Import of exotic and zoonotic trematodes (Heterophyidae: Centrocestus sp.) in Xiphophorus maculatus: Implications for ornamental fish import control in Europe

Ornamental fish, Xiphophorus maculatus, were imported from Singapore to Denmark for distribution to local aquarists. Importers observed lethargic and erratic swimming patterns among fish and forwarded a total of 30 fish for pathological examination to a university diagnostic service. All fish were diagnosed infected with encysted Centrocestus sp. metacercariae in gills (prevalence of 100% and mean intensity of 454.5 ± 161.9 parasites per fish). Metacercariae were identified by morphological and molecular methods. Cysts (mean length 163.3 ± 13.7 μm and mean width 113.3 ± 10.6 μm) contained a bent metacercaria with an X-shaped excretory bladder. PCR amplification of a rDNA region (5.8S rRNA gene, ITS-2, 28S rRNA gene) and subsequent sequencing confirmed the diagnosis. Metacercariae were found in gill filaments adjacent to the cartilage associated with cartilage hypertrophy, epithelial and mucous cell hyperplasia, clubbing and lamellar fusion. Host cell encapsulation of cysts comprised several layers of leucocytes, chondroblast-like and fibroblast-like cells. The observations raise concerns with regard to veterinary inspection and quarantine procedures. The zoonotic potential of these trematodes and a possible spread of the parasites in natural habitats in Europe should be regarded as a public health issue. So far, several cases of human infections have been reported only in Asia, but the potential intermediate host snail, Melanoides tuberculata, has been recorded in Germany. Accordingly, establishment of the parasite in Europe with climate changes should be considered a risk.