Follicular demodicosis



October 2019

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 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


·      Lesions are extremely painful and pruritic, and may expand rapidly

·      A distinct foul odor maybe detected



·      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


·      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


1.     Bowman DD. Arthropods. In: Georgis’ Parasitology for Veterinarians. 10th ed. Philadelphia, PA: WB Saunders; 2014: 26-29.

2.     Buisch WW, Hyde JL, Mebus CA. Screwworm myiasis. In: Foreign Animal Diseases. Richmond, VA: Carter Printing Company; 1992: 333-342.

3.     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.

4.     Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 477, 564.

5.     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.

6.     Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissue. Washington, DC: Armed Forces Institute of Pathology; 1999: 56-61.

7.     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.

8.     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.

9.     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.

10.  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: 669-670.

11.  Miller WH, Griffin CE, Campbell KL. Parasitic skin diseases. In: Muller and Kirk’s Small Animal Dermatology. 7th ed. St. Louis, MO: Elesevier Inc.; 2013: 331-332, 381.

12.  Scott DW. Parasitic diseases. In: Large Animal Dermatology. Philadelphia, PA: WB Saunders; 1988: 245-251.

13.  Urquhart GM, Armour J, Duncan JL, Dunn AM, Jennings FW. Veterinary Parasitology. 2nd ed. London, England: Blackwell Science; 1996: 141-169.

14.  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.

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