Natural infections and distributions of parasitic mermithids (Nematoda: Mermithidae) infecting larval black flies (Diptera: Simuliidae) in tropical streams of Malaysia

•Nine of 42 simuliid species in Malaysia were parasitized by mermithid nematodes.•The number of mermithids per simuliid host larva was 1–3, with a mean of 1.10.•Mermithid distributions were patchy among simuliid hosts and streams.•Parasitism was significantly associated with environmental factors, e.g. temperature.•Patchy parasite distributions suggest strong host–parasite–environment interactions.