Manual for Diagnosis of Screw-worm Fly - xcs consulting

4- Myiasis: Differential Diagnosis
(Plates 2-6)
Although the end results of unrestricted oviposition by SWF are spectacular, this cannot be
said of more recently established myiases, which may be insidious and readily overlooked,
even following close examination (see Fig. 33). This is particularly important in relation to
quarantine and the detection of invasion into hitherto uninfested areas, and should be drawn
to the attention of all people in Australia associated with livestock, including graziers, animal
health officers and veterinarians. At the quarantine level, infestation by C. bezziana may not
be evident initially in epizootic proportions, as the term ‘exotic disease’ might be taken to
imply.
The clinical syndrome, pathogenesis, pathology and differential diagnosis of C. bezziana
infestations have been studied and described by Humphrey et al. (1980) and Spradbery and
Humphrey (1988).
Predisposing conditions
Infestations are generally associated with traumatic injury, erosive or ulcerative lesions or
haemorrhage. Differences in occurrence and site of myiasis between animal species probably
reflect behavioural, environmental and husbandry factors rather than innate differences in
susceptibility.
Infestation commonly follows parturition. The navel of the new born and the vulval or
perineal region of the dam, particularly when traumatised, are principal sites of infestation.
Husbandry procedures such as dehorning, castration, branding, docking and ear tagging are
also common sites of infestation. Myiasis associated with otitis externa has been seen in
dogs, and myiasis associated with foot abscess has been seen in sheep and cattle. In
Australia, the technique of ‘mulesing’ sheep would provide an ideal medium for screw-worm
fly myiasis. Traumatic injuries due to barbed wire or other penetrating objects are also
commonly infested. Skin punctures caused by cattle tick and the lesions associated with
buffalo fly infestations are attractive to screw-worm fly and provide ideal sites for myiasis
in areas of Australia where the screw-worm fly would be expected to survive throughout the
year. A particularly important feature of the disease in sheep, with major consequences for
the Australian sheep industry, is the ability of C. bezziana to invade the intact perineal region
of ewes in the absence of overt trauma or haemorrhage (Plate 5).
Course of myiasis
Agriculture, Fisheries and Forestry - Australia
Myiasis by C. bezziana begins with the early larval invasion of the disrupted epidermis,
where the larvae aggregate in small cavities up to 5-10mm in diameter. There the larvae
are bathed in small quantities of serous fluid and are visibly active. Within 24 hours, the
cavities enlarge and extend laterally and deeply into the subcutaneous tissue and muscle. A
sero-sanguinous exudate is evident at this stage. Progressive liquefactive necrosis of muscle
and skin continues, associated with larval growth and invasion until a large cavernous lesion
with irregular ragged edges is present. The depths of the lesion contain a seething, pulsating
mass of larvae immersed in copious quantities of necrotic, fibrino-purulent or liquefied tissue
and blood (see wound biopsy, Plate 4). Haemorrhage from the lesion may be severe and the
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