Abnormal Helminth Egg Development, Strange Morphology, and the Identification of Intestinal Helminth Infections

Abstract

Occasionally, abnormal forms of parasitic helminth eggs are detected during routine diagnostics. This finding can prove problematic in diagnosis because morphologic analysis based on tightly defined measurements is the primary method used to identify the infecting species and molecular confirmation of species is not always feasible. We describe instances of malformed nematode eggs (primarily from members of the superfamily Ascaridoidea) from human clinical practice and experimental trials on animals. On the basis of our observations and historical literature, we propose that unusual development and morphology of nematode and trematode eggs are associated with early infection. Further observational studies and experimentation are needed to identify additional factors that might cause abnormalities in egg morphology and production. Abnormal egg morphology can be observed early in the course of infection and can confound accurate diagnosis of intestinal helminthiases.

Keywords: Ascarididae; Schistosomatidae; diagnostics; egg development; helminths; morphology; nematodes; parasites; trematodes.

Figure 1

Abnormalities of Ascaris lumbricoides eggs from patients in the Solomon Islands, visualized on Kato-Katz. A) Giant egg with irregular indented shape. B) Giant egg with 2 morulae. Arrows indicate eggs of normal morphology. Original magnification ×400.

Figure 2

Abnormalities of Schistosoma spp. eggs visualized in wet preparation. A) Normal S. haematobium egg (≈150 µm long). B) S. haematobium egg with reduced terminal spine. C) S. haematobium egg with irregular, indented shape. D) Normal S. mansoni egg (≈150 µm long). E) S. mansoni egg with 2 lateral spines. Original magnification ×400. Panel E image courtesy of John Goldsmid, University of Tasmania (Hobart, Tasmania, Australia).

Figure 3

Abnormalities of Baylisascaris procyonis eggs shed by experimentally inoculated dogs and raccoons, visualized on fecal flotation. A) Triangular egg. B) Pear-shaped egg. C) Almond-shaped egg. D) Triangular egg with indented edge. E) “Immature” egg with underdeveloped morula and no cortex or proteinaceous coat. F) Budded egg. G) Twin conjoined eggs with separate morulae and vitelline membranes. H) Triplet conjoined eggs with distinct morulae; vitelline membrane might be shared between 2 eggs. Arrows indicate eggs of normal morphology (65–75 µm). Original magnification ×400.

Figure 4

Conjoined Trichuris vulpis eggs from a domestic dog, visualized on fecal flotation. Arrow indicates a morphologically normal egg. Original magnification ×400. Photograph by Danielle E. Preston and courtesy of Mani Lejeune, both of the Cornell University College of Veterinary Medicine (Ithaca, NY, USA).