Abstract
The present work contributs to the knowledge on the aquatic mermithids (Nematoda, Mermithidae) occurring in black flies - an insufficiently studied group of parasitic nematodes. Isomermislairdi Mondet, Poinar & Bernadou, 1977, described from larvae of Simuliumdamnosum Theobald, 1903 in Western Africa, is reported to occur in Bulgaria. The species was isolated from larvae of Simuliumornatum Meigen, 1818 in a local population of simuliids in a mountain stream near Jeleznitsa Village, Sofia district. Postparasitic juveniles of mermithids were obtained from the hosts and reared to the adult stage. The species was identified by morphological and morphometrical characters of postparasitic juveniles, and of male and female individuals. In the summer of 2012 a relatively high rate of mermithid infection in a local host population was detected (prevalence up to 44.1%). In August of the next year host abundance had considerably declined and other simuliid species, Simuliumvariegatum Meigen, 1818 and Simuliumreptans (Linnaeus, 1758), predominated in the investigated locality. In West Africa, Isomermislairdi is considered to be a potential biological agent for reducing the population density of the Simuliumdamnosum complex - the main vector of human onchocerciasis. In Europe, species of the Simuliumornatum complex are among the vectors of onchocerciasis of cattle and deer. The mermithids presumably play a certain role in the epidemiology of these diseases. A brief discussion on the taxonomy of the genus Isomermis Coman, 1953, and of the feasibility of molecular methods in mermithid taxonomy is provided. The species Isomermislairdi is reported for the first time from Europe.