Descriptive Pathological Study of Avian Schistosomes Infection in Whooper Swans (Cygnus cygnus) in Japan

SIMPLE SUMMARY: Avian schistosomes are a group of parasites responsible for most of the reported cases of cercarial dermatitis outbreaks. Among others, Trichobilharzia is considered the largest genus of avian Schistosomatidae, and it infects more than 40 avian species. The present study involves a descriptive pathological study of avian schistosome in 54 whooper swans (Cygnus cygnus) from various rescue/rehabilitation centers in Honshu, Japan. Interestingly, adult schistosomes were detected in the lumen of mesenteric, serosal, portal, and testicular veins, in the capillaries of the intestinal lamina propria, and in the sinusoids of the adrenal gland, spleen, and liver of 23 (42.59%) swans. Schistosomes were assumed to be Allobilharzia visceralis based on the morphological characteristics of the worm and eggs found at histopathological examination of internal organs, along with suggestive pathological findings as well as the pathological findings. Collectively, the present study provides novel descriptive pathological data about schistosome infection in whooper swans with new insights on their role in the transmission and spreading of avian schistosomes in Japan. ABSTRACT: Cercarial dermatitis, or Swimmer’s itch, is one of the emerging diseases caused by the cercariae of water-borne schistosomes, mainly Trichobilharzia spp. Since the zoonotic potential of Allobilharzia visceralis is still unknown, studies on this schistosome would be helpful to add knowledge on its possible role in causing human infections. In the present study, 54 whooper swans (Cygnus cygnus) from rescue/rehabilitation centers in Honshu, Japan, were necropsied to identify the cause of death. Grossly, 33 (61.11%) swans were severely emaciated and 23 (42.59%) had multiple reddened areas throughout the length of the intestine with no worms detected in the internal organs. Microscopically, adult schistosomes were found in the lumen of the mesenteric, serosal, portal, and testicular veins, in the capillaries of the intestinal lamina propria, and in the sinusoids of the adrenal gland, spleen, and liver of 23 (42.59%) swans. Hypertrophy of veins containing adult worms was identified in 15 (27.77%) swans, and vascular lumen obliteration was observed in 8 (14.81%) swans. Mild to severe villous atrophy and superficial enteritis were observed in 8 birds (14.81%), whereas bile pigments and hemosiderin were detected in the livers of 14 (25.92%) and 18 (33.33%) swans, respectively. In three swans (5.55%), schistosome parasites were found in the subcapsular veins of the testes. The schistosomes in the present study were assumed to be A. visceralis based on the microscopical and histological evidence of adult schistosomes found in the lumen of veins as well as the infection pathology, which was very similar to the schistosome-induced pathology previously reported in swans infected by A. visceralis in Europe and Australia. The swans examined herein most likely died from obstructive phlebitis associated with A. visceralis, but further molecular confirmation is required for identification of this species. However, the present study does not provide new data on the zoonotic potential, but only on the pathogenic potential of this schistosome in swans. Furthermore, our study provides a novel contribution to the description of the pathological effects of avian schistosomes infection in whooper swans in Japan.