The forgotten exotic tapeworms: a review of uncommon zoonotic Cyclophyllidea

Abstract

As training in helminthology has declined in the medical microbiology curriculum, many rare species of zoonotic cestodes have fallen into obscurity. Even among specialist practitioners, knowledge of human intestinal cestode infections is often limited to three genera, Taenia, Hymenolepis and Dibothriocephalus. However, five genera of uncommonly encountered zoonotic Cyclophyllidea (Bertiella, Dipylidium, Raillietina, Inermicapsifer and Mesocestoides) may also cause patent intestinal infections in humans worldwide. Due to the limited availability of summarized and taxonomically accurate data, such cases may present a diagnostic dilemma to clinicians and laboratories alike. In this review, historical literature on these cestodes is synthesized and knowledge gaps are highlighted. Clinically relevant taxonomy, nomenclature, life cycles, morphology of human-infecting species are discussed and clarified, along with the clinical presentation, diagnostic features and molecular advances, where available. Due to the limited awareness of these agents and identifying features, it is difficult to assess the true incidence of these 'forgotten' cestodiases as clinical misidentifications are likely to occur. Also, the taxonomic status of many of the human-infecting species of these tapeworms is unclear, hampering accurate species identification. Further studies combining molecular data and morphological observations are necessary to resolve these long-standing taxonomic issues and to elucidate other unknown aspects of transmission and ecology.

Keywords: Bertiella; Cestodes; Cyclophyllidea; Dipylidium; Inermicapsifer; Mesocestoides; Raillietina; Zoonoses.

Fig. 1.

Generalized life cycles for Bertiella, Dipylidium, Raillietina and Inermicapsifer spp. Cestode stages shown on the outside: (A) scolex of an adult; (B) gravid proglottids with egg(s); (C) cysticercoid; representative definitive hosts (DH) and intermediate hosts (IH) on the inside (Drawings by SGH Sapp).

Fig. 2.

Specimens of Bertiella studeri. (A) Carmine-stained scolex; (B) single elongate, gravid proglottid (scale bar = 1 cm), (C) multiple eggs showing pyriform apparati; (D) singular egg, showing oncosphere with hooklets (arrow). Photos courtesy of DPDx, Centers for Disease Control and Prevention.

Fig. 3.

Specimen of Inermicapsifer madagascariensis from Cuba. (A) Gravid proglottid (4 mm long); (B) egg capsule liberated from gravid proglottid (scale bar = 100 μm); (C) portion of strobila, showing median genital pores (arrows) (~30 ×  magnification); (D) unarmed scolex (scale bar = 200 μm). Photos courtesy of DPDx, Centers for Disease Control and Prevention.

Fig. 4.

Proposed life cycle schemes for Mesocestoides spp., showing two-host (dotted line) and three-host (dashed line) hypotheses. Cestode stages shown on the outside: (A) scolex of an adult; (B) gravid proglottids with oncosphere; (C) unknown cysticercoid or first larval stage; (D) tetrathyridium; representative definitive host (DH) and intermediate hosts (IH) on the inside (Drawings by SGH Sapp).