JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

November 2022

I-P13 (NP)

 

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 either coagulative or lytic necrosis. Lytic necrosis is 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. Coagulative necrosis is characterized by retention of architecture with loss of differential staining. Within necrotic foci are frequent, cross and tangential sections of dipteran arthropod larvae up to 8x4 mm. The arthropod larvae have a 40-50µm thick chitinous cuticle with short, yellow, anisotropic 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 and increased clear space (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 µm 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 screwworm)

 

CONDITION:  Cutaneous myiasis

 

GENERAL DISCUSSION:  

• 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 Keys September 2016-March 2017, 1^st 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

 

PATHOGENESIS:

• 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
• Cutaneous myiasis is more common in previously damaged skin as a secondary invader

 

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

ADDITIONAL DIAGNOSTICS:

• Speciation:  Screwworm larvae must be differentiated from other dipterid larvae due to the clinical importance
  1. Gross appearance of larvae:  Larvae can be identified by the dark pigmentation of their tracheal trunk through the last 3 or 4 segments  

 

DIFFERENTIAL DIAGNOSIS:

• 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

 

COMPARATIVE PATHOLOGY:

Other causes of cutaneous myiasis:

• Hypoderma myiasis; H. bovis and H. lineatum occur in cattle, occasionally horses and humans in Northern hemisphere. Deposits “butcher’s jelly” in fascial planes from larval migration. “Warbles” are larval lifestage in skin with central pore. 
• Cuterebra myiasis; obligate parasites of rodents and lagomorphs; occasional abberant infections in cats, dogs, pigs, howler monkeys, and humans
  • In cats, aberrant migration is the likely cause of feline ischemic encephalopathy (N-M23)

Other selected larval dipterids:

•  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
• In anurans (frogs/toads), multiple genera of dipteran flies cause myiasis of SQ lymph sacs or nasal cavities: Notocaeta, Batrachomyia, Bufolucillia
• Wohlfahrtia opaca: traumatic fatal umbilical myiasis in hatchling ducklings
• Cobboldia sp., Platycobboldia sp.:gastric myiasis in elephants

 REFERENCES:

1. Cantile C, Youssef, S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 391.
2. Caswell JL, Williams KJ.  Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6^th ed. Philadelphia, PA: Elsevier Saunders; 2016: 477, 564.
3. Fenton H, McManamon R, Howerth EW. Anseriformes, Ciconiiformes, Charadriiformes, and Gruiformes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 717.
4. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissue. Washington, DC: Armed Forces Institute of Pathology; 1999: 56-61. 
5. Hennessey MJ, His DJ, Davis JS, et. al.  Use of a multiagency approach to eradicate New World screwworm flies from Big Pine Key, Florida, following an outbreak of screwworm infestation (September 2016–March 2017). J Am Vet Med Assoc. 2019;255(8):908-914.
6. Hock SH, Peik YT, Hock BY, Hooi ML. Canine and feline cutaneous screw-worm myiasis in Malaysia: clinical aspects in 76 cases. Vet Dermatol 2018; 29: 442–e148.
7. Howerth EW, Nemeth NM, Ryser-Degiorgis MP. Cervidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 175. 
8. Mauldin EA, Peters-Kennedy J. Integument system.  In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders ; 2016: 668-670.
9. Pessier AP. Amphibians. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 947.
10. Strait K, Else JG, Eberhard ML. Parasitic diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T. Nonhuman Primates in Biomedical Research Volume 2: Diseases. 2nd ed. London, UK: Academic Press; 2012:263.
11. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:43, 209.
12. Welle MM, Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1183-1184.

 

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