Study of a New Species of Echinostome Cercaria, from the Freshwater Snail, Bellamya dissimilis (O.F. Müeller) in Palakkad, Kerala, India

Abstract

Freshwater snails serve as first intermediate hosts for numerous larval digeneans, and documentation of their cercarial fauna supports the assessment of parasite diversity and host–parasite relationships. This study examined the echinostome cercariae shed by Bellamya dissimilis collected from paddy wetlands in Palakkad district, Kerala, India. A total of 168 snails were collected from Kavassery and Erattakulam between May and August 2024 and screened individually for cercarial emergence. Fresh cercariae were examined by light microscopy, morphometric measurements were obtained from 20 heat-killed and formalin-fixed specimens, and five specimens were examined by scanning electron microscopy. Natural infection was detected in five snails, giving an overall prevalence of 2.98%; locality-specific prevalence was 2.25% at Kavassery and 3.80% at Erattakulam. Cercariae emerged mainly between 5:00 and 7:00 p.m., displayed positive phototaxis, and remained alive for approximately 16 h in well water. The cercarial form possessed a large elongate-oval aspinose body, a cylindrical aspinose tail with a dorsoventral fin fold, prominent oral and ventral suckers, three pairs of penetration glands, narrow caeca extending to the excretory bladder, and 15 pairs of flame cells. Rediae contained 15–20 developing cercariae. Comparison with selected related forms indicated several morphological distinctions, and the cercaria was provisionally designated Cercaria sp. XXVIII Malabar n. sp. Molecular characterisation and completion of the life cycle are required to support definitive species-level recognition.