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Chapter 24: Helminthic Diseases

Blaine A. Mathison; Raymond L. Barnhill

INTRODUCTION

The parasitic worms (helminths) are multicellular animals, some of which have complex life cycles involving more than one host. Parasitic helminths of humans can be grouped into 4 main categories: nematodes (roundworms), cestodes (tapeworms), trematodes (flukes), and acanthocephalans (thorny-headed worms); all but the acanthocephalans may be seen in dermatological specimens (see Tables 24-1, 24-2, and 24-3).

TABLE 24-1Comparison of Nematodes Likely to Be Encountered in Dermatopathologic Specimens
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TABLE 24-1 Comparison of Nematodes Likely to Be Encountered in Dermatopathologic Specimens
SPECIES [STAGE(S) USUALLY OBSERVED] KEY MORPHOLOGIC FEATURES PRIMARY MORPHOLOGICAL DIFFERENTIAL DIAGNOSES

Onchocerca volvulus, Onchocerca spp.

[adults, microfilariae]

-females 400 µm in diameter; males 200 µm in diameter

-muscle cells weak and few per quadrant

-microfilariae may be present in utero

-transverse external cuticular ridges

-microfilariae in skin up to 315 µm long; unsheathed with anucleate tail

 Dirofilaria spp., Loa loa; Mansonella streptocerca

Wuchereria bancrofti

[adults]

-females 300 µm in diameter; males 150 µm in diameter

-muscle cells coelomyarian, low and few per quadrant

-microfilariae may be present in utero

-smooth cuticle

 Onchocerca spp., Loa loa

Loa loa

[adults]

-females 500 µm in diameter; males 400 µm in diameter

-muscle cells coelomyarian, tall and numerous, 8–12 per quadrant

-cuticle with bosses (may be hard to discern in biopsy specimens)

-uterine tubes may contain developing eggs or microfilariae

 Onchocerca volvulus, Dirofilaria spp.

Mansonella ozzardi

[adults]

-females 150 µm in diameter; males 70 µm in diameter

-muscle cells coelomyarian, 8–12 per quadrant

-cuticle smooth

-microfilariae may be present in very large uterine tubes

 Onchocerca volvulus, Loa loa

Mansonella streptocerca

[adults, microfilariae]

-females 85 µm in diameter; males 50 µm in diameter

-muscle cells coelomyarian, 8–12 per quadrant

-cuticle smooth

-microfilariae in skin up to 240 µm in length, unsheathed, with nuclei to tip of tail

 Onchocerca volvulus

Dirofilaria spp.

[immature adults]

-variable in size depending on species, females 360–700 µm in diameter; males 300–490 µm in diameter

-muscle cells coelomyarian/polymyarian, very tall and numerous per quadrant

-large lateral chords with internal cuticular ridge at base

-external longitudinal cuticular ridges

-females rarely gravid in human host

 Onchocerca spp., Lagochilascaris minor

Strongyloides stercoralis

[filariform larvae]

-length up to 600 µm in diameter

-most organ systems undifferentiated

-double lateral alae, pointed (visible in cross-section only)

-obsolete buccal canal

-long, slender esophagus

 Onchocerca volvulus (microfilariae), Pelodera strongyloides

Pelodera strongyloides

[filariform larvae]

-long, tubular buccal canal

-rhabditiform esophagus

-low, dull double lateral alae (visible in cross-section only)

 Strongyloides stercoralis

Gnathostoma spinigerum

[third-stage larvae]

-strongly muscled esophagus

-very large, voluminous lateral chords

-large, boxy intestinal cells with 3–7 nuclei per cell and a brush border

-numerous muscle cells per quadrant

-cephalic spines

 Dioctophyme renale

Dioctophyme renale

[third-stage larvae]

-prominent ventral nerve chord with U- or V-shaped arrangement of nuclei

-pseudocoelomic membranes connecting intestine to body wall

-strongly muscled triradiate esophagus

-rectangular ...

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