Nodular onchocercosis in red deer (Cervus elaphus) in Sweden

The presence of subcutaneous nodular onchocercosis was investigated at slaughter of 151 red deer (Cervus elaphus) (107 juveniles and 44 adults) between October–December 2015. The prevalence of subcutaneous nodules was 56%. Nodules were located in the lumbar region of the back in 96% of the cases, and 38% of the infected red deer had additional parasitic nodules in other body locations, such as rump, thorax, forelimbs and neck. The number of nodules per deer was 1–10 in two-thirds of the affected animals, and only 2% had more than 50 nodules. Significant differences in prevalence of nodule presence were found between adult (81%) and juvenile red deer less than two years of age (45%) (p = 0.0001). Species confirmation and identification was done on 14 randomly selected nodules taken from the lumbar region of 14 deer of different geographical origin, by sequencing the mitochondrial 12S, 16S and nad5 gene fragments. The sequences were more or less identical (12S = 99–100%; 16S = 93–100%; nad5 = 92–100%) to previously published sequences for Onchocerca flexuosa. The high prevalence of nodular onchocercosis caused by O. flexuosa in red deer in Sweden shows that the investigated areas in the southern third of Sweden have favorable vector (simuliids and ceratopogonids) conditions and suitable environment for the maintenance of the parasite. To our knowledge, this is the first systematic study of O. flexuosa in red deer in Sweden.