JPC SYSTEMIC PATHOLOGY
Signalment (JPC# 21474-16): 7-year-old German shepherd dog
HISTORY: A draining skin lesion from a dog from South Vietnam.
HISTOPATHOLOGIC DESCRIPTION: Fibroadipose tissue: Affecting approximately 80% of the section are multifocal to coalescing areas of lytic necrosis characterized by loss of tissue architecture with replacement by abundant eosinophilic cellular and karyorrhectic debris admixed with fibrin, hemorrhage, and edema as well as multifocally abundant viable and degenerate neutrophils, eosinophils, fewer plasma cells, lymphocytes, and macrophages. Within necrotic foci are frequent, large, cross and tangential sections of dipteran arthropod larvae up to 8x4 mm. The arthropod larvae have a 40-50 um thick chitinous cuticle with short, yellow spines; striated skeletal muscle; a hemocoel; chitinized tracheal rings; a large, tortuous digestive tract; and gonads. Collagen bundles are occasionally brightly eosinophilic and hyalinized (degeneration) and rarely replaced by granular, basophilic mineral. There are multifocal ectatic lymphatics (edema). There are abundant reactive fibroblasts admixed with numerous small caliber blood vessels lined by reactive endothelium (granulation tissue). Multifocally, vessels contain fibrin thrombi. Within necrotic areas, there are occasional aggregated colonies of 1um basophilic cocci.
MORPHOLOGIC DIAGNOSIS: Fibroadipose tissue: Cellulitis, necrosuppurative and eosinophilic, chronic, diffuse, severe, with dipteran larvae, German shepherd dog, canine.
ETIOLOGIC DIAGNOSIS: Cutaneous chrysomyiasis
ETIOLOGY: Chrysomya bezziana (Asian and African screw worm)
CONDITION: Cutaneous myiasis
· Myiasis: infestation of tissue of live animals with larvae of dipterous flies (larvae also referred to as maggots or grubs); may be facultative or obligate parasites; families of veterinary importance include: Cuterebridae, Sarcophagidae (Wohlfartia spp.), Gasterophilidae, Oestridae, and Calliphoridae (blowflies)
· Primary myiasis refers to requiring a living host for the larvae to feed on; obligate parasite
· Cutaneous myiasis:
· Screw worms (AKA screwworms, screw-worms): larvae of certain species of Cochliomyia (syn. Callitroga) including C. hominivorax and C. macellaria in North, Central, and South America and Chrysomya bezziana in Africa and Asia
· Reportable foreign animal disease with potential devastating effects on the livestock industry
· Eradicated from the US in 1982, with ongoing eradication campaigns using sterile insect technique in Central America and the Caribbean
· Outbreak in Florida key September 2016-March 2017, 1st sustained population of New World screwworm in US since 1966 (Hennessey, J Am Vet Med Assoc 2019)
· Blowfly (Calliphoridae) larvae: see discussion in differential diagnosis section
· The females lay their eggs in fresh, uninfected wounds (such as those caused by castration, dehorning, branding, accidental injuries), navels of recently calved cows, and tick bites on live hosts > deposit 200 eggs in tidy rows > eggs hatch within a day > maggots feed in groups and use proteolytic enzymes to penetrate and liquefy the live host tissue > larvae leave the host in 5-7 days and enter the soil to pupate
· If the wound is disturbed while the larvae are feeding, the larvae will burrow or "screw" deeper into the flesh
· Death probably results from secondary toxicity and/or secondary infections
TYPICAL CLINICAL FINDINGS:
· Lesions are extremely painful and pruritic, and may expand rapidly
· A distinct foul odor maybe detected
TYPICAL GROSS FINDINGS:
· Screw worm myiasis: Larvae are tightly packed, vertically oriented, usually deep within a wound and rarely evident crawling on the surface
· The wound oozes blood-stained fluid that contains incompletely digested shreds of tissues and clusters of voraciously feeding larvae
· Common locations include areas around nose, eye, mouth, anus, genitalia, umbilicus, or adjacent to neglected wounds
· Canine and feline cutaneous screw-worm myiasis in Malaysia (Hock Vet Dermatol 2018), most common anatomical sites of infestation:
· Dogs - external ear canals, followed by the perineum and medial canthus
· Cats - paws and tail; five cats with screw-worm myiasis were concurrently infected with sporotrichosis
TYPICAL LIGHT MICROSCOPIC FINDINGS:
· Variable severity from a larva confined in a subcutaneous cyst formed by connective tissue infiltrated by neutrophils and eosinophils (particularly with warbles, not screwworms) to severe suppurative, pyogranulomatous, or granulomatous inflammation with numerous eosinophils and larval segments
· Dipterid larvae: body cavity (hemocoel), striated musculature, chitinous exoskeleton, trachea (often with cuticular rings)
· Speciating fly larvae in tissue sections is extremely difficult; typically requires gross examination of intact larvae
· Speciation: Screwworm larvae must be differentiated from other dipterid larvae due to the clinical importance
o Molecular diagnostics: Sequence-characterized amplified region (SCAR) marker is superior to other molecular techniques for the positive identification of screwworm (J Vet Diagn Invest. 2015;27(1):25-30)
o Gross appearance of larvae: Larvae can be identified by the dark pigmentation of their tracheal trunk through the last 3 or 4 segments
· Obligate and facultative larvae may be found around the same wound
· Calliphorine (blowfly) myiasis: Cutaneous ulcers with irregular scalloped edges; pruritic; larvae easily seen on the surface of the lesion; feed only on dead tissue
· Cuterebra myiasis; see Comparative Pathology section
· Screwworms are not host species-specific
Other causes of cutaneous myiasis:
· Hypoderma myiasis; H. bovis and H. lineatum occur in cattle, occasionally horses and humans
· Cuterebra myiasis; obligate parasites; usually affect rodents and lagomorphs; occasionally cats, dogs, pigs, and humans
· In cats, aberrant migration is the likely cause of feline ischemic encephalopathy (N-M23, Cantile Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals 2016)
Other selected larval dipterids of veterinary importance:
· Oestrus ovis: nasal bot of sheep, goats, and nondomestic cervids; causes sinusitis, and rarely penetrates into cranial cavity or results in secondary bacterial spread causing meningitis
· Gasterophilus sp.: most common gastric parasite of horses; larvae of botflies; G. intestinalis is most common, affects the squamous mucosa of the gastric cardia; G. nasalis affects the pyloric mucosa and proximal duodenum
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