New insights into the life cycle of Gnathostoma turgidum Stossich, 1902 (Nematoda: Gnathostomatidae).

In this work, the life cycle of Gnathostoma turgidum was studied both in natural temporary water bodies and under experimental conditions, with materials collected from the municipality of San Francisco Ixhuatán, Oaxaca. Adult nematodes inhabit the interstitial layers of the stomach in the definitive host (Didelphis virginiana) and release their eggs into the environment via faeces, typically in a dry environment. Under these conditions, the eggs remain quiescent until the onset of the rainy season. Development of early third-stage larvae occurs in cyclopoid copepods. Fry acts as transport and/or intermediate hosts, while frogs (Lithobates forreri) serve as obligatory intermediate hosts for advanced third-stage larvae, ultimately transmitting the parasites to the definitive host. Notably, the precocity phenomenon, typical of larval development in intermediate hosts, occurs in this species within the definitive host. Precocity is likely a response to host behaviour or other ecological factors that restrict transmission to narrow spatial and temporal windows. Adults die at the end of the rainy season. The marked seasonality of this species is mainly attributed to the combination of two factors: (i) the seasonal predation of frogs by the definitive host, and (ii) expulsion of adult stages as immune-mediated 'self-cure'. This study represents the first documentation of birds acting as paratenic hosts for this nematode.