

   1. Characteristic clinical appearance.
   2. Demonstration of larvae inside the nodules.
   3. The season, location of the lesion(s), and the presence of a breathing pore is diagnostic.
   4. The most important differential diagnosis in horses is collagenolytic granuloma. The nodules of collagenolytic granuloma are similar in appearance and location and also tend to occur at the same time of the year but they lack a breathing pore.
   5. Dermoid cysts and neoplasia should also be considered as differentials.

5. Treatment

   1. Bovine:
      1. Topical organophosphate treatments in the form of spray, dip or pour-on can be used. Examples include fenthion, famfur, phosmet, ronnel, trichlorfon, crufomate, and coumaphos.
      2. Ivermectin is effective against all three larvae stages. Topical dose: 0.5% solution poured down the back at a dose of 0.5 mg/kg. Long-acting ivermectin injection dose: 1mL/50 kg of 3.15 % w/v ivermectin (Ivomec® Gold, Merial).
      3. A report showed that a single application of doramectin pour-on at the dosage of 0.5 mg/kg (1 ml/10 kg) was very efficacious against Hypoderma bovis and Hypoderma lineatum.
      4. The time of treatment is correlated with the end of the fly season.
      5. Death of Hypoderma lineatum larvae in the wall of the esophagus may cause esophagitis and bloat.
      6. Death of Hypoderma bovis larvae in the spinal canal may result in meningitis, periostitis, osteomyelitis, and posterior paralysis.
      7. Therapy includes the administration of glucocorticoids, phenylbutazone and antibiotics if secondary bacterial infection is present.
      8. Animals that recover usually do so within 3 to 5 days.
   2. Equine:
      1. In most cases, the simple enlarging of the breathing pore with a scalpel blad and extraction of the larva is all that is needed.
      2. Do not break the larva since local inflammation or an anaphylactic response may occur.
      3. In parts of the country with high incidence of the disease, treatment with organophosphates (13.4% cruformate, 30 ml/45 kg) after all eggs have hatched may be indicated as a preventive. Moreover, oral ivermectin at the anthelmintic dose of 200 µg/kg has shown to prevent hypodermosis in horses.

References

Borges F, Sybrecht GW, von Samson-Himmelstjerna G. First reported case of Hypoderma diana Brauer, 1985 (Diptera: Oestridae)-associated myiasis in a horse in Germany. Equine Vet Educ 2019; 31: 122-125.

Boulard C. Durably controlling bovine hypodermosis. Vet Res 2002; 33: 455-464.

Otranto D, Johnson G, Syvrud K et al. Treatment and control of bovine hypodermosis with ivermectin long-acting injection (IVOMEC® GOLD). Parasit Vectors 2016; 9: 551.

Rooney KA, Illyes EF, Sunderland SJ et al.  Efficacy of a pour-on formulation of doramectin against lice, mites, and grubs of cattle. Am J Vet Res 1999;  60:402-404.

Scholl PJ. Biology and control of cattle grubs. Annu Rev Entomol 1993; 39: 53-70.

Scott DW. Parasitic Diseases.  Large Animal Dermatology. 1st edn. Philadelphia, PA: W.B. Saunders, 1988;  245-248.