Morphological description and multilocus genotyping of Onchocerca spp. in red deer (Cervus elaphus) in Switzerland

Onchocercosis is a parasitic disease caused by over 30 Onchocerca spp. (Nematoda: Filarioidea) and predominantly affecting ungulates. Four Onchocerca spp. have been described in the European red deer (Cervus elaphus). Onchocerca flexuosa and Onchocerca jakutensis form subcutaneous nodules in the back region. The other two species, Onchocerca skrjabini and the lesser-known Onchocerca garmsi, are found freely in the subcutaneous tissue of carpal and tarsal joints, and the sternal region, respectively. The presence of Onchocerca spp. in eight red deer shot in the hunting season during September 2020 in the Grisons region, Switzerland, was investigated by analysing nodules and free worms in the subcutaneous tissue. The obtained worms were morphologically and genetically identified as O. jakutensis, O. flexuosa and O. skrjabini. The latter two are first reports from Switzerland, and morphological redescriptions of these two species are presented. Onchocerca skrjabini and O. jakutensis are newly described from the sternal region of deer. One female of O. jakutensis was found free in the subcutaneous tissue of the sternal region, an atypical presentation for this species. Phylogenetic analyses were based on four mitochondrial and one nuclear loci, revealing that O. jakutensis belongs to a clade which so far only included non-cervid Onchocerca spp. Analysis of sequences from this study and GenBank entries revealed two distinct subpopulations of O. skrjabini: one from European red deer and another from Japanese serow and sika deer. Morphological identification can be challenging, also because worm location in the host is less strictly determined than previously described. Genetic identification is straightforward for O. flexuosa, O. jakutensis and O. skrjabini for which complete data of five loci are now available whereas genetic data of O. garmsi is still lacking.