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: Diffusely, the tissue is replaced by abundant eosinophilic cellular and karyorrhectic debris (necrosis), admixed with abundant viable and degenerate neutrophils, eosinophils, fewer plasma cells, lymphocytes and macrophages. Multifocally there is profuse hemorrhage separating and surrounding collagen fibers and adipocytes and frequently there are large cross and tangential sections of arthropod larvae up to 8x4 mm. The arthropod larvae have a 40 to 50 um chitinous cuticle with short, yellow spines, striated skeletal muscle, a hemocoelom, chitinized tracheal rings, a large tortuous digestive tract, and gonads (dipteran). Collagen bundles are frequently separated by either clear space or a pale eosinophilic, beaded to fibrillar material (fibrin) with multifocal ectatic lymphatics (edema). There are large numbers of reactive fibroblasts admixed with numerous small caliber blood vessels, hemorrhage, and edema (granulation tissue). Multifocally, vessels contain fibrin thrombi. Within necrotic areas surrounding the arthropod larvae, there are high numbers of previously described inflammatory cells admixed with aggregated colonies of 1um basophilic cocci.
MORPHOLOGIC DIAGNOSIS: Fibroadipose tissue: Cellulitis, suppurative and eosinophilic, chronic, diffuse, severe, with dipteran larvae, German shepherd dog, canine.
ETIOLOGIC DIAGNOSIS: Cutaneous chrysomyiasis
ETIOLOGY: Chrysomya bezziana (Asian and African screw worm)
- Screw worm refers to larvae of certain species of Cochliomyia (syn. Callitroga) including hominivorax and C. macellaria in North, Central and South America and Chrysomya bezziana in Africa and Asia
- Primary myiasis refers to requiring a living host for the larvae to feed on; obligate parasite
- Using the sterile insect technique of competitive exclusion, the United States officially eradicated the screw worm in 1982; ongoing campaigns are currently in effect in Central America and the Caribbean
- Screw worms are a reportable foreign animal disease
- The females lay their eggs in fresh, uninfected wounds (such as those caused by castration, dehorning, branding or 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 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
- Lesions are extremely painful and may expand rapidly leading to the death of the animal
TYPICAL GROSS OBSERVATIONS:
- Screw worm myiasis: Larvae are tightly packed, vertically oriented, usually deep within a wound and rarely evident crawling on the surface
- A wound that oozes with blood-stained fluid and 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
- Calliphorine myiasis: Cutaneous ulcers with irregular scalloped edges; pruritic; larvae easily seen on the surface of the lesion
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Histopathologic findings vary from a larva confined in a subcutaneous cyst formed by connective tissue infiltrated by neutrophils and eosinophils, (particularly with warbles) to severe suppurative, pyogranulomatous, or granulomatous reactions containing numerous eosinophils and larval segments
- Typical features: body cavity, striated musculature, chitinous exoskeleton, trachea (often with cuticular rings)
- Though readily recognizable as arthropod parasites, identifying specific fly larvae in tissue sections is extremely difficult and requires intact larvae
- Sequence-characterized amplified region (SCAR) marker is superior to other molecular techniques for the positive identification of screwworm
- It is critical to differentiate screwworm larvae from other maggots because of the clinical importance, the fact that it is a reportable disease and the potential devastating effects on the livestock industry
- 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
- Cuterebra myiasis; obligate parasites; usually affect rodents and lagomorphs; occasionally cats, dogs, pigs and humans
- Hypoderma myiasis; bovis and H. lineatum occur in cattle, occasionally horses and humans
- Cerebrospinal cuterebriasis is the likely cause of feline ischemic encephalopathy
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- Christen JA, Skoda SR, Heng-Moss TM, Lee DJ, Foster JE. Sequence-characterized amplified regions that differentiate New World screwworms from other potential wound-inhabiting flies. J Vet Diagn Invest. 2015;27(1):25-30.
- Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissue. Washington, DC: Armed Forces Institute of Pathology; 1999: 56-61.
- Mauldin EA, Peters-Kennedy J. Integument system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. New York, NY: Elsevier Limited; 2016: 669-670.
- Scott DW. Parasitic diseases. In: Large Animal Dermatology. Philadelphia, PA: WB Saunders; 1988: 245-251.
- Scott DW, Miller WH, Griffin CE. Parasitic skin diseases. In: Muller and Kirk’s Small Animal Dermatology. 6th ed. Philadelphia, PA: WB Saunders; 2001: 500-505.
- Urquhart GM, Armour J, Duncan JL, Dunn AM, Jennings FW. Veterinary Parasitology. 2nd ed. London, England: Blackwell Science; 1996: 141-169.
- Williams KJ, Summers BA, De Lahunta A. Cerebrospinal cuterebriasis in cats and its association with feline ischemic encephalopathy. Vet Pathol. 1998; 35:330-343.