Bovis ISSN 2398-2993

Warble fly (hypoderma bovis)

Synonym(s): Gad flies, cattle grub

Contributor(s): Mike Taylor , Andrew Forbes

Introduction

Classification

  • Phylum: arthropoda.
  • Class: insecta.
  • Family: oestridae.
  • Subfamily: hypodermatinae.

Active Forms

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Clinical Effects

Epidemiology

Habitat

  • Cattle are infected at pasture during the peak fly season.
  • The adult flies are active only in warm weather, with the peak period usually in mid-summer.
  • Adult H. lineatum usually emerge about one month before H. bovis.

Lifecycle

  • Female flies attach their eggs to hairs on the lower parts of the body and on the legs below the hocks.
  • H. bovis attaches its eggs singly to hairs on the lower parts of the body.
  • H. lineatum attaches its eggs in rows of 5-15 on individual hairs below the hocks.
  •  One female may lay 100 or more eggs on an individual host.  
  • The first-stage larvae, hatch in a few days and crawl down the hairs, penetrate the hair follicles and migrate in the body, following species-specific pathways (see below).
  •  The larvae feed as they travel to the species-specific resting sites, which are reached in late autumn, where they spend the winter.
  • H. bovis larvae migrate along nerves until they reach the epidural fat of the spine in the region of the thoracic and lumber vertebrae, where they overwinter.
  • Larvae of H. lineatum migrate along connective tissue between muscles, towards the diaphragm, reaching the sub-mucosa of the oesophagus where they spend the winter.
  • The moult to the second stage occurs at these resting sites.
  •  Migration resumes from January onwards and the larvae arrive under the skin of the back (January/February to April for H. lineatum and March to June for H. bovis), where they moult to third stage larvae, which appear as distinct swellings (‘warbles’) .
  • The larvae make a hole in the skin and breathe by applying their spiracles to the aperture.    
  • After about 4-6 weeks in this site they emerge in May-June fall to the ground, where they pupate for about five weeks.
  • The precise timings and duration of events vary depending on latitude and ambient temperature and the whole life cycle takes about 1 year.

Transmission

  • Hatched first-stage larvae, crawl down the hairs and penetrate the hair follicles.

Pathological effects

  • Larval presence in the sub-dermis causes the production of a thickened connective tissue lined cavity, surrounding the larva, filled with inflammatory cells, particularly eosinophils and lymphocytes.
Larvae that die in the spinal canal release a toxin that may cause paraplegia.

Other Host Effects

  • Poor growth and decreased milk yield in some cases.
  • The presence of larvae causes characteristic visible fluid-filled swellings (‘warbles’) in the dermis of the back.

Control

Control via chemotherapies

  • Warble fly larvae are highly susceptible to topically active Organophosphorus (OP) insecticides and to the injectable or pour-on macrocyclic lactones - Ivermectin Ivermectin, doramectin, Eprinomectin Eprinomectin and Moxidectin Moxidectin.
  • In control schemes in Europe, a single annual treatment has been recommended, preferably in September, October or November before the larvae of H. bovis have reached the spinal canal, so that there was no risk of spinal damage from disintegration of dead larvae.

Control via environment

  • No environmental control options reported.

Vaccination

  • No vaccines are available.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Tarry D, Sinclair I & Wassall D (1992) Progress in the British hypodermosis eradication programme: the role of serological surveillance. Veterinary Record 131, 310-312 PubMed.
  • Sinclair I J, Tarry D W & Wassall D A (1990) The serological incidence of Hypoderma bovis in cattle in England and Wales in spring 1989. Veterinary Record 126, 327-329 PubMed.
  • Webster K A, Giles M & Dawson C (1987) A competitive ELISA for the serodiagnosis of hypodermosis. Veterinary Parasitology 68, 155-164 PubMed.
  • Sinclair I J & Wassall D A (1983) Enzyme-linked immunosorbent assay for the detection of antibodies to Hypoderma bovis in Cattle. Research in Veterinary Science 34, 251.
  • Tarry D W (1980) Warble fly infestation and climate. Veterinary Record 106, 559-560.
  • Andrews A H (1978) Warble fly - the life cycle, distribution, economic losses and control. Veterinary Record 103, 348-353 PubMed.

Other sources of information

  • Taylor M A, Coop R L & Wall R L (2016) Chapter 8 - Parasites of Cattle. In: Veterinary Parasitology. 4th edn. John Wiley & Sons, Chichester, West Sussex, UK. pp 380-382.
  • Nilssen A C (2006) Pupal biology and metamorphosis behaviour. In: The oestrid flies: biology. Host-parasite relationships, impact and management. Eds: Coldwell D D, Hall M J R & Scholl P J. CAB International. pp 123-138.
  • Taylor M A (2004) Chemotherapeutics of Ectoparasiticides: Large Animals. Merck Veterinary Manual. 9th edn. Merial Whitehouse Station, NJ, USA. pp 2158-2168.

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