Halicephalobus gingivalis (Stefanski, 1954) from a fatal infection in a horse in Ontario, Canada with comments on the validity of H. deletrix and a review of the genus

Abstract

Halicephalobus gingivalis (Stefanski, 1954), from a fatal infection in a horse in Ontario, Canada, was cultured and restudied. Although the original description given by Stefanski (1954) was satisfactory, Anderson & Bemrick (1965), in describing H. deletrix (= Micronema deletrix), claimed Stefanski's description was «inadequate» and the species a «species inquirenda». Thus, infections in horses and humans have been assigned to H. deletrix. We believe the species reported in horses and humans is H. gingivalis and that H. deletrix is its synonym. H. gingivalis is separated herein from forms found free-living. The genital tract in the advanced fourth stage of H. gingivalis is didelphic and amphidelphic and terminal ends of the horns are reflected, the anterior one ventrally, the posterior one dorsally. In the adult parthenogen the latter forms a short ovary, whereas most of the anterior horn forms a combined uterus-oviduct as a receptacle for a single large egg which is laid in the 2-cell or multi-cell stage. Eggs in the 2-cell stage embryonated to larvae in 17 hours at 28°C but did not hatch until an additional 24 hours. First-stage larvae were unusually large and variable in length (136-199 μm × = 168). Inactive third-stage larvae were 180-240 μm (× = 203) in length. The possible route of infection in horses and humans is briefly discussed.