Sinoatrial contracaeciasis in Johnny Darters (Etheostoma nigrum) and Plains Topminnow (Fundulus sciadicus) from the South Platte drainage, Colorado

Intra-sinoatrial nematodes were incidentally recognized in wild-caught Colorado Johnny Darters (Etheostoma nigrum, JD) in 2020–2021 and in Colorado Plains Topminnow (Fundulus sciadicus, PTM) in 2023-2024. PTM and JD were evaluated histologically. Nematodes dissected from PTM were used for morphologic evaluation and molecular identification. The first and second internal transcribed spacers (ITS1 and ITS2) of ribosomal DNA were sequenced. Sinoatrial nematodes were found in two of 1232 JD (0.2%) and nine of 110 PTM (8.2%). Worms caused dilation or aneurysm of the sinus venosus. One JD had severe sinus venosus phlebitis. Morphologic, histologic and molecular features were diagnostic for Contracaecum spp. This is the first identification of larval Contracaecum in PTM, the first record of an intravascular nematode in this species, and the first documentation of vascular localization of Contracaecum larvae in JD. Vascular pathology could result in increased susceptibility to predation and favour the completion of the nematode life cycle. Parasites could become detrimental to population survival, especially those that are stressed by ecological and anthropogenic factors.