Myiasis

Also Known As: Fly-Strike — Wound Myiasis — Maggots — Strike — Fly Infestation — Wool Maggots

Introduction

Cat with myiasis. Wikimedia Commons

Larval form of Gasterophilus spp stomach botfly. Wikimedia Commons

Myiasis is caused by infestation of live hosts with dipterous larvae. Small and large ruminants and poultry can be affected. These larvae then feed on the host’s living and dead tissue.

Myiasis Producing Flies can be obligate or facultative.

Myiasis Causing Flies

Obligate

Obligate flies occur exclusively in or on living vertebrates.

Oestrus ovis, Gasterophilus spp., Hypoderma spp., Dermatobia spp, Wohlfahrtia spp., Cochliomyia, Chrysomya bezziana and Cordylobia.

Facultative

Facultative flies are free-living and usually found in detritus or carrion.

Cochliomyia macellaria, Chrysomya megacephala, rufifacies and albiceps, Lucilla sericata and cuprina, Phormia spp., Protophormia spp. and Caliphora spp.

For more information, see Myiasis Producing Flies.

Distribution

Myiasis Producing Flies are found in most regions of the world.

Signalment

Presence of wounds, wet fleece in sheep cases, recent surgery, bacterial wool/skin contamination and faecal contamination are the main predisposing factors for myiasis.

Density of stock will determine size and viability of the fly population.

Clinical Signs

Can be classified as cutaneous, nasopharyngeal, intestinal or urogenital. Pain, irritation, discomfort, alopecia and pruritus are common signs locally. There is also likely to be direct tissue damage, haemorrhage, hyperpigmentation and secondary infection.

Nasal myiasis causes irritation and epistaxis.

Aural myiasis can cause deafness, discharge and foul exudates.

Gastrointestinal myiasis caused by Gasterophilus spp. often causes ulceration, GI bleeding, weight loss, diarrhoea and pupae are voided in faeces.

Warble flies cause cysts along the midline of the back.

Lucilia sericata tends to cause lesions on the inner thighs and perineum due to faecal soiling.

Wohlfahrtia' spp. cause genital lesions on the vulva and prepuce.

Loss of feathers and soiling of the vent is seen in poultry.

In severe cases, anaemia, anaphylaxis and toxaemia may be fatal. Likely consequence is reduced feeding and resulting weight loss and infertility.

Diagnosis

Diagnosis is primarily dependent on observation of larvae on the host or in the faeces.

Larvae may also be observed in the carcass at post-mortem.

Gastroscopy may be used in the case of gastric myiasis.

ELISA is also available for Hypoderma spp. and Oestrus ovis. PCR is available for Cochliomyia spp.

Treatment

Treatment with ectoparasiticides is usually effective. Administration can be oral, topical or by subcutaneous injection and the type of myiasis should be considered when deciding upon route of administration.

Ivermectin and doramectin are both effective in the control of nasal myiasis ,warble fly and Dermatobia when injected.

Moxidectin is the main drug for oral treatment. It is effective against Gasterophilus spp.

Topical treatment by pour-on or dipping is most effective against cutaneous/subcutaneous myiasis. A huge range of products are available. Resistance should be considered and monitored.