Parasites of the Skin, Subcutaneous and Connective Tissue

By | 2011-11-07

Nematodes

ACANTHOCHEILONEMA RECONDITUM

• Distributed in areas of Europe, Africa, and North America in canids; low significance except for need to differentiate from microfilariae of D. immitis.

Life Cycle

• Indirect.

• Intermediate host: fleas (Ctenocephalids felis, C. canis, Pulex irritans); also, ticks (Rhipicephalus sanguineus) and lice (Heterodoxus spiniger).

• Adults produce microfilariae (mff) that circulate in peripheral blood; flea ingests microfilariae with blood meal; infective L3 develops in 1-2 weeks and is passed to definitive host with next blood meal.

• Prepatent period is 2-3 months.

Pathogenesis and Clinical Signs

• Generally nonpathogenic.

Diagnosis

ANTEMORTEM

• Microfilariae may be found on wet mount of blood or on stained, direct smear; Knott’s technique or commercial filter kits may be used to concentrate microfilariae (preferred); must be differentiated from D. immitis (see Table Comparisons of Morphological and Other Characteristics of Microfilariae

of D. immitis and A. reconditum).

POSTMORTEM

• Adults are very small and generally missed at necropsy.

Treatment and Control

• Generally do not treat dogs for infection.

• Flea control will reduce the prevalence of this parasite.

STEPHANOFILARIA STILESI

• Distribution includes cattle in North America; low significance.

Life Cycle

• Indirect.

• Intermediate host: horn fly (Haematobia irritans).

• Adults live in dermis of inidventral abdomen; produce microfilariae that are ingested as flies feed; infective L3 develops in 18-21 days; definitive host infected when the fly feeds again.

• Prepatent period is 6-8 weeks.

Pathogenesis and Clinical Signs

• Causes circumscribed dermatitis that begins in calves 8-10 months old and lasts for years; lesions start as small papules that coalesce to form larger lesions that are moist and/or hemorrhagic; as lesions resolve, the skin becomes thickened, dry, and hairless; eventually, the skin regains normal texture but remains hairless.

• Open lesions are usually present in cattle <3 years of age.

• Economic loss from condemnation of hides.

Diagnosis

ANTEMORTEM AND POSTMORTEM

• Usually based on typical lesions on ventral midline between navel and brisket; can find microfilariae in a deep skin scrape or microfilariae and adults in biopsy or tissue sections.

Treatment and Control

• Generally do not treat cattle for infection.

• Fly control is more important in preventing transmission.

ONCHOCERCA SPP.

• Includes Onchocerca cervicalis in horses and donkeys, and O. gutturosa and O. lienalis in cattle.

• Worldwide distribution; low significance.

Life Cycle

• Indirect.

• Intermediate hosts: blackflies (Simulium spp.) for O. gutturosa and O. lienalis; biting midges (Culicoides spp.) for O. cervicalis.

• Intermediate host ingests mff; develops to infective L3 in 2-3 weeks; definitive host infected when infected arthropod feeds again; larvae migrate to final sites and develop to adults.

• Prepatent period is approximately 16 months for O. cervicalis.

Pathogenesis and Clinical Signs

• Adults are nonpathogenic as are microfilariae in cattle.

• In horses, dying microfilariae release antigens resulting in a hyper-sensitivity reaction; clinical signs may include alopecia, erythema, crusting with or without pruritis; must be differentiated from hypersensitivity reaction to biting flies.

• Aberrant infections in or near the eye of dogs have been reported; a few zoonotic cases have been reported in humans in North America.

Diagnosis

ANTEMORTEM

• Full-thickness skin biopsy soaked in warm saline for 6-12 hours; larvae leave samples and can be found in the sediment.

• Microfilariae are found in the dermis of the ventral thorax and abdomen as well as head, neck, withers, and eye (O. cervicalis), the neck and back (O. gutturosa), or the ventral midline, especially near the umbilicus (O. Iwnalis).

POSTMORTEM

• Adults are slender, white, up to 5.5 cm in length; found in the ligamentum nuchae (O. cervicalis, O. gutturosa) or gastrosplenic ligament (O. lienalis).

Treatment and Control

• Generally do not treat domestic animals for infection.

• A single SQ injection of ivermectin at 0.2 mg per kg was effective in controlling dermatitis by killing microfilariae in horses; post-treatment reactions to dying microfilariae may occur; moxidectin at 0.3-0.5 mg per kg will eliminate microfilariae from the blood of infected horses.

DRACUNCULUSINSICNIS

• Distributed in North America in raccoons, mink, dogs, and cats; low significance.

Life Cycle

• Indirect.

• Intermediate host: copepod.

• Paratenic host: tadpoles/frogs.

• First-stage larvae are released into the water and ingested by the copepod; develop to infective L3 in 13-16 days.

• Definitive host acquires infection by drinking water containing infected copepods or ingesting paratenic host (may be the most common route); larvae penetrate the small intestine and migrate to the subcutaneous tissues of the thorax, abdomen, and inguinal area; molt and mature in approximately 65 days; males die soon after mating; females may migrate to the subcutanous tissues of the limbs; nodules form that ulcerate at the location of the anterior end of the worm; whenever ulcer contacts water, the worm projects out of the ulcer, prolapses the uterus, and releases larvae into the water.

• Prepatent period approximately 200 days.

Pathogenesis and Clinical Signs

• Clinical signs include pruritic, painful, chronic nodules or abscesses, especially on the limbs or abdomen with or without ulceration; urticaria and fever may also occur.

Diagnosis

ANTEMORTEM AND POSTMORTEM

• May find L1 in impression smears of discharge from lesions; larvae are 500-760 um in length, with esophagus, intestinal tract, and anus, with distinctly prominent long, pointed tails; characteristics allow differentiation from microfilariae (mff).

• Can remove adults from nodules; females are up to 120 cm in length; presence of characteristic larvae in utero will allow differentiation from other subcutaneous nematodes.

Treatment and Control

• Treatment consists of removal of the worm; chemotherapy is ineffective.

• Prevent predation and scavenging whenever possible.