Parasitic skin infections - Pathology
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Fleas
- Occurs in cats and dogs mainly
- Irritation caused by enzymes, anticoagulants and histamine-like substances
- Hypersensitivity to flea saliva may develop
- Young or small animals may suffer from aneamia
- Lesions normally in dorsolumbosacral region, caudomedial thighs, ventral abdomen, flank, neck (cats)
- Grossly:
- Red papules and secondary excoriations
Flies
- Bites may cause irritation, anaemia, toxicity or hypersensitivity
- Grossly:
- Wheals and papules around puncture wound
- Hair loss, scales, crusts, erythema
- Microscopically:
- Superficial perivascular dermatitis
- Eosinophilic folliculitis, +/- furunculosis
- Intraepidermal pustules
- Eosinophils and necrosis
Myiasis
- Infection by dipterous flies in moist body areas (eg. perineum)
- Commonly affects sheep - ovine fleece rot
- Grossly:
- Matted hair or wool, may be in wounds
- Multiple holes or ulcers
- Malodour
- May result in death due to septicaemia or toxaemia
- Cuterebra myiasis
- Caused by Cuterebra sp
- Young animals come into contact with eggs on stones or plants
- Nodules with larvae in subcutis + respiration openings to surface
- Hypoderma myiasis
- Caused by Hypoderma lineatum or H. bovis
- Mainly affects cattle
- Skin of legs is penetrated -> migration through oesophagus or vertebral canal to subcutis of back
- Cause subcutaneous nodules with respiration openings
- Larvae in cavity with fibrin and eosinophils surrounded by granulation tissue with eosinophil clusters
- Screwworm myiasis
- Occurs in Africa, Asia, Central and South America
- Larvaae are deposited in wounds
- Penetrate and liquify tissue
Helminths
Cutaneous habronemiasis
- In horses (also calles summer sores)
- Caused by Habronema sp. or Draschia sp. - spread by house flies or stable flies
- In traumatised or moist soft skin
- Grossly:
- Single or multiple tomorous masses that proliferate and ulcerate
- Red to brown, on section have small gritty yellow to white foci
- Microscopically:
- Nodular dermatitis
- Eosinophils, epithelioid macrophages, multinucleated giant cells
- Larvae and necrotic tissue
- Granulation tissue with neutrophils along ulcerated surface
Hookworms
- Caused by Ancylostoma or Uncinaria
- Grossly:
- Red papules -> lichenified alopecic areas
- Mainly of dog feet
- Foot pads may soften with keratinised area separation
- Possible secondary bacterial dermatitis and paronychia
- Microscopically:
- Hyperplastic spongiotic perivascular dermatitis
- Eosinophils
- Crusts
- Migration tracts
Onchocerciasis
- Caused by Onchocerca spp.
- Affects horses, cattle, donkeys, mules, sheep, goats
- Transmitted by Simulidae and Ceratopogonidae
- Hypersensitivity varies with individuals, may occur without any inflammation
- Grossly:
- Adult parasites within connective tissue nodules
- Microfilaria in dermis, especially ventral midline
- Patchy alopecia, erythema, scaling, crusting, change in pigmentation
- Circular areas of dermatitis on forehead appears in some horses
- May also involve keratitis, uveitis and conjunctivitis
- Microscopically:
- Normal to superficial to deep perivascular dermatitis
- Eosinophils, lymphocytes, microfilariae
Stephanofilariasis
- Caused by Stephanofilaria stilesi
- Occurs in cattle, buffalo, goats
- Transmitted by flies
- Affects ventral midline (other species affect different areas of body)
- Grossly:
- Foci extending to large areas
- Errect hairs, epidermal haemorrhage, serum exudate -> crusts
- Microscopically:
- Superficial and deep perivascular dermatitis
- Eosinophils, adult and microfilarial parasites
- Epidermal hyperkeratosis, parakeratosis, acanthosis, eosinophilic microabscesses and crusts
Lice
- Infestation is called pediculosis
- May cause anaemia (sucking lice), weakness, discomfort and damage to hair or wool
- More common in low temperatures when hair is longer
- Host-specific
- Spread by direct contact
- Most lesions are due to self-inflicted trauma
- Grossly:
- Papules, crusts, secondary excoriations
- Eggs and lice visible in lesions
Mites
Cheyletiellosis
- Caused by Cheyletiella sp.]]
- Occurs in dogs, cats, rabbits, humans and wild animals
- In dogs and cats: hyperkeratosis - dry, white, scaly dandruff along back
- May be asymptomatic
- Grossly:
- Focal, multifocal or generalised red papules or crusts
- Microscopically:
- Superficial perivascular dermatitis
- Eosinophils
Chorioptic mites
- Host-specific
- In cattle, horses, goats
- Caused by Chorioptes spp.
- Grossly:
- Erythematous, papular, crusty lesions
- Hairless, thickened skin
- Areas usually affected:
- Horses: lower hind legs
- Cattle: lower hind legs, scrotum, perineum, tail, udder, thigh
- Sheep: lower hind legs and scrotum
- Goats: lower hind legs, hindquarters, abdomen
Demodicosis
- Demodex are normal inhabitants of hair follicles and sebaceous glands
- Have to have increased number of immature mites to classify as infestation
- Host-specific
- Occurs in dogs, cattle, sheep, goats, pigs, horses and cats
- Transmits during nursing from dam to offspring
- Demodex canis in dogs:
- Generalised or localised
- Mainly young dogs affected or adults with other disease (hyperadrenocorticism, hypothyroidism)
- Often inherited in pure bred
- Grossly:
- Localised: scaly, erythematous, macular, alopecic areas; usually face or front legs
- Generalised: large, coalescing lesions, patches, erythema, alopecia, scales, crusts
- Microscopically:
- Localised: lymphoplasmacytic perifolliculitis with hyperkeratinisation, sebaceous adenitis, low pigment, intraluminal mites
- Generalised: perifolliculitis and follicular hyperkeratosis, follicular pluggingf, bacterial proliferation, bacterial neutrophilic folliculitis
- May cause folliculat rupture -> furunculosis, cellulitis, lymphadenitis and septicaemia
Notoedric mites
- Rare but very contagious
- Caused by Notoedres cati
- Grossly:
- Erythematous papular rash -> scales, crusts, alopecia -> lichenification when chronic
- Start on neck and ears -> head, face, paws -> generalised
- Microscopically:
- Epidermal hyperplasia, spongiosis, crusts
- Superficial perivascular dermatitis
- Eosinophils
Otodectic mites
- Caused by Otodectes cynotis
- In external ear canals of carnivores, occasionally elsewhere
Psoroptic mites
- Occurs in cattle, sheep, horses, goats, rabbits and other animals
- Host-specific
- Psoroptes cuniculi in ear canals of rabbits, horses, goats and sheep
- P.equi at base of mane, ttail and forelock in horses
- P.ovis in sheep (sheep scab)and cattle
- Thickened skin and dry scales and crusts
- Starts at withers and spreads due to self trauma
- Microscopically:
- Spongiotic, hyperplastic or exudative superficial perivascular dermatitis
- Eosinophils
Scabies
- Caused by Sarcoptes scabiei
- Highly contagious
- Mainly in pigs, dogs, also horses, cattle, sheep, goats and cats
- Intense pruritus due hypersensitivity to mites borrowing through stratum corneum
- Usually starts with external pinnae -> head -> neck -> generalised
- Grossly:
- Erythematous macules, papules, crusts
- If chronic -> lichenified, hairless
- Microscopically:
- Hyperplastic, spongiotic, superficial perivascular dermatitis
- Crusting, eosinophil infiltration
- Mites are not commonly seen, but eggs and feces may be found in stratumn corneum
Trombiculidiasis
- Harvest mite infestation (chiggers) - Trombicula sp.
- Form tunnels in epidermis by injecting saliva - eat digested tissue fluid
- Intense pruritus
- Grossly:
- Small, red papules or crusts
- Orange to red larvae
- On skin close to plants or the ground
- Microscopically:
- Hyperplastic, superficial perivascular dermatitis
- Eosinophils, mast cells
- Intraepidermal mites
Protozoa
- Cutaneous infection may become a part of systemic infection
- Mostly caused by Leishmania
- Transmitted by sandflies
- Occurs in dogs, cats and rodents (also humans)
- Grossly in dog:
- Generalised alopecia
- Silvery white scales
- Nodules and ulcers if more severe
- Mainly on muzzle, ears and eyes where flies feed
- Microscopically:
- Hyperkeratosis, parakeratosis, crusts
- Granulomatous nodules in dermis
- Macrophages, some lymphocytes and plasma cells - periadnexal pattern, may affect sebaceous glands
- Organisms may be found extra or intracellularly
Ticks
- Local reaction depends on host hypersensitivity and tick's secretions
- Grossly:
- Focal erosions, erythema, crusted ulcers, sometimes alopecia and nodules
- Microscopically:
- Epidermal and dermal necrosis
- Perivascular of diffuse inflammation and necrotic margins
- Eosinophils, macrophages and lymphocytes in exudate
- May form granulomas containing collagenous fibres and lymphoid follicles within dermis