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674 Myiasis

harbor flies causing myiasis are predisposed. • Irregularly ulcerated to crateriform lesions Differential Diagnosis
are characteristic.
Some dipteran larvae can penetrate normal skin. • Several lesions may coalesce to form large Direct visualization of maggots in representa-
VetBooks.ir CONTAGION AND ZOONOSIS • Tracts dissecting through nearby soft-tissue Specific species identification is not usually
tive cutaneous lesions confirms the diagnosis.
soft-tissue defects.
• Emerging adult flies are a potential public
required to treat most cases of myiasis.
health risk because they serve as fomites for
structures may cause fistulation.
bacteria found in animal waste. • Parasitized skin may present as a focal Initial Database
• People with open wounds may be at risk for fistulated subcutaneous nodule. • Direct visualization of maggots
myiasis. • Aberrant larval migration can cause signs • Cytologic evaluation of wounded and
• Other implications: accidental reintroduction specific to other tissues. infested skin to identify secondary bacterial
of screwworm (Cochliomyia hominivorax), • Findings associated with incontinence or infection
which is mainly eradicated from the United debilitation (i.e., musculoskeletal, neurologic,
States, is an ongoing threat because of the or internal disease) might be identified in Advanced or Confirmatory Testing
aggressiveness of infestations. select cases. Severely infested animals may • Other diagnostic tests are done at the discre-
• Superficial wounds are a point of entry for be in shock. tion of the clinician, but in most patients,
screwworm larvae, which voraciously feed a CBC, biochemistry profile, and urinalysis
on living flesh and have fatally infected or Etiology and Pathophysiology are beneficial to exclude concurrent and
“have completely eaten away the facial bones” • Eggs are laid on the moist skin of debilitated predisposing conditions.
of unconscious human victims (Bowman, or wounded animals. Emerging larvae • Bacterial culture and susceptibility testing
2013). Veterinarians play a key role in screw- (maggots) secrete proteolytic enzymes for lesions that fail to heal rapidly or in
worm prevention through identification of that liquefy cutaneous tissue, creating septicemic animals
larvae. full-thickness skin defects within hours. • Exam of the spiracle and stigmal plates on
Occasionally, the initial larval infestation the maggot can aid in species identification.
GEOGRAPHY AND SEASONALITY favors the strike of other myiasis-causing • Larvae can be submitted to a regional veteri-
Myiasis has a worldwide distribution, but flies, resulting in disease propagation. nary office for precautionary identification
larvae-specific infestations do occur in some • Many dipteran flies (i.e., house, stable, horn, and confirmation if a suspicion of screwworm
countries. In the Northern Hemisphere, myiasis and black flies) can infest the skin of animals myiasis exists.
occurs during the summer months in temperate, with risk factors (above). These flies cause the
moist areas. typical myiases seen in routine small animal TREATMENT
practice.
Clinical Presentation • Blowflies (common in large animals), Treatment Overview
HISTORY, CHIEF COMPLAINT screwworm flies (reportable in many parts The goals are to remove and kill maggots,
A foul, putrid-smelling animal may be the of the world), and flesh flies rarely infest manage the wound(s), and identify and treat
only reason an owner seeks veterinary care, the skin of small animals. However, their underlying predisposing conditions.
although occasionally, malodor may be absent presence is alarming because of contagion
or unnoticed. Other owners are aware that implications. Acute General Treatment
maggots are taking residence in their animal’s skin. • Although more common in rabbits and • Stabilize the patient if needed (e.g., fluid
Occasionally, owners present an animal because rodents, Cuterebra flies can cause myiasis resuscitation if hypotensive).
of incontinence, debilitation, paresis, or skin fold in companion animals. • Clip, clean, and surgically debride lesions
dermatitis, and myiasis is an incidental finding. (general anesthesia required).
DIAGNOSIS • Mechanically remove larvae.
PHYSICAL EXAM FINDINGS • Apply a non–alcohol-based pyrethrin or
• Maggot infestation typically occurs in moist Diagnostic Overview pyrethroid (dogs but not cats) spray on
locations: around the eyes, nose, mouth, The diagnosis is confirmed based on the obser- lesions to kill maggots.
genitalia, anus, or adjacent to wounded vation of maggots in the skin and surrounding • Systemically administered avermectins
(traumatic or surgical) skin. soft tissue. (ivermectin 0.2-0.4 mg/kg PO or SQ, may

A B

MYIASIS A, Numerous maggots packed into the open wound of a stray dog. B, The maggots have been removed,
leaving a deep central ulcer with numerous satellite ulcers. (From Hnilica KA, et al: Small animal dermatology: a color
atlas and therapeutic guide, ed 4, Philadelphia: 2017, Elsevier.)

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